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Probing Exoplanet Obliquity

It’s always a shock for me when the soft air and fecund smells of spring slam into a parched and baked July, but seasonal change is inevitable. At least it is on Earth. We get such seasonal changes because of Earth’s obliquity, the angle of its spin axis relative to the plane of its orbit. For Earth, the angle has stayed pretty close to 23 degrees for a long time, although the tilt’s direction wobbles over cycles of thousands of years. And this very constancy of obliquity turns up in exoplanet discussions at times because it affects conditions on a planetary surface.

Some have argued that without the gravitational effects of the Moon, the tilt of the Earth would be changed by the gravitational pull of the Sun and planets, producing a potentially high degree of obliquity. Contrast our situation with that of Uranus, where we find a 90-degree tilt that leaves one pole in sunlight for half the Uranian year as the other remains in darkness. Without knowing how long the Moon has been able to stabilize Earth’s axial tilt, we can’t say how apparent equatorial ice sheets some 800 million years ago fit into this view of the Moon’s effect.

But obliquity as a factor in habitability continues to energize exoplanetary researchers. At Georgia Tech, a team led by Gongjie Li, working with graduate student Yutong Shan (Harvard-Smithsonian Center for Astrophysics) has developed computer simulations to analyze the spin axis dynamics of two exoplanets, Kepler 186f and Kepler 62f, two planets considered to be in or close to the habitable zone of their stars. The paper argues that without our Moon, Earth’s obliquity variation would range from 0 to 45 degrees over billion-year timescales.

Thus obliquity is an interesting data point. Bear in mind that so far, we have no reliable values for exoplanet obliquity, although ways to infer it from light curves and from high-contrast direct imaging have been proposed in the literature. The authors make the assumption that in both exoplanet systems studied, all planets have been identified. They then go on to study the evolution of the two five-planet systems. The ‘secular analytical framework’ they arrive at allows them to factor in planetary rotation rates, additional planets and satellites, and regions where resonant interactions within the system can produce large obliquity variations. For various realizations of planetary systems, the paper thus describes an ‘obliquity evolution.’

We know that Mars and Earth interact strongly with each other, as do Mercury and Venus; other than Earth, none of these worlds has a large moon. The authors point out that the orientation angle of a planet’s orbit around its host star can be made to oscillate through gravitational interactions. If the orbit oscillates at the same pace as the precession of the planet’s spin axis, large obliquity variations can be induced, the kind of thing our Moon dampens out.

Image: An artist’s depiction of Kepler-62f. Credit: NASA Ames/JPL-Caltech/T.Pyle.

For these two exoplanet systems, we get an interesting result, for even without a stabilizing moon (if none is present), these two planets could be experiencing relatively low changes in their axial tilt:

“It appears that both exoplanets are very different from Mars and the Earth because they have a weaker connection with their sibling planets,” said Li. “We don’t know whether they possess moons, but our calculations show that even without satellites, the spin axes of Kepler-186f and 62f would have remained constant over tens of millions of years. That’s not to say either exoplanet has water, let alone life. But both are relatively good candidates. Our study is among the first to investigate climate stability of exoplanets and adds to the growing understanding of these potentially habitable nearby worlds.”

As Li has just pointed out, we have no knowledge of surface conditions on either of these planets, making the lovely image above nothing more than a guess, and an optimistic one at that. The ‘super Earth’ Kepler 62f, about 40 percent larger and with a mass 2.8 times that of our planet, is in the constellation of Lyra, the outermost of the five planets orbiting a K2-class star some 1200 light years from Earth. Kepler-186f orbits a red dwarf about 550 light years out, part of a five-planet system in the constellation Cygnus. A stable axial tilt would make it likely that both worlds experience regular seasons and thus a stable climate.

But are large obliquity values necessarily inimical to life? Some recent work, considered by the authors, shows that variability in obliquity can keep a planet’s global temperature higher than it would otherwise have been, extending the outer edge of the habitable zone. But it does appear that obliquity variations can produce sharp transitions between climate states. From the paper:

Recently, Kilic et al. (2017) mapped out the various equilibrium climate states reached by an Earth-like planet as a function of stellar irradiance and obliquity. They find that, in this parameter space, the state boundaries (e.g. between cryo- and aqua-planets) are sharp and very sensitive to the climate history of the planet. This suggests that a variable obliquity can easily move the planet across state divisions, as well as alter the boundaries themselves, which would translate into a dramatic impact on instantaneous surface conditions and long-term climate evolution.

Planets with highly irregular seasons aren’t necessarily destined to be lifeless, but if we become capable of determining planetary obliquity, such a value could help us narrow the target list for future space telescopes. The authors also suggest that their framework can provide input parameters for existing global climate models as we analyze habitability in multi-planet systems.

The paper is Shan and Li, “Obliquity Variations of Habitable Zone Planets Kepler-62f and Kepler-186f,” <em>Astronomical Journal</em> Vol. 155, No. 6 (17 May 2018). Abstract / preprint.



The Dipole Drive: A New Concept for Space Propulsion

One reason we look so often at sail technologies in these pages is that they offer us ways of leaving the propellant behind. But even as we enter the early days of solar sail experimentation in space, we look toward ways of improving them by somehow getting around their need for solar photons. Robert Zubrin’s work with Dana Andrews has helped us see how so-called magnetic sails (magsails) could be used to decelerate a craft as it moved into a destination system. Now Zubrin looks at moving beyond both this and solar wind-deflecting electric sails toward an ingenious propellantless solution. Zubrin presented the work at last April’s Breakthrough Discuss meeting, and today he fills us in on its principles and advantages. Read on for a look at a form of enhanced electric sail the author has christened the Dipole Drive.

by Robert Zubrin


The dipole drive is a new propulsion system which uses ambient space plasma as propellant, thereby avoiding the need to carry any of its own. The dipole drive remedies two shortcomings of the classic electric sail in that it can generate thrust within planetary magnetospheres and it can generate thrust in any direction in interplanetary space. In contrast to the single positively charged screen employed by the electric sail, the dipole drive is constructed from two parallel screens, one charged positive, the other negative, creating an electric field between them with no significant field outside. Ambient solar wind protons entering the dipole drive field from the negative screen side are reflected out, with the angle of incidence equaling the angle of reflection, thereby providing lift if the screen is placed at an angle to the plasma wind. If the screen is perpendicular to the solar wind, only drag is generated but the amount is double that of electric sail of the same area. To accelerate within a magnetosphere, the positive screen is positioned forward in the direction of orbital motion. Ions entering are then propelled from the positive to the negative screen and then out beyond, while electrons are reflected. There are thus two exhausts, but because the protons are much more massive than the electrons, the thrust of the ion current is more than 42 times greater than the opposing electron thrust, providing net thrust. To deorbit, the negative screen is positioned forward, turning the screen into an ion reflector. The dipole drive can achieve more than 6 mN/kWe in interplanetary space and better than 20 mN/kWe in Earth, Venus, Mars, or Jupiter orbit. In contrast to the electric sail, the ultimate velocity of the dipole drive is not limited by the speed of the solar wind. It therefore offers potential as a means of achieving ultra-high velocities necessary for interstellar flight.


The performance of rockets as propulsion systems is greatly limited by their need to carry onboard propellant, which adds to the mass which must be propelled exponentially as the extent of propulsive maneuvers is increased. For this reason, engineers have long been interested in propulsion systems that require no propellant.

The best known propellantless system is the solar sail, which derives its thrust by reflecting light emitted by the Sun. Solar sails are limited in their performance however, by their dependence upon sunlight, which decreases in strength with the square of the distance, and the laws of reflection, which dictate that the direction of thrust can only lie within 90 degrees of the vector of sunlight. Moreover, because photons move so swiftly, the amount of thrust that can be derived by reflecting light is at best 0.0067 mN/kW (at 100% reflectance, full normal incidence), which means that very large sails, which necessarily must have significant mass and be difficult to deploy, must be used to generate appreciable thrust. As a result, while solar sails have been studied since the time of Tsiolokovsky [1], we are only now beginning to experiment with them in space.

An alternative to the solar sail is the magnetic sail, or magsail, which was first proposed by Zubrin and Andrews in 1988, and subsequently analyzed extensively by them in a variety of further papers [2,3] in the 1990s. The magnetic sail uses a loop of superconducting wire to generate a magnetosphere to deflect the solar wind. Assuming the development of high temperature superconducting wire with the same current density as existing low temperature superconductors, a magsail should be able to generate significantly higher thrust to weight than is possible with solar sails. However such wire has yet to be developed.

Another propellantless propulsion system of interest is the electric sail [4], which like the magsail operates by deflecting the solar wind, in its case by using an electrostatic charge. As a result, like the magsail, the classic electric sail (electric sail) cannot operate inside of a planetary magnetosphere other than as a drag device, has its thrust decrease with distance from the Sun, and is limited in the potential direction of its thrust. Because of the low momentum density of the solar wind, electric sails must be even bigger than solar sails. However, because only sparsely spaced thin wires are needed to create sail area, higher thrust to mass ratios can be achieved than are possible using solar sails which require solid sheets of aluminized plastic.

Electrodynamic tethers [5] have also been proposed, which use the interaction of a current in a tether with the Earth’s geomagnetic field to produce thrust. In addition to facing a variety of engineering and operational issues, however, such systems can only operate in a planetary magnetic field and can only thrust in a direction normal to the field lines, a consideration which limits their applicability.

Finally, we note recent claims for a system called the EM Drive [6], which according to its proponents can generate about 1 mN/kWe, in any direction, without the use of propellant, an external light source or plasma wind, or magnetic field. Such performance would be of considerable interest. However, as it appears to contradict the laws of physics, there is reason to suspect that the measurements supporting it may be erroneous.

As a result, there clearly remains a need for a new type of propellantless propulsion system, which can operate both inside and outside of a planetary magnetosphere, can thrust in a multitude of directions, and which is not dependent upon sunlight or the solar wind as a momentum source. The dipole drive is such a system.

