The Plasma Magnet Drive: A Simple, Cheap Drive for the Solar System and Beyond

Can we use the outflow of particles from the Sun to drive spacecraft, helping us build the Solar System infrastructure we’ll one day use as the base for deeper journeys into the cosmos? Jeff Greason, chairman of the board of the Tau Zero Foundation, presented his take on the idea at the recent Tennessee Valley Interstellar Workshop. The concept captured the attention of Centauri Dreams regular Alex Tolley, who here analyzes the notion, explains its differences from the conventional magnetic sail, and explores the implications of its development. Alex is co-author (with Brian McConnell) of A Design for a Reusable Water-Based Spacecraft Known as the Spacecoach (Springer, 2016), focusing on a new technology for Solar System expansion. A lecturer in biology at the University of California, he now takes us into a different propulsion strategy, one that could be an enabler for human missions near and far.

by Alex Tolley

Suppose I told you that a device you could make yourself would be a more energy efficient space drive than an ion engine with a far better thrust to weight ratio? Fantasy? No!

Such a drive exists. Called the plasma magnet, it is a development of the magnetic sail but with orders of magnitude less mass and a performance that offers, with constant supplied power, constant acceleration regardless of its distance from the sun.

At the recent Tennessee Valley Interstellar Workshop (TVIW), Jeff Greason presented this technology in his talk [1]. What caught my attention was the simplicity of this technology for propulsion, with a performance that exceeded more complex low thrust systems like ion engines and solar sails.

What is a plasma magnet?

The plasma magnet is a type of magsail that creates a kilometers wide, artificial magnetosphere that deflects the charged solar wind to provide thrust.

Unlike a classic magsail [9] (figure 1) that generates the magnetic field with a large diameter electrical circuit, the plasma magnet replaces the circular superconducting coil by inducing the current flow with the charged particles of the solar wind. It is an upgraded development of Robert Winglee’s Mini-Magnetospheric Plasma Propulsion (M2P2) [7, 8], a drive that required injection of charged particles to generate the magnetosphere. The plasma magnet requires no such injection of particles and is therefore potentially propellantless.

Figure 1. A triple loop magsail is accelerated near Jupiter. Three separate boost beams transfer momentum to the rig, carefully avoiding the spacecraft itself, which is attached to the drive sail by a tether. Artwork: Steve Bowers, Orion’s Arm.

Developed by John Slough and others [5, 6], the plasma magnet drive has been validated by experimental results in a vacuum chamber and was a NIAC phase 1 project in the mid-2000s [6]. The drive works by initially creating a rotating magnetic field that in turns traps and entrains the charged solar wind to create a large diameter ring current, inducing a large scale magnetosphere. The drive coils of the reference design are small, about 10 centimeters in diameter. With 10 kW of electric power, the magnetosphere expands to about 30 kilometers in diameter at 1 AU, with enough magnetic force to deflect the solar wind pressure of about 1 nPa (1 nN/m2) which produces a thrust in the direction of the wind of about 1 newton (1N). Thrust is transmitted to the device by the magnetic fields, just as with the coupling of rotation in an electric motor (figure 2).

For a fixed system, the size of the induced magnetosphere depends on the local solar wind pressure. The magnetosphere expands in size as the solar wind density decreases further from the sun. This is similar to the effect of Janhunen’s electric sail [2] where the deflection area around the charged conducting wires increases as the solar wind density decreases. The plasma magnet’s thrust is the force of the solar wind pushing against the magnetosphere as it is deflected around it. It functions like a square-rigged sail running before the wind.

Figure 2. Plasma magnetic sail based on rotating magnetic field generated plasma currents. Two polyphase magnetic coils (stator) are used to drive steady ring currents in the local plasma (rotor) creating an expanding magnetized bubble. The expansion is halted by solar wind pressure in balance with the magnetic pressure from the driven currents (R >= 10 km). The antennas (radius ~ 0.1 m) are shown expanded for clarity. [6]

The engine is little more than 2 pairs of charged rotating coils and is therefore extremely simple and inexpensive. The mass of the reference engine is about 10 kg. Table 1 shows that the plasma magnet has an order higher thrust to weight ratio than an ion engine and 2 orders better than a solar sail. However, as the plasma magnet requires a power source, like the ion engine, the comparison to the solar sail should be made when the power supply is added, reducing is performance to a 10-fold improvement. [ A solar PV array of contemporary technology requires about 10 kg/kW, so the appropriate thrust/mass ratio of the plasma magnet is about 1 order of magnitude better than a solar sail at 1 AU]

The plasma magnet drive offers a “ridiculously high” thrust to weight ratio

The plasma magnet, as a space drive, has much better thrust to weight ratio than even the new X-3 Hall Effect ion engine currently in development. This ratio remains high when the power supply from solar array is added. Of more importance is that the plasma magnet is theoretically propellantless, providing thrust as long as the solar wind is flowing past the craft and power is supplied.

NameTypeThrust/weight (N/kg)
Engine mass only
Thrust/weight (N/kg) with power supply
plasma magnetosphereElectro-magnetic0.1.01
NSTAR-1Ion (Gridded)0.0040.002
X-3Ion (Hall Effect)0.020.004
Solar SailPhoton Sail0.001 (at 1 AU)N/A

Table 1. Comparison of thrust to mass ratios of various types of propulsion systems. The power supply is assumed to be solar array with a 10 kg/kW performance.

The downside with the plasma magnet is that it can only produce thrust in the direction of the solar wind, away from the sun, and therefore can only climb up the gravity well. Unlike other propulsion systems, there is little capability to sail against the sun. While solar sails can tack by directing thrust against the orbital direction, allowing a return trajectory, this is not possible with the basic plasma magnet, requiring other propulsion systems for return trips.

Plasma magnet applications

1. Propulsion Assist

The most obvious use of the plasma magnet that can only be used to spiral out from the sun is as a propellantless assist. The drive is lightweight and inexpensive, and because it is propellantless, it can make a useful drive for small space probes. Because the drive creates a kilometers sized magnetosphere, scaling up the thrust involves increased power or using multiple drives that would need to be kept 10s of kilometers apart. Figure 3 shows a hypothetical gridded array. Alternatively, the plasma magnets might be separated by thrusters and individually attached to the payload by tethers.

Figure 3. Plasma magnets attached to the nodes in a 2D grid could be used to scale up the thrust. The spacecraft would be attached by shroud lines as in a solar sail with a trailing payload. Scaling up the power supply to create a larger magnetosphere is also possible.

For a mixed mode mission, the plasma magnet engine is turned on for the outward bound flight, with or without the main propulsion system turned on. The use of power to generate thrust without propellant improves the performance of propellant propulsion systems where the accumulated velocity exceeds the performance cost of the power supply mass or reduced propellant. For an ion engine as the main drive, the plasma magnet would use the same power as 4 NSTAR ion engines but provide 3x the thrust.

2. Moving Asteroids for Planetary Defense

The propellantless nature of the plasma magnet drive makes it very suitable for pushing asteroids for planetary defense. Once turned on, the drive provides steady thrust to the asteroid, propelling it away from the sun and raising its orbit. Because the drive does not need to be facing any particular direction, it can be attached to a tumbling asteroid without any impact on the thrust direction.

3. Charged particle radiation shield for crewed flights

The magnetosphere generated by the engine makes a good radiation shield for the charged particles of the solar wind. It should prove to be a good solution for the solar wind, solar flares and even coronal mass ejections (CME). This device could, therefore, be used for human flight to reduce radiation effects. For human crewed flights, the 1N of thrust is insufficient for the size of the spacecraft and would have a marginal propulsion compared to the main engines. Given the plasma magnet’s small size and mass, and relatively low power requirements, the device provides a cost-effective means to protect the crew without resorting to large masses of physical shielding. The plasma magnet would appear to be only effective for the charged solar wind, leaving the neutral GCRs to enter the craft. However, when an auxiliary device is used in the mode of aerobraking, the charge exchange mechanism should reduce the galactic cosmic ray (GCR) penetration (see item 8 below).

4. Asteroid mining

The plasma magnet thruster might be a very useful part of a hybrid solution for automated mining craft. The hybrid propulsion would ally the plasma magnet thruster with a propellant system, such as a chemical or ion engine. The outward bound trip would use the plasma magnet thruster to reach the target asteroid. The propellant tanks would be empty saving mass and therefore improving performance. The propellant tanks would be filled with the appropriate resource, e.g. water for an electrothermal engine, or for a L2/O2 chemical engine. This engine would be turned on for the return trip towards the inner system. The reverse would be used for outward bound trips to the inner system

5. Interstellar precursor using nuclear power

A key feature of the plasma magnet is that the diameter of the magnetosphere increases as the density of the solar wind decreases as it expands away from the sun. The resulting expansion exactly matches the decrease in density, ensuring constant thrust. Therefore the plasma magnet has a constant acceleration irrespective of its position in the solar system.

As the solar wind operates out to the heliopause, about 80 AU from the sun, the acceleration from a nuclear powered craft is constant and the craft continues to accelerate without the tyranny of the rocket equation. Assuming a craft with an all up mass of 1 MT (700 kg nuclear power unit, 10 kg engine, and the remaining in payload), the terminal velocity at the heliopause is 150 km/s. The flight time is 4.75 years, which is a considerably faster flight time than the New Horizons and Voyager probes.

Slough assumed a solar array power supply, functional out to the orbit of Jupiter at 5 AU. This limited the velocity of the drive, although the electrical power output of a solar array at 1 AU is about 10-fold better than a nuclear power source, but rapidly decreases with distance from the sun. Assuming a 10 kW PV array, generating decreasing power out to Jupiter, the final velocity of the 1 MT craft is somewhere between 5 and 10 km/s, but with a much larger payload.

