by Paul Gilster | Mar 6, 2019 | Advanced Rocketry |
When he was considering white dwarfs and neutron stars in the context of what he called ‘gravitational machines,’ Freeman Dyson became intrigued by the fate of a neutron star binary. He calculated in his paper of the same name (citation below) that gradual loss of energy through gravitational radiation would bring the two neutron stars together, creating a gravitational wave event of the sort that has since been observed. Long before LIGO, Dyson was talking about gravitational wave detection instruments that could track the ‘gravitational flash.’
Image: Artist conception of the moment two neutron stars collide. Credit: LIGO / Caltech / MIT.
Observables of this kind, if we could figure out how to do it (and we subsequently have) fascinated Dyson, who was in this era (early 1960s) working out his ideas on Dyson spheres and the capabilities of advanced civilizations. As to the problematic merger of neutron stars in a ‘machine,’ he naturally wondered whether astrophysical evidence of manipulations of these would flag the presence of such cultures, noting that “…it would be surprising if a technologically advanced species could not find a way to design a nonradiating gravitational machine, and so to exploit the much higher velocities which neutron stars in principle make possible.”
He goes on in the conclusion to the “Gravitational Engines” paper to say this: “In any search for evidences of technologically advanced societies in the universe, an investigation of anomalously intense sources of gravitational radiation ought to be included.”
Searching for unusual astrophysical activity is part of what would emerge as ‘Dysonian SETI,’ or in our current parlance, the search for ‘technosignatures.’ It’s no surprise that since he discusses using binary black holes as the venue for his laser-based gravity assist, David Kipping should also be thinking along these lines. If the number of black holes in the galaxy were large enough to support a network of transportation hubs using binary black holes, what would be the telltale sign of its presence? Or would it be observable in the first place?
Remember the methodology: A spacecraft emits a beam of energy at a black hole that is moving towards it, choosing the angles so that the beam returns to the spacecraft (along the so-called ‘boomerang geodesic’). With the beam making the gravitational flyby rather than the spacecraft, the vehicle can nonetheless exploit the kinetic energy of the black hole for acceleration. Huge objects up to planetary size could be accelerated in such a way, assuming their mass is far smaller than the mass of the black hole. No fuel is spent aboard the spacecraft which, using stored energy from the beam, continues to accelerate up to terminal velocity.
Image: Simulated image of the two merging black holes detected by LIGO, viewed face-on. LIGO’s gravitational-wave detection is the first direct observation of such a merger. Credit: LIGO / AAS Nova.
Kipping likes to talk about the process in terms of a mirror. Because light loops around the approaching black hole and returns to the spacecraft, the black hole exhibits mirror-like behavior. Thus on Earth, if we bounce a ping-pong ball off a mirror, the ball returns to us. But if the mirror is moving towards us quickly, the ball returns faster because it has picked up momentum from the mirror. Light acts the same way, but light cannot return faster than the speed of light. Instead, in gaining momentum from the black hole, the light blueshifts.
We exploit the gain in energy, and we can envision a sufficiently advanced civilization doing the same. If it can reach a black hole binary, it has gained an essentially free source of energy for continued operations in moving objects to relativistic speeds. Operations like these at a black hole binary result in certain effects, so there is a whisper of an observable technosignature.
I discussed the question with Kipping in a recent email exchange. One problem emerges at the outset, for as he writes: “The halo drive is a very efficient system by design and that’s bad news for technosignatures: there’s zero leakage with an idealized system.” But he goes on:
The major effect I considered in the paper is the impact on the black hole binary itself. During departure, one is stealing energy from the black binary, which causes the separation between the two dead stars to shrink slightly via the loss of gravitational potential energy. However, an arriving ship would cancel out this effect by depositing approximately the same energy back into the system during a deceleration maneuver. Despite this averaging effect, there is presumably some time delay between departures and arrivals, and during this interval the black hole binary is forced into a temporarily contracted state. Since the rate of binary merger via gravitational radiation is very sensitive to the binary separation, these short intervals will experience enhanced infall rates. And thus, overall, the binary will merge faster than one should expect naturally. It may be possible to thus search for elevated merger rates than that expected to occur naturally. In addition, if the highway system is not isotropic, certain directions are preferred over others, then the binary will be forced into an eccentric orbit which may also lead to an observational signature.
