by Paul Gilster | Mar 25, 2022 | Missions |
Epsilon Eridani has always intrigued me because in astronomical terms, it’s not all that far from the Sun. I can remember as a kid noting which stars were closest to us – the Centauri trio, Tau Ceti and Barnard’s Star – wondering which of these would be the first to be visited by a probe from Earth. Later, I thought we would have quick confirmation of planets around Epsilon Eridani, since it’s a scant (!) 10.5 light years out, but despite decades of radial velocity data, astronomers have only found one gas giant, and even that confirmation was slowed by noise-filled datasets.
Even so, Epsilon Eridani b is confirmed. Also known as Ægir (named for a figure in Old Norse mythology), it’s in a 3.5 AU orbit, circling the star every 7.4 years, with a mass somewhere between 0.6 and 1.5 times that of Jupiter. But there is more: We also get two asteroid belts in this system, as Gerald Jackson points out in his new paper on using antimatter for deceleration into nearby star systems, as well as another planet candidate.
Image: This artist’s conception shows what is known about the planetary system at Epsilon Eridani. Observations from NASA’s Spitzer Space Telescope show that the system hosts two asteroid belts, in addition to previously identified candidate planets and an outer comet ring. Epsilon Eridani is located about 10 light-years away in the constellation Eridanus. It is visible in the night skies with the naked eye. The system’s inner asteroid belt appears as the yellowish ring around the star, while the outer asteroid belt is in the foreground. The outermost comet ring is too far out to be seen in this view, but comets originating from it are shown in the upper right corner. Credit: NASA/JPL-Caltech/T. Pyle (SSC).
This is a young system, estimated at less than one billion years. For both Epsilon Eridani and Proxima Centauri, deceleration is crucial for entering the planetary system and establishing orbit around a planet. The amount of antimatter available will determine our deceleration options. Assuming a separate method of reaching Proxima Centauri in 97 years (perhaps beamed propulsion getting the payload up to 0.05c), we need 120 grams of antiproton mass to brake into the system. A 250 year mission to Epsilon Eridani at this velocity would require the same 120 grams.
Thus we consider the twin poles of difficulty when it comes to antimatter, the first being how to produce enough of it (current production levels are measured in nanograms per year), the second how to store it. Jackson, who has long championed the feasibility of upping our antimatter production, thinks we need to reach 20 grams per year before we can start thinking seriously about flying one of these missions. But as both he and Bob Forward have pointed out, there are reasons why we produce so little now, and reasons for optimism about moving to a dedicated production scenario.
Past antiproton production was constrained by the need to produce antiproton beams for high energy physics experiments, requiring strict longitudinal and transverse beam characteristics. Their solution was to target a 120 GeV proton beam into a nickel target [41] followed by a complex lithium lens [42]. The world record for the production of antimatter is held by the Fermilab. Antiproton production started in 1986 and ended in 2011, achieving an average production rate of approximately 2 ng/year [43]. The record instantaneous production rate was 3.6 ng/year [44]. In all, Fermilab produced and stored 17 ng of antiprotons, over 90% of the total planetary production.
Those are sobering numbers. Can we cast antimatter production in a different light? Jackson suggests using our accelerators in a novel way, colliding two proton beams in an asymmetric collider scenario, in which one beam is given more energy than the other. The result will be a coherent antiproton beam that, moving downstream in the collider, is subject to further manipulation. This colliding beam architecture makes for a less expensive accelerator infrastructure and sharply reduces the costs of operation.
The theoretical costs for producing 20 grams of antimatter per year are calculated under the assumption that the antimatter production facility is powered by a square solar array 7 km x 7 km in size that would be sufficient to supply all of the needed 7.6 GW of facility power. Using present-day costs for solar panels, the capital cost for this power plant comes in at $8 billion (i.e., the cost of 2 SLS rocket launches). $80 million per year covers operation and maintenance. Here’s Jackson on the cost:
…3.3% of the proton-proton collisions yields a useable antiproton, a number based on detailed particle physics calculations [45]. This means that all of the kinetic energy invested in 66 protons goes into each antiproton. As a result, the 20 g/yr facility would theoretically consume 6.7 GW of electrical power (assuming 100% conversion efficiencies). Operating 24/7 this power level corresponds to an energy usage of 67 billion kW-hrs per year. At a cost of $0.01 per kW-hr the annual operating cost of the facility would be $670 million. Note that a single Gerald R. Ford-class aircraft carrier costs $13 billion! The cost of the Apollo program adjusted for 2020 dollars was $194 billion.
Science Along the Way
Launching missions that take decades, and in some cases centuries, to reach their destination calls for good science return wherever possible, and Jackson argues that an interstellar mission will determine a great deal about its target star just by aiming for it. Whereas past missions like New Horizons could count on the position of targets like Pluto and Arrokoth being programmed into the spacecraft computers, the preliminary positioning information uploaded to the craft came from Earth observation. Our interstellar craft will need more advanced tools. It will have to be capable of making its own astrometrical observations, sending its calculations to the propulsion system for deceleration into the target system and orbital insertion, thus refining exoplanet parameters on the fly.