In his TVIW talk [1], Greason suggested that the 10kW power supply could propel a 2500 kg craft with an acceleration of 0.5g, reaching 400-700 km/s in just half a day. Greason [i] suggested that with this acceleration, the FOCAL mission for gravitational lens telescopes requiring many craft should be achievable. *

6. Mars Cycler

Greason suggested that the plasma magnet might well be useful for a Mars cycler, as the small delta v impulse needed for each trip could be easily met.[1]

7. Deceleration at target star for interstellar flight

For interstellar flights, deploying the plasma magnet as the craft approaches the target star should be enough to decelerate the craft to allow loitering in the system, rather than a fast flyby. Again, the high performance and modest mass and power requirements might make this a good way to decelerate a fast interstellar craft, like a laser propelled photon sail.[1]

8. Magnetoshell Aerocapture (MAC)

While the studies on the plasma magnet seemed to have stalled by the late 2000s, a very similar technology development was gaining attention. A simple dipole magnet magnetosphere can be used as a very effective aerocapture shield. The shield is just the plasma magnet with coils that do not rotate, creating a magnetosphere of a diameter in meters, one that requires the injection of gram quantities of plasma to be trapped in the magnetic field. As the magnetosphere impacts the atmosphere, the neutral atmosphere molecules are trapped by charge exchange. The stopping power is on the order of kilonewtons, allowing the craft to achieve orbit and even land without a heavy, physical shield. The saving in mass and hence propellant is enormous. Such aerobraking allows larger payloads, or alternatively faster transit times. Because the magnetoshell is immaterial, heat transmission to the shield is not an issue. The mass saving is considerable and offers a very cost-effective approach for any craft to reduce mass, propellant requirements or increase payloads. This approach is suitable for Earth return, Mars, outer planets, and Venus capture. Conceivably aerocapture might be possible with Pluto.

Figure 4. A dipole magnet creating a small diameter magnetic field is injected with plasma. As the magnetosphere impacts the atmosphere, charge exchange result in kilonewton braking forces. The diagram at left shows the craft with the training magnetosphere impacting the atmosphere. The painting on the right shows what such a craft might look like during an aerobraking maneuver. Source: Kirtley et al [3].

Making the plasma magnet thrust directional

A single magnetosphere cannot deflect the solar wind in any significant directional way, which limits this drive’s navigational capability. However, if the magnetosphere could be shaped so that its surface could result in an asymmetric deflection, it should be possible to use the drive for tacking back to the inner system.

Figure 5 shows an array of plasma magnets orientated at an angle to the solar wind. The deflection of the solar wind is no longer symmetric, with the main flow across the forward face of the array. Under those conditions, there should be a net force against the grid. This suggests that like a solar sail, orientating the grid so that the force retards the orbital velocity, the craft should be able to spiral down towards the Sun, offering the possibility of a drive that could navigate the solar system.

Figure 5. A grid of plasma magnets deflects the flow of the solar wind, creating a force with a component that pushes against the grid. If the grid is in orbit with a velocity from right to left, the force will reduce the grid’s velocity and result in a spiral towards the Sun.

Pushing the Boundaries

The size of the magnetic sail can be increased with higher power inputs, or increasing the antenna size. Optimization will depend on the size of the craft and the mass of the antenna. Truly powerful drives can be considered. Greason [12] has calculated that a 2 MT craft, using a superconducting antenna with a radius of 30 meters, fed with a peak current of 90 kA, would have a useful sail with a radius of 1130 km and an acceleration of 2 m/s2, or about 0.2g. As the sail has a maximum velocity of that of the solar wind, a probe accelerating at 0.2g would reach maximum velocity in a few days, and pass by Mars within a week. To reach a velocity of 20 km/s, faster than New Horizons, the Plasma magnet would only need to be turned on for a few hours. Clearly, the scope for using this drive to accelerate probes and even crewed ships is quite exciting.

Coupling a more modest velocity of just 10’s of km/s with the function of a MAC, a craft could reach Mars in less than 2 months and aerobrake to reach orbit and even descend to the surface. All this without propellant and a very modest solar array for a power supply.

An Asteroid, a tether and a Round Trip Flight

As we’ve seen, the plasma magnet can only propel a craft downwind from the Sun. So far I have postulated that aerobraking and conventional drives would be needed for return flights. One outlandish possibility for use in asteroid mining might be the use of a tether to redirect the craft. On the outward bound flight, the craft driven by the plasma magnet makes a rapid approach to the target asteroid which is being mined. The mined resources are attached to a tether that is anchored to the asteroid. As the craft approaches, it captures the end of the tether to acquire the new payload, and is swung around the asteroid. On the opposite side of the asteroid, the tether is released and the craft is now traveling back towards the Sun. No propellant needed, although the tether might cause some consternation as it wraps itself around the asteroid.


The plasma magnet as a propulsion device, and the same hardware applied for aerocapture, would drastically reduce the costs and propellant requirements for a variety of missions. Coupled with another drive such as an ion engine, a craft could reach a target body with an atmosphere and be injected into orbit with almost no propellant mass. The return journey would require an engine delivering just enough delta V to escape that body and return to Earth, where aerocapture again would allow injection into Earth orbit with no extra propellant. If direction deflection can be achieved, then the plasma magnet might be used to navigate the Solar System more like a solar sail, but with a far higher performance, and far easier deployment.

Using a steady, nuclear power or beamed power source, such a craft could accelerate to the heliopause, allowing interstellar precursor missions, such as Kuiper belt exploration and the FOCAL mission within a short time frame.

The technology of the plasma magnet combined with a MAC could be used to decelerate a slowish interstellar ship and allow it to achieve orbit and even land on a promising exoplanet.

The size of the magnetic sail can be extended with few constraints, allowing for considerably increased thrust that can be applied to robotic probes and crewed spacecraft. For crewed craft, the magnetosphere also provides protection from the particle radiation from the sun, and possibly galactic cosmic rays.

Given the potential of this drive and relatively trivial cost, it seems that testing such a device in space should perhaps be attempted. Can a NewSpace billionaire be enticed?

* These numbers are far higher than those provided by Winglee and Slough in their papers and so I have used their much more conservative values for all my calculations.


Greason, Jeff “Missions Enabled by plasma magnet Sails”, Presentation at the Tennessee Valley Interstellar Workshop, 2017.

Janhunen, P., The electric sail – a new propulsion method which may enable fast missions to the outer solar system, J. British Interpl. Soc., 61, 8, 322-325, 2008.

Kelly, Charles and Shimazu, Akihisa “Revolutionizing Orbit Insertion with Active Magnetoshell Aerocapture,” University of Washington, Seattle, WA, 98195, USA.

Kirtley, David, Slough, John, and Pancotti, Anthony “Magnetoshells Plasma Aerocapture for Manned Missions and Planetary Deep Space Orbiters”, NIAC Spring Symposium, Chicago, Il., March 12, 2013

Slough, John. “The plasma magnet for Sailing the Solar Wind.” AIP Conference Proceedings, 2005, doi:10.1063/1.1867244.

Slough, John “The plasma magnet” (2006). NASA Institute for Advanced Concepts Phase 1 Final Report.

Winglee, Robert. “Mini-Magnetospheric Plasma Propulsion (M2P2): High Speed Propulsion Sailing the Solar Wind.” AIP Conference Proceedings, 2000, doi:10.1063/1.1290892.

Winglee, R. M., et al. “Mini-Magnetospheric Plasma Propulsion: Tapping the Energy of the Solar Wind for Spacecraft Propulsion.” Journal of Geophysical Research: Space Physics, vol. 105, no. A9, Jan. 2000, pp. 21067-21077., doi:10.1029/1999ja000334.

Zubrin, Robert, and Dana Andrews. “Magnetic Sails and Interplanetary Travel.” 25th Joint Propulsion Conference, Dec. 1989, doi:10.2514/6.1989-2441.

Greason, Jeff. Personal communication.


Solar System Formation near a Massive Star

An unusual type of star may be showing us something about the origin of our own Solar System. Wolf-Rayet stars display unusual spectra, prominent in which are heavy elements as well as broad emission lines of ionized helium, nitrogen and carbon. These are massive objects 40 to 50 times the size of our Sun, with surface temperatures ranging up to 200,000 K. Have a look at one of these, showing another Wolf-Rayet trait, the strong stellar winds ejecting material into nearby space. A bubble with a dense shell forms around such stars, trapping gas and dust that could form into new stars.

Image: Here we see the spectacular cosmic pairing of the star Hen 2-427 — more commonly known as WR 124 — and the nebula M1-67 which surrounds it. Both objects, captured here by the NASA/ESA Hubble Space Telescope, are found in the constellation of Sagittarius and lie 15,000 light-years away. The star Hen 2-427 shines brightly at the very centre of this explosive image and around the hot clumps of gas that are ejected into space at over 150,000 kilometres per hour. Hen 2-427 is a Wolf-Rayet star, named after the astronomers Charles Wolf and Georges Rayet. Wolf-Rayet are super-hot stars characterized by a fierce ejection of mass. The nebula M1-67 is estimated to be no more than 10,000 years old — just a baby in astronomical terms — but what a beautiful and magnificent sight it makes. A version of this image was released in 1998, but has now been re-reduced with the latest software. Credit: ESA/Hubble.

Vikram Dwarkadas (University of Chicago) and colleagues believe that Wolf-Rayet stars can unlock the mystery of how our Solar System emerged. The researchers are hoping to update the older view that our system formed in the vicinity of a relatively conventional supernova, noting peculiarities in the proportion of two isotopes in the early Solar System. One of these is aluminium-26, which turns up in relatively high proportion in our system compared with the rest of the galaxy.

The other issue is with iron-60, which earlier work by Nicolas Dauphas, a co-author on the current paper, suggests is found in smaller amounts than we would expect. We couple this with the interesting fact that Wolf-Rayet stars release a good deal of aluminium-26, but are not associated with iron-60. Add into the mix the giant stars’ ability to shed mass through intense stellar winds. We wind up with a bubble structure with a dense shell, a potential star-making factory. Dwarkadas and team estimate that 1 to 16% of Sun-like stars could form in this way.


Image: This simulation shows how bubbles form over the course of 4.7 million years from the intense stellar winds off a massive star. UChicago scientists postulated how our own Solar System could have formed in the dense shell of such a bubble. Credit: V. Dwarkadas & D. Rosenberg.