Tricky business, this, for a black hole binary in this formulation can be used not only for acceleration but deceleration. The latter potentially undoes the distortions caused by the former, though Kipping believes elevated merger rates between the binary pair will persist. Our technosignature, then, could be an elevated binary merger rate and excess binary eccentricity.
I was also interested in directionality — was the spacecraft limited in where it could go? I learned that the halo drive would be most effective when moving in a direction that lies along the plane of the binary orbit. Traveling out of this plane is possible, though it would involve using onboard propellant to attain the correct trajectory. The potential of reaching the speed of the black hole itself remains, but excess stored energy would then need to be applied to an onboard thruster to make the course adjustment. Kipping says he has not run the numbers on this yet, but from the work so far be believes that a spacecraft could work with angles as high as 20 degrees out of the plane of the binary orbit and still reach an acceleration equal to that of the black hole.
For more on the halo drive, remember that Kipping has made available a video that you can access here. The other citations are Kipping, “The Halo Drive: Fuel-free Relativistic Propulsion of Large Masses via Recycled Boomerang Photons,” accepted at the Journal of the British Interplanetary Society (preprint); and Dyson, “Gravitational Machines,” in A.G.W. Cameron, ed., Interstellar Communication, New York: Benjamin Press, 1963, Chapter 12.
by Paul Gilster | Mar 5, 2019 | Advanced Rocketry |
One of the interesting things about gravitational assists is their ability to accelerate massive objects up to high speeds, provided of course that the astrophysical object being used for the assist is moving at high speeds itself. Freeman Dyson realized, as we saw yesterday, that a pair of tightly rotating white dwarfs could offer such an opportunity, while a binary neutron star carried even more clout. When Dyson was writing his “Gravitational Machines” paper, neutron stars were still a theoretical concept, so he primarily focused the paper on white dwarfs.
Get two neutron stars in a tight enough orbit and the speeds they achieve would make it possible to accelerate a spacecraft making a gravity assist up to a substantial percentage of lightspeed. But what an adventure that close pass would be — the tidal forces would be extreme. I don’t recall seeing a neutron star propulsive flyby portrayed in science fiction (help me out here), though Gregory Benford offers a variant on the white dwarf idea in his early work Deeper than the Darkness (1970, later re-done as The Stars in Shroud, which ran in Galaxy in 1978), where a neutron star and an F-class star comprise a system he calls a ‘Flinger,’ with potential uses both for acceleration and deceleration.
David Kipping’s thoughts on extending Dyson to black holes were partially triggered by a fascinating paper from William Stuckey in 1993 entitled “The Schwarzschild Black Hole as a Gravitational Mirror” (citation below). The gravitational mirror in the title is what happens when photons skim near the event horizon and return to the source, a photon ‘boomerang’ that gains propulsive impact because the returning light rays receive a blueshift thanks to the black hole’s relative motion.
Thus the Kipping concept: Use laser beaming technologies to beam toward a precisely targeted member of a black hole binary. The light is returned in blue-shifted form to the spacecraft, the extra energy being used to push the craft. What’s especially intriguing here is that in this scenario, it is not the spacecraft that is making the messy plunge between two rapidly moving, tightly orbiting black holes, but rather the photon beam that is used for propulsion.
It is the light beam that gets the gravitational assist as photons are re-emitted and re-absorbed, for as the paper puts it, “…the halo drive transfers kinetic energy from the moving black hole to the spacecraft by way of a gravitational assist.” A less harrowing experience for any crew, to be sure, and one in which extreme time dilation can be avoided, not to mention the dangers of tidal disruption and radiation referred to above.
