Centauri Dreams
Imagining and Planning Interstellar Exploration
HOEE: The Starshade and the Ground
I always keep an eye on the Phase I and Phase II studies in the pipeline at the NASA Innovative Advanced Concepts (NIAC) program. The goal is to support ideas in their early stages, with the 2022 awards going out to 17 different researchers to the tune of a combined $5.1 million. Of these, 12 are Phase I studies, which deliver $175,000 for a nine-month period, while the five Phase II awards go to $600,000 over two years. We looked at one of the Phase I studies, Jason Benkoski’s solar-thermal engine and shield concept, in the last post. Today we go hunting exoplanets with a starshade.
This particular iteration of the starshade concept is called Hybrid Observatory for Earth-like Exoplanets (HOEE), as proposed by John Mather (NASA GSFC). Here the idea is to leverage the resources of the huge ground-based telescopes that should define the next generation of such instruments – the Giant Magellan Telescope, the Extremely Large Telescope, etc. – by using a starshade to block the glare of the host star, thus uncovering images of exoplanets. Remember that at visible wavelengths, our Sun is 10 billion times brighter than the Earth. The telescope/starshade collaboration would produce what Mather believes will be the most powerful planet finder yet designed.
Image: Three views of a starshade. Credit: NASA / Exoplanet Exploration Program.
Removing the overwhelming light of a star can be done in more than one way, and we’ve seen that an internal coronagraph will be used, for example, with the Nancy Grace Roman Space Telescope. It’s what NASA describes as “a system of masks, prisms, detectors and even self-flexing mirrors” that is being built at the Jet Propulsion Laboratory for the mission.
In conjunction with a space telescope, a starshade operates as a separate spacecraft, a large, flat shade positioned tens of thousands of kilometers away. Starshades have heretofore been studied in this configuration, so the innovation in Mather’s idea is to align the starshade with instruments on the ground. His team believes that we could detect oxygen and water on an Earth-class planet using a 1-hour spectrum out to a distance of 7 parsecs (roughly 23 light years. In an ASTRO2020 white paper, Mather described a system like this using a different orbit for each target star, with the orbit being a highly eccentric ellipse. Thrust is obviously a key component for adjusting the starshade’s position for operations.
From the white paper:
An orbiting starshade would enable ground-based telescopes to observe reflected light from Earth-like exoplanets around sun-like stars. With visible-band adaptive optics, angular resolution of a few milliarcseconds, and collecting areas far larger than anything currently feasible for space telescopes, this combination has the potential to open new areas of exoplanet science. An exo-Earth at 5 pc would be 50 resolution elements away from its star, making detection unambiguous, even in the presence of very bright exo-zodiacal clouds. Earth-like oxygen and water bands near 700 nm could be recognized despite terrestrial interference…
And what a positioning challenge this is in order to maximize angular resolution, sensitivity and contrast, with the starshade matching position and velocity with the telescope from an orbit with apogee greater than ~ 185,000 km, thus casting a shadow of the star, while leaving the light of its planets to reach the instrument below. In addition to the active propulsion to maintain the alignment, the concept relies on adaptive optics that will in any case be used in these ground instruments to cope with atmospheric distortion. Thus low-resolution spectroscopy becomes capable of analyzing light that is actually reflected from Earth-like planets.
Mather’s team wants to cut the 100-meter starshade mass by a factor of 10 to support about 400 kg of thin membranes making up the shade. Thus the concept of an ultra-lightweight design that would be assembled – or perhaps built entirely – in space. It’s worthwhile to remember that the starshade concept in orbit is a new entry in a field that has seen study at NASA GSFC as well as JPL’s Team X, with suitability considered for various missions including HabEx, WFIRST, JWST, New Worlds Explorer, UMBRAS and THEIA. The Mather plan is to create a larger, more maneuverable starshade, as it will indeed have to be to make possible the alignments with ground observatories contemplated in the study.
It’s an exciting prospect, but as Mather’s NIAC synopsis notes, the starshade is not one we could build today. From the synopsis:
The HOEE depends on two major innovations: a ground-space hybrid observatory, and an extremely large telescope on the ground. The tall pole requiring design and demonstration is the mechanical concept of the starshade itself. It must satisfy conflicting requirements for size and mass, shape accuracy and stability, and rigidity during or after thruster firing. Low mass is essential for observing many different target stars. If it can be assembled or constructed after launch, it need not be built to survive launch. We believe all requirements can be met, given sufficient effort. The HOEE is the most powerful exoplanet observatory yet proposed.