Remember that what we are considering is a hybrid mission, using one form of propulsion to attain interstellar cruise velocity, and antimatter as the method for deceleration. You might recall, for example, the starship ISV Venture Star in the film Avatar, which uses both antimatter engines and a photon sail. What Jackson has added to the mix is a deep dive into the possibilities of antimatter for turning what would have been a flyby mission into a long-lasting planet orbiter.
Let’s consider what happens along the line of flight as a spacecraft designed with these methods makes its way out of the Solar System. If we take a velocity of 0.02c, our spacecraft passes the outgoing Voyager and Pioneer spacecraft in two years, and within three more years it passes into the gravitational lensing regions of the Sun beginning at 550 AU. A mere five years has taken the vehicle through the Kuiper Belt and moved it out toward the inner Oort Cloud, where little is currently known about such things as the actual density distribution of Oort objects as a function of radius from the Sun. We can also expect to gain data on any comparable cometary clouds around Proxima Centauri or Epsilon Eridani as the spacecraft continues its journey.
By Jackson’s calculations, when we’re into the seventh year of such a mission, we are encountering Oort Cloud objects at a pretty good clip, with an estimated 450 Oort objects within 0.1 AU of its trajectory based on current assumptions. Moving at 1 AU every 5.6 hours, we can extrapolate an encounter rate of one object per month over a period of three decades as the craft transits this region. Jackson also notes that data on the interstellar medium, including the Local Interstellar Cloud, will be prolific, including particle spectra, galactic cosmic ray spectra, dust density distributions, and interstellar magnetic field strength and direction.
Image: This is Figure 7 from the paper. Caption: Potential early science return milestones for a spacecraft undergoing a 10-year acceleration burn with a cruise velocity of 0.02c. Credit: Gerald Jackson.
It’s interesting to compare science return over time with what we’ve achieved with the Voyager missions. Voyager 2 reached Jupiter about two years after launch in 1977, and passed Saturn in four. It would take twice that time to reach Uranus (8.4 years into the mission), while Neptune was reached after 12. Voyager 2 entered the heliopause after 41.2 years of flight, and as we all know, both Voyagers are still returning data. For purposes of comparison, the Voyager 2 mission cost $865 million in 1973 dollars.
Thus, while funding missions demands early return on investment, there should be abundant opportunity for science in the decades of interstellar flight between the Sun and Proxima Centauri, with surprises along the way, just as the Voyagers occasionally throw us a curveball – consider the twists and wrinkles detected in the Sun’s magnetic field as lines of magnetic force criss-cross, and reconnect, producing a kind of ‘foam’ of magnetic bubbles, all this detected over a decade ago in Voyager data. The long-term return on investment is considerable, as it includes years of up-close exoplanet data, with orbital operations around, for example, Proxima Centauri b.
It will be interesting to see Jackson’s final NIAC report, which he tells me will be complete within a week or so. As to the future, a glimpse at one aspect of it is available in the current paper, which refers to what the original NIAC project description referred to as “a powerful LIDAR system…to illuminate, identify and track flyby candidates” in the Oort Cloud. But as the paper notes, this now seems impractical:
One preliminary conclusion is that active interrogation methods for locating 10 km diameter objects, for example with the communication laser, are not feasible even with megawatts of available electrical power.
We’ll also find out in the NIAC report whether or not Jackson’s idea of using gram-scale chipcraft for closer examination of, say, objects in the Oort has stood up to scrutiny in the subsequent work. This hybrid mission concept using antimatter is rapidly evolving, and what lies ahead, he tells me in a recent email, is a series of papers expanding on antimatter production and storage, and further examining both the electrostatic trap and electrostatic nozzle. As both drastically increasing antimatter production, as well as learning how to maximize small amounts, are critical for our hopes to someday create antimatter propulsion, I’ll be tracking this report closely.
by Paul Gilster | Mar 3, 2022 | Missions |
As we saw recently with the analogy of salt grains for stars, the scale of things cosmic stuns the imagination. But we don’t have to go to galactic scale. We can stay much closer to home and achieve the same effect. Because at our current technological levels, getting even as far as the outer planets taxes our capabilities. The least explored types of planet in our Solar System are the dwarf worlds, places like Ceres, Pluto and Charon, not to mention the enigmatic Triton. It takes years to reach them.
Beyond these objects we have a wide range of other dwarfs that merit study, at distances that push us ever farther. In a description of their NIAC Phase I study, just announced as a selection for 2022, Jason Benkoski and colleagues at Johns Hopkins University look into a combination heat shield and solar propulsion system that would perform a close Solar pass and use the Sun’s gravity to slingshot outwards at the highest possible velocity. It’s a maneuver familiar to Centauri Dreams readers, and one recently examined by the Interstellar Probe team at JHU’s Applied Physics Laboratory.
Benkoski is a materials scientist who has been working with the APL team, envisioning a tight solar pass around the Sun followed by the firing of a thruster to enhance the craft’s acceleration. This will require the probe to move within 1.6 million kilometers of the Sun’s surface, actually four times closer than the Parker Solar Probe plans to reach by 2025. In a 2021 article in Johns Hopkins Magazine, Benkoski explained the concept, which will preserve the heat shield by using channels filled with hydrogen gas that are built into the bulk of the shield itself. As the article puts it:
During the probe’s searing slingshot around the sun, the gas would heat up, expand, and course through the channels that all lead to a single exhaust nozzle. “The idea is to absorb all this heat with hydrogen,” Benkoski says, “and shoot it out the back of the probe.” In this way, the cooling setup also opportunistically doubles as an engine, thus supplying the thrust needed to complete the Oberth maneuver in the first place. “It’s like hitting two birds with one stone,” Benkoski says.