Image: David Kipping (Columbia University), creator of the ‘halo drive’ concept.
Kipping described the idea to me in an email recently:
The idea is to essentially to perform a Dyson slingshot remotely, by firing a collimated particle/energy beam just to the side of the event horizon of a Schwarzchild black hole. If you choose the angle carefully, the beam loops around (like a halo) and comes back to you. If the black hole is moving towards you (I envisaged a compact binary like Dyson), then the beam returns blue shifted. When you initially fire the beam, your ship receives a small momentum kick and when the beam returns and strikes your ship you get another. This is how the ship is propelled, much like a light sail. But the beam actually returns with more energy than it departed, since it siphoned some of the kinetic energy from the black hole. So not only did you accelerate, but your ship actually gained stored energy.
Do this right and the speed of your spacecraft eventually matches that of the black hole, but the cumulative blue shifts allow the craft to continue firing the laser past that point. No new ‘free’ energy is gained but energy from the stored cells aboard the craft can keep the acceleration going up to, the author calculates, 4/3 the speed of the black hole. It’s interesting to note that we are not limited to small masses in such a calculation. Unlike the Breakthrough Starshot energy issue we discussed yesterday, we can drive arbitrarily large masses up to potentially relativistic speeds. All of this by exploiting the fantastic energies available through astrophysical objects.
Image: This is Figure 1 from the paper. Caption: Figure 1. Outline of the halo drive. A spaceship traveling at a velocity ?i emits a photon of frequency ?i at a specific angle ? such that the photon completes a halo around the black hole, returning shifted to ?f due to the forward motion of the black hole, ?BH. Credit: David Kipping.
As you would imagine, for the gravitational mirror to function in this way, the beam must be precisely oriented. From the paper:
In order for the deflection to be strong enough to constitute a boomerang, this requires the light’s closest approach to the black hole to be within a couple of Schwarzschild radii, RS ? 2GM/c2. Light which makes a closest approach smaller than 3GM/c2 becomes trapped in orbit, known as the photon sphere, and thus typical boomerang geodesics skim just above this critical distance [italics mine].
Kipping calls this a ‘halo drive’ because the photons returning to the craft appear as a halo around the black hole. Single black holes as well as binaries can be used (and evidently Kerr black holes as well as Schwarschild black holes, though the author plans future work on this), but the paper notes that the potential for tight configurations at relativistic speeds makes binaries preferable. 10 million binary black holes are thought to exist in the galaxy [I’ve seen this figure reduced to 1 million recently — clearly, the issue is still open], raising the possibility of a network of starship acceleration points or, for that matter, deceleration stations.
A range of possible uses for binary black holes emerges, as the paper notes:
Although not the focus of this work, it is worth highlighting that halo drives could have other purposes besides just accelerating spacecraft. For example, the back reaction on the black hole taps energy from it, essentially mining the gravitational binding energy of the binary. Similarly, forward reactions could be used to not only decelerate incoming spacecraft but effectively store energy in the binary like a fly-wheel, turning the binary into a cosmic battery.
Another possibility is that the halos could be used to deliberately manipulate black holes into specific configurations, analogous to optical tweezers. This could be particularly effective if halo bridges are established between nearby pairs of binaries, causing one binary to excite the other. Such cases could lead to rapid transformation of binary orbits, including the deliberate liberation of a binary.
A natural question is why an interstellar civilization, one already capable of reaching a black hole binary and manipulating it in this manner would need to establish a galactic network of transit points. A possible answer is that the amount of energy liberated from such a black hole binary is, as with other kinds of gravitational assist, arriving essentially ‘free’ at the spacecraft. Thus a transportation network on the cheap could be established between specific locations, a mechanism for cost-savings that may well be too efficient to ignore.
Freeman Dyson’s interest in using astrophysical objects for propulsion included, of course, his abiding fascination with the technosignatures of advanced civilizations. Would a galactic infrastructure at work in the galaxy exhibit evidence of its use to distant astronomers? In my next post, I’ll look into the possibility.