The Dipole Drive

The principle of operation of the dipole drive while accelerating a spacecraft within a planetary magnetosphere is illustrated in Fig. 1 below.

Fig. 1. The Dipole Drive Accelerating within a Magnetosphere.

In Fig. 1 we see two parallel screens, with the one on the left charged positive and the one on the right charged negative. There is thus an electric field between them, and effectively no field outside of them, as on the outside the field of each screen negates the other. There is also a voltage drop between the two, which for purposes of this example we will take to be 64 volts.

Protons entering the field region from the left are accelerated towards the right and then outward through the right-hand screen, after which they escape the field and experience no further force. Protons entering from the right are reflected towards the right, adding their momentum to that generated by the protons accelerated from left to right. There is thus a net proton current from left to right, and a net proton thrust towards the left.

In the case of electrons, the situation is exactly the opposite, with a net electron current from right to left, and a net electron thrust towards the right. Note that while electrons entering from the right will be greatly accelerated by the field, reflected electrons will only be reflected with their initial velocity. There will also be an electron current through the outside plasma to neutralize the net proton flow to the right.

Because space plasmas are electrically neutral, the number density of both electrons and ions (which for the moment we will consider to be protons, but may which – advantageously – be heavier species, as we shall discuss later) will be the same, so the proton and electron electrical currents will be equal, as will the power associated with each of them. However because the mass of a proton is about 1842 times as great as the mass of an electron, the thrust of the proton current will be about 43 times greater than the opposing electron current thrust (because the momentum of particles of equal energy will scale as the square root of their mass, sqrt(1842)=43) and the system will generate a net thrust. The acceleration of the electrons is a form of drag, which is provided for by loss of spacecraft kinetic energy. It therefore could, in principle be used to generate electric power, partially compensating for the power consumed to accelerate the protons. In the following examples, however, we will assume that there is no provision for doing this, i.e. that the efficiency of any such energy recovery is zero.

To see what the performance of a dipole drive might be, let us work an example, assuming a 500 W power source to drive the system. The electron current negates about 2% of the thrust (1/43rd) produced by the proton current. The maximum possible jet power is thus about 490 Wj. Assuming additional inefficiencies, we will round this down to 400 Wj, for a total system electrical to jet power efficiency of 0.8.

A Coulomb of protons has a mass of 0.011 milligrams. If the jet power is 400 W, and the potential difference is 64 V, so the proton current will be 6.25 A, and have a mass flow of 0.0652 mg/s.

The relationship of jet power (P) to mass flow (m) and exhaust velocity (c) is given by:

P = mc2/2                                                                         (1)

Taking P = 400 W and m = 0.0652 mg/s, we find that c= 110,780 m/s. Since thrust (T) is given by T=mc, we find:

T = mc = 7.2 mN                                                            (2)

This is a rather striking result. It will be recalled that the electrical power driving this system is 500 W. So what we are seeing here is thrust to power ratio of 14.4 mN/kWe, more than ten times better than that claimed for the EM Drive, but done entirely within the known laws of physics!

If it is desired to deorbit (decelerate) a spacecraft, the direction of the screens would be reversed, with the negative screen leading in the direction of orbital motion. In this case, the screens would become a proton reflector. An electric sail could also be used as a drag device to serve the same purpose. However, because the dipole drive doesn’t merely create drag against passing protons, but reflects them, it would create twice the drag of an electric sail of the same area. If the dipole drive is positioned obliquely to the wind angle, it can reflect protons, with the angle of incidence equaling the angle of reflection. For example, if it is tilted 45 degrees to the wind, a force will be generated perpendicular to the wind, that is “lift” will be created. Such maneuvers could also be done with the dipole drive in acceleration mode, deflecting protons to combine lift with thrust. Using this capability, a dipole drive propelled spacecraft in orbit around a planet could execute inclination changes.

To summarize, in contrast to the electric sail which can only create drag against the wind to lower its orbit, the dipole drive can thrust in any direction, raising or lowering its orbit or changing its orbital inclination. In addition, when used as a drag device, the dipole drive can create twice the drag per unit area as the electric sail.

The Dipole Drive in Planetary Orbit

Let us therefore analyze the system further. The dipole drive exerts no field outside of its screens, so the only plasma it collects is the result of its own motion through the surrounding medium. So how big does its screen need to be?

We consider first the case of the above described dipole drive system operating in LEO at an altitude of 400 km, being used to thrust in the direction of orbital motion. It is moving forward at an orbital velocity of 7760 m/s. The average density of ions at this altitude is about 1,000,000 per cc. Assuming (conservatively) that all the ions are protons, the required ion mass flow of 0.0652 mg/s would be swept up by a screen with a radius of 127 m.

It may be noted however, that at 400 km altitude there are also O+ ions, each with a mass 16 times that of a proton, with a numerical density of about 100,000/cc. These therefore more than double the ion mass density provided by the protons alone. If these are taken into account, the required scoop radius would drop to about 80 m.

Another way to reduce the scoop size would be by going to higher voltage, so that more power can be delivered to a smaller number of ions. If, for example, we quadrupled the voltage to 256 volts, the exhaust velocity would double, to 222 km/s, allowing us to cut the mass flow by a factor of four, and the scoop radius by a factor of two, to just 40 m. The thrust, however, would be cut in half, giving us 3.6 mN/kWe.

As we go up in altitude, the plasma density decreases, as does the orbital velocity, requiring us to go to larger scoops. Examples of 500 W dipole drive systems operating at a variety of altitudes are provided in Table 1. In Table 1, Vo and C are orbital velocity and exhaust velocity, in km/s.

Table 1. Dipole Drive Systems Operating in Earth Orbit (Power=500 W)

It can be seen that the dipole drive is a very attractive system for maneuvering around from LEO to MEO orbits, as the high ion density makes the required scoop size quite modest. It should be emphasized that the above numbers are for a 500 W system. If a 5 W dipole drive thruster were employed by a microsatellite, the required scoop areas would be reduced by a factor of 100, and the radius by a factor of 10.

It may be noted that Mars, Venus and Jupiter all have ion densities in low orbit comparable to those above. For example, Mars has 500,000/cc at 300 km, Venus has 300,000/cc at 150 km, and Jupiter has 100,000/cc at 200 km, making the dipole drive attractive for use around such planets as well. Many of the moons of the outer planets also have ionospheres, and the dipole drive should work very well in such environments.

As one ascends to higher orbits, the density of ions decreases dramatically, while the orbital speed decreases as well. For example, in GEO, the ion density is only about 20/cc, while the orbital velocity is 3 km/s. These two factors combine to make much larger scoops necessary. So, for example, in GEO, a 500 W dipole drive operating at 1024 volts would need a scoop 3.6 km in radius.

Because the effectiveness of the dipole drive decreases at higher altitudes while operating within the magnetosphere, the best way for a dipole drive propelled spacecraft to escape the Earth is not to continually thrust, as this would cause it to spiral out to trans GEO regions where it would become ineffective. Rather, what should be done is to only employ it on thrust arcs of perhaps 30 degrees around its perigee, delivering a series of perigee kicks that would raise its apogee on the other side of its orbit higher and higher until it escaped the magnetosphere and became able to access the solar wind.

The Dipole Drive in Interplanetary Space

The dipole drive can also operate in interplanetary space. Compared to planetary orbit, the ion densities are lower, but this is partially compensated for by much higher spacecraft velocities relative to the plasma wind. As a result, the required scoop sizes are increased compared to planetary orbital applications, but not by as much as considerations of ion density alone might imply.

Let us consider the case of a dipole drive traveling in heliocentric space at 1 AU, positioned at an angle of 45 degrees to the wind, with its negative screen on the sunward side. It would thus reflect solar wind protons 90 degrees, thereby accelerating itself forward in the direction of orbital motion. A diagram showing the dipole drive operating as a sail in interplanetary space is shown in Fig. 2.

Fig. 2 The Dipole Drive Operating as a Sail in Interplanetary Space.

The solar wind has a velocity of 500 km/s, so to insure reflection, we employ a voltage of 2028 volts, sufficient to reverse the motion of a proton moving as fast as 630 km/s. With a density of 6 million protons per cubic meter, the wind has a dynamic pressure of 1.25 nN/m2. As the sail is positioned 45 degrees obliquely to the wind, its effective area will be reduced by a factor of 0.707, with the thrust reduced to 0.9 nN/m2. In this case, virtually all of the protons hitting the sail will be coming from the sunward side, and since they are reflected without adding any kinetic energy, no power is required to drive them. However, we still have an electron current coming from the sunward side being accelerated outward. This requires power. With 500 W, total radial thrust would be 1.27 mN, with 1.27 mN also delivered in the direction of orbital motion, for a L/D ratio of 1. The total effective screen area would therefore need to be 1,414,000 m2, with an actual area of 2,000,000 m2, requiring a radius of 798 m. Total thrust to power would be 3.6 mN/kWe.

If instead we had not concerned ourselves with obtaining complete deflection of each particle, we could have used a lower voltage. This would increase the thrust per unit power, but increase the required sail area for a given amount of thrust. So, for example, if we chose 512 volts, we would have a total thrust of 3.6 mN, for a thrust/power ratio of 7.2mN/kWe, but need a sail radius of 1127 m.

It may be noted that all of these results are for a 500 W dipole drive. A microsatellite might employ a 5 W dipole drive, in which case the required scoop radii would drop by a factor of 10.

The thrust and diameter of a 1 kWe dipole drive system operating as a solar wind sail in interplanetary space at 1 AU is shown in fig. 3.

Fig. 3. Thrust and Diameter of a 1 kWe dipole drive system operating as a solar wind sail in interplanetary space.