Image: Graphic depiction of Hybrid Observatory for Earth-like Exoplanets (HOEE). Credit: John Mather.
Centauri Dreams readers will know that Ashley Baldwin has covered starshade development extensively in these pages. His WFIRST: The Starshade Option is probably the best place to start for those who want to delve further into the matter, although the archives contain further materials. Also see my Progress on Starshade Alignment, Stability.
For more, see Peretz et al., “Exoplanet imaging performance envelopes for starshade-based missions,” Journal of Astronomical Telescopes, Instruments, and Systems 7(2), 021215 (2021). Abstract. And for an overview: Arenberg et al., “Special Section on Starshades: Overview and a Dialogue,” Journal of Astronomical Telescopes, Instruments, and Systems 7(2), 021201 (2021). Abstract.
Engineering the Oberth Maneuver
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.
Lowering the Laser Barrier
The continuing release of papers related to or referring to the Breakthrough Starshot sail concept is good news for the entire field. Interstellar studies as an academic discipline has never had this long or sustained a period of activity, and the growing number of speakers at space-related conferences attests to the current vitality of starflight among professionals and the general public alike.
Not all interstellar propulsion concepts involve laser-beaming, of course, and we’ll soon look at what some would consider an ever more exotic concept. But today I’m focusing on a paper from Ho-Ting Tung and Artur Davoyan, both in the Mechanical and Aerospace Engineering Department at UCLA. You could say that these two researchers are filling in some much needed space between the full-bore interstellar effort of Breakthrough Starshot, the Solar System-oriented laser work of Andrew Higgins’ team at McGill, and much smaller, near-term experiments we could run not so far from now.
Of the many potential show-stoppers faced by a mission to another star at our stage of development is the need to develop the colossal laser array envisioned by Starshot. The Higgins array is at a smaller scale, as befits a concept with nearby targets like Mars. What Tung and Davoyan envision are tiny payloads (here they parallel Breakthrough), some no more than a gram in mass, but the authors push the sail with a 100 kW array about a meter in size. Compare this with Breakthrough’s need for a gigantic square-kilometer array of 10 kW lasers with a combined output of up to 100 GW.
Image: In this illustration, a low-power laser (red cone) on Earth could be used to shift the orbit (red lines) of a small probe (grey circle), or propel it at rapid speeds to Neptune and beyond. Credit: Ho-Ting Tung et al.
The UCLA work takes us to a consideration of operations with spacecraft in Earth orbit as well as payloads sent on interplanetary trajectories. Thus we are in the realm of the kind of missions that today would demand chemical or electric propulsion, and we are looking at a system that might be used, for example, for orbital adjustment of Earth satellites after launch, or in the case of chip-class payloads, interplanetary missions with surprising velocities, up to 5 times that of New Horizons. As noted, the needed laser aperture is, by the standards of the missions we’ve discussed earlier, small:
…a sail with w = 1 m would require a laser with an aperture D ? 26 m (compare with the 30 m diameter primary mirror of the Thirty Meter Telescope under construction). However, we stress that most practical scenarios are limited to low and medium Earth orbits that require a much shorter operation range (z ? 1000km), and therefore a significantly smaller laser array.
Indeed, an array a meter in size could be efficient, maneuvering small satellites in Earth orbit, or being used to bring small chip-craft up to Solar System escape velocity. Thus we have the potential to create laser propulsion experiments and missions with array powers of ? 100 kW and array sizes that do not require kilometers of desert for their construction. Payloads can range from 1 to 100 grams depending on the mission, though the focus here is wafer-scale, on the order of 10 centimeters.
As to sail materials, the authors calculate that for maximum reflectivity coupled with rapid cooling, silicon nitride and boron nitride are the materials of choice:
Broadband spectral emissivity of silicon nitride…results in a better heat rejection (i.e., lower temperature) as compared [to] narrow band BN thermal emitters. However, boron nitride being lighter than silicon nitride allows design of very light-weight light-sails, which eventually translates onto higher velocity gain, ?v.