The Dyson paper is “Gravitational Machines,” in A.G.W. Cameron, ed., Interstellar Communication, New York: Benjamin Press, 1963, Chapter 12. The Kipping paper is “The Halo Drive: Fuel-free Relativistic Propulsion of Large Masses via Recycled Boomerang Photons,” accepted at the Journal of the British Interplanetary Society (preprint). The Stuckey paper is “The Schwarzschild Black Hole as a Gravitational Mirror,” American Journal of Physics Vol. 61, Issue 5 (1993), pp. 448-456.
by Paul Gilster | Mar 4, 2019 | Advanced Rocketry |
Let’s start the week by talking about gravitational assists, where a spacecraft uses a massive body to gain velocity. Voyager at Jupiter is the classic example, because it so richly illustrates the ability to alter course and accelerate without propellant. Michael Minovitch was working on this kind of maneuver at UCLA as far back as the early 1960s, but it was considered even before this, as in a 1925 paper from Friedrich Zander. It took Voyager to put gravity assists into the public consciousness because the idea enabled the exploration of the outer planets.
Can we use this kind of maneuver to help us gain the velocity we need to make an interstellar crossing? Let’s consider how it works: We’re borrowing energy from a massive object when we do a gravity assist. From the perspective of the Voyager team, their spacecraft got something for ‘free’ at Jupiter, in the sense that no additional propellant was needed. What’s really happening is that the spacecraft gained energy at the expense of the planet. Jupiter being what it is, the change in its own status was invisible, but it lent enough energy to Voyager to prove enabling.
According to David Kipping (Columbia University), the maximum speed increase equals twice the velocity of the planet we’re using for the maneuver, and when you look at Jupiter’s orbital speed around the Sun (around 13.1 kilometers per second), you can see that we’re only talking about a fraction of what it would take to get us to interstellar speeds. But the principle is enticing, because traveling with little or no propellant is a longstanding goal, one that drives research into solar sails and their fast cousins, beamed lightsails. And it has been much on Kipping’s mind.
For gravitational assists from planets are only one aspect of the question, there being other kinds of astrophysical objects that can help us out. Depending on their orbital configuration, some of these are moving fast indeed. In the early 1960s, Freeman Dyson went to work on the physics of gravitational assists around binary white dwarf stars — he would ultimately go on to consider the case of neutron star binaries (back when neutron stars were still purely theoretical). Such concepts obviously imply an interstellar civilization capable of reaching the objects in the first place. But once there, the energies to be exploited would be spectacular.
While I want to begin with Dyson’s ideas, I’ll move tomorrow to Kipping’s latest paper, which addresses the question in a novel way. Kipping, well known for his work in the Hunt for Exomoons with Kepler project, has been pondering Dyson’s notions but also applying them to what would seem, on the surface of things, to be an entirely different proposition: Beamed propulsion. How he combines the two may surprise you as much as it did me, as we’ll see in coming days.
Image: An artist’s conception of two orbiting white dwarf stars. Credit: Tod Strohmayer (GSFC), CXC, NASA, Illustration: Dana Berry (CXC).
Nature of the Question
If we talk about manipulating astrophysical objects, a natural objection arises: Why should we study things that are impossible for our species today? After all, we can get to Jupiter, but getting to the nearest white dwarf, much less a white dwarf binary, is beyond us.
But big ideas can be productive. Consider Daedalus, conceived in the 1970s as the first serious design for a starship. The idea was to demonstrate that a spacecraft could be designed using known physics that could make a journey to another star. The massive two-stage Daedalus (54,000 tonnes) seems impossible today and doubtless will never be built. Was it worth studying?
The answer is yes, because once you’ve established that something is not impossible, you can go to work on ways to engineer a result that may differ hugely from the original. Breakthrough Starshot is built around the idea of using lasers to propel a different kind of spacecraft, not of 54,000 tonnes but of 1 gram, carried by a small lightsail, and designed to be sent not as a one-off mission but as a series of probes driven by the same laser installation.