It’s an interesting explanation because we would expect both isotopes to be produced in the kind of supernova long held to have provided materials for the infant Solar System. Given the proportions we actually find in meteorites from the early system, the question becomes why one isotope is found in the days of system formation while the other was not. Says Dwarkadas:

“The idea is that aluminum-26 flung from the Wolf-Rayet star is carried outwards on grains of dust formed around the star. These grains have enough momentum to punch through one side of the shell, where they are mostly destroyed—trapping the aluminum inside the shell.”

Over time, the shell begins to collapse inward due to gravity, forming our Solar System. The original Wolf-Rayet star is long gone, doubtless through a supernova explosion or, the authors note, through direct collapse into a black hole. The latter would produce little iron-60, while the former could have trapped any iron-60 formed in the supernova within the bubble walls.

Image: Slices of a simulation showing how bubbles around a massive star evolve over the course of millions of years (moving clockwise from top left). Credit: V. Dwarkadas & D. Rosenberg.

The paper is Dwarkadas et al., “Triggered Star Formation inside the Shell of a Wolf-Rayet Bubble as the Origin of the Solar System,” Astrophysical Journal Vol. 851, No. 2 (22 December 2017). Abstract available. The earlier paper on iron-60 is Tang & Dauphas, “Low 60FE Abundance in Semarkona and Sahara 99555,” Astrophysical Journal Vol. 802, No. 1 (17 March 2015). Abstract available.


Dragonfly: Contemplating a Return to Titan

Our continuing interest in Titan as a possible venue for life was energized last year with the publication of a paper by Martin Rahm and Jonathan Lunine, working with colleagues David Usher and David Shalloway (all at Cornell University). I’ve written about this one before (see Prebiotic Chemistry on Titan?) and won’t revisit the details, but the gist is that hydrogen cyanide produced in Titan’s atmosphere can condense into aerosols that are transformed into interesting polymers on the surface. Of these, the most intriguing seems to be polyimine.

The authors see polyimine as capable of producing complex, ordered structures that absorb light, producing energy that can be used to catalyze prebiotic chemistry. Rather than looking in Titan’s seas, the authors think we’ll find hydrogen cyanide reactions in tidal pools on the shores near seas and lakes. It’s an interesting proposition, and like so many notions about Titan, it requires us to get a payload back to the surface, as we did in 2005 with Huygens. But this time, we’ll want to have extended survivability on Titan and a full suite of instruments.

Image: This composite was produced from images returned on 14 January 2005, by ESA’s Huygens probe during its successful descent to land on Titan. It shows the boundary between the lighter-coloured uplifted terrain, marked with what appear to be drainage channels, and darker lower areas. These images were taken from an altitude of about 8 kilometres with a resolution of about 20 metres per pixel. Credits: ESA/NASA/JPL/University of Arizona.

Thus the news that Dragonfly has won approval as a finalist concept for a robotic launch to Titan in the mid-2020s is encouraging. Dragonfly offers not just a useful instrument package but mobility on the surface in the form of a rotorcraft that could explore numerous sites on the moon. We have to be creative indeed in imagining life that would exist at -180 degrees Celsius in an environment that gets a tenth of one percent of the sunlight Earth’s surface receives. But as Rahm, Lunine and colleagues have reminded us, mechanisms may exist to make it happen.

Elizabeth Turtle (JHU/APL) is lead investigator on Dragonfly, with APL providing project management. The concept involves an eight-bladed drone — two rotors at each of its four corners — capable of sampling widely. Dragonfly would be able to look at prebiotic chemistry of the kind Rahm and Lunine have studied, selecting sites with varying geology and surface composition.

Another key issue for the lander: Is there exchange of organics between the surface and Titan’s interior ocean? Using a Multi-mission Radioisotope Thermoelectric Generator (MMRTG) for power, Dragonfly should be capable of remaining operational not just for months but for years in answering these questions.

Image: Dragonfly is a dual-quadcopter lander that would take advantage of the environment on Titan to fly to multiple locations, some hundreds of kilometers apart, to sample materials and determine surface composition to investigate Titan’s organic chemistry and habitability, monitor atmospheric and surface conditions, image landforms to investigate geological processes, and perform seismic studies. Credit: NASA.

NASA’s competitive peer review process whittled a dozen proposals submitted under the New Frontiers program announcement of opportunity down to a final two, the other being a Comet Astrobiology Exploration Sample Return (CAESAR). Here we’re talking about a return to 67P/Churyumov-Gerasimenko, following up the European Space Agency’s highly successful Rosetta mission. Both CAESAR and Dragonfly will receive funding through the end of 2018. One of the concepts will be selected the following year for subsequent mission phases.

Expect more on CAESAR in an upcoming article. The Rahm et al. paper mentioned above is “Polymorphism and electronic structure of polyimine and its potential significance for prebiotic chemistry on Titan,” published online by Proceedings of the National Academy of Sciences 4 July 2016 (full text). Matt Williams interviews Elizabeth Turtle about Dragonfly in a fine Universe Today article from May of this year.


Interstellar Communication Using Microbes: Implications for SETI

Mention Robert Zubrin’s name and the planet Mars invariably comes up, given his long-time work on finding ways to establish a human presence there. Dr. Zubrin is the originator of Mars Direct, the author of The Case for Mars, and founder of the Mars Society. But his work on interstellar matters is likewise significant, including the analysis, with Dana Andrews, of the Bussard ramjet, which taught us much about magsails and drag, offering a useful way to re-think starship braking at destination. Another key Zubrin creation is the nuclear salt water rocket concept. With over 200 papers and five books to his credit, he runs Pioneer Astronautics, where he continues to focus on innovative aerospace technologies. Today’s essay goes in another direction, with a fresh look at interstellar communications using microscopic data carriers. Ponder now how information can be conveyed star to star, and how we might find it by methods far removed from conventional SETI.

by Robert Zubrin


Since the dawn of the SETI effort in 1960, it has been generally assumed that the transmission of information across interstellar distances can most practically be accomplished using electromagnetic waves, with the most popular candidate method being radio in the 21-cm wavelength (1.42 GHz) range. Accordingly, a series of searches based on this assumption have been conducted, thus far without any success. In this paper, we will advance a hypothesis that the reason for this failure is because radio is, in fact, a very inefficient means of interstellar communication between species, and that a superior alternative is available. Specifically, we will show that communication between species can be much more effectively accomplished over interstellar distances using microscopic high density data storage packages sized between 1 and 10 microns. Such packages have already been detected. They are, in fact all around us, and within us, in vast numbers and varieties. Generally known as bacteria, these spaceflight-capable data storage systems are carrying enormous amounts of information, only a small fraction of which has any identifiable purpose. Could interstellar messages be found encoded within the genomes of microbes? Could records of such past transmission be found within the genomes of multicellular organisms? In this paper we shall explore the possibilities, discuss how such transmissions could be efficiently sent, and propose methods by which such a hypothesis might be falsified or verified.


It is a general principle of science that the laws of nature should apply equally well throughout the universe. Specifically, since the dawn of modern science in the Renaissance under the philosophical banner “as above, so below” it has been taken as axiomatic that the laws of nature that prevail elsewhere in the universe should also apply on Earth, and those that apply on Earth should apply equally well elsewhere. This being the case, it necessarily follows that if life and intelligence could develop from physics and chemistry via natural processes on Earth, it should also have done so in innumerable other equally satisfactory physical and chemical environments throughout the cosmos. Indeed, the early Earth at the time of life’s first appearance immediately following the end of the heavy bombardment was in no way exceptional. Furthermore, the processes by which life can develop from simple forms to complexity and intelligence are, in broad outline, well understood. While the universe is vast in space, it also is so in time, so that a spacecraft traveling at a velocity of 0.0001 c (the speed of the Earth in its travel around the Sun) could in 4.5 million years (0.1% the age of the Earth) travel 450 light years, a radius encompassing approximately 1 million stars, with surely enough candidates for many additional origins for life and civilization. Despite these favorable odds, no such extraterrestrials have been detected, a mystery leading the physicist Enrico Fermi to ask his famous paradoxical question at a 1950 Los Alamos lunchtime meeting: “So then, where are they?”

One possible reply to the Fermi Paradox is that they are out there, but that if you want to find them you need to look for them in the right way. In 1959, Cornell physicists Phil Morrison and Giuseppe Cocconi (Cocconi and Morrison, 1959) proposed that extraterrestrials might be communicating across space using 1.42 GHz radio, as the emissions of hydrogen gas at that frequency make it the most listened-to band in radio astronomy. Moreover, it is approximately the same frequency as the L-band and S-band radio systems that were becoming state of the art for spacecraft communication at that time, a fact which added credence to the Morrison-Cocconi hypothesis, and made it readily testable as well. Accordingly, shortly thereafter astronomer Frank Drake attempted to detect such signals from Tau Ceti, without success. Undeterred, Drake, his co-workers, competitors, and successors continued with many such search searches, from that time down to the present, where the SETI Institute, among others, is continuing the effort on a greatly expanded basis with vastly improved instrumentation, but no better results.

Given this failure, it is appropriate to revisit the assumptions behind the Morrison-Cocconi hypothesis suggesting interstellar communication via S-band. Certain of its supports have already been falsified by technological progress, in that, a mere 60 years later, S-band is already obsolete. Instead our spacecraft now communicate at higher frequencies, such as X-band (10 GHz) and Ka-band (30 GHz), as these higher frequencies allow for higher bandwidths for spacecraft communication systems of a given size and power.

But while the Morrison-Cocconi hypothesis, and resulting search, can be adjusted to take into account such improvements, there are deeper problems. The first of these is that communicating effectively across interstellar distances via radio is incredibly hard and inefficient. To see this, let us consider what it would take to design such a system.