Image: Graphic depiction of combined heat shield and solar thermal propulsion system for an Oberth maneuver. Credit: Jason Benkoski.
The team believes that advances in materials science and engineering make their solar thermal engine concept a workable model for development. The 20 x 20 cm prototype they designed and fabricated is at benchtop scale, using liquid helium as coolant and propellant. The new study will extend this work, taking the concept into the realm of realistic materials and propellants. No small challenge, that, given that the contemplated Oberth maneuver would subject the probe to temperatures of 2500 degrees C, enough to melt even the Parker Solar Probe’s heat shield.
Benkoski points out that neither of our Voyagers was designed for observing the interstellar medium through which it now passes, while of course the Pioneers have long since ceased to function. New Horizons remains thankfully robust but will ultimately succumb to dwindling power levels and lose communications with Earth. The numbers are daunting: The Voyagers managed 3.6 AU per year, while even a full-stack SLS (which will never fly this mission) would push a 1 tonne spacecraft only to 8 AU per year.
The latter would require not just a working SLS but a Jupiter gravity assist, limiting the fly-out direction of our probes. Hence the need for a solar Oberth maneuver, in Benkoski’s thinking, which would be capable of surviving temperatures of 2800 K and use propellants now under study to widen the range of potential mission targets:
We…therefore propose a full trade study of alternate propellants in order to determine the maximum escape velocity for a given total system mass, including spacecraft, heat shield, propellant storage, and attitude control system. The main propellants of interest include H2, LiH, Li, CH4, NH3, and H2O. Methods: First we would determine material compatibility for each propellant with respect to its proposed storage system. We then calculate the efficiency (specific impulse) as a function of temperature for each propellant using Chemical Equilibrium Analysis (CEA).
Benkoski intends to discover how the mass and storage volume of the tank scale with the quantity of propellant to produce a series of realistic tank designs, devising an equation for the heat shield area and maximum propellant fraction that can be achieved given the limitations of existing heavy boosters. We’ll see how this study fares in producing a full-scale heat shield/heat exchanger design with robust long-term cryogenic storage. A tight Oberth maneuver is not going to be easy. See Assessing the Oberth Maneuver for Interstellar Probe for some of the myriad reasons why.
by Paul Gilster | Feb 18, 2022 | Missions |
In Part 2 of Andrew Higgins’ discussion of laser-thermal rocketry and fast missions to Mars, we look more deeply at the design and consider its potential for other high delta-V missions. Are we looking at a concept that could help us build the needed infrastructure to one day support expansion beyond the Solar System?
by Andrew Higgins
We now turn to the detailed design our team at McGill University came up with for a laser-thermal mission capable of reaching Mars in 45 days. Our team took the transit time and payload requirement (1 ton) from a NASA announcement of opportunity that appeared in 2018 that was seeking “Revolutionary Propulsion for Rapid Deep Space Transit”. Although being in Canada made us ineligible to apply to this program, we adopted this mission targeted by the NASA announcement for our design study; being in Canada also means we are used to working without funding.
Image: McGill University students responsible for the design of the laser-thermal mission to Mars.
The NASA-defined payload of 1 ton would be a technology demonstration mission (what we call Mission Mars 1 in our study). Placing a premium on minimizing the transit time presumably reflects NASA’s eventual interest in lessening astronaut exposure to galactic cosmic rays, which increases sharply once a spacecraft leaves the Earth’s protective magnetosphere. Once on the surface of Mars, data from the Curiosity rover have shown that the radiation environment there appears to be more benign, comparable to or even less than the radiation exposure encountered on the ISS. Throwing regolith to cover the habitat on Mars would lower the radiation risk further, so astronauts leading a hobbit-like existence on Mars should stay healthy, provided they get there quickly.
Our Mars 1 mission starts with our spacecraft already in medium Earth orbit (MEO), so that it remains in view of the ground-based laser during the entire laser-powered burn, which takes about an hour. Given the ongoing revolution in space access, we did not bother to explore using laser propulsion to get to orbit. Chemical propulsion is well-suited for reaching orbit, so we selected a Falcon 9 to bring our vehicle to MEO and focused on using the laser for the transit to Mars.
Image (click to enlarge): The concept of operations for a rapid transit to Mars mission using laser-thermal propulsion. Note the use of a burn-back maneuver to bring the laser-thermal stage back to medium Earth orbit after sending the payload to Mars.
The laser array on Earth is about 10 m by 10 m, comparable to a volleyball court, and for the 1 ton payload mission, the laser would operate at 100 MW output for an hour, using power taken from the grid or generated via solar and then stored in a battery farm. (It is worth noting that a battery farm capable of providing 100 MW for an hour was built in South Australia in 2017 from scratch in just 60 days, in response to a taunt posted in a tweet [1]. So, powering the laser is not a problem.