Once again we’re stretching our thinking, but here the technologies to do such a thing may (or may not, depending on what Breakthrough Starshot’s analyses come up with) be no more than a few decades away. The current Breakthrough effort is all about finding out what is feasible.
Again we’re designing something before we’re sure we can do it. The challenges are obviously immense. Consider: To go interstellar with cruise times of several decades, we need to ramp up velocity, and that takes enormous amounts of energy. Kipping calculates that 2 trillion joules — the output of a nuclear power plant running continuously for 20 days — would be needed to send the Breakthrough Starshot ‘chip’ payload to Proxima Centauri. And that’s just for one ‘shot’, not for the multiple chips envisioned in what might be considered a ‘swarm’ of probes.
Working with Massive Objects
Are there other ways to generate such energies? Freeman Dyson’s extraordinary white dwarf binary gravitational assist appears in “Gravitational Machines,” a short paper that ran in a book A.G.W. Cameron edited called Interstellar Communication (New York, 1963). Conventional gravity assists aren’t sufficient because to be effective, a gravitational ‘machine’ would have to be built on an astronomical scale. Fortunately, the universe has done that for us. So we should be thinking about the principles involved, and what they imply:
…if our species continues to expand its population and its technology at an exponential rate, there may come a time in the remote future when engineering on an astronomical scale will be both feasible and necessary. Second, if we are searching for signs of technologically advanced life already existing elsewhere in the universe, it is useful to consider what kinds of observable phenomena a really advanced technology might be capable of producing.
Dyson’s considers the question in terms of binary stars, specifically white dwarfs, but goes on to address even denser concentrations of matter in neutron stars. Now we’re talking about a kind of gravitational assist that has serious interstellar potential. A spacecraft could be sent into a neutron star binary system for a close pass around one of the stars, to be ejected from the system at high velocity. If 3,000 kilometers per second appears possible with a white dwarf binary, fully 81,000 kilometers per second could occur — 0.27 c — with a neutron star binary.
Hence the ‘Dyson slingshot.’ (As an aside, I’ve always wondered what it must be like to have a name so famous in your field that everything from ‘Dyson spheres’ to ‘Dyson dots’ are named after you. The range of Dyson’s thinking on these matters certainly justifies the practice!).
The slingshot isn’t particularly effective with stars of solar class, where what you gain from a gravitational assist is still outweighed by the possibility of using stellar photons for propulsion. But as Dyson shows, once you get into white dwarf range and then extend the idea down to neutron stars, you’re ramping up the gravitational energy available to the spacecraft while at the same time reducing stellar luminosity. An advanced civilization, in ways Dyson explores, might tighten the orbital distance until the binary’s orbital period reached a scant 100 seconds.
Now a gravity assist has serious punch. In other words, there is the potential here for a civilization to manipulate astrophysical objects to achieve a kind of galactic network, where binary neutron stars offer transportation hubs for propelling spacecraft to relativistic speeds. As you would imagine, this plays to Dyson’s longstanding interest in searching for technological artifacts, and we’ll be talking about that possibility as we get into David Kipping’s new paper.
For Kipping will take Dyson several steps further, by looking not at neutron stars but black hole binaries and coming up with an entirely novel way of exploiting their energies, one in which a beam of light, rather than the spacecraft itself, gets the gravitational assist and passes those energies back to the vehicle. Kipping calls his idea the ‘Halo Drive,’ and we’ll begin our discussion of it, and a novel insight that inspired it, tomorrow.