The Mars Reconnaissance Orbiter (MRO), launched in 2005, has a modern 100 W X-band communication system. Equipped with a 2-m diameter dish, it can transmit at a rate of 6 Mb/s to a 70-m receiving system on Earth at a distance of 100 million km. A modest range for effective interstellar communication would be 10 light years, or 100 trillion km. At this million-fold greater distance, the MRO communication system would have its data rate reduced by a factor of a trillion, from 6 Mb/s to 6 microbits/s, or 200 b/year. This would appear to be too slow for practical purposes, so let’s upgrade the transmitter power to 1 GW and its dish size to 70 m. Taken together these upgrades would increase the data rate by a factor of 10 billion, to 60 kb/s, which would be fine. The upgraded system would have a capability of 600 b/s at 100 ly, which is still sufficient to be useful, as those who can remember working with computer modems in the 1980s can readily attest.

Assuming that the transmitter was X-band, the diameter of the beam at 10 ly would be about 2500 AU, so it would encompass the whole solar system, and then some, but the closest neighbor solar system would be 100 times further away that the width of the beam. So ET’s 1 GW transmitter could only be used to signal one solar system at a time. How would they know who to signal?

A good place to start would be to only send signals to planets manifesting a biosphere. These could be detected using astronomical techniques by observing the spectral signal of free oxygen in the planet’s atmosphere. But Earth, for example, would have provided a positive answer to such a search criterion for the past 500 million years, but only has only possessed a species able to receive and detect such a signal for the past 50 years. Based on these odds, ET would have to set up 1 million transmitters to living planets have a 10% chance that one of its signal units would be sending at the right time. This would require a transmitter system power of 1000 TW, about two orders of magnitudes higher than the total power produced by human civilization today. If they wanted to keep their odds as good, but reduce the number of transmitters, they could do so, but only at the cost of continuing to power the transmission program for millions of years. Furthermore, to receive the signal, the inhabitants of the target planet would have to be focusing their 70-m dish at precisely the right star at the right time, be listening at the right frequency, and be technologically and intellectually prepared to recognize and decipher the signal. Using lasers instead of radio to transmit would reduce the power requirement significantly, but it would still be huge, and furthermore, it would impose a requirement that the receiving civilization have a giant telescope focused on the transmitter’s narrow beam, at the right time, with its optics limited to the transmitting frequency to avoid having the transmitter outshined by its home system’s star. This seems like a rather farfetched hope upon which to expect ET to spend such a large investment in infrastructure and energy.

Surely there must be a more efficient approach. What could it be?

Interstellar Data Transmission by Microbial Storage Drives

One problem of data transmission by radio is that it occurs in real time, leaving no record behind. Once the transmission is over, it’s gone. If the intended recipients miss it the first time, they’ve missed it for all time. As a result, the transmitting party is forced to transmit repeatedly, perhaps endlessly, in the hope that one at least one occasion, someone might be listening.

Imagine that you want to tell a story to children. So you walk onto the front porch of a house and tell the story, whether any children are there are not. In fact, they are usually out running around, so you probably have missed your audience. But the odds against you are worse, because while the house is suitable for children, they may not have been born yet, or those that are there may be too young to understand your story, or they could have grown up and moved out. Now, you could increase your odds by going from porch to porch, reading the story again and again in the hope that someone might be there to listen. But this could get exhausting. A better strategy would be to go about slipping story books into houses through their mail slots. Even if most of them ended up on the shelf or sold to used book stores, sooner or later many of them would likely get read. The only problem with this strategy is its cost; giving out a lot of storybooks could be expensive. But if you could get them for free, and have them delivered for you cheaply, it could be a very good approach, allowing you to reach many children not only today, but for decades and generations to come.

In short, I am suggesting, in agreement with Davies (Davies, 2010), that rather than transmit information across interstellar distances using radio waves, that solid objects containing records be used instead. This may seem like it would be very inefficient, but in fact every planetary civilization orbiting a star has available to it an engine that it can use to send information across space at little or no energy cost. This is the star itself.

Let us consider our own Sun as a case in point. At 1 AU, the sun attracts objects to it with a gravitational acceleration of 0.006 m/s2. It also repels objects from it via its light pressure with a force of 0.000009 N/m2. If these two forces are equal, and object will feel no attraction from the Sun, and fly out of the solar system in a straight line with the Earth’s velocity of 30 km/s. (Both of these forces change with the inverse square of their distance from the Sun, so if they are equal at 1 AU, they remain equal at any distance.) Assuming the object has a radius r and a density d, and setting these two forces as being equal we find:

If d = 1000 kg/m3 (water density), we find that r = 1 micron. This means that such an object would have a diameter of 2 microns. However, it is not necessary to cancel all the force of gravity to escape the solar system. If half of the gravitational attraction is cancelled, an object with the orbital velocity for the full gravity object will escape. In the case above, this would imply a maximum diameter of 4 microns.

If the star were brighter than the Sun compared to its gravity, which would be the case with F stars, the objects could be somewhat larger. If it were dimmer than the Sun compared to its gravity, as would be the case with K stars, the objects would be need to be smaller. If their initial orbits were elliptical, rather than circular, the objects could be bigger. So, depending on assumed conditions, the diameter of the objects could range from 1 to 10 microns, and be readily projectible across interstellar distances using no other mechanism than the pressure of the star’s light. This is precisely the size range of many typical bacteria. (Arrhenius, 1908) It is also possible that the solar wind, which moves at a velocity of about 500 km/s, could be used to propel particles out of the solar system at very high velocity. However, in order for a particle to interact effectively with the solar wind it would need be strongly magnetized, allowing it to function as a miniature magnetic sail. (Zubrin and Andrews, 1989.) Under normal circumstances even such a strongly magnetized particle would be propelled away from the Sun by the solar wind with less force than that provided by light pressure. During solar flare events, which greatly increase the force of the plasma wind emanating from the Sun, however, this could change radically.

In Table 1, we show the relationship between star type, population fraction, star mass, luminosity, orbital distance and velocity and maximum particle diameter for stellar system escape for stars of various types. In each case the launch planet is assumed to be in a circular orbit in the given star type’s habitable zone. It can be seen that for type F, G, and K stars, collectively amounting to about 22.5% of the stellar population, that starlight can readily propel bacteria sized objects to system escape velocity. At half the diameters shown the objects will be projected outward from the stellar system at the orbital velocity given, at less than half, still greater velocity. Objects larger than that shown will not escape the system by light pressure alone, but could still do so if the light pressure is enough to drive them into an elliptical orbit that intersects a large planet capable of delivering a gravity assist.

Upon reaching a destination solar system, the bacteria-sized particles could be decelerated using the same mechanism.

Table 1 Interstellar Particle Transmission Capabilities of Star Types.

In Table 1, the particles are assumed to be simple spheres. If more complex shapes are used, for example shapes with a spherical core surrounded by thinner wings, the core spheres can be thicker and still achieve the same velocities. For example, a sphere with a diameter of 4 microns surrounded by a disc with a diameter of 8 microns and a thickness of 1 micron would have the same surface/mass ratio as a simple sphere with a diameter of 1.2 microns, and would therefore be able to escape a K star. Such “microsailcraft” designs could be readily mass-produced artificially or potentially created through natural crystallization processes, as exemplified by snowflakes. They could be stabilized in a manner to effectively serve as sails either by spinning (as some snowflakes do), or by having inherently stable shapes, as exemplified by a badminton birdie.

So, bacteria can be projected across interstellar space at essentially no power cost to the transmitting party, beyond that required to launch them to planetary escape velocity. The latter could be accomplished either by artificial technological means that are well within our means today – and therefore clearly feasible for advanced extraterrestrials – or potentially through natural processes such as asteroidal impacts. They also may be cheaply mass produced. (McDowell, 2003) But can they carry useful information?

The answer is most certainly yes. The genetic material of individual common bacteria is estimated to contain between 130 kB to 14 MB of information. Current estimates that bacteria can be used to store data with a density of 900 Terabytes per gram, about 500 times the current state of the art electromagnetic hardware. This means that a bacterium 5 microns on a size could store about 120 kB of information. (Wilkins 2010, Herkewitz 2016) Taking 60 kB as typical, this means that a single bacterium can carry a record of information about equal in size to a 10,000-word (~30 page) booklet. In experiments done to date, scientists have demonstrated such capabilities by encoding entire books in DNA, and showing that bacteria can be made to replicate encoded information when they reproduce.(Ayre 2012). Most recently, researchers at Columbia University and the New York Genome project have shown that they can encode information with a density 215,000 Terabytes of information per gram in DNA, with some of the items successfully encoded including a movie, a computer operating system, and an Amazon gift card. (Service, 2017)

Traveling at a velocity of 30 km/s (0.0001 c), bacteria would take 100,000 years to fly 10 light years. This would expose them to cosmic ray doses between 1 and 10 Mrads, which is close to the limit for survivability of hardy microcrobial species such as radiodurans. This need not be a show stopper. A message sent using bacteria storage would no doubt use billions of individuals, and if even a few survived the trip the message could still get through. While ultraviolet light would kill unshielded bacteria in days, effective shielding against this hazard can be provided by a half micron of soot. (Hoyle, 1981). Such protection would be provided by design in any artificial microsailcraft, but could conceivably also occur naturally.

Such long trips might not be necessary, however. Once they reach a planet, bacteria will multiply to vast numbers. They can then be ejected again into space via cometary impact. Such impacts are most likely to occur during periods when a foreign star is passing through the Oort cloud of the bacteria’s home star, as such a passage would destabilize Oort cloud object orbits orbiting both stars, and thereby causing impacts to occur. Like frigates in the age of fighting sail, which could span the globe with their movements but only reach a few hundred yards with their guns, roving solar systems discharge their broadsides at each other only during rare close approaches. As a result, ejected bacteria might typically only need to travel distances on the order of 0.1 ly to reach a new planetary home, with radiation doses accordingly reduced by two orders of magnitude compared to those postulated above. Given the density of stars in our own region of the galaxy, and assuming a random velocity of stars relative to each other of 5 km/s, it can be shown that a star is likely to experience such a close encounter about once every 20 million years, a frequency strikingly close to the observed mean time of 26 million years between of mass extinctions on Earth. (Zubrin, 2001). It may be noted that microbes traveling embedded in impact debris would be well shielded from ultraviolet and soft x-rays, thereby increasing their survival odds. (Melosh, 1988, Hornek, Klaus, and Mancinelli, 2010)

The Milky Way galaxy is 13 billion years old. Allowing 3 billion years for several generations of early stars to seed the place with heavy elements, that leaves 10 billion years, or 500 stellar close-encounter doubling times of 20 million years each for life to spread from its first planet of origin to everywhere else. So the answer to Fermi’s paradoxical question is almost certainly this: They’re here.