When the laser beam arrives at the spacecraft, it is focused into the propellant heating chamber by a large, inflatable reflector—a balloon that is transparent on one half and reflective on the other. Inflatable space structures like this are fairly mature, including a demonstration of an inflatable antenna that flew on the Space Shuttle in 1996; a comprehensive overview of this technology was given by Jamey Jacob at the 6th TVIW in Wichita [2]. Inflatable collectors such as these have shown sufficient optical quality for our purposes. While the laser flux on the inflatable is intense, we found fluorinated polyimide films have sufficiently low absorptivity to avoid overheating.
Image: Inflatable Antenna Experiment deployed from the Space Shuttle Endeavor (STS-77).
Image Source: https://apod.nasa.gov/apod/ap960525.html
The inflatable reflector focuses the laser into the heating chamber, raising the temperature of the hydrogen flowing through the chamber to greater than 10,000 K. Keeping the walls of the chamber cool is the central challenge of the design, but our team found a combination of regenerative cooling (cool hydrogen flowing through the walls), transpiration cooling (injecting hydrogen through porous walls), and seeding the hydrogen (to trap thermal radiation in the propellant, similar to the greenhouse effect) should be sufficient to keep the walls cool. The heat absorbed via regeneration is used to power the turbopumps needed to pump the hydrogen via an expander cycle. The fully ionized hydrogen propellant is then exhausted through a conventional bell nozzle to generate thrust. Based on our own calculations and prior work on laser thermal propulsion and gas-core NTRs from the 1970s, a specific impulse of 3000 s appears feasible.
Image: Details of the propellant heating chamber and associated propellant feed and cooling systems.
The laser propulsion hardware is just dead mass once the spacecraft exceeds the focal length of the laser (which is about 50,000 km), so our team proposed bringing the laser thermal propulsion stage back to Earth via a flip-and-burn-back maneuver while still within range of the laser in cis-Lunar space. Once the propulsion stage is brough back to low or medium Earth orbit, it can be refilled and readied for use again. This would allow a single laser-thermal stage to throw multiple payloads to Mars over the duration of a given launch window.
The 14 km/s Delta-V laser thermal burn sends the spacecraft to Mars on a nearly straight line trajectory: no need for looping ellipses and Venus flybys. Our astrodynamicist optimized the trajectory for a 2020 departure. Even though our design had the launch two months after Perseverance, the vehicle would arrive at Mars three months before the newest Mars rover, overtaking it on the way.
Image: 45-day transfer orbit to Mars via laser thermal propulsion, in comparison to the 7-month journey of the Perseverance rover.
When the spacecraft arrives at Mars, there is no laser to perform a laser-assisted deceleration burn (at least, not yet) and at the high approach velocity, aerocapture appears the best option. At an approach speed of 16 km/s, aerocapture is going to be harsh and is another critical link in the mission design. The heat flux will be intense, but the new Heatshield for Extreme Entry Environment Technology (HEEET) developed by NASA in recent years appears to be rated to withstand even greater heat flux. The vehicle entering the Martian atmosphere would need to use lift pointed down (toward the surface of Mars) to keep the vehicle in a trajectory that skims the atmosphere. This maneuver is a delicate balance between heat load, the g-load, and the lift and ballistic coefficients of the spacecraft, which we first modelled analytically and then backed-up with full three-degree-of-freedom simulations. The g-load limit was set at 8-gees for our study; for the scaled-up design with astronauts, the g-load will be severe and sustained for several minutes, but within the limits of what humans can tolerate. (Relevant to note that, at the recent Interstellar Symposium in Tucson, Esther Dyson reported from her centrifuge training at Star City that, “8-gees going through you was actually a lot of fun” [3]). The aerocapture would be a wild ride, for sure.
Image: Details of model used for aerocapture upon arrival at Mars.
The scaled-up version of our design (Mission Mars 2a) intended for crewed missions used a 40-ton spacecraft derived from the Orion capsule and European Service Module. The greater payload requires a more powerful (4 GW) laser to effectuate the same 45-day transit to Mars, but the laser array occupies the same 10-m footprint on earth.
The other mission we considered was a cargo mission (Mission Mars 2b). Robert Zubrin often makes the point that—even if advanced propulsion capable of high thrust and high specific impulse was available—he would still opt for a 6-month free-return trajectory and use the enhanced propulsion capability to bring more payload. So, the Mars 2b mission uses the performance of laser thermal propulsion to maximize the amount of cargo that could be brought to Mars with a Hohmann-like transfer, and shows that the payload could be increased by a factor of more than 10 over what a Centaur upper stage—with the same mass of propellant—could throw to Mars.
Image: Final design of laser-thermal propulsion spacecraft capable of reaching Mars in 45 days.
While a more thorough vetting of our design is called for and much work remains to be done, one encouraging finding is that the specific power of the laser thermal propulsion design is so good—an “alpha” on the order of 0.001 kg/kW—that even if the mass of the entire propulsion system were to increase by a factor of ten, the increased mass would not significantly affect the overall performance or payload capacity of the design. There is sufficient margin in the concept to accommodate the inevitable upward creep in mass that occurs as the design is refined.