The Dyson paper is “Gravitational Machines,” in A.G.W. Cameron, ed., Interstellar Communication, New York: Benjamin Press, 1963, Chapter 12. The Kipping paper is “The Halo Drive: Fuel-free Relativistic Propulsion of Large Masses via Recycled Boomerang Photons,” accepted at the Journal of the British Interplanetary Society (preprint). For those who want to get a head start, Dr. Kipping has also prepared a video on the Halo Drive that is available here.
by Paul Gilster | Jan 25, 2019 | Advanced Rocketry |
Most of my research for Centauri Dreams involves looking at papers and presentations on matters involving deep space, whether propulsion systems, closed loop life support, or even possible destinations. That’s why it’s a huge help to get an article like the one below, giving us an overview of current thinking and an analysis of what is either in the pipeline or under consideration for the near-term. How do we get from here to the kind of Solar System infrastructure we’ll need to go interstellar? Ioannis Kokkinidis has an impressive background: He holds a Master of Science in Agricultural Engineering from the Department of Natural Resources Management and Agricultural Engineering of the Agricultural University of Athens. He went on to obtain a Mastère Spécialisé Systèmes d’informations localisées pour l’aménagement des territoires (SILAT) from AgroParisTech and AgroMontpellier and a PhD in Geospatial and Environmental Analysis from Virginia Tech. Today Ioannis surveys the lay of the land, from near-future manned missions to robotic forays to Jupiter, Saturn, the ice giants and beyond.
by Ioannis Kokkinidis
Introduction
This post was inspired from a reader’s comment that came in my review of Andy Weir’s Artemis at this website: most people are not aware of what are the planned future plans for space exploration beyond the highly publicized plans of SpaceX. I noted then that I might write in the future about the future of space exploration as it is outlined in the current plans, rather than just the dreams of visionaries. Furthermore 2019 is the 50th anniversary of the first moon landing, and people are probably wondering what is supposed to come next. In this post I review what the current plans to explore space from earth orbit to interstellar space look like currently, both for robotic and human spaceflight. My standard caveat that these are the views of a learned amateur, not a professional in the sector, applies as always.
Earth orbit
Since this is a blog on interstellar travel, I will keep my analysis on earth orbit short. Suffice to say that for robotic exploration even a simple listing of all the future satellites that are planned to be put up by various public and private agencies can fill volumes of books. For human spaceflight it is certain that this particular destination will not be abandoned. The International Space Station is currently authorized until 2024 and there has been legislative action to extend it all the way to 2030. This is something to consider when evaluating the feasibility of the Planetary Society’s Mars in the 2030s plan, which expects that ISS money will go to Mars Exploration after the station is abandoned in 2024 or 2028 at the latest. Even if ISS is abandoned in 2030, NASA will not abandon the destination. There are currently very preliminary plans about a successor space station that will not be owned by NASA, but by a private contractor with NASA leasing space for use by its astronauts. This is a similar model to what ESA uses, which plans in the future to keep sending its astronauts to earth orbit stations it will not own. Russia still has several modules to add to its segment of the International Space Station, starting with Nauka MLM-U. When the time comes to abandon the current ISS, Russia will detach its new modules and turn them into an autonomous space station. China also intends to keep sending its own space stations to low earth orbit, and ESA has signed a cooperating agreement to send its own astronauts there. India has its own plans to achieve orbit autonomously.
Maintaining a station in earth orbit makes sense from a deep space perspective: you would want to first test how your astronauts will react to space in the cloistered environment near earth before sending them off to farther destinations where they will face increased risks. There are plans to put up private space stations as hotels and, if the ISS proved anything, is that there are several people willing to pay an 8 digit price tag to experience space. Whether these plans actually come to fruition remains to be seen. My understanding is that private trips to the ISS ended with the retirement of the Space Shuttle and its logistics capacity, even though all private travelers came by Soyuz. The problem with private space stations is that the current market will not support a privately owned station. The ISS receives some $100 million in private income per year; there is a figure on the web that private space research is about $500 million per year which sounds a lot until you consider that for Fiscal Year 2018 US Congress appropriated over $4 billion for ISS operations and related activities such as commercial cargo and crew. To that we would need to add spending by the other ISS partners. Future space stations would need to function more cheaply and attract more funding just to cover operation costs, let alone assembly costs.