Use of the Dipole Drive for Interstellar Flight

In contrast to the electric sail, the dipole drive can be used to accelerate a spacecraft at velocities greater than that of the solar wind. For example, consider a spacecraft moving away from the Sun at a velocity of 1000 km/s. The solar wind is following it at a velocity of 500 km/s, so relative to the spacecraft there is a wind moving inward towards the sun at a velocity of 500 km/s. In this case, to accelerate the spacecraft would direct its positive screen away from the sun. This would cause it to accelerate protons sunward, while reflecting electrons outward, for a net outward thrust. At 500 km/s the protons are approaching the spacecraft with a kinetic energy equal to 1300 volts. It can be shown that employing a screen voltage difference that is about triple the kinetic voltage produces an optimal design for an accelerating system, while one using a voltage difference equal to the kinetic voltage is optimal for deceleration. This is illustrated in figs 4 and 5 which respectively show the kinetic voltage as a function of velocity, and the relative power/ thrust and area/thrust ratios of the spacecraft as a function of the dimensionless parameter Z, where Z=(engine voltage)/(kinetic voltage.)

Fig 4. Kinetic Voltage as a function of spacecraft velocity.

Fig 5. Relative Power/Thrust and Area/Thrust as a function of Z=(engine voltage)/(kinetic voltage.) There is a step factor of 2 increase in thrust during deceleration when Z reaches 1, because protons are reflected. For acceleration, Power/Thrust ~ 1 + sqrt(1+Z), while Area/Thrust ~ 1/(-1 + sqrt(1+Z)).

If we add 3900 volts to the incoming protons, quadrupling their energy, we will double their velocity relative to the spacecraft, thereby providing an effective exhaust velocity of 500 km/s. The solar wind has a density of 6 million protons/m3 at 1 AU, with ambient density decreasing to 1 million/m3 in interstellar space. If we take the former value, we get a thrust of (1.67e-27 kg/proton)(500,000m/s)2(6,000,000/m3) = 2.5 nN/m2. If we take the latter value, it would be 0.42 nN/m2. The proton current at the smaller value would be 80 nA/m2, which at 3900 volts works out to 0.312 mW/m2. The thrust to power ratio would therefore be 1.35 mN/kW. (This ratio would also hold true at the 1 AU value, but the magnitudes of both the thrust and power per unit area would be six times greater.)

If a dipole drive powered spacecraft were receding 500 km/s directly away from the Sun, it would see no relative wind and thus produce no thrust. However, like a modern sailboat that can sail faster crosswind than downwind, because it can generate lift, the dipole drive can get to speeds above 500 km/s by sailing across the wind. As the spacecraft’s crosswind speed increases, it becomes advisable to turn the sail to ever greater angles to the solar wind and increasingly normal to the crosswind. As this occurs, the L/D resulting from solar wind reflection increases while the total solar wind thrust decreases. At the same time, however, thrust resulting from the acceleration through the screens of crosswind protons increases, maintaining total thrust constant at ever higher L/D (relative to the solar wind) levels. Once the crosswind velocity exceeds the solar wind velocity the solar wind becomes increasingly irrelevant and the dipole drive becomes a pure acceleration system, driving the incoming crosswind plasma behind it to produce thrust,

As the speed of the spacecraft increases relative to the wind, it is necessary to increase the voltage in order maintain thrust/power ratio efficiency. For example, let’s say we want to achieve 3000 km/s, or 0.01c. Then the kinetic energy equivalent voltage of the approaching protons would be 47 kV. So, to double this velocity we need to quadruple the total voltage, or add a sail voltage drop of 141 kV. The proton current would have a value of 480 nA/m2, with a power of 68 mW/m2. The thrust would be 15.1 nN/m2, for a thrust to power ratio of 0.22 mN/kW.

It may be observed that since the necessary voltage increases as the square of the velocity, with power increasing with voltage but thrust increasing with velocity, the thrust to power ratio of the dipole drive decreases linearly with velocity. This puts limitations on the ultimate velocity achievable. For example, the most optimistic projections for advanced large space nuclear power systems project a mass to power ratio of 1 kg/kW. If we accept this number, then, neglecting the mass of any payload or the dipole drive system itself, then the system described in the previous paragraph performing with a thrust to power ratio of 0.22mN/kilowatt at 3000 km/s would have an acceleration of 0.00022m/s2, or 7 km/s per year. The average acceleration getting up to 3000 km/s would be twice this, so the spacecraft would take 214 years to reach this speed. During this time it would travel 1.07 light years. To reach 6000 km/s (0.02 c) starting from negligible velocity would require 857 years, during which time the spacecraft would travel 8.57 light years. The performance of such a system is shown in Table 2. Note 63,000 AU = 1 light year. The performance shown assumes an advanced 1 kg/kWe power supply. If a more near-term power system with a higher mass/power is assumed, the time to reach any given distance increases as the square root of the mass/power ratio. So for example, if we assume a conservative near-term space nuclear power reactor with a mass/power ratio of 25 kg/kW, the time required to reach any given distance would increase by a factor of 5.

Table 2. Advanced Dipole Drive Performance for Ultra High-Speed Missions (1 kg/kW power)

It can be seen that advanced dipole drive spacecraft could be quite promising as a method of propulsion for missions to near interstellar space, for example voyages to the Sun’s gravitational focus at 550 AU. Unless much lighter power systems can be devised than currently anticipated however, they would still require centuries to reach the nearest stars. Power beaming may provide an answer. However such technologies are outside the scope of this paper.

If a spacecraft has been accelerated to interstellar class velocities, whether by means of the dipole drive or any alternative technology, the dipole drive provides a means of deceleration without power (it could actually generate power) by creating drag against the relative plasma wind. This feat can also be done by a magnetic sail or an electric sail. However because it can also create lift as well as drag, the dipole drive offers much greater maneuverability during deceleration as well as a means to freely maneuver within the destination solar system after arrival.

Dipole Drive Design Issues

Let us consider the case of a 2 kg microsatellite operating in LEO, with 5 W of available power to drive a dipole drive. (Note, a typical CubeSat has a mass of 1.3 kg. At 20 kg/kWe, a 5 W solar array should have a mass of about 0.1 kg.) If we operate it with a voltage of 16 Volts, it will produce 28.8 mN/kWe, or 0.144 mN thrust over all. It would have an acceleration of 0.000072 m/s2. This would allow it to generate a ΔV of 2288 m/s in a year, sufficient to provide extensive station keeping propulsion, substantially change its inclination, or to raise it from a 400 km altitude orbit to a 700 km orbit in 1.6 months. To generate this much thrust at 400 km would require a scoop with a radius of 16 m, while doing so at 700 km would require a scoop with a radius of 58 m. Let us assume that the scoop is made of aluminum wire mesh, using wires 0.1 mm in diameter separated by distances of 2 m. Each square meter of mesh would thus have about 1 m length of wire. This needs to be doubled as there are two meshes, one positive and one negative. Therefore, a scoop with a radius of 16 m would have a mass of 32 grams. If the propulsion system were used simply for station keeping, inclination change, or deorbit functions at the 400 km altitude, that’s all that would be needed. To operate at 700 km, a 116 gram scoop would be required. From these examples we can see that the use of the dipole drive to provide propulsion for microsatellites in LEO could potentially be quite attractive, as the modest scoop sizes required do not pose major deployment challenges.

Now let us consider a 100 kg interplanetary spacecraft in interplanetary space, operating with 500 W at a voltage of 2028 volts. From the discussion above it can be seen that this would generate about 2.54 mN of thrust in the direction of orbital motion. The scoop would need to have a radius of about 800 m. In interplanetary space, the Debye shielding length is ~60 m, and so a screen with a 20 m mesh would suffice. Such a screen would have a mass of about 8.5 kg, which would be well within the spacecraft mass budget. The 2.54 mN thrust would accelerate the spacecraft at 0.000025 m/s2. It could thus impart a V to the spacecraft of about 804 m/s per year. Higher accelerations could be provided by increasing the spacecraft power to mass ratio.

The deployment of large scoops composed of two parallel, oppositely charged meshes poses operational and design issues. Prominent among these is the fact that the two opposite charged screens will attract each other. However the total force involved is not that large. For example, let us consider a configuration consisting to two sails of 500 m radius separated by 500 m with a 2 kV potential difference. Then the electric field between them will be 4 volts/m. The area of each screen will be 785,400 m2. From basic electrostatics we have EA = Q/ε, so Q, the charge of each screen will be given by Q=(4)(785,400)(8.85 e-12) = 0.000028 coulombs. The electrostatic force on each sail is given by F=QE, so the total electrostatic force of each sail will be 0.1 mN. This is about a tenth the thrust force exerted by the screens themselves. Nevertheless, as small as they are, both of these forces will need to be negated. This can be done either with structural supports or by rotating the spacecraft and using artificial gravity to hold the sails out perpendicular to the axis of rotation. An alternative is to use the self-repulsion of the charge of each sail to help hold it out flat. In such a configuration two sails held separate from each other by a boom attached to their centers could be expected to curve towards each other at their edges until the stiffening self-repulsive force on each sail from its own charge balanced the bending forces exerted by the spacecraft’s acceleration, the push of the wind, and the attractive force of the opposite sail.

One way to avoid such issues would be to design the system as a literal dipole, with a rod holding a positive charge at its end to the front of the spacecraft, and a rod holding the negative charge pointing to the rear of the spacecraft. Seen from a distance, such a configuration is electrically neutral and would exert negligible field. However, in the zone between the charges, there is a strong field from one pole to the other. Particles entering this field along the rod center lines would experience the full voltage drop. Particles entering the field at some distance from this central axis would experience a lower voltage drop. The overall functional voltage of such a system, from the point of view of power consumption and exhaust velocity, would be an average over many particles entering the dipole field at all distances from its axis. This is obviously a more complex configuration to analyze than that of the two parallel screens discussed so far, but it may be much simpler to implement in practice on an actual spacecraft.