Laser thermal propulsion may be well suited to other high Delta-V missions, such as flybys of interstellar comets, the mission to the solar gravitational focus, and a probe to the hypothetical Planet 9—if it is found. There is no reason the laser-thermal approach cannot be combined with laser electric propulsion or other techniques such as an Oberth maneuver. Perhaps it is best to think of laser thermal propulsion as a dragster that burns a lot of propellant quickly to get you up to speed, but from there, you can invoke laser electric propulsion that is well suited to the diminishing laser flux as the spacecraft exceeds the focal length of the laser. Appendix A in our paper details where we calculate the tradeoff between laser thermal and laser electric propulsion occurs. Hopefully, the laser-thermal concept can contribute to a further appreciation of directed energy as a disruptive technology for high-velocity missions in the solar system and beyond.
The complete details of our study can be found in our published paper: Duplay et al, “Design of a rapid transit to Mars mission using laser-thermal propulsion,” Acta Astronautica Volume 192 (March 2022), pp. 143-156 (abstract / preprint).
A browser-friendly version of the paper is available here: https://ar5iv.org/html/2201.00244
References
1. https://www.popularmechanics.com/science/a31350880/elon-musk-battery-farm/
2. J. D. Jacob, B. Loh, Inflatable technologies for interstellar missions, in: P. Gilster (Ed.), Proceedings of the 6th Tennessee Valley Interstellar Workshop, 2020.
3. https://www.youtube.com/watch?v=nHnUeM8RovE
by Paul Gilster | Feb 15, 2022 | Missions |
As far back as the 1960s, aerospace engineer John Bloomer published on the idea of using an external laser as the energy source for a rocket, using the incoming beam to fire up an onboard electrical propulsion system. And it was in a 1971 speech that Arthur Kantrowitz, looking toward the technologies that would succeed chemical rockets, suggested using lasers to heat a propellant within a rocket. This is laser-thermal propulsion, in which hydrogen (the assumed propellant) is heated to produce an exhaust stream. The hybrid method would be studied extensively in the 1970s.
So when Al Jackson and Daniel Whitmire took up the idea in a 1978 paper, they were in tune with an area that had already provoked some research interest. But Jackson and Whitmire had ideas that would refine the ramjet design introduced by Robert Bussard. They were pondering ways to power a starship, one that would carry its own reaction mass. Uneasy about the core Bussard design, the duo had, the year before, published on the idea of using a laser to augment the ramjet. Bussard sought to ignite fusion in reaction mass gathered by a magnetic ramscoop, gathering its own fuel as it roamed the galaxy. It was a dazzling idea, but problems had soon become apparent.
Image: Al Jackson, whose laser-powered ramjet and laser-powered interstellar rocket concepts, developed with Daniel Whitmire, refined the Bussard ramjet design and illustrated its shortcomings.
For the Bussard design, as we’ve seen not long ago in Peter Schattschneider work with Jackson (see John Ford Fishback and the Leonara Christine), runs into serious problems, including igniting the proton/proton fusion reaction Bussard advocated. We would go on to learn that the vast magnetic ramscoop of the ramjet generates far too much drag to be practical.
So Jackson and Whitmire proceeded first to come up with a laser-augmented ramjet that applied beamed energy from a transmitting installation in the Solar System, one that would interact with hydrogen collected by the starship ramscoop. They then turned their attention to beaming energy to a craft that operated using its own reaction mass rather than mass drawn from the interstellar medium.
It’s that latter idea that has the most resonance. Robert Forward in this period had been talking about beaming laser energy to space sails. Now the laser idea goes into the service of a hybrid propulsion concept that loses at least one Bussard showstopper. I’ve described Jackson and Whitmire’s idea in the past as ‘rocketry on a beam of light.’ It’s an ingenious solution, even if it does not permit us to leave the propellant behind.
Image: Daniel Whitmire, collaborator with Al Jackson on the laser-powered interstellar ramjet and rocket concepts, and author of important work on Carbon Nitrogen Oxygen cycle (CNO) fusion possibilities for the ramjet. Credit: University of Louisiana at Lafayette.
And it’s this ‘laser-powered rocket,’ as opposed to a ramjet, that should get more attention in the community than it has, given that subsequent studies of the interstellar medium have cast doubt on how even the most efficient ramscoop could collect enough reaction mass given variations in the distribution of hydrogen in the galaxy. In other words, you might have to get up to relativistic speeds in the first place just to ignite a ramjet, if indeed it could be ignited, and you would have to reckon with varying supplies of interstellar material along the way. Poul Anderson’s wonderful take on the interstellar ramjet in Tau Zero (1970) becomes highly problematic!
The laser-powered interstellar rocket contains the added advantage of being able to accelerate not only away from the laser beam but towards it, for the beam is conceived purely as an energy source, not a source of momentum. This has immediate benefits in mission planning. One of the great challenges of interstellar flight is that once you’ve managed to get your craft up to relativistic speeds, you’d like to do more upon arrival at destination than simply blitz through a planetary system at 20 percent of c. The laser-powered interstellar rocket, however, operates efficiently in both acceleration and deceleration phases. No need for Robert Forward’s ‘staged sails’ when using this take on a starship, or for the deployment of a magnetic sail as a brake.
Image: Laser-thermal propelled spacecraft in Earth orbit awaiting its departure. Credit: Creative Commons Attribution 4.0 International License.
The history of interstellar studies has involved conceiving of ideas that do not break physics and then probing them to find out whether they work. Like the original Bussard ramjet and so many of Robert Forward’s ideas, Jackson and Whitmire’s concept is highly futuristic, but I love what Al said in a reminiscence on the matter in these pages: “The importance is in showing that the physics allows an opening for the engineering physics. There is no exotic physics here, only – so to speak – exotic technology.”