The paper offers possible ways to create these structures, including using metamaterials formed into nanostructured architectures with nanometer-scale ‘sandwich’ panels between material layers, or using ‘micro pillars’ within the photonic structure.
The broader picture is that we’re mapping out how to experiment with lasers and materials that may begin moving up the ladder of mission complexity. There are innumerable issues to be overcome, but the early theoretical work is crucial to making what may become an interplanetary infrastructure a reality. These examinations should also feed into the ambitious work on projects that aim at interstellar missions.
The paper is Ho-Ting Tung et al, Low-Power Laser Sailing for Fast-Transit Space Flight, Nano Letter,” Nano Letters 22, 3 (31 January 2022), 1108–1114 (abstract).
Delving into the Interstellar Sail
One of the benefits of a project like Breakthrough Starshot is that it moves the ball forward in terms of the academic research that underpins advances in technologies. I seriously doubt that Starshot will result in an Alpha Centauri probe reaching these stars within the next 50 years, given among other things the conundrum of data retrieval from a fleet of chip-sized micro-craft. But we all gain from the fact that scientists are tackling these issues in a well-funded and coordinated way. The research library grows.
As a field, interstellar studies has always been resource-starved, not to mention winning scant attention among the larger community of scientists and engineers at conferences and in publications. But it has drawn on a consistent thread of interest that now gains new energies. That benefits the entire effort. And let’s not forget the power of looking far into the future to get a conception of what we can do with scaled-down projects in the near term, as for example Andrew Higgins’ laser-fed fast missions to Mars.
Starshot, of course, takes the laser concept into the interstellar realm, using a massive ground-based array that would likely be based in Chile’s Atacama Desert. The laser array is used to push meter-scale sails, making Starshot’s committee on sail design a major component of the effort. The infusion of funding into sail technologies is welcome, as it leads to new insights into a sail’s shape, its size and its materials.
Image: An artist’s conception of the Starshot Lightsail spacecraft during acceleration by a ground-based laser array. Previous conceptions of lightsails have imagined them being passively pushed by light from the sun, but Starshot’s laser-based approach requires rethinking the sail’s shape and composition so it won’t melt or tear during acceleration. Credit: Masumi Shibata, courtesy of Breakthrough Initiatives.
Thus the significance of Igor Bargatin’s work. An associate professor at the University of Pennsylvania in the Department of Mechanical Engineering and Applied Mechanics, Bargatin and colleagues at the university as well as at UCLA have just published two papers going through fundamental sail issues and specifications. Remember that the projected sails, perhaps three-meters wide and a thousand times thinner than a sheet of paper, are to be subjected to a light intensity millions of times that of the Sun.
The team sees these sails as being made of ultrathin sheets of aluminum oxide and molybdenum disulfide, constructed in a parachute shape rather than a flat surface. The structure would be about as deep as it is wide, allowing the greatest ability under these calculations of withstanding the strain of the sudden acceleration, which is expected to reach tens of thousands of g’s. A ‘billowing’ sail should hold up to the strain better than a tight, flat one, providing a surface that is more resistant to tears.
Matthew Campbell is a postdoctoral researcher in Bargatin’s group and lead author of the paper covering the sail’s shape:
“Laser photons will fill the sail much like air inflates a beach ball. And we know that lightweight, pressurized containers should be spherical or cylindrical to avoid tears and cracks. Think of propane tanks or even fuel tanks on rockets.”
Image: Campbell et al. show that the diameter and radius of curvature of a circular light sail should be comparable in magnitude, both on the order of a few meters, in optimal designs for gram-scale payloads. Credit: Campbell et al.
The second paper, led by UCLA engineer Aaswath Raman, examines sail materials, offering insights into how heat will be dissipated under the powerful laser beam. Here the idea is to use nano-scale patterning within the material to manage the heat. Says Raman:
“If the sails absorb even a tiny fraction of the incident laser light, they’ll heat up to very high temperatures. To make sure they don’t just disintegrate, we need to maximize their ability to radiate their heat away, which is the only mode of heat transfer available in space.”