A critical issue is the material to be used to create the dipole drive. In his original paper on the classic electric sail [4], Pekka Janhunen suggested using copper wires with diameters between 2.5 and 10 microns. This is not an optimal choice, as copper has a much lower strength to mass ratio than aluminum, and such thin strands would be quite delicate. For this reason, in the above examples we specified aluminum wire with 100-micron diameters. A potentially much better option, however, might be to use aluminized Spectra, as spectra has about 10 times the yield strength of aluminum, and roughly 1/3 the density (Aluminum 40,000 psi, 2700 kg/m3, compared to Spectra 400,000 psi, 970 kg/m3.). Spectra strands with 100-micron diameters and a coating of 1 micron of aluminum could thus be a far superior material for dipole drive system, and classic electric sails as well. An issue however is Spectra’s low melting point of 147 C. Kevlar, however, with a yield strength of 200,000 psi, a density of 1230 kg/m3, and a melting point of 500 C could provide a good compromise. Still another promising option might be aluminized strands made of high strength carbon fiber, such as the T1000G (924,000 psi, 1800 kg/m3) produced by Toray Carbon Fibers America.

Some options for dipole drive spacecraft configurations are show in in Fig. 6. As can be seen, small dipole drive systems can be used for spacecraft control, for example as an empennage. Such small dipole drive units could also be used for attitude control on non-dipole drive spacecraft, such as solar sails.

Fig. 6. Options for dipole drive spacecraft configuration. Small dipole drive systems can be used for attitude control.

As with the electric sail, the dipole drive must deal with the issue of sail charge neutralization caused by the attraction of ambient electrons to the sail’s positive screen. In reference 4, P. Janhunen showed that the total such current that an electric sail would need to dispose of would be modest, entailing small power requirements if ejected from the spacecraft by a high voltage electron gun. In the case of the dipole drive, the current would be still smaller because the spacecraft has no net charge. In addition electrons acquired by the positive screen could be disposed of by using the power source to transport them to the negative screen. Alternatively, if an electron gun were used, its required voltage would be less than that needed by an electric sail because external to the screens, the dipole drive’s field is much weaker and falls off much more quickly. For these reasons, the issue of sail charge neutralization on the dipole drive should be quite manageable.

Because the dipole drive does not interact with plasma outside of the zone between its screens, the issue of Debye shielding of its screen system to outside charges is not a concern. Debye shielding of its individual wires within screens can be dealt with by means of adequately tight wire spacing. As shown by Janhunen [4], such spacing may be quite liberal (~60 m in near Earth interplanetary space), enabling sails with very low mass to area ratios. [7]


The dipole drive is a promising new technological concept that offers unique advantages for space propulsion. Requiring no propellant, it can be used to thrust in any direction, and both accelerate and decelerate spacecraft operating within planetary magnetospheres, in interplanetary space, and interstellar space. Unlike magnetic sails and electric sails, it can generate both lift and drag, and its maximum velocity is not limited by the speed of the solar wind. Near-term dipole drives could be used to provide a reliable, low cost, low mass technology to enable propellantless movement of spacecraft from one orbit to another, to provide station keeping propulsion, or to deorbit satellites, as required. Then dipole drive could also be used as a method of capturing interplanetary spacecraft into orbit around destination planets, or of lowering the orbits of spacecraft captured into initial elliptical orbits using high thrust propulsion. The latter application is particularly interesting, because it could enable a small lightweight lunar ascent vehicle to carry astronauts home from the Moon by launching directly from the lunar surface to trans-Earth injection and then subsequently lower itself to LEO to rendezvous with a space station or reentry capsule spacecraft without further use of propellant. Such an approach could potentially reduce the mass of a manned lunar mission to within the launch capacity of a single Falcon Heavy. Because it needs no propellant, the dipole drive offers the unique advantage of being able to provide its propulsion service to any spacecraft indefinitely. While the dipole drive is most attractive in orbital space whether ambient plasma is thickest, it can be used in interplanetary space and even enable interstellar missions as well, becoming more attractive for such applications as ancillary technologies, such as power generation evolve.

There are many technical issues that need to be resolved before practical dipole drive spacecraft can become a reality. However both the theory of dipole drive operation and it potential benefits are clear. Work should therefore begin to advance it to flight status. The stars are worth the effort.


1. Jerome Wright (1992), Space Sailing, Gordon and Breach Science Publishers

2. D. G. Andrews and R. Zubrin, “Magnetic Sails and Interstellar Travel”, IAF-88-553, 1988

3. R. Zubrin and D.G Andrews, “Magnetic Sails and Interplanetary Travel,” AIAA-89-2441, AIAA/ASME Joint Propulsion Conference, Monterey, CA July 1989. Published in Journal of Spacecraft and Rockets, April 1991.

4. Pekka Janhunen, “Electric Sail for Spacecraft Propulsion,” J. Propulsion, Vol. 20, No. 4: Technical Notes, pp763-764. 2004.

5. Cosmo, M.L., and Lorenzini, E.C., Tethers in Space Handbook, NASA Marshall Space Flight Center, 1997

6. D. Hambling, “The Impossible EM Drive is Heading to Space,” Popular Mechanics, September 2, 2016.

7. “Debye Length,” Plasma Universe.com, https://www.plasma-universe.com/Debye_length accessed Feb 18, 2018.



‘Oumuamua: New Data Point to a Comet

New evidence for the nature of interstellar object ‘Oumuamua is in, making it far more likely that the unusual interloper is a comet rather than an asteroid. The data come from an array of instrumentation — the Hubble Space Telescope, the Canada-France-Hawaii Telescope, ESO’s Very Large Telescope and the Gemini South Telescope — and show that `Oumuamua is slowing down slightly less than expected. We are talking about a tiny force, about 1/1000 as strong as the pull of the Sun’s gravity, according to this overview of new work in Nature.

The science paper on this work, which also appears in Nature, looks at a variety of possible explanations for the velocity change. The one the authors think most likely is that `Oumuamua (pronounced “oh-MOO-ah-MOO-ah”), now moving at some 114,000 kilometers per hour, has vented material during its pass through our system, behaving the way many comets do. Marco Micheli (ESA), lead author of the paper, puts it this way: “We can see in the data that its boost is getting smaller the farther away it travels from the Sun, which is typical for comets.”

Co-author Karen Meech (University of Hawaii) concurs. It was Meech who led the initial discovery team characterisation in 2017. Although the scientists found no visual evidence for outgassing, it remained true that the composition of its surface resembled a cometary nucleus. Meech adds: “We think that ‘Oumuamua may vent unusually large, coarse dust grains.” If passage through interstellar space had eroded smaller dust grains on the surface of the object, a cloud of larger particles would not have been bright enough for Hubble to detect.

Image: This artist’s impression shows the first interstellar object discovered in the Solar System, ʻOumuamua. Observations made with the NASA/ESA Hubble Space Telescope, CFHT, and others, show that the object is moving faster than predicted while leaving the Solar System. Researchers assume that venting material from its surface due to solar heating is responsible for this behavior. This outgassing can be seen in this artist’s impression as a subtle cloud being ejected from the side of the object facing the Sun. Because outgassing is a behavior typical for comets, the team thinks that ʻOumuamua’s previous classification as an interstellar asteroid should be changed to a comet. Credit: ESA/Hubble, NASA, ESO, M. Kornmesser.

For my part, I like the description offered by Michele Bannister (Queen’s University Belfast) in the article cited above. Bannister compares the object to a “‘baked Alaska’ dessert, with a frozen heart and warm exterior.” An object like this, approaching the Sun, would begin outgassing, though the rate here is tiny. Let me quote from the Nature article:

The outgassing rate is small compared to what typical comets experience, says Jessica Agarwal, an astronomer at the Max Planck Institute for Solar System Research in Göttingen, Germany. ‘Oumuamua also emits relatively little debris, perhaps because its dust particles are too large and heavy for the weak outgassing to carry aloft. That could explain why ‘Oumuamua never developed a visually stunning, comet-like tail.

Comets can be affected by non-gravitational accelerations, however, as ‘Oumuamua now apparently shows. From the paper:

After ruling out solar-radiation pressure, drag- and friction-like forces, interaction with solar wind for a highly magnetized object, and geometric effects originating from ‘Oumuamua potentially being composed of several spatially separated bodies or having a pronounced offset between its photocentre and centre of mass, we find comet-like outgassing to be a physically viable explanation, provided that ‘Oumuamua has thermal properties similar to comets.

The paper also considers solar radiation pressure, the Yarkovsky effect (in which thermal variations on the surface of a rotating object like an asteroid can lead to asymmetric forces), and the possibility of a collision with another object, none of which fit the bill. The unlikely idea that `Oumuamua is an alien spacecraft is rejected because the object is tumbling on all three axes. Our short-term interstellar guest, in any case, has been nudged a bit faster than expected.

Assuming we’re dealing with a comet, the outgassing may make our attempts to trace its home star that much more difficult. The new observations were carried out to help make that call, but team member Olivier Hainaut (European Southern Observatory, Germany) now wonders whether we will ever know its true home. As to its apparently cometary nature, he adds:

“It was extremely surprising that ʻOumuamua first appeared as an asteroid, given that we expect interstellar comets should be far more abundant, so we have at least solved that particular puzzle. It is still a tiny and weird object that is not behaving like a typical comet, but our results certainly lean towards it being a comet and not an asteroid after all.”

The paper is “Non-gravitational acceleration in the trajectory of 1I/2017 U1 (`Oumuamua)”, Nature 27 June 2018 (abstract).



Hayabusa 2 Arrives at Ryugu

The asteroid game is heating up. The Japanese probe Hayabusa 2 has arrived at asteroid 162173 Ryugu, the plan being to reach the surface with landers later this year and bring back samples in 2020. We also have ORISIS-REx, launched in 2014, on course to 101955 Bennu in December, with a sample return planned for 2023. Assuming both missions are successful, scientists will have the opportunity to compare the composition of the two. Both are C-type (carbonaceous) asteroids, darker than previously explored asteroid Itokawa. The current Hayabusa is similar to the probe that first returned an Itokawa sample to Earth in 2010.

JAXA confirmed the arrival of Hayabusa 2 at 9:35 (Japan time) on the 27th: “The National Research and Development Corporation Japan Aerospace Exploration Agency (JAXA) announces that we have confirmed the arrival at asteroid Ryugu (Ryugu) of the asteroid explorer ‘Hayabusa 2′”, adding that the distance between the spacecraft and the asteroid is about 20 kilometers.