I sometimes forget how venerable some of these ideas are, for even while Jackson and Whitmire were doing their work on laser beaming variants to adapt the Bussard design, George Marx had already published a paper in Nature in 1966 with the provocative title “Interstellar Vehicle Propelled by Terrestrial Laser Beam.” Laser lightsails were under active discussion, and now there was a laser rocket. We have had half a century to ponder these ideas, and I see that another variant on beaming power to a spacecraft with onboard fuel has just emerged. While it’s a system advocated for fast transit to Mars, it plays upon motifs that can turn interstellar.
In a paper appearing in Acta Astronautica, lead author Emmanuel Duplay (McGill University, Montreal) and colleagues take a near-term look at what such methods can achieve. But they also give a nod to interstellar prospects, pointing out that trends like the emergence of inexpensive fiber-optic laser amplifiers and the possibility of phase-locking large arrays of such amplifiers to operate as a single element are now under active study. Moreover, adaptive optics methods can smooth beam distortions if moving through the atmosphere, allowing such an array to beam energy to a spacecraft from the surface of the Earth. Long-term, a space-based array offers huge advantages, but deep space missions do not depend on this.
Indeed, the new work responds to a recent NASA solicitation looking for propulsion concepts for rapid interplanetary missions capable of making the Earth-Mars crossing in no more than 45 days, and reaching a distance of 5 AU in no more than a year, or 40 AU (in the realm of Pluto/Charon) in no more than five years. As Mars is a feasible target for human crews in the not distant future, such a capability would mitigate the risk to astronauts of exposure to galactic cosmic rays and dangerous solar activity.
I’m intrigued by the idea that beamed propulsion can become a major factor in creating a system-wide infrastructure, one that will along the way develop the needed technologies for missions to another star. So in the next two posts, I want to turn things over to Andrew Higgins (McGill University), who is at the heart of the work in Montreal. Rapid transport to Mars is the baseline design here, but it’s a metric that not only allows us to compare competing propulsion methods but also look ahead to the deep space missions it enables.
The Jackson and Whitmire paper is “Laser Powered Interstellar Rocket,” Journal of the British Interplanetary Society, Vol. 31 (1978), pp.335-337. The Bloomer paper is “The Alpha Centauri Probe,” in Proceedings of the 17th International Astronautical Congress (Propulsion and Re-entry), Gordon and Breach. Philadelphia (1967), pp. 225-232. The paper we’ll look at next is Duplay et al, “Design of a rapid transit to Mars mission using laser-thermal propulsion,” Acta Astronautica Volume 192 (March 2022), pp. 143-156 (abstract / preprint).
by Paul Gilster | Dec 22, 2021 | Missions |
The fact that we have three functioning spacecraft outside the orbit of Pluto fills me with holiday good spirits. Of the nearest of the three, I can say that since New Horizons’ January 1, 2019 encounter with the Kuiper Belt Object now known as Arrokoth, I have associated the spacecraft with holidays of one kind or another The July 14, 2015 flyby of Pluto/Charon wasn’t that far off the US national holiday, but more to the point, I was taking a rare beach vacation during the last of the approach phase, most of my time spent indoors with multiple computers open tracking events at system’s edge. It felt celebratory, like an extended July 4, even if the big event was days later.
Also timely as the turn of the year approaches is Alan Stern’s latest PI’s Perspective, a look at what’s ahead for the plucky spacecraft. Here January becomes a significant time, with the New Horizons team working on the proposal for another mission extension, the last of which got us through Arrokoth and humanity’s first close-up look at a KBO. The new proposal aims at continued operations from 2023 through 2025, which could well include another KBO flyby, if an appropriate target can be found. That search, employing new machine learning tools, continues.
Image: Among several discoveries made during its flyby of the Kuiper Belt object Arrokoth in January 2019, New Horizons observed the remarkable and enigmatic bright, symmetric, ring-like “neck” region at the junction between Arrokoth’s two massive lobes. Credit: NASA/Johns Hopkins APL/Southwest Research Institute.
But what happens if no KBO is within reach of the spacecraft? Stern explains why the proposed extension remains highly persuasive:
If a new flyby target is found, we will concentrate on that flyby. But if no target is found, we will convert New Horizons into a highly-productive observatory conducting planetary science, astrophysics and heliospheric observations that no other spacecraft can — simply because New Horizons is the only spacecraft in the Kuiper Belt and the Sun’s outer heliosphere, and far enough away to perform some unique kinds of astrophysics. Those studies would range from unique new astronomical observations of Uranus, Neptune and dwarf planets, to searches for free floating black holes and the local interstellar medium, along with new observations of the faint optical and ultraviolet light of extragalactic space. All of this, of course, depends on NASA’s peer review evaluation of our proposal.
Our only spacecraft in the Kuiper Belt. What a reminder of how precious this asset is, and how foolish it would be to stop using it! Here my natural optimism kicks in (admittedly beleaguered by the continuing Covid news, but determined to push forward anyway). One day – and I wouldn’t begin to predict when this will be – we’ll have numerous Kuiper Belt probes, probably enabled by beamed sail technologies in one form or another as we continue the exploration of the outer system, but for now, everything rides on New Horizons.