While earlier research maximized heat dissipation through a photonic crystal design that deployed regularly spaced holes in the sail material, the new work suggests adding a grid-like pattern for the ‘fabric,’ with the spacing of the holes matching the wavelength of light, and the swatches of sail material forming the grid spaced to match the wavelength of the thermal emission. The result is a stronger sail, one that could endure a higher initial thrust and therefore need less time under the beam.
The mathematics involved here is of a complexity far above my pay grade. I’ll have to send you to the paper for the details. But I think we can visualize the sail internals as a kind of ‘scaffolding’ that is apparent in the image below. The reference to ‘Mie resonant features’ in the caption to this image points to the work of Gustav Mie, who described what we now call ‘Mie scattering,’ showing the behavior of light of various wavelengths as it strikes particular kinds of structures. Mie resolved the intricate mathematics of these interactions.
Image: This is Figure S4 from the paper. Caption: Continuous Mie structure design. The black outlines serve to highlight the positions of the Mie resonant features against the continuous reflective green layer, but would not exist in the actual design. Credit: Brewer et al.
UCLA’s Deep Jariwala, who was involved with both papers, comments:
“A few years ago, even thinking or doing theoretical work on this type of concept was considered far-fetched. Now, we not only have a design, but the design is grounded in real materials available in our labs. Our plan for the future would be to make such structures at small scales and test them with high-power lasers.”
Thus the theoretical work continues. Exactly when it pays off in hardware and actual missions is something we cannot know.
The papers are Campbell et al., “Relativistic Light Sails Need to Billow,” Nano Letters 22, 1 (2022), 90-96 (abstract); and Brewer et al., “Multiscale Photonic Emissivity Engineering for Relativistic Lightsail Thermal Regulation,” Nano Letters 22, 2 (2022), 594-601 (abstract).
Galaxies Like Grains of Salt
I’m riffing on a Brian Aldiss title this morning, the reference being the author’s 1959 collection Galaxies Like Grians of Sand, which is a sequence of short stories spanning millions of years of Earth’s future (originally published as The Canopy of Time). But sand is appropriate for the exercise before us today, one suggested by memories of the day my youngest son told me he had to construct a model of an atom and we went hunting all over town for styrofoam balls. It turns out atoms are easy.
Suppose your child comes home with a project involving the creation of a scale model of the galaxy. Pondering the matter, you announce that grains of salt can stand in for stars. Sand might work as well, but salt is easier because you can buy boxes of salt at the grocery. So while your child goes outside to do other things, you and your calculator get caught up in the question of modeling the Milky Way. Just how much salt will you need?
Most models of the galaxy these days come in at a higher number than the once canonical 100 billion stars. In fact, 200 billion may be too low. But let’s economize by sticking with the lower number. So you need 100 billion grains of salt to make your scale model accurate. A little research reveals that the average box of Morton salt weighs in with about five million grains. Back to the calculator. You will need 20,000 boxes of salt to make this work. The local grocery doesn’t keep this much in stock, so you turn to good old Amazon, and pretty soon a semi has pulled up in front of your house with 20,000 blue boxes of salt.
But how to model this thing? I wouldn’t know where to begin, but fortunately JPL’s Rich Terrile thought the matter through some time back and he knows the answer. If we want to reflect the actual separation of stars just in the part of the galaxy we live in, we have to separate each grain of salt by eleven kilometers from any of its neighbors. Things get closer as we move in toward the bulge. Maybe your child has lots of friends to help spread the salt? Let’s hope so. And plenty of room to work with for the model.
I mention all this because I was talking recently with Nate Simpson, lead developer of Kerbal Space Program 2, and colleague Jon Cioletti. This is the next iteration of the remarkable spaceflight simulation game that offers highly realistic launch and orbital physics capabilities. We were talking deep space, and the salt box comparison came naturally, because these guys are also in the business of reaching a broader audience with extraordinary scales of time and space.
I strongly recommend Kerbal, by the way, because I suspect Kerbal Space Program has already turned the future career path of more than a few young players in the direction of aerospace, just as, say, science fiction novels or Star Trek inspired an earlier generation in that direction. Watching what develops as Kerbal goes into version 2 will be fascinating.