From a JAXA statement:

The confirmation of the Hayabusa2 rendezvous made at 9:35 a.m. (Japan Standard Time, JST) is based on the following data analyses:

* The thruster operation of Hayabusa2 occurred nominally
* The distance between Hayabusa2 and Ryugu is approximately 20 kilometers
* Hayabusa2 is able to maintain a constant distance to asteroid Ryugu
* The status of Hayabusa2 is normal

Itokawa is an S-type asteroid, so this will be our first chance to sample a C-type, the closest previous encounter with the latter being the NEAR Shoemaker flyby of 253 Mathilde in 1997, which could not close to less than 1000 kilometers. Hayabusa 2 has already released an image of Ryugu from about 40 kilometers out. I liked the way project manager Yuichi Tsuda described the boulder-filled object:

“The shape of Ryugu is now revealed. From a distance, Ryugu initially appeared round, then gradually turned into a square before becoming a beautiful shape similar to fluorite [known as the ‘firefly stone’ in Japanese]. Now, craters are visible, rocks are visible and the geographical features are seen to vary from place to place. This form of Ryugu is scientifically surprising and also poses a few engineering challenges.”

Image: Closing on destination. This is asteroid 162173 Ryugu from a distance of roughly 40 kilometres. The image was taken by the spacecraft’s ONC-T (Optical Navigation Camera – Telescopic) on June 24, 2018 at around 00:01 JST. Credit : JAXA, University of Tokyo, Kochi University, Rikkyo University, Nagoya University, Chiba Institute of Technology, Meiji University, Aizu University, AIST.

I like the idea of a ‘firefly stone,’ and also note that the name Ryugu means ‘dragon’s palace’ in Japanese, giving the imagination plenty to play with. The science payoff from both Ryugu and Bennu could be significant. C-type asteroids are intriguing because they are assumed to be the source of carbonaceous chondrite meteorites and are thought to contain organic material as well as water. With a diameter of about 900 meters and a low-reflectance surface, Ryugu may not win any beauty contests, but it is conceivable that asteroids like this could be sources of water or oxygen that future space missions will tap.

Moreover, asteroids like these are expected to give us insights into objects in the early Solar System, and the samples gained by Hayabusa 2 and its landers should be able to tell us whether the asteroid’s dark surface is as rich in carbon as we assume. Of particular interest is the question of water. The isotopic and chemical analysis of Ryugu should help us gain insights into the formation of Earth’s oceans through incoming asteroids or comets. Landing site selection will be critical as the mission teams plans a 30 cm per second touchdown in October.

Tsuda goes on to note some of the challenges the early imaging is beginning to reveal. While the rotation axis of the asteroid is perpendicular to its orbit, there is a peak near the equator and various small craters, factors that will have to be considered as landing sites are assessed. The object’s unusual shape also introduces some issues:

Globally, the asteroid also has a shape like fluorite (or maybe an abacus bead?). This means we expect the direction of the gravitational force on the wide areas of the asteroid surface to not point directly down. We therefore need a detailed investigation of these properties to formulate our future operation plans.

Image: It looks like a big space diamond — but with craters. It’s 162173 Ryugu (Dragon’s Castle), and Japan’s robotic Hayabusa 2 mission is now arriving at this near-Earth asteroid. Ambitious Hayabusa 2 is carrying an armada of separable probes, including two impactors, four small close-proximity hoverers, three small surface hoppers, and the Mobile Asteroid Surface Scout (MASCOT) which will land, study, and move around on Ryugu’s surface. Most of these are equipped with cameras. Moreover, Hayabusa 2 itself is scheduled to collect surface samples and return these samples to Earth for a detailed analysis near the end of 2020. Pictured, a series of approach images shows features suggestive of large boulders and craters. Credit & Copyright: ISAS, JAXA, Hayabusa 2 Team.

Hayabusa 2’s distance from Earth is now 1.9 AU, with a round-trip light time of 1895 seconds.



Exoplanet Hunt: Speeding Up the Data Pipeline

The K2 mission’s C16 and C17 observing campaigns — each containing observations of one patch of the sky for an 80-day period — have proven fruitful for astronomers at MIT. The institution’s Ian Crossfield, working with graduate student Liang Yu, has brought new software tools developed at MIT to work, producing results just weeks after the mission’s raw data for these observing runs were made available. Now we have nearly 80 new exoplanet candidates from C16, but we also have a method of fast analysis that should benefit future missions.

Let’s pause on method. The idea here is to speed up the analysis of light curves, the graphs showing the intensity of light from a star. Between them, C16 and C17 tracked about 50,000 stars, the analysis of whose light would normally take at least several months and perhaps as long as a year. Speed is of the essence because a faster planet identification process makes it possible for quick ground-based radial velocity follow-ups that might otherwise prove challenging.

The reason: K2’s position in a trailing orbit as Earth moves around the Sun means that, depending on its orientation, some stars are not observable by scientists on the ground until the Earth returns to the same patch of sky. So-called ‘rear-facing’ campaigns, as this MIT news release points out, have little need for fast data analysis because the follow-up cannot happen for almost a year. But C16 and C17, in ‘forward facing mode,’ examined stars that remained within Earth’s field of view for several more months. The confirmation process could begin.

Image: The Kepler Space Telescope, now operating in its K2 extended mission, diagrammed here in ‘forward-facing mode.’ “Forward-facing” implies looking towards Earth in the spacecraft’s orbit, in the direction of the spacecraft’s velocity vector. Credit: This is a slide from a talk by Geert Barentsen and Tom Barclay for the 20th Microlensing Workshop in Paris in 2016.

The researchers’ goal, then, was to work through the light curves, which had been released on February 28, to let their algorithms narrow the field. In the case of C16, which is discussed in a paper on this work in The Astronomical Journal, that means paring 20,647 targets down to 1,097 stars of particular interest. The paper presents a catalog of interesting C16 targets identified through photometry, candidates for rapid identification and follow-up. 30 high-quality planet candidates emerged from the process, along with 48 lower-quality candidates, 164 eclipsing binaries, and 231 other periodically-variable astrophysical sources.

One find stands out as particularly interesting, a planet around the star HD 73344 that orbits its primary every 15 days. HD 73344 is a high proper-motion F6 star, its planet (if confirmed) orbiting the brightest planet host K2 has yet discovered. Crossfield and Yu’s data point to a planet of 2.5 Earth radii and 10 Earth masses circling its star every 15 days — Crossfield refers to it as “a smaller, hotter version of Uranus or Neptune.” At 114 light years from Earth, HD 73344 b could be a prime candidate for atmospheric composition studies.

The paper argues that fast planet searches should be of value for TESS (Transiting Exoplanet Survey Satellite), which will study nearby stars in 30-day periods, ultimately covering 85% of the sky. The data trove from this mission should be enormous, and as it is received, there will be a period of months before the stars examined during that month set for the year. Says Crossfield:

“If we get candidates out quickly to the community, everyone can start immediately observing systems discovered by TESS, and doing a lot of great planetary science. So this [analysis] was really a dress rehearsal for TESS.”

And this is from the paper:

The release of planet catalogs has occurred only irregularly during the K2 mission, but this paradigm will change once TESS operations begin in earnest. Data from TESS will be released and processed on a 27-day rhythm for most of the two-year mission duration. With the shorter observing windows, ephemeris decay is also a much larger problem for TESS and therefore the importance of securing planet candidates in the same season is even higher. If interesting objects could be rapidly gleaned from TESS data and circulated to the community, follow-up observations and analyses could begin a full season earlier and so the full impact of that mission could more quickly be achieved.

As to TESS itself, it’s hard to believe we’re already 68 days past the launch, a period in which the spacecraft has been undergoing commissioning operations before beginning the collection of scientific data.

The paper is Yu et al., “Planetary Candidates from K2 Campaign 16,” The Astronomical Journal Vol. 156, No. 1 (21 June 2018). Abstract / preprint.



The Importance of an Eclipsing Charon

The quality of the image below isn’t very high, but consider what we’re looking at. This is the ‘night side’ of Pluto’s moon Charon as viewed against a star field by the New Horizons spacecraft. We’re looking at reflected light from Pluto –’Plutoshine’ — as the sole illumination of most of the surface. Who would have thought, in the 88 years since Clyde Tombaugh’s discovery of Pluto, that we would see a Plutonian moon’s dark side by Pluto’s light?

I wonder if there would have been a mission to Pluto at all if it hadn’t been for James Christy. Working with astronomer Robert Harrington at the U.S. Naval Observatory Flagstaff Station (Arizona), Christy was situated just miles away from Lowell Observatory, where Pluto was discovered, when he noticed a strange elongation in images of the world. That was forty years ago, on June 22, 1978, during an effort to tighten up estimates of Pluto’s orbit around the Sun.

I suppose an astronomical analogue to this odd ‘blob’ on Christy’s plates would be the elongation Galileo puzzled over when he looked at Saturn with his earliest optics. In both cases, it would take a bit of imagination and better viewing to make out the real cause, which in Christy’s case was the moon that would be called Charon. It was Christy who proposed the name, and it stuck because the ferryman who carried souls across the river Acheron fit right in with the Plutonian milieu. Acheron was one of five rivers in mythical Pluto’s underworld.

Image: Jim Christy points to the photographic plate on which he discovered Pluto’s largest moon, Charon, in 1978. Credit: U.S. Naval Observatory.

A careful scientist, Christy compared other images to note that the ‘bump’ on Pluto seemed to move from one side to another. Bear in mind that it had been almost a half century since the discovery of Pluto, and in that time no moon had been found. Finding still more images showing an ‘elongated’ Pluto, Christy and Harrington worked out orbital parameters of a moon that could explain what they were seeing, and subjected them to yet more data via a confirmation by the Naval Observatory’s 61-inch telescope. The discovery was announced on July 7, 1978.