The Purpose of Interstellar Communication

At this point, we need to reexamine the question of what might be the purpose of interstellar communication. Certainly, if a species were spacefaring and had sent out expeditions which established settlements in nearby solar systems, it would want to maintain communication to exchange or trade information between its various worlds. For such purposes, high-gain directed electromagnetic transmissions would be the most practical, as they are the fastest possible, the most secure, and all the required technological and linguistic conventions would be known and mutually understood between the parties involved.

But if we are talking about communication between different species originating in different and distant worlds, what is the point? In speculative SETI literature, it is frequently supposed that there are intelligent aliens out there, who for some reason want others to know that they exist, and therefore transmit signals such as the value of into the void, so that other smart folks won’t confuse them with astrophysical phenomenon. Then, assuming that someone picks up the signal, they will transmit back the value of e, or the golden mean, or some other special number, thereby completing the freemasonic handshake. This done, the two parties could then proceed to methodically expand their mutual vocabulary, eventually allowing them to exchange QST cards, recipes, celebrity gossip, novels, scientific theories, starship designs, and treaties of alliance against the barbarians from the Galactic rim.

However, as noted above, both experimental searches and theoretical considerations suggest that such a picture does not correspond to reality. Microbial data transmission is possible, but it does not lend itself to conversations of the types described above. Rather it is a superior method of interstellar broadcasting.

So the question is, what kind of information is really worth broadcasting – that is distributed as widely as possible to people who we don’t know and are not likely to hear back from? If human experience is any guide, the answer is propaganda. Think of Radio Free Europe, or its Cold War opposite number, Radio Moscow, for example, and their persistent messaging: “We are good. You should admire us. You should be like us. You should join us.” Another example would be the Gideons, placing Bibles in hotel rooms in the hope that their unknown readers would see the light and become Christians. We also try to broadcast ourselves to worlds we will never see, by using art to send our message across deep time. Thus Pericles at the Parthenon: “Future ages will wonder at us, even as the present age wonders at us now.”

The key to propaganda is in the root of the word itself: propagate. Through propaganda we seek to propagate ourselves across both space and time. This can be in spirit, as in the cases described above, or in the flesh, through physical reproduction. Indeed, while only a relative handful of people have been able to message the future through monuments, literary works, or art, the great majority of those of the past world who have sent us something of themselves have done so by propagating themselves biologically. Using this method, they have transmitted to our world not only their genotypes and phenotypes, but even their languages, beliefs, and traditions as well. Propagation is propaganda. Propaganda is propagation. It is the most desired form of communication. It is how the past has communicated with us, and how we seek to communicate with the future. This is a key point, because interstellar communication through any means must perforce be communication across time.

So, it should be clear. If we are going to transmit across the ages, we need to send instructions on how to make ourselves. Such messages are not sent via radio. They are sent using genes.

The code of life is the code of the cosmos.

Panspermia or Geospermia?

It is a striking fact that, despite several centuries of microbial research by thousands of competent investigators, no free-living organisms have been found on Earth that are simpler than bacteria. This is truly remarkable because, as simple as bacteria may be compared to more complex organisms, they are certainly not simple in any absolute sense, incorporating as they do, among other things, the entire elegant double-helix scripted language of DNA. Believing that bacteria were the first life forms to emerge from chemistry is like believing that the iPhone was the first human-invented machine. This is incredible. Just as the development of the iPhone had to be preceded by the development of computers, radio, telephones, electricity, glassware, metallurgy, and written and spoken language, to name just a few necessary technological predecessors, so the creation of the first bacterium had to be preceded by the evolution of a raft of preceding biological technologies. But we see no evidence of any such history. We still see devices all around us that use one or more of the iPhone’s ancestor technologies – telephones, light bulbs, batteries, glass windows, and steel knives, for example – but we see no pre-bacteria organisms. This observation has led many investigators, dating back to Arrhenius (Arrhenius 1908) over a century ago, to postulate that life on Earth is an immigrant phenomenon. According to this “panspermia” hypothesis, bacteria did not originate on Earth, but came here from space, after which they gave rise via generally understood evolutionary processes to all other life forms.

The panspermia hypothesis is generally disliked by origin of life researchers, because it completely ducks their central question of how life originated from chemistry in the first place. This is particularly the case for the original form of the panspermia hypothesis offered by Arrhenius, who believed that the universe and life had existed eternally, thus making the question of the origin of either meaningless. However, if the panspermia hypothesis is taken to simply open the question as to the location of life’s planet of origin, then it is by no means useless. Consider: an investigator seeking to explain the origin of Americans would be crippled in his or her research if he or she had to accept as axiomatic the conceit that humans evolved independently in North America (and even more so if Golden, CO were specified.). No, the fact is that humans originated in Africa, and only came to the Americas much later. This is why we can find evidence of humanity’s closest relatives, primate ancestors, and earliest cultures and technologies in Africa but not in North America. Knowing this, an investigator would not be bound to try to explain the independent origin of humans from native North American (or Goldenian) fauna, but instead could focus on conditions and biological foundations that were present in Africa in the relevant period. Similarly, there have been origin of life experiments, such as the famed Miller-Urey experiment, that have been discounted because they postulated conditions that did not exist on the early Earth. If the possibility of an extraterrestrial origin of life is accepted, such objections lose their force.

Indeed, insistence on geospermia by assumption puts origin of life researchers in the same absurd position as the above described unfortunate paleontologist, whose assumption of a local origin for humanity forces him or her to reject the theory that humans evolved from higher primates because there were no such species in evidence in Golden, CO at the time of humanity’s appearance. There are innumerable planets where the spontaneous formation of amino acids from chemistry, as demonstrated by the Miller-Urey experiment, could readily have occurred, as opposed to the early Earth, where it could not. Science needs to follow the data, not defy it. Therefore it is the Miller-Urey experimental results that discredit the assumption of geospermia, rather than the reverse.

Further support is offered to the panspermia hypothesis by discoveries of bacterial fossils known as stromatolites, dating back approximately 3.5 billion years, and residues of biological activities dating back 3.8 billion years, that is practically right back the end of the heavy bombardment that previously made the early Earth uninhabitable. In fact, as this is being written, a team of researchers have just reported microfossils that date back 4.28 billion years, that is to the middle of the heavy bombardment. (Drake 2017) In short, life appeared on our planet virtually as soon as it possibly could (and possibly several times, before it could last), suggesting that it was already around, waiting to land and spread as soon as conditions on the ground were acceptable.

The primary counter argument offered against the panspermia hypothesis is that there may once have been prebacteria on Earth, but that they have since been wiped out by their more developed descendants. While this may be possible, it is not consistent with the history of life on Earth, in which simpler forms generally continue to exist in abundance even after they give rise to higher or more complex varieties. In any case, this argument is only an excuse for the lack of any evidence for any prebacterial history of life on Earth. Accordingly, it has no power or potential to falsify the panspermia hypothesis.

Furthermore, it needs to be understood that the conceit that life originated on Earth is quite extraordinary. There are over 400 billion of stars in our galaxy, with multiple planets orbiting many of them. There are 51 billion hectares on Earth. The probability that life first originated on Earth, rather than another world, is thus comparable to the probability that the first human on our planet was born on any particular 0.1 hectare lot chosen at random, for example my backyard. It really requires evidence, not merely an excuse for lack of evidence, to be supported.

The panspermia hypothesis could be falsified however, if we were to send explorers to Mars and find either a) no evidence of any past or present life, b) evidence for the presence of prebacteria or c) evidence of life of sufficiently different type as to imply a second genesis. Condition (a) would falsify panspermia because Mars had liquid water on its surface during the period when life appeared on Earth, so that if Earth were seeded via panspermia, Mars should have been seeded too. Condition (b) would refute panspermia directly by revealing the prior evolutionary history of Earth life on Mars, from whence it could readily have been transmitted here via meteoric impact. Condition (c) would refute panspermia by showing two independent origins. However, if none of these conditions are met, and we find evidence of past or present bacteria on Mars similar in structure to Earth bacteria dating back to the planet’s early history, with no evidence of prebacteria, the panspermia hypothesis would be strongly supported.

In the absence of falsification, we are presented with three possibilities for interstellar microbial transmission.

1. The transmission is natural, being the result of ejection of material from microbe-inhabited planets following meteoric impacts.

2. The transmission is artificial, being the result of intentional dispersal by intelligent extraterrestrials of microbes carrying imprinted encoded information.

3. The dispersal is both artificial and natural, being the result of both processes listed above going on simultaneously.

With respect to the above listed possibilities, the one that seems most difficult to defend is (2), because if bacteria can survive interstellar trips, there will be natural transmission, regardless of whether artificial transmission is also going on. Indeed, it is hard to escape the conclusion that natural transmission has been going on for at least 3.6 billion years, if from no other original source than the Earth. If the average time between close stellar encounters is 20 million years, with number of microbe inhabited system doubling each time, we could expect 2180 solar systems in our galaxy to have been Earth-progeny-microbe-invaded by now, which is to say all of them, many times over. (This being the case, the probability that the Earth was actually the first of these billions of microbe-inhabited worlds would be vanishingly small.)

So, the question is not whether interstellar microbial transmission is going on; it almost certainly is. Further, even if is not occurring naturally, it is still clearly possible through artificial means. So indeed, far from being a necessary condition for microbial SETI, even the possibility of natural panspermia creates difficulties, as it introduces the potential for noise that could drown out the signal.

The key question for microbial SETI is whether there is artificial intelligent input being inserted into the vast flood of genetic information traveling around the Earth. Are there any real letters of importance to be found in the deluge of junk mail? If so, how could we pick them out?