The ongoing analysis of what New Horizons found at Pluto/Charon is a reminder that no mission slams to a halt when one or another task is completed. For one thing, it takes a long time to get data back from New Horizons, and we learn from Stern’s report that a good deal of the flyby data from Arrokoth is still on the spacecraft’s digital recorders, remaining there because of higher-priority transmission needs as well as scheduling issues with the Deep Space Network. We can expect the flow of publications to continue. 49 new scientific papers came out this year alone.
That Arrokoth image above is still a stunner, and the inevitable naming process has begun not only here but on Pluto as well. The KBO’s largest crater has been christened ‘Sky,’ while Ride Rupes (for astronaut Sally Ride) and Coleman Mons (for early aviator Bessie Coleman) likewise will begin to appear on our maps of Pluto. All three names have been approved by the International Astronomical Union. ‘Rupes’ is the Latin word for ‘cliff,’ and here refers to an enormous feature near the southern tip of Pluto’s Tombaugh Regio. Ride Rupes is between 2 and 3 kilometers high and about 250 kilometers long, while Coleman Mons is a mountain, evidently recently created and thus distinctive in a region of older volcanic domes.
Image: Close-up, outlines of Ride Rupes (left) and Coleman Mons on the surface Pluto. Credit: NASA/Johns Hopkins APL/Southwest Research Institute/SETI Institute/Ross Beyer.
As the New Horizons team completes the mission extension proposal, it also proceeds with uploading another instrument software upgrade, this one to the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) charged-particle spectrometer. And while spacecraft power levels have continued to decline, as is inevitable given the half-life of the nuclear battery’s plutonium, Stern says the spacecraft should be able to maintain maximum data transmission rates for another five years. That new power-saving capability, currently being tested, should strengthen the upcoming proposal and bodes well for any future flyby.
Those of you with an investigative bent should remember that 2021’s data return, along with six associated datasets, is available to researchers whether professional or working in a private capacity, within NASA’s Planetary Data System. This is an active mission deeply engaged with the public as well as its natural academic audience, as I’m reminded by the image below. Here the New Horizons spacecraft has captured a view taken during departure from Pluto, seeing however faintly the ‘dark side’ that was not illuminated by the Sun during the approach.
Image: Charon-lit-Pluto: The image shows the dark side of Pluto surrounded by a bright ring of sunlight scattered by haze in its atmosphere. But for a dark crescent zone to the left, the terrain is faintly illuminated by sunlight reflected by Pluto’s moon Charon. Researchers on the New Horizons team were able to generate this image using 360 images that New Horizons captured as it looked back on Pluto’s southern hemisphere. A large portion of the southern hemisphere was in seasonal darkness similar to winters in the Arctic and Antarctica on Earth, and was otherwise not visible to New Horizons during its 2015 flyby encounter of Pluto.
Credit: NASA/Johns Hopkins APL/Southwest Research Institute/NOIRLab.
This is Pluto’s southern hemisphere during the long transition into winter darkness; bear in mind that a winter on the distant world lasts 62 years. The all too faint light reflecting off Charon’s icy surface allows researchers to extract information. Tod Lauer (National Optical Infrared Astronomy Research Observatory, Tucson), lead author of a paper on the dark side work, compares available light here to moonlight on Earth:
“In a startling coincidence, the amount of light from Charon on Pluto is close to that of the Moon on Earth, at the same phase for each. At the time, the illumination of Charon on Pluto was similar to that from our own Moon on Earth when in its first-quarter phase.”
That’s precious little to work with, but the New Horizons Long Range Reconnaissance Imager (LORRI) made the best of it despite the fierce background light and the bright ring of atmospheric haze. We’ll have to wait a long time before the southern hemisphere is in sunlight, but for now, Pluto’s south pole seems to be covered in material darker than the paler surface of the northern hemisphere, with a brighter region midway between the south pole and the equator. In that zone we may have a nitrogen or methane ice deposit similar to the Tombaugh Regio ‘heart’ that is so prominent in the flyby images from New Horizons.
For more, see Lauer et al., “The Dark Side of Pluto,” Planetary Science Journal Vol. 2, No. 5 (20 Ocober 2021), 214 (abstract).
Of course, there is another mission that will forever have a holiday connection, at least if its planned liftoff on Christmas Eve happens on schedule. Dramatic days ahead.
by Paul Gilster | Dec 16, 2021 | Missions |
We’ve been learning about the solar wind ever since the first interplanetary probes began to leave our planet’s magnetosphere to encounter this rapidly fluctuating stream of plasma. Finding a way to harness the flow could open fast transport to the outer Solar System if we can cope with the solar wind’s variability – no small matter – but in any case learning as much as possible about its mechanisms furthers our investigation of possible propulsive techniques. On this score and for the sake of solar science, we have much reason to thank the Parker Solar Probe and its band of controllers as the spacecraft continues to tighten its approaches to the Sun.
The spacecraft’s repeated passes by the Sun, each closer than the last, take advantage of speed and a heat shield to survive each perihelion event, and the last for which we have data was noteworthy indeed. During it, the Parker Solar Probe moved three separate times into and out of the Sun’s corona. This is a region where magnetic fields dominate the movement of particles. The Alfvén critical surface, which the spacecraft repeatedly crossed, is the boundary where the solar atmosphere effectively ends and the solar wind begins. Solar material surging up from below reaches a zone where gravity and magnetic fields can no longer hold it back. Breaking free, the solar wind effectively breaks the connection with the solar corona once across the Alfvén boundary.