And the personal note: We’re told that Christy first chose the name because its first four letters matched the name of his wife, Charlene, known as ‘Char’ to her friends..”A lot of husbands promise their wives the moon,” Charlene Christy would later say, “but Jim actually delivered.”

Image: Forty years after his important discovery, Jim Christy holds two of the telescope images he used to spot Pluto’s large moon Charon in June 1978. A close-up photo of Charon, taken by the New Horizons spacecraft during its July 2015 flyby, is displayed on his computer screen. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Art Howard/GHSPi.

What a fine tribute that the New Horizons science operations center at JHU/APL, dedicated in 2002, is named after Christy. For the discovery of Charon marked a huge step in getting New Horizons bumped along the difficult terrain of mission approval and launch. Charon is aligned to make eclipses of Pluto possible (for a period of several years) just twice every 248 year Plutonian orbit of the Sun. The eclipses would start in 1985, allowing good science to flow.

Surface brightness could be measured, compositions inferred, possible atmospheres. As David Grinspoon and Alan Stern note in Chasing New Horizons:

In the pantheon of strange and lucky coincidences, the fact that Charon was discovered just before it was about to swing into place for this mutual event season — after all, another such set of eclipses wouldn’t occur for more than a century — is right up there with the grand-tour alignment of the planets appearing just when humans were ready to take advantage of it by mastering spaceflight.

Suddenly a six-year observing window was opening, one that would put Pluto at the top of every major scientific conference, and given the trials of getting New Horizons flown, it’s more than likely that without this string of events, the mission would never have been built. As it is, those of us who were startled to read about Christy’s discovery years ago now have the opportunity to look at images like the one below, showing what we saw then and what we see now.

Image: What a difference 40 years makes. An enhanced color image of Charon from data gathered by the New Horizons spacecraft in 2015 shows a range of diverse surface features, significantly transforming our view of a moon discovered in 1978 as a “bump” on Pluto (inset) in a set of grainy telescope images. Credit: U.S. Naval Observatory; NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

The Charon image that follows shows the fractures and canyons of Charon’s Pluto-facing hemisphere, a belt that stretches fully 1600 kilometers across the face of the moon and most likely onto the far side. Stark evidence of geological upheaval. the terrain is four times as long as the Grand Canyon and in places twice as deep, as this JHU/APL news release points out. New Horizons deputy project manager Cathy Olkin (SwRI) sees this tectonic belt as evidence that Charon once had a subsurface ocean whose eventual freezing cracked the surface.

Image: This great canyon system stretches more than 1,600 kilometers across the entire face of Charon and likely around onto Charon’s far side. Four times as long as the Grand Canyon, and twice as deep in places, these faults and canyons indicate a titanic geological upheaval in Charon’s past. Credit: JHU/APL.

So this post is all about who we were 40 years ago and who we are now, what we knew back then and how dogged persistence and extraordinary coincidence could propel us to today’s stunning views of Charon. “When you go from this little blur in which you don’t actually see anything, to the enormous detail New Horizons sent back,” says Christy, “it’s incredible.”



On Galactic Migration

Yesterday I looked at the prospect of using technology to move entire stars, spurred on by Avi Loeb’s recent paper “Securing Fuel for Our Frigid Cosmic Future.” As Loeb recounts, he had written several papers on the accelerated expansion of the universe, known to be happening since 1998, and the resultant ‘gloomy cosmic isolation’ that it portends for the far future. It was Freeman Dyson who came up with the idea that a future civilization might move widely spaced stars, concentrating them into a small enough volume that they would remain bound by their own gravity. This escape from cosmic expansion has recently been explored by Dan Hooper, who likewise considers moving stellar populations.

Image: Harvard’s Avi Loeb, whose recent work probes life’s survival at cosmological timescales.

I gave a nod yesterday to the star-moving ideas of Leonid Shkadov, who suggested a ‘Shkadov thruster’ that would use the momentum of stellar photons to operate, but Loeb pointed out how inefficient the process would be. Better to harvest stellar energy more directly, as Hooper was proposing. This reminds me of Fritz Zwicky’s own ideas about moving stars. In his book Discovery, Invention, Research through the Morphological Process (Macmillan, 1969) the physicist developed ideas he had first presented in lectures at Oxford University in 1948 on how to reach Alpha Centauri.

The fiercely independent Zwicky coined the term ‘stellar propulsion’ at Oxford and went on to describe using the matter of the Sun itself as nuclear propellant. In his later work, he followed up on the idea, the plan being to turn our planet, pulled along with the Sun, into the ultimate generation ship. I have to pause to quote Zwicky on this, from a June, 1961 article in Engineering and Science called “The March Into Inner and Outer Space”:

In order to exert the necessary thrust on the sun, nuclear fusion reactions could be ignited locally in the sun’s material, causing the ejection of enormously high-speed jets. The necessary nuclear fusion can probably best be ignited through the use of ultrafast particles being shot at the sun. To date there are at least two promising prospects for producing particles of colloidal size with velocities of a thousand kilometers per second or more. Such particles, when impinging on solids, liquids, or dense gases, will generate temperatures of one hundred million degrees Kelvin or higher-quite sufficient to ignite nuclear fusion. The two possibilities for nuclear fusion ignition which I have in mind do not make use of any ideas related to plasmas, and to their constriction and acceleration in electric and magnetic fields.

Like Loeb, Zwicky (1898-1974) liked to think big. The discoverer of 122 supernovae, he came to be interested in galactic clusters, and in particular the Coma cluster. Here his reputation for being ahead of his time is on full display, for he discovered that the mass of the cluster was far too little to produce the gravitational effects observed. In other words, something was keeping the cluster together beside visible matter. This anticipation of what we today call ‘dark matter’ was one Zwicky suggested could be studied by another cutting edge idea, gravitational lensing.

I wish I had known Zwicky, who surely would have jumped into the ideas in Avi Loeb’s paper with gusto. Loeb argues that moving stars to concentrate them into smaller regions is ultimately not necessary. If we want to avoid the cosmic fate awaiting us, with galaxies winking out in the distant future as they move beyond the visible universe, we should think in terms of locating the places where stars are already the most concentrated, the huge galactic clusters. Zwicky, the force behind a six-volume catalog of 30,000 galaxies based on the Palomar Observatory Sky Survey, was just the man to appreciate this insight, and doubtless to add a few of his own.

The Coma cluster that was the subject of Zwicky’s observations on ‘dark matter’ is about six times further away than the Virgo cluster, but both are laden with resources. Given that accelerated cosmic expansion should be detectable by any sufficiently advanced civilization, these galactic clusters — massive reservoirs of fuel, as Loeb calls them — should be desirable places for migration, just as in our own history civilizations settled around rivers and lakes.

The benefits of moving a civilization into a galactic cluster are numerous, writes Loeb:

Once settled in a cluster, a civilization could hop from one star to another and harvest their energy output just like a butterfly hovering over flowers in a hunt for their nectar. The added benefit of naturally-produced clusters is that they contain stars of all masses, much like a cosmic bag that collected everything from its environment. The most common stars weigh a tenth of the mass of the Sun, but are expected to shine for a thousand times longer because they burn their fuel at a slower rate. Hence, they could keep a civilization warm for up to ten trillion years into the future.

As Loeb goes on to point out, nearby red dwarfs like Proxima Centauri and TRAPPIST-1 have already been found to have rocky, Earth-sized planets around them in or near the habitable zone. If this is the case with nearby stars we have just begun to examine, the implication is that planets are likely around most. This kind of star, the M-dwarf comprising up to 80 percent of all stars in the Milky Way, appears made to order for civilizations dependent on liquid water. Note the vivid image above, by the way: A civilization harvesting energy output like a butterfly hovering over flowers. Like fellow astronomer Greg Laughlin, Loeb is an uncommonly fine wordsmith.

Image: Almost every object in the above photograph is a galaxy. The Coma Cluster of galaxies pictured here is one of the densest clusters known – it contains thousands of galaxies. Each of these galaxies houses billions of stars – just as our own Milky Way galaxy does. Although nearby when compared to most other clusters, light from the Coma Cluster still takes hundreds of millions of years to reach us. In fact, the Coma Cluster is so big it takes light millions of years just to go from one side to the other. Most galaxies in Coma and other clusters are ellipticals, while most galaxies outside of clusters are spirals. The nature of Coma’s X-ray emission is still being investigated. Credit: Russ Carroll, Robert Gendler, & Bob Franke; Dan Zowada Memorial Observatory.

Naturally we are talking about very long-term solutions to a far-distant problem when we discuss moving a civilization to the most useful galactic cluster. The question comes down to whether it would be possible to travel, say, a hundred million light years within the age of the universe. To do this, Loeb says, it would be necessary to exceed one percent of the speed of light. At these speeds, no relativistic time dilation can shorten the journey for its participants. These would be civilization-spanning journeys by cultures capable of surviving on geological timescales.

But let’s mimic Fritz Zwicky and let our imaginations loose. Zwicky proposed that a moving Sun could reach Alpha Centauri in approximately 50 human generations. Loeb ratchets up the challenge in a grand way, though leaving the method of travel up to future scientists. A bonus in going where the fuel is: We might expect to find other civilizations that have made the same decision, with whom we could share cultures and technologies. Like individual species, perhaps all life-forms capable of making the journey will want to congregate around watering holes like these, a far future echo of the history of life on a planet we may or may not be taking with us.

The paper is Loeb, “Securing Fuel for Our Frigid Cosmic Future” (preprint).



Cosmic Engineering and the Movement of Stars

Avi Loeb’s new foray into the remote future had me thinking of the Soviet physicist Leonid Shkadov, whose 1987 paper “Possibility of Controlling Solar System Motion in the Galaxy” (citation below) discussed how an advanced civilization could get the Sun onto a new trajectory within the galaxy. Why would we want to do this? Shkadov could imagine reasons of planetary defense, a star being moved out of the way of a close encounter with another star, perhaps.