Distinguishing ET’s Messages from Junk Mail

Radio SETI researchers record electromagnetic waves received from space, and examine them with algorithms to try to detect something that seems too organized to be merely the result of astrophysical phenomenon. In principle, microbial SETI researcher could take a similar approach, using gene sequencing technology to process large numbers of bacterial genomes, looking for something that just doesn’t seem natural. Perhaps buried amidst the junk DNA of some bacterial species is a sequence of amino acids that an expert cryptanalyst can decode to read; “Here is the value of π, 3.14159265…This is just our way of saying hello. You will find the primer for our language on the next gene, and starship design instructions on the gene after that.” Perhaps that is a somewhat facetious way of putting it, but certainly if we believe that extraterrestrials want to send us some such signals, a direct attempt to sort through the mail pile using gene-sequencing and cryptanalytical tools to see if anything interesting pops up might be warranted.

That said, there is, as noted above, grounds for skepticism that this is the form of communication that intelligent species would find interesting. If there is a field of life throughout the galaxy, initiated on innumerable worlds by natural panspermia, it could be expected to evolve in multitudes of new and unpredictable directions through natural processes including mutation and natural selection, driven by chance and diverse environmental conditions. It seems to me that the most portentous form of communication that intelligent extraterrestrials could undertake would be to try to propagate themselves by sending out genetic information to influence this chaotic process in their own direction. Genomes can contain dormant plans for complex traits, as evidenced by recent work in which scientists activated what had been considered junk DNA in chickens to produce long-lost dinosaur features, like teeth. (Hoggenboom 2015, Bhuler 2015) Well, chickens are descended from dinosaurs, so perhaps it’s not too astonishing to discover that they still keep some of the old body plans on file. Such plans could come in handy if new conditions require a radical evolutionary leap. But could it be possible that some such genetic plans were sent here intentionally by bacterial conveyance? Could some have been used, and others be still awaiting their chance? There are a variety of physiological features, such as the complex eye, or bird wings, whose origin is hard to explain in terms of incremental natural selection, as they appear to be completely non-functional in partially-developed form. The raising of such paradoxes has long been a stock in trade of theists arguing the case for supernatural “intelligent design.” But while there are, by definition, no supernatural phenomenon, there really are many things – including not only buildings and ships, but also domesticated animals and plants – that are the product of intelligent design. Bacteria can transfer genes among themselves, and to and from macrofauna and macroflora (Yong, 2016, Margulis and Sagan, 2008). Instead of sending us greetings and saluting us with the value of π, could extraterrestrials be sending us microbial messages for the purpose of guiding the evolution of our biosphere?

As fantastical as it sounds, I believe this is a testable hypothesis. Specifically, one could search for genetic material being carried by bacteria that can be inserted into animals or plants and result in the production of striking adaptations that have not yet manifested themselves in living species on Earth. Birds once had teeth, but they never had radar. No species is known which communicates telepathically using bioradio. There are any number of useful adaptations that are physically possible but which never have manifested themselves in terrestrial biology. Furthermore, there are traits that we do see in some species, but not in their ancestors. Mice were once fish, but fish were never mice. The fact that mice still carry fish traits has been clear since the 19th Century, when it was observed that mammal embryos exhibit fish traits such as gills. But do fish carry in their inactive DNA advance plans for mammalian traits? It is possible that such traits could be induced by the transference of bacterial genes. But from what source? If such were found, could they have been sent by extraterrestrials in the distant past, to either fish or their ancestors? Or did fish get them from mammals by natural local bacterial transference much more recently? How could these two possibilities be sorted out?

We could also consider the past. There were certain periods when a massive amount of evolutionary innovation occurred within an extremely brief period of time, with the Cambrian Explosion and the early Eocene immediately following the KT extinction immediately coming to mind. Can the sudden appearance of so many radically new traits be best explained by the prior existence of readily available genetic plans? Or perhaps could it be the result of the opening of expanded channels of communication between genetic plans? (Hoyle, 1981)

Current evolutionary neo-Darwinian “fundamental dogma” holds that all traits of a species are only passed on genetically, so that traits acquired in life are not inherited. Furthermore, new genetic traits are only created through mutation and passed down within a species own line of descent, and that only if they pass the test of natural selection within that species line. While useful in understanding biological evolution, this theory is clearly false when applied to human social evolution. This is so because in contrast to what seems to be the case with other animal species, humans can inherit acquired traits, such as technologies, and furthermore, can inherit such traits from unrelated persons or groups. As a result, such theories as national social Darwinism, (Bernhardi, 1912, Hitler, 1941) which postulate history as a battle of nations for limited resources, with advances occurring via the resulting elimination of the less militarily proficient, are not only morally reprehensible, but scientifically wrong, since inventions made in any nation can (and generally do) ultimately benefit all nations. Indeed, were it the case that technological innovations were not laterally transferable from person to person, tribe to tribe, and nation to nation, it is doubtful that humanity would ever have advanced beyond the old stone age.

Conversely, it is easy to see the quantum acceleration of human progress following the establishment of global communications through the development of the printing press and long-distance sailing ships circa 1500, and their further improvement via railroads, steamboats, telegraphs, telephones, radio, TV and the internet in the period since. Furthermore, it is unquestionable that the growth of human population has contributed to this trend, since the more people there are, the more inventors there can be, and inventions are cumulative. This is why, contrary to Malthus, as the world’s population has gone up, the standard of living has gone up, not down. For similar reasons, it is in the self-interest of intelligent species to promote the creation of and communication with as many intelligent species elsewhere as possible. The more sources of transferable invention that there are, the more inventions each will receive, and the greater will be the power of each to add further inventions in turn. Furthermore, most intelligent species must be aware that their self interest lies in increasing, rather than decreasing, the number and effectiveness of the creative agents that they draw upon, because if they were not, they could not survive for long.

This said, can it really be true that the biosphere is so defective that it is incapable of allowing analogous transfer of useful genes between species? The creation of new useful traits by random mutation is a slow process. Clearly it would benefit all species, and thus the biosphere as a whole, were each to be able to draw to some degree on the genetic innovations of the rest. A community of life that had such ability would have greatly improved odds of survival, and enjoy tremendous adaptive advantage over any that did not. In fact, contrary to the neo-Darwinian fundamental dogma, the biosphere has exactly such a capability. Such transfers from species to species are enabled by bacteria, (Hotopp 2011). Such bacterial transfers of entire genes allow the evolution of valuable new inheritable traits to appear in species not over millennia, but within the lifetime of a single individual. Yong (2016) reports many such instances, for example species of woodrats that upon acquiring certain bacteria, immediately gain the ability to digest various plants, such as creosote or cactus, that they did not have before, thereby becoming able to live and prosper in environments dominated by such species. These valuable bacterial-derived traits are then passed from mother to child during birth, along with samples of the maternal bacteria. But since the rats are constantly broadcasting their bacteria through exhalation, these traits can be transferred to other unrelated individuals and even other species. Other bacteria have been shown to provide various species with inheritable defenses against parasites such as nematodes. Margulis and Sagan (2008) report many additional examples, including cases where the acquired genes eventually move from the bacterial microbiome carried by the animal into the animal cells themselves.

To use an analogy, the genotype of an animal’s cells is its hardware, but an animal’s characteristics are also determined by its bacterial software, or microbiome, which can be rapidly changed in and out. Evolution occurs not by random changes in the hardware circuitry, but by adding or subtracting software programs, which are constantly being developed and exchanged in vast amounts by the extremely prolific bacteria. If found by natural selection to be valuable over the long haul, these programs can end up being written into the hardware.

If the software is found to be useful, the animal’s chances of survival are improved, and the trait is passed on to her descendants. But not only that, she and each of her progeny become agents for spreading the useful trait throughout the biosphere through their exhalations and excretions. As you read this article, dear reader, you are exhaling half a million of your bacteria every minute, providing every creature around you with samples of what you and your ancestors have found to be useful. Your dog or cat is broadcasting similar service. Thus the web of life on our planet shares its inventions.

Can bacteria perform the same role from world to world? If they could, they would be a tremendously positive influence on evolution throughout the galaxy, transferring entire encyclopedias of hard won biological knowledge between planetary biospheres. But while benefiting the progress of life, it is hard to see how such capabilities benefit the bacteria themselves. Could they be serving the purposes of others? Perhaps they are, particularly since it is easy to see how others could be readily sending them.

It may be objected that ET’s wishing to control evolution elsewhere in order to reproduce themselves on other planets could hardly accomplish such as objective by spreading microbes carrying their genes, as evolutionary processes occurring on destination worlds would certainly carry matters forward in unpredictable directions. This is unquestionably true. However, returning to our example of trying to talk to the children in the neighborhood by leaving books around, we can imagine a more general method of communication. Let’s say the book is Harry Potter and the Sorcerer’s Stone. There are several layers of messaging in the book. These include:

1. Being a wizard beats being a muggle. So try to get into Hogwarts, and be sure to choose a Nimbus 2000 for your broomstick.

2. Virtue will be rewarded, vice will be punished, and good will triumph over evil in the end.

3. Reading is fun!

As a (1) method for transforming children into wizards, Harry Potter fails. It can contribute to (2) moral instruction, however, and succeeds brilliantly as (3) a way to get children to become readers. After that, there is no limit to what they might learn or become.

By analogy, the purpose of microbial data transmission might not be to direct evolution in particular ways, but simply to encourage evolution to be more creative.

Finding the Most Likely Suspects

Given these observations, how can we make use of them to detect ET? A good place to start might be to focus study on those bacteria which show the strongest signs of most recent extraterrestrial origin. These would be those best adapted for spaceflight. In nature, adaptations for particular purposes always come at a cost, and therefore generally disappear over time if they become unnecessary. Some bacteria, such as radiodurans, have excellent astronautical adaptations, including extreme resistance to hard radiation and vacuum. If people exiting an airplane landing in a warm city are observed to be wearing winter coats, the chances are good that they came from someplace cold. Similarly, if certain types of bacteria are adapted to space, there is reason to suspect they came from space. Accordingly, samples containing large varieties of bacteria could be exposed to space conditions prevailing in space. Those which survive the best could then be cultured and identified for further focused study. Evolution since arrival could have erased any past encoded genetic message, but remnants might still be found. There is only one way to find out.