Unexpected Interstellar Targets
But I was also thinking about the salt box analogy because it can be so difficult to get interstellar distances across to the average person, who may know on some level that a galaxy is a very big place, but probably doesn’t have that deep awe that a real acquaintance with the numbers delivers. I think about craft trying to navigate these immensities, and also about objects like ‘Oumuamua and 2I/Borisov, the first two known interstellar objects we have detected. What vast oceans of interstellar space such tiny objects have drifted through! Clearly, as our ability to observe them grows, we’ll find many more such objects, and one of these days we’ll get a mission off to study one up close (probably designed by Andreas Hein and team).
Image: This Hubble Space Telescope image of 2I/Borisov shows the first observed rogue comet, a comet from interstellar space that is not gravitationally bound to a star. It was discovered in 2019 and is the second identified interstellar interloper, after ‘Oumuamua. 2I/Borisov looks a lot like the traditional comets found inside our solar system, which sublimate ices, and cast off dust as they are warmed by the Sun. The wandering comet provided invaluable clues to the chemical composition, structure, and dust characteristics of planetary building blocks presumably forged in an alien star system. It’s rapidly moving away from our Sun and will eventually head back into interstellar space, never to return. Credit: NASA, ESA, and D. Jewitt (UCLA).
I was heartened to learn over the weekend that the James Webb Space Telescope will likely have a role to play in further detection efforts. Indeed, there is now a Webb Target of Opportunity program that homes in on just such discoveries. Here is how a Target of Opportunity is defined on the JWST website:
A target for JWST observation is deemed a Target of Opportunity (ToO) if it is associated with an event that may occur at an unknown time, and in this way ToOs are distinct from time constrained observations.
Sounds made to order for interstellar interlopers. But we can add this:
ToO targets include objects that can be identified in advance, but which undergo unpredictable changes (e.g., some dwarf novae), as well as objects that can only be identified in advance by class (e.g., novae, supernovae, gamma ray bursts, newly discovered comets, etc.). ToOs are generally not suitable for observations of periodic phenomena (e.g., eclipsing binary stars, transiting planets, etc.). ToO proposals must provide a clear definition of the trigger criteria and present a detailed plan for the observations to be performed in the technical justification of the PDF submission if the triggering event occurs. A ToO activation may consist of a single observation or of a set of observations executed with a pre-specified cadence.
Martin Cordiner (NASA GSFC/Catholic University of America) is principal investigator of the Webb Target of Opportunity program to study the composition of an interstellar object:
“The supreme sensitivity and power of Webb now present us with an unprecedented opportunity to investigate the chemical composition of these interstellar objects and find out so much more about their nature: where they come from, how they were made, and what they can tell us about the conditions present in their home systems, The ability to study one of these and find out its composition — to really see material from around another planetary system close up — is truly an amazing thing.”
Image: This artist’s illustration shows one take on the first identified interstellar visitor, 1I/’Oumuamua, discovered in 2017. The wayward object swung within 38 million kilometers of the Sun before racing out of the solar system. 1I/’Oumuamua still defies any simple categorization. It did not behave like a comet, and it had a variety of unusual characteristics. As the complex rotation of the object made it difficult to determine the exact shape, there are many models of what it could look like. Credit: NASA, ESA, and J. Olmsted and F. Summers (STScI).
When astronomers detect another interstellar interloper, they’ll first need to confirm that it’s on a hyperbolic orbit, and if JWST is to come into play, that its trajectory intersects with the telescope’s viewing field. If that’s the case, Cordiner’s team will use JWST’s Near-Infrared Spectrograph (NIRSpec) to examine gasses released by the object due to the Sun’s heat. The spectral resolution available here should allow the detection of molecules ranging from water, methanol, formaldehyde and carbon dioxide to carbon monoxide and methane. The Mid-infrared instrument (MIRI) will track any dust or solid particles produced by the object.
The near- and mid-infrared wavelength ranges will be used to examine interstellar interlopers for the first time with this program, making this fertile ground for new discoveries. The assumption being that such objects exist in vast numbers, the Webb Target of Opportunity program should find material to work with, and likely soon, especially given JWST’s ability to detect incoming objects at extremely faint magnitudes. Are most such discoveries likely to be comet-like, or do we have the possibility of finding other objects as apparently anomalous as ‘Oumuamua?
Laser Thermal Propulsion for Rapid Transit to Mars: Part 2
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
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