All of this may remind science fiction readers of Robert Metzger’s novel CUSP (Ace, 2005), which sees the Sun driven by a massive propulsive jet. A more recent referent is Gregory Benford and Larry Niven’s novels Bowl of Heaven (Tor, 2012) and ShipStar (2014), in which a star is partially enclosed by a Dyson sphere and used to explore the galaxy. In 1973, Stanley Schmidt would imagine Earth itself being moved to M31 as a way of avoiding an explosion in the core of the Milky Way that threatens all life (Sins of the Fathers, first published as a serial in Analog).

Loeb’s paper mentions technologies for moving stars because several recent proposals involve this kind of ‘cosmic engineering’ to change civilizational outcomes. The accelerated expansion of the universe will, after the universe has aged by a factor of 10, make all stars outside the Local Group of galaxies disappear as they recede. The process continues with continued acceleration. Wait long enough and we fall prey to cosmic winter, and as Loeb writes:

Following the lesson from Aesop’s fable “The Ants and the Grasshopper,” it would be prudent to collect as much fuel as possible before it is too late, for the purpose of keeping us warm in the frigid cosmic winter that awaits us. In addition, it would be beneficial for us to reside in the company of as many alien civilizations as possible with whom we could share technology, for the same reason that animals feel empowered by congregating in large herds.

There are places where we might do this, says Loeb, and I’ll talk about his ideas on them tomorrow. For today, note that in response to his previous papers on the oncoming ‘cosmic isolation,’ Freeman Dyson proposed to Loeb his own project that would move stars, bringing large-scale formations of stars down to a more manageable volume so that they will be bound by their own gravity. Closely spaced, the stellar collection avoids being dissipated in the accelerated expansion of the cosmos. And it turns out that Dan Hooper (University of Chicago) has also been pondering induced stellar motion, using the energy output of stars to cluster large numbers of them into an astronomically tight radius so that their energies can be harvested.

Image: A Shkadov thruster as conceived by the artist Steve Bowers.

Like Dyson and Loeb, Hooper has his eye on the acceleration of cosmic expansion and its consequences. So what kind of ‘stellar engines’ can we envision that could move objects as large as stars? Leonid Shkadov suggested using a star’s radiation pressure. The Shkadov thruster extracts energy from the star by using a vast mirror to take advantage of photon momentum. Let me turn back to an earlier post to reprint a diagram that Duncan Forgan uses in describing a Shkadov thruster. Forgan (University of Edinburgh) points out the difference between these thrusters and Dyson spheres, the latter being spherical and shaped so as to balance gravitational forces on the sphere by way of collecting a maximum amount of stellar energy. But here is the Shkadov thruster as diagrammed by Forgan:

Image: This is Figure 1 from Duncan Forgan’s paper “On the Possibility of Detecting Class A Stellar Engines Using Exoplanet Transit Curves.” Caption: Diagram of a Class A Stellar Engine, or Shkadov thruster. The star is viewed from the pole – the thruster is a spherical arc mirror (solid line), spanning a sector of total angular extent 2ψ. This produces an imbalance in the radiation pressure force produced by the star, resulting in a net thrust in the direction of the arrow. Credit: Duncan Forgan.

Thus the whole idea of the Shkadov thruster is not balance but imbalance. And Forgan goes on to say this about the idea:

In reality, the reflected radiation will alter the thermal equilibrium of the star, raising its temperature and producing the above dependence on semi-angle. Increasing ψ increases the thrust, as expected, with the maximum thrust being generated at ψ = π radians. However, if the thruster is part of a multi-component megastructure that includes concentric Dyson spheres forming a thermal engine, having a large ψ can result in the concentric spheres possessing poorer thermal efficiency.

Efficient or not, Shkadov thrusters interest Forgan as a possible SETI detection (Hooper also notes the possibility). Like Dyson spheres, their sheer scale and unusual features could make them visible in a lightcurve, perhaps with the aid of radial velocity follow-ups. Arguing against the idea, though, is the fact that the Shkadov thruster is probably not the technology our hypothetical future civilization would use to move its star (or stars).

I found this out when I wrote Avi Loeb in reaction to his new paper and mentioned the Shkadov idea. Loeb found Dan Hooper’s ideas (in “Life Versus Dark Energy: How An Advanced Civilization Could Resist the Accelerating Expansion of the Universe,” citation below) to be a better solution to the problem. Here is what Loeb told me in our email exchange yesterday:

The use of the momentum associated with the radiation emitted by the star for its propulsion, as envisioned in Shkadov’s thruster, is much less efficient than using the energy associated with same radiation. The radiation momentum equals its energy divided by the speed of light, c. However, the momentum gained by converting this energy to the kinetic energy of a massive object moving at a speed v is larger by a factor of 2(c/v). This is a huge factor of which Dan Hooper is taking advantage to argue that Sun-like stars can reach a percent of the speed light, 0.01c, in a billion years. If he would have used the momentum of the light emitted by the star as in Shkadov’s thruster, then the attainable speed would have been a hundred times smaller, only of order 30 km/s (similar to the speed of chemical rockets), and the journey being contemplated would have not been feasible. During the age of the Universe (10 billion years) one would only be able to traverse a million light years and not leave the Local Group of galaxies. This is not sufficient for gaining more fuel than available within the Local Group of galaxies.

Thus the problem of the Shkadov thruster: By using the momentum of stellar photons, Shkadov loses efficiency. Loeb adds: “One way to improve the efficiency of Shkadov’s thruster (by employing the energy and not the momentum of the star’s radiation) is to harvest the energy from the star through a Dyson sphere and then use it to ablate its surface on one side, generating a rocket effect.” This seems to be what Dan Hooper has in mind in his paper. The civilization in question would harvest the energy of stars through Dyson spheres that, quoting Hooper, “…use the collected energy to propel the captured stars, providing new and potentially distinctive signatures of an advanced civilization in this stage of expansion and stellar collection.”

Hooper has a SETI prospect in mind, while also thinking about collecting sources of energy, maximizing its amount in the form of starlight by a factor of several thousand. It is this prospect that interests Loeb. His paper makes the case that we already have massive reservoirs of fuel in the visible universe in the form of galactic clusters, each containing the equivalent of 1000 Milky Ways. Perhaps, then, we need no star-moving and collection project a la Hooper or Dyson, but rather need to think about reaching a galactic cluster for our fuel. Given the magnitude of that challenge, whether or not we take our home star along is inconsequential.

The speculative buzz I get from this is science fictional indeed. The nearest cluster is Virgo, whose center is some 50 million light years away, with the Coma cluster six times farther still. Thinking in terms of a civilization that could cross such gulfs takes us into Olaf Stapledon territory, a region of spacetime with which I share Loeb’s obvious fascination. I’m running out of time today, but want to look deeper into this with the help of Loeb’s paper tomorrow. I’ll also have more thoughts on ‘stellar engines’ and their origins.

Avi Loeb’s new paper is “Securing Fuel for Our Frigid Cosmic Future” (preprint). Leonid Shkadov’s paper on the Shkadov thruster is “Possibility of controlling solar system motion in the galaxy,” 38th Congress of IAF,” October 10-17, 1987, Brighton, UK, paper IAA-87-613. The Forgan paper is “On the Possibility of Detecting Class A Stellar Engines Using Exoplanet Transit Curves,” accepted at the Journal of the British Interplanetary Society (preprint). The Hooper paper is “Life Versus Dark Energy: How An Advanced Civilization Could Resist the Accelerating Expansion of the Universe” (2018). Preprint.



On Potentially Habitable Moons

Looking through a recent Astrophysical Journal paper on gas giants in the habitable zone of their stars, I found myself being diverted by the distinction between a conservative habitable zone (CHZ) and a somewhat more optimistic one (OHZ). Let’s pause briefly on this, because these are terms that appear frequently enough in the literature to need some attention.

The division works like this (and I’ll send you to the paper for references on the background work that has developed both concepts): The OHZ in our Solar System is considered to be roughly 0.71 to 1.8 AU, which sees Venus as the inner cutoff (a world evidently barren for at least a billion years) and Mars as the outer edge, given that it appears to have been habitable in the early days of the system, perhaps some 3.8 billion years ago. ‘Habitable’ in both HZ categories is defined as the region around a star where water can exist in a liquid state on a planet with sufficient atmospheric pressure (James Kasting has a classic 1993 paper on all this).

The CHZ’s inner edge is considered to be at the ‘runaway greenhouse limit,’ where the breakdown of water molecules by solar radiation allows free hydrogen atoms to escape, drying out the planet at approximately 0.99 AU in our own system. Its outer edge, says the paper:

…consists of the maximum greenhouse effect, at 1.7 AU in our solar system, where the temperature on the planet drops to a point where CO2 will condense permanently, which will in turn increase the planet’s albedo, thus cooling the planet’s surface to a point where all water is frozen (Kaltenegger & Sasselov 2011).

It goes without saying that boundaries like these are going to vary from one planetary system to another, and it’s likewise clear that most of our thinking about habitable zone planets has gone in the direction of small rocky worlds as we mount the search for Earth analogues. What Stephen Kane (University of Southern Queensland), working with an undergraduate student at the university named Michelle Hill as well as colleagues at the University of California, Riverside has done is to identify 121 giant planets in Kepler data that could host habitable moons.

To be sure, the gas giants themselves aren’t considered candidates for life as we know it (though obviously we can’t rule out exotic species adapted to extreme conditions, like Edwin Salpeter’s ‘gasbags,’ free-floating lifeforms that might populate dense atmospheres — see Edwin Salpeter and the Gasbags of Jupiter for more). But the real focus is on those rocky moons that occur in such abundance in our own system.

“There are currently 175 known moons orbiting the eight planets in our solar system. While most of these moons orbit Saturn and Jupiter, which are outside the Sun’s habitable zone, that may not be the case in other solar systems,” says Kane. “Including rocky exomoons in our search for life in space will greatly expand the places we can look.”

Image: This is an artist’s illustration of a potentially habitable exomoon orbiting a giant planet in a distant solar system. Credit: NASA GSFC: Jay Friedlander and Britt Griswold.