But bacteria can evolve quickly, thereby potentially erasing essential information about the past of those who joined Earth’s Melting Pot some time ago. Therefore the most convincing place to look for microbial astronauts would be in space itself. Spacecraft carrying aerogels or other suitable capture media could be deployed with the mission of trying to gather spacebugs in flight. In principle they could be sent anywhere, with perhaps the most promising location being the vicinity of a comet as it outgases volatiles through its trip through the inner solar system. This could be a favorable location for space microbe collecting because it is possible that Oort Cloud objects might collect such interstellar voyagers over time, and then, when heated during close solar approach, release them in large numbers along with the vapors of the frozen volatiles that preserved and held them until that time. (Hoyle 1981). However the problem with this approach is that the characteristic relative velocities of objects moving in various orbits in space exceed several kilometers per second, so that microbes on one such trajectory slamming into anything remotely as dense as an aerogel on another would almost certainly be destroyed on impact. What is needed is an extremely diffuse medium of large expanse that can be used to slow the fast-moving microbes down to a near halt to so that they can be gathered without harm on a solid collecting surface. Fortunately, there are such media available. They are called planetary atmospheres.

Atmospheric entry from space is ordinarily thought of at a very high temperature affair, and it certainly can be. But the temperature reached will be a function of the object’s mass to surface area ratio, or ballistic coefficient. Objects like the Space Shuttle or Dragon capsule typically hit the atmosphere with ballistic coefficients between 100 and 1000 kg/m3, and these glow incandescent during reentry. However, as design studies for the German Mars Society’s ARCHIMEDES mission showed in considerable detail, a balloon with a ballistic coefficient on the order of 1 kg/m2 might well survive atmospheric entry at Mars without ever exceeding the temperature limits of Mylar. A spherical bacteria with a radius of 1 micron would have a ballistic coefficient three orders of magnitude lower still, and readily be able to avoid incineration after entry at speeds on the order of the Earth’s 11 km/s escape velocity. After being slowed down in such a manner, such a microbe would continue to fall downward, with a terminal velocity of about 3 m/s at 40 km altitude, decreasing to 1.5 m/s at 30 km altitude, and still slower at yet lower altitudes. Larger bacteria would have faster terminal velocities, with falling speed increasing as the square root of the microbe radius. For similar reasons, any microsailcraft with a low enough ballistic coefficient to escape its solar system using light pressure would have no problem safely entering the atmosphere of practically any planet.

It is possible to fly balloons at 40 km altitude, and use them to try to collect microbes. In the 1960s this was actually done by (later) Viking chief scientist Jerry Soffen, who did indeed discover viable microbes at that altitude, in such numbers that the results were considered so counterintuitive that the experiments were discontinued. (Hoyle 1981). While it is clear that any microbes found in the Earth’s upper stratosphere could be the result contamination from the biosphere below, such upward delivery by upwelling is made difficult by the strong temperature inversion in the stratosphere. This inversion places very cold (-50 C at 25 km) air below warm air (30 C at 40 km), and thereby suppresses upward convection. As a result, there is reason to suspect that microbes collected at 40 km altitude might be from space.

But how could we know? One way is by looking at the nature of the organisms collected themselves. The earliest microbes of Earth were anaerobic archaea. These can no longer live on the surface of the Earth, because they cannot tolerate the presence of oxygen. However, if life on Earth were begun via panspermia, such organisms would have been right stuff to get the ball rolling, because the surface of the prebiotic Earth was a suitable habitat for them. Furthermore, if anyone – be it ET or Nature – is still trying to seed life on prebiotic worlds today, it is these organisms, rather than the Earth’s surface biosphere currently dominant types, who would be the required pioneers. So, in short, what we should look for with our balloon-borne microbe collecting systems are anaerobes, who may exist within the Earth, but not on it, and who have no business flying around the stratosphere unless they just got off the boat.

It is also possible to fly balloons in the atmospheres of Venus and Mars. Both of these planets have sterile surfaces, effectively ruling out contamination via upwelling from below. Venus is especially attractive in this regard, as its thick atmosphere readily facilitates ballooning (two Soviet Vega balloons were successfully flown in the Venusian atmosphere in 1985) and its hot surface precludes indigenous life entirely. Any microbes collected by balloon-borne platforms floating in the atmosphere of Venus – and arguably Mars – clearly would have to come there from space, although the ultimate source of many of them might still well be the Earth.

Perhaps a particularly interesting time to conduct such experiments, whether in Earth’s atmosphere or elsewhere, might be during cometary events. If collecting balloons were flown at various times, including both normal conditions and during or shortly after cometary encounters, the difference in results could be instructive.

Using such techniques, suspects for either natural panspermia or ETI influenced microbial data transmission could be identified. The genome of these top suspects should be sequenced first, identifying which genes play a role in the functioning of the microorganism itself, and which appear to be simply along for the ride. The first set pertains to the messenger, but the second could contain the message. Perhaps it can be decoded, or if not decoded, at least examined for a format suggestive of an artificial design.

Of course, if we captured not merely microbes, but actual microsailcraft, the case for ETI initiated microbial data transmission would be settled out of hand. In that case, the key question would move directly to decryption.

If the transmitter is biological in nature, it would appear to be most logical that the receiver should be as well. If messages are being sent using genes, perhaps their meaning can best be found by inserting them in samples of terrestrial life to see what comes forth. The best terrestrial organisms to use as receivers might be bacteria themselves, as of all life here, they are most adept at adopting and putting to use new genetic information. These could be used to both receive and amplify the signal and then deliver it to more complex organisms. Perhaps novel traits could be made to appear. While radar-wielding birds are not to be expected, there are large numbers of potential animal body plans that are not currently in use of Earth. Many such plans, representing whole phyla of animal life, were briefly exhibited on our planet during the period of first flourishing of multicellular life known as the Cambrian Explosion, some 550 million years ago, only to go extinct shortly thereafter. If genes carried by suspected astrobacteria were found to induce the appearance of traits representative of such extinct phyla or other unknown animal or plant types in current life, that would be very exciting.

Of course, we might not be so lucky. A mother seeking to promote the intellectual development of her child might leave works of literature for a 16 year old, chapter books for an 8 year old, picture books for a 4 year old, and letter blocks for a 2 year old about the house in places where her child might find them. By the same logic, if ET wanted to promote evolution, he might send types of microbes adapted to successive stages of biospheric evolution containing only the information needed for the next steps, rather than the whole library of potential plans right from the start. After all, it would be futile to leave a copy of War and Peace in the nursery of a 2 year old. But even letter blocks are a dead giveaway for developmental intent. So perhaps rather than finding the genes for creating fish, trilobites, edicaria, or even eukaria in anaerobes arriving from space, we might hope to find plans for just the next step, for example chlorophyll. Any microbes carrying plans for further steps might be designed to make their way after arrival in more developed (i.e. oxygenated) phases of the biosphere, and thus be harder to identify as extraterrestrial messengers.

Still, any genetic information of a forward-looking nature carried by microbes strongly suspected of recent arrival should be a Wow signal for the ETI search. If such organisms were found to induce the appearance of such traits either uniquely or with markedly greater effectiveness than more mundane microbes, the case for evolutionary influence from space would be proven.

But there might be an even simpler way to search, because the biosphere has been acting as a giant receiver and amplifier for such messages for the past 3 billion years. That is, the history of such messages may be recorded in the biosphere itself. Consider this: the mustangs of the American west are well-adapted to their current environment, and might appear to a naïve biologist to be a product of local Darwinian evolution. But, while plausible, this conclusion would not be entirely correct. In fact, mustangs are descended from horses that escaped the Spanish Conquistadors, and their ancestors were the products of selective breeding to enable them to carry medieval knights. No doubt the genes for such past incarnations as a breed of heavily-muscled carriers of armored knights are still to be found in the cells of mustangs today, and could be activated to reveal themselves as actual traits in a mustang colt by an appropriate program. This would prove that the mustangs had a previous form that was actually a product of “intelligent design.” Furthermore, it is probably the case that if someone wanted to breed mustangs back into knight-carriers, they could do it much faster than by using horses with no such genetic history in their ancestral line. In short, both their history and some potential forward-looking traits are encoded in their genome.

This brings us back to the question of the mice and the fish. We know there are fish traits encoded in mice. No surprise there; mice evolved from fish, so naturally they carry the record of their previous evolutionary career. But do fish carry in their genome plans for any of the noteworthy traits of mice? They very well might, but perhaps only because bacteria can move genetic material around from one species to another. But if this were all there were to it, they might very well carry similar amounts of genetic material transmitted to them from species, such as insects, that are not fish descendants. However, if fish were found to be carrying genes for not only mammalian traits, but the whole roadmap of amphibians, reptiles, and mammal-like reptiles leading from fish to mice, – or even just the first essential steps on that path – that would show that somebody had been doing some serious advance planning, at least taking the trouble to shout some useful advice into the arena.

Could potential draft plans for future biospheric evolution be preprogrammed into the genes of space-traveling microbial messengers? Could the history of such past messages be recorded in the genomes of species all around us? Let’s have a look and see.

Predictions and Conclusions

To be useful, any scientific theory needs to be testable and falsifiable. While the balloon borne microbe or microsailcraft collection experiments described above could potentially offer strong evidence in favor of panspermia, they cannot disprove it because a negative result could be explained away by the argument that the flux of microbes from space is simply too low to be detected by such means. Similarly, the genomic search for forward-looking traits in terrestrial organisms could reveal directed panspermia, but not disprove it. Such experiments should be done regardless because they are cheap and might produce profound results. But if they are unsuccessful in producing useful data, a more muscular approach to settling the matter will be necessary. This can be done through the exploration of Mars.