So, as with our Voyagers moving past the heliopause and into interstellar space, we’ve accomplished another boundary crossing of consequence. A crossing into and back out of the corona helps define the location of the Alfvén critical surface, which turns out to be close to earlier estimates. These targeted a range between 10 and 20 solar radii. In its most recent passes by the Sun, the Parker Solar Probe has been below 20 solar radii, and on April 28 of this year, at 18.8 solar radii, it penetrated the Alfvén surface.
Nour Raouafi is a Parker project scientist at the Johns Hopkins Applied Physics Laboratory (JHU/APL):
“Flying so close to the Sun, Parker Solar Probe now senses conditions in the magnetically dominated layer of the solar atmosphere – the corona – that we never could before. We see evidence of being in the corona in magnetic field data, solar wind data, and visually in images. We can actually see the spacecraft flying through coronal structures that can be observed during a total solar eclipse.”
Image: As Parker Solar Probe passed through the corona on encounter nine, the spacecraft flew by structures called coronal streamers. These structures can be seen as bright features moving upward in the upper images and angled downward in the lower row. Such a view is only possible because the spacecraft flew above and below the streamers inside the corona. Until now, streamers have only been seen from afar. They are visible from Earth during total solar eclipses. Credit: NASA/Johns Hopkins APL/Naval Research Laboratory.
It’s clear from Parker data that the Alfvén surface is anything but smooth, and the spacecraft’s crossing into the corona did not in fact occur at perihelion on this particular pass, an indication of the varied structures within the region. As seen above, streamers and so-called pseudostreamers are found here, large magnetic-field structures streaming out of regions of the same magnetic polarity that are separated by an inner zone of opposite polarity. Caltech’s Christina M. S. Cohen explains the situation this way in a useful overview of the coronal crossing that notes the spacecraft’s fleeting passage through the boundary:
The center of a pseudostreamer is a region of enhanced magnetic field and reduced plasma density. This combination can push the Alfvén surface higher up in the corona, explaining why PSP’s orbit was able to cut across it…The period of time PSP spent below the Alfvén surface was too short to fully characterize the boundary and explore the inner region. Researchers expect that such a full characterization will require multiple expeditions carried out over different magnetic configurations and solar conditions.
It’s interesting to learn that we’re behind in acquiring data from the Parker Solar Probe because its high-gain antenna cannot be pointed toward Earth until it is far enough from the Sun on its current close pass to protect the equipment. Thus while the current data are from April of 2021, there was a likely crossing of the Alfvén critical surface again in November, when the probe reached a perihelion of 13.6 solar radii. This is close enough to suggest a longer period within the corona, something we won’t know until data download of that pass in late December.
Image: The solar corona during a total solar eclipse on Monday, August 21, 2017, above Madras, Oregon. The red light is emitted by charged iron particles at 1 million degrees Celsius and the green are those at 2 million degrees Celsius. On April 28, 2021, NASA’s Parker Solar Probe crossed the so-called Alfvén surface, entering, for the first time, a part of the solar corona that is “magnetically dominated.” Credit: M. Druckmuller / Christina M. S. Cohen.
Just as the Alfvén critical surface is anything but smooth, so too is the solar wind full of structure as it moves into the realm of the planets. So-called switchbacks were first detected by the Ulysses probe in the 1990s, what NASA describes as “bizarre S-shaped kinks in the solar wind’s magnetic field lines” which deflect charged particle paths as they move away from the Sun. The Parker Solar Probe discovered just how common these switchbacks were back in 2019, with later data showing that at least some switchbacks originate in the photosphere.
Switchbacks also align with magnetic funnels that emerge out of the photosphere between the convection cell structures called supergranules. It may be, then, that we can trace the origins of the solar wind at least partially to these magnetic funnels, as Stuart Bale (University of California, Berkeley) suggests:
“The structure of the regions with switchbacks matches up with a small magnetic funnel structure at the base of the corona. This is what we expect from some theories, and this pinpoints a source for the solar wind itself. My instinct is, as we go deeper into the mission and lower and closer to the Sun, we’re going to learn more about how magnetic funnels are connected to the switchbacks.”
Whether switchbacks are produced by the process of magnetic reconnection at the boundaries of magnetic funnels, or are produced by moving waves of plasma, is a question scientists hope the Parker Solar Probe will be able to answer. Just how the solar wind connects to switchbacks may help to explain how the corona is heated to temperatures far above that of the solar surface below. Bear in mind that the corona itself will be expanding as the Sun goes through its normal eleven year activity cycle, so we’ll have more opportunities for the Probe to pass through it.
Parker will eventually reach 8.86 solar radii, a scant 6.2 million kilometers from the solar surface, so this is a story that is far from over. The next flyby will be in January of 2022.
Findings from the recent Parker Solar Probe milestone will be published in The Astrophysical Journal, and are also examined in a paper by Kasper et al., “Parker Solar Probe Enters the Magnetically Dominated Solar Corona,” Physical Review Letters 127 (14 December 2021), 255101 (abstract).