The Moon
The moon is the most accessible destination beyond earth, 600 times closer than Mars. China has given an emphasis on the moon with its Chang’e program, which can lead to human landings. Russia intends to return to the moon with the Luna-Glob program, which will be missions Luna 25 through 31. South Korea intends to send a rover to the moon, with US participation. India has sent an orbiter, Chandrayaan-1, which had US participation, and which it intends to follow up. SpaceIL of Israel is about to send the first private mission to the Moon, funded by philanthropy rather than government appropriations. The US intends to send robots to the lunar surface and has solicited the private sector for lunar landing architectures. The program is called the Commercial Lunar Payload Services program and has awarded 10 contracts, initially for a user manual to the landing architecture, that can reach up to $6.2 billion in awards. Many of those participating have come out of the defunct Google Lunar X Prize. Those plans though are dwarfed in excitement by the plans for a return of people to the moon.
Orbit
Image: Plan of the Lunar Orbital Gateway. Credit: From the December 7 2018 presentation to the NASA Advisory Council Human Exploration committee.
The next great frontier of human space exploration is currently planned to be Earth’s Moon. Of course this can sound a bit oxymoronic: didn’t we first orbit the Moon 50 years ago with Apollo 8, and go on to orbit it another 8 times? That is very true, but we have not been there since Apollo 17 in 1972. The Soviet Union abandoned its moonshot after the fourth N-1 rocket exploded in 1974, and by then NASA had focused on the earth because of the backlash to Apollo. A plan for a partially crewed station in lunar orbit emerged at the start of this decade, proposed by the contractors of the International Space Station when their work assembling it had finished. During the Obama presidency it was sold as proving ground for the Journey to Mars: a prototype of the Mars transfer habitat loosely connected to the earth but not very far away. Currently the Moon itself is the final destination: rather than have the Altair lander that would be carried with Orion per the Constellation program plan, another lander is to be used that will go from the currently dubbed Lunar Orbital Platform Gateway (LOP-G) to the lunar surface and back. The Gateway will be in a Near Rectilinear Halo Orbit and will be crewed only part of the time, just as Skylab and the early Salyut stations were. The first section will be the Power and Propulsion Element which will be an adaptation of current spacecraft buses for geostationary satellites; it is expected to proceed from the conceptual to the development stage in 2019 and launch in 2022. While it will be launched by a commercial rocket to earth orbit and proceed using its own high power electric propulsion engines to the Moon, that will not be the case for the other modules. Starting from Exploration Mission 3 each Orion launch to the Moon will carry with it a module that will be dropped off at the station until it is assembled.
Image: Overview of the Gateway plan. Credit: From the December 7 2018 presentation of Jason Crusan to the NASA Advisory Council.
The Gateway has many critics. For starters many people are opposed to it because it will use the Space Launch System and the Orion Spacecraft: “Any money not spent on SpaceX is a waste, SLS is a waste, Orion is a waste, government is waste, private sector forever”. It is true that the origins of the Space Launch System is the Senate; the Obama administration wished to abolish all of the George W Bush era Constellation program and let the private sector do everything. The Senate stopped this continued the Orion spacecraft as a deep space vehicle and created the SLS as a continuation of the Ares V rocket. Congress did not allow Nixon to abolish NASA because it had become a jobs program; this was also the case for the end of Constellation. The Congressional delegation of Alabama is not about to send thousands of jobs and millions in funding from Huntsville to California, nor are the delegations of any other of the 41 states that have SLS contractors. At least Jeff Bezos has been wise enough to put his rocket factories in established rocket manufacturing areas, Elon Musk did not. If anything, the cancellation of Space Shuttle proved that Congress will not automatically re appropriate money saved to other NASA programs: NASA was not given saved shuttle funding and it was used instead to pay other government priorities. NASA funding rose again only after The Martian film came out and Congress got more interested in human space exploration.
Another set of critics point out that it would be more advantageous to launch bigger pieces of the Gateway on their own dedicated SLS launches rather than try to fit them behind the Orion. This is a very valid point; Mir was sent up separately from the cosmonauts that assembled it. Yet another