Mars was once a warm and wet planet, which could have hosted life on its surface, and there is strong evidence that there is still liquid water to be found underground on Mars, which could serve as a habitable environment for microbial life today. If there ever was life on the surface of Mars, it is reasonable to assume that it retreated into the groundwater when conditions on the surface of the planet deteriorated, much as the anaerobic archaea did on Earth after the oxygenation of the atmosphere made the surface here inhospitable for their kind. If we could go to Mars and sample the groundwater, what we find, or fail to find, therein would be very informative. There are four primary possibilities.

1. We find no life in Mars groundwater. This is a very unlikely result, because Mars almost certainly had life on its surface at one time, if from no other source than the Earth. But if that should be the finding, it would prove the case for not only geospermia, but unique geospermia, occurring on Earth but not on the similar early Mars, indicating that life is rare in the universe.

2. We find life in Mars groundwater, which uses the same biochemistry as Earth life, but including more primitive free living representatives ancestral to bacteria. This would refute panspermia as an origin of life theory, instead showing that life on Earth originated on Mars. It would also support the conjecture that life is common in the universe, as apparently it could evolve from chemistry readily on a primitive terrestrial type planet.

3. We find life in Mars groundwater which uses a different biochemistry than what we find on Earth, i.e. a second genesis. This would refute natural panspermia, but prove that life is very common and quite diverse in the universe, as it would be seen that life could originate from chemistry, de novo, two different ways, on two out of two typical primitive terrestrial planets.

4. We could find life in the groundwater of Mars that uses the same biochemistry as Earth life, manifesting similar bacteria forms, with no more primitive free-living representatives evident. This is what natural panspermia would predict. It may be argued that the same result could be achieved by the origin of life on either Earth or Mars, with subsequent transfer between them as well as extinction of the ancestral forms on both worlds. But this means that the current alibi of the geospermians – “the origin did happen here, really it did, we believe that sincerely, even though experiments show that conditions here were unfavorable, we’ve just lost all the evidence” – would need to be stretched to two worlds.

It should be noted that while operation of robotic rovers on the surface of Mars might serve to falsify alternative 1 (which is fantastical in any case as it requires accepting the conceit that not only is Earth uniquely capable of originating life, but that life from Earth cannot spread to other habitable places nearby) by discovering fossils, it cannot affirm it. More importantly, such robotic exploration techniques would be incapable of distinguishing between alternatives 2,3, or 4, which is the most critical scientific question. Resolving between these alternatives, which have profound implications for the nature of life and the universe, will require drilling to depths on the order of a kilometer to reach groundwater, bringing up samples, culturing them, and subjecting them to biological analysis. From a practical point of view, this can only be done by sending human explorers to Mars.

But which of these alternatives is most likely? I predict that number 4 is what we will find. The reason is simply this: The Milky Way galaxy predates the Earth by 8 billion years. The early Earth was not exceptional in any significant way, so that if life could evolve here, it could have evolved first on hundreds of billions of other possible locations. Furthermore, the first life that did so which developed adaptations allowing it to survive interstellar flight would necessarily spread throughout the galaxy in less than a billion years by natural collisional processes. This would result in the appearance of life on any planet as soon as conditions there were suitable, which is exactly what we observe in the fossil record on Earth.

For these reasons I believe that natural panspermia is extremely probable, and that the results of Mars exploration will prove to be consistent with it. But what about directed panspermia? While the history of life on Earth has demonstrated the ability of biospheres to evolve increasingly intelligent species (Morell, 2013), what reason is there to believe that such intelligent extraterrestrials should want to spread their kind around? None, except this: If there were any such species, it would be their kind that would get spread around. If we find any forward-looking traits encoded in the Martian biota, their handiwork would be there for all to see.

Finally, it may be observed that a program of directed panspermia using microsailcraft is well within humanity’s current technological means. If it turns out that no one else has been doing the noble work of spreading life throughout the universe, perhaps we should get the ball rolling ourselves.


I wish to thank Chris McKay of NASA Ames Research Center and Paul Davies of Arizona State University for useful comments on early drafts of this manuscript.


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A Pulsar Habitable Zone?

Life and pulsars don’t seem to mix. But science fiction hasn’t shied away from making the connection, as witness Robert Forward’s Dragon’s Egg (Ballantine, 1980). In the novel, a species called the cheela live on the surface of a neutron star, coping with a surface gravity 67 billion times stronger than that of Earth. An interesting consequence: The cheela live at an accelerated rate, going from the development of agriculture to high-tech in little more than a month, as perceived by the human crew observing the course of their rapid development.

Now we have news that two astronomers are considering habitable planets in orbits around pulsars, a venue that to my knowledge Forward never considered, but perhaps more recent science fiction writers have (let me know if you have any references). Alessandro Patruno (Leiden University), working with Mihkel Kama (Leiden and Cambridge University) see reasons for thinking that life might emerge in such an environment, though the kind of atmosphere that would sustain it would be like nothing we’ve yet encountered.

The paper defines three categories of neutron star planets while explaining the conditions they would be subjected to:

Neutron star planets can be first-, second- or third-generation. First generation planets would be formed in the usual manner, as a by-product of the star formation process, and would likely be ablated or unbound during stellar death. Second generation objects would form in the supernova fallback disk around a freshly-formed neutron star. Third generation planets would form from a disk consisting of a disrupted binary companion (possibly previously overflowing its Roche lobe), thought to be essential for producing millisecond pulsars such as B1257+12. The supernova explosion, the accretion from a companion for millions up to billion years that MSPs [millisecond radio pulsars] undergo, and the emission of high energy X-ray/γ-ray radiation and MeV-TeV particles (the pulsar wind) are all disruptive processes that might destroy planets or disrupt their orbits.

In any case, neutron stars deal out bursts of X-rays and other particles, accreting matter around them and boasting huge magnetic fields. This is a very dicey environment, one would think, to be talking in terms of habitable zones. But in their paper in Astronomy & Astrophysics, Patruno and Kama find room for a habitable zone as large as 1 AU in breadth. To make this work, the planet must be a super-Earth with a mass between one and ten times that of Earth. Also required: An atmosphere a million times as thick as Earth’s.

Daunting conditions indeed. The work draws on studies of the pulsar PSR B1257+12, famous for its three known planets, which were the first exoplanets ever discovered, in 1992 (the third was found in 1994, still a year before the discovery of 51 Pegasi b). Aleksander Wolszczan and Dale Frail will forever be associated with the discovery. Patruno and Kama used the Chandra space telescope to study PSR B1257+12, which is 2300 light years out in Virgo.

Image: This artist’s concept depicts the pulsar planet system discovered by Aleksander Wolszczan in 1992. Wolszczan used the Arecibo radio telescope in Puerto Rico to find three planets – the first of any kind ever found outside our solar system – circling a pulsar called PSR B1257+12. Pulsars are rapidly rotating neutron stars, which are the collapsed cores of exploded massive stars. They spin and pulse with radiation, much like a lighthouse beacon. Here, the pulsar’s twisted magnetic fields are highlighted by the blue glow. All three pulsar planets are shown in this picture; the farthest two from the pulsar (closest in this view) are about the size of Earth. Radiation from charged pulsar particles would probably rain down on the planets, causing their night skies to light up with auroras similar to our Northern Lights. One such aurora is illustrated on the planet at the bottom of the picture. Credit: NASA/JPL-Caltech/R. Hurt (SSC).

What we have around this pulsar are two super-Earths with masses between 4 and 5 times that of Earth, orbiting the pulsar at 0.36 and 0.46 AU respectively; the third, innermost planet is about twice as massive as the Moon. The pulsar itself shows a mass of 1.4 times the Sun’s, with a radius estimated to be in the range of 10 kilometers. All three planets are close enough to be heated by the pulsar, a daunting thought given the X-ray radiation and relativistic ‘pulsar wind,’ which could have devastating effects on a planetary atmosphere.

Nonetheless, the paper continues:

… the two Super-Earths may have retained their atmosphere for at least a hundred million years provided they contain a large atmospheric fraction of the total planet mass, with the atmosphere possibly still being present to these days. We also find that if a moderately strong planetary magnetosphere is present, the atmospheres can survive the strong pulsar winds and reach survival timescales of several billion years. The same argument applies to possible pulsar planets around more powerful objects than PSR B1257+12.

We are talking about a planet that would have an atmosphere accounting for about 30 percent of the planet’s mass. In this news release, the authors liken conditions on the surface of such a world to the deep sea floor here on Earth. Says Patruno: “According to our calculations, the temperature of the planets might be suitable for the presence of liquid water on their surface. Though, we don’t know yet if the two super-Earths have the right, extremely dense atmosphere.”

That pulsar wind remains tricky on several levels. It is not an indefinite process, but one that turns off once the pulsar reaches a slow enough rate of spin. The paper points out that young pulsars turn off the pulsar wind within about a million years, while millisecond radio pulsars do the same in about a billion years. Losing the pulsar wind turns off the planet’s energy source and would cause a dramatic drop in temperature, unless tidal heating, radiogenic effects or X-ray radiation can step in in a process called Bondi-Hoyle accretion, analyzed in the paper:

Isolated neutron stars are directly exposed to the interstellar medium and it is expected that all of them would accrete some of this material. Such accretion process generates extra power due to the conversion of the accreted gas rest mass into energy, with a typical efficiency of the order of 10-20%. This so-called Bondi-Hoyle accretion process should be continuous and might be the main source of power for these type of systems.

I’m thinking science fiction writers among our audience (of which there are more than a few) might want to look at this paper to see what kind of scenarios they can tease out of it. Bear in mind that to this date, we’ve found but five pulsar planets, out of some 3000 pulsars studied. But exotica are what science fiction thrives on, and the kind of habitable zone depicted here is made to order for the hard science fiction writer willing to dig into this paper’s equations.

Addendum: Didn’t Alastair Reynolds deal with a neutron star planet in the first book of the Revelation Space sequence? I need to revisit the series. Wonderful stuff.

The paper is Patruno & Kama, “Neutron Star Planets: Atmospheric Processes and Irradiation,” Astronomy & Astrophysics Vol. 608, A147, published online 19 December 2017 (full text).