As we consider the different dimensions of habitable zones around other stars, we should also keep in mind the fact that the moons that may emerge in these systems can be as various as our own. Earth’s Moon, for example, seems to be the result of a giant impact early in the system’s formation. Most moons are thought to have formed by accretion within the dust disks around planets, but others can be captured by a planet’s gravitational pull — Triton seems to be an example of this. Thus we could find moons of considerably different composition than their host planet. Considering how many moons we see orbiting our gas giants, the assumption that moons exist around other such worlds in exoplanetary systems seems reasonable.

We still have no exomoon detections, but the search continues, and I always scan the latest papers from the Hunt for Exomoons with Kepler project that David Kipping runs with anticipation, along with those of exomoon theorist René Heller. Having a database of the giant planets we’ve identified thus far as being in the habitable zone of their star may help us target future observations to refine the expected properties of their moons, assuming these exist. Such moons would receive energy from the primary star, of course, but would also receive reflected radiation from the planet they orbit. René Heller has proposed that exomoons in a habitable zone could provide a better environment for life than Earth itself. Let me quote the Hill paper:

Exomoons have the potential to be what [Heller] calls ”super-habitable” because they offer a diversity of energy sources to a potential biosphere, not just a reliance on the energy delivered by a star, like earth. The biosphere of a super-habitable exomoon could receive energy from the reflected light and emitted heat of its nearby giant planet or even from the giant planet’s gravitational field through tidal forces. Thus exomoons should then expect to have a more stable, longer period in which the energy received could maintain a livable temperate surface condition for life to form and thrive in.

Discussing the difficulties of exomoon detection, such as the fact that multiple moons around a single planet may eliminate a useful transit timing signal (this is Jean Schneider’s work) and the problems of direct imaging, it’s interesting to see that microlensing remains a candidate. It’s also intriguing to ponder the fate of exomoons, as this paper does, in terms of migrating gas giants and the likelihood that their moons will be lost. We still have much to learn about the movement of giant planets and the effect of their migration upon their own moons as well as other planets.

Once we have a firm exomoon detection, we can begin to characterize the possibilities. As we await improvements in our technology, deepening our knowledge of potential exomoon host planets is the best we can do, and that would begin, as this paper suggests, with radial velocity follow-up observations on gas giant habitable zone candidates like the ones compiled by the authors.

The paper is Hill et al., “Exploring Kepler Giant Planets in the Habitable Zone,” The Astrophysical Journal, 2018; 860 (1): 67. Abstract / preprint. The Kasting paper mentioned above is “Habitable Zones around Main Sequence Stars,” Icarus Vol. 101, Issue 1 (1993), pp. 108-128 (abstract). For René Heller’s work on ‘superhabitable’ moons, see Heller & Armstrong, “Superhabitable Worlds,” Astrobiology January 2014 (preprint). Jean Schneider’s paper on exomoon detection problems is Schneider & Sartoretti, “On the detection of satellites of extrasolar planets with the method of transits,” Astronomy & Astrophysics. Suppl. Ser. Vol. 134, No. 3 (1 February), pp. 553-560 (abstract).



Marc Millis on Mach Effect Thruster, EmDrive Tests

Marc Millis spent the summer of 2017 at the Technische Universität Dresden, where he taught a class called Introduction to Interstellar Flight and Propulsion Physics, a course he would also teach at Purdue University last November. The former head of NASA’s Breakthrough Propulsion Physics project and founding architect of the Tau Zero Foundation, Marc participated in the SpaceDrive project run by Martin Tajmar in Dresden, an effort that has been in the news with its laboratory testing of two controversial propulsion concepts: The Mach Effect Thruster and the EmDrive. Marc’s review comments on modeling for the former were almost as long as Tajmar’s draft paper. Described below, the SpaceDrive project is a wider effort that includes more than these two areas — neither the EmD or MET thruster had reached active test phase during the summer he was there — but the ongoing work on both occupies Millis in the essay that follows.

by Marc Millis

You may have noticed a renewed burst of articles about the EmDrive. What prompted this round of coverage was an interim report, part of the progress on Martin Tajmar’s ‘SpaceDrive’ project to carefully test such claims. Tajmar’s conference paper [citation below] is one of the early steps to check for false-positives. I expect more papers to follow, each progressing to other possibilities. It might take a year or so more before irrefutable results are in. Until then, treat the press stories about certain conclusions as highly suspect.

On Tajmar’s work, this quote from his conference paper:

Within the SpaceDrive project [6], we are currently assessing the two most prominent thruster candidates that promise propellantless propulsion much better than photon rockets: The so-called EMDrive and the Mach-Effect thruster. In addition, we are performing complementary experiments that can provide additional insights into the thrusters under investigation or open up new concepts. In order to properly test the thruster candidates, we are constantly improving our thrust balance facility as well as checking for thruster-environment interactions that can lead to false thrust measurements.

The Mach Effect Thruster is a different approach to the goal of a non-rocket spacedrive, but one that is rooted in unsolved questions in physics where there is a chance for new discoveries. Its theory led to a testable prediction that then evolved into an idea for a propulsive effect.

The unsolved physics question is: “What is the origin of inertial frames?” One attempt to answer that is called “Mach’s Principle” (term coined by Einstein to describe Ernst Mach’s perspective), which is roughly: “inertia here, because of matter out there.” The idea is that the phenomenon of inertia is an interaction between that mass and all the surrounding mass in the universe (presumed gravitational in nature). Jim Woodward picked up on a version of this from Dennis Sciama, and noticed that the inertial mass of an object can fluctuate if its energy fluctuates (think energy in a capacitor). That led to an idea for a propulsive effect by varying the distance between two fluctuating inertias. Unlike the EmDrive, this idea has been in the peer-reviewed literature from the beginning, with some of the more relevant papers being:

Woodward, J. F. (1990). A New Experimental Approach to Mach’s Principle and Relativistic Gravitation, in Foundations of Physics Letters, 3(5): 497-506.

Woodward, J. F. (1991). Measurements of a Machian Transient Mass Fluctuation, in Foundations of Physics Letters, 4(5): 407-423.

Woodward, J (1994), “Method for Transiently Altering the Mass of an Object to Facilitate Their Transport or Change their Stationary Apparent Weights,” US Patent # 5,280,864.

Woodward, J. (2012). Making Starships and Stargates, Springer.

Fearn, H. & Wanser, K. (2014). Experimental Tests of the Mach Effect Thruster. Journal of Space Exploration, 3: 197-205.

Martin Tajmar’s laboratory results can be summarized this way: False positive thrusts were observed under conditions where there should be no thrusting or only minor thrusting. More systematic checks have to be made prior to testing the thrusters at their nominal and maximum operating parameters. The mismatch was more pronounced for the EmDrive than for the Mach Effect Thruster. In both cases it is premature to reach definitive conclusions since this is a work in progress. And if any thrusters do pass all those tests, then more tests will commence to figure out how the thrusters operate (varying conditions to see which affect the thrust levels).

In the case of the EmDrive, only 2 W of the more normal 60 W of power was made available to the thruster. Even at that low power level, thrusts of about 4 µN were observed, which is more than the 2.6 µN expected from the claims from Sonny White’s tests. The more revealing observations were that thrusts were observed when the EmDrive was not supposed to be thrusting. When the EmDrive was pointed to a non-thrusting direction, thrusts were still observed. When the power to the thruster was sent to an attenuator to further reduce the power to the thruster by a factor of 10,000, thrusting at the prior level was still observed.

These observations do not bode well for the EmDrive’s claims of real thrust, but it is too early to firmly dismiss the possibilities. One suspect for the false positive is the interaction with the current to the device and the Earth’s magnetic field, where a current of 2-amps in a few cm of wires can produce a thrust in the µN range. Further tests are planned after adding more magnetic shielding and operating over different power levels.

In the case of the Mach Effect Thruster – which by the way, none of the press articles mentioned – the findings were less pessimistic. Again there were thrusts measured in excess of what was expected for the low power levels (0.6 versus 0.02 µN). Unlike the EmDrive’s mismatch, no thrust was observed when the Mach Effect Thruster was pointed to a non thrusting direction. There was, however, a case where the thrust direction did not change when the thruster direction was flipped. The suspected causes to be further investigated include both magnetic and thermal (expansion) effects.

A word of advice: if you plan to look at Tajmar’s paper. When I tried my usual “rush read” through the paper by reading the abstract and scanning the figures, I misled myself. Read the full text that accompanies the figures to know what you are really looking at. It’s a short article.

Regarding some representative press articles, here is a quick assessment

(1) David Hambling, New Study Casts Doubt on the “Impossible” EmDrive, But this weird propulsion idea isn’t dead yet

This one goes into more detail than the other articles about what was actually done and not done and does link to its information sources. It does not mention the Mach Effect Thruster.

(2) Mike Wall, ‘Impossible’ EmDrive Space Thruster May Really Be Impossible

This one mentions the doubt, but leaves the door open just a bit. Although it does not mention the Mach Effect Thruster also under test, it does at least give a link to the core article and mentions where it came from.

(3) Ethan Siegel, The EmDrive, NASA’s ‘Impossible’ Space Engine, Really Is Impossible: Many tests have reported an ‘anomalous thrust’ where there should be none. A researcher has finally shown where everyone else has messed up

This article talks more about the old claims and expectations than what was really in the new paper. It does not mention the Mach Effect Thruster.

(4) Mike Wehner, NASA’s ‘impossible’ fuel-free engine is actually impossible

More short-hand opinion, and again, no mention of the Mach Effect Thruster.

The takeaway: Science does not proceed by proclamation. Despite what headlines may say, laboratory work is a matter of refining techniques and bringing precision to bear on prior claims. At the moment, evaluation of the EmDrive and Mach Effect thruster continues, with no guarantee that either of these effects may prove genuine, but let’s let the process play out.

The Tajmar paper is Tajmar et al., “The SpaceDrive Project – First Results on EMDrive and Mach-Effect Thrusters,” presented at the Space Propulsion 2018 conference in Seville, Spain (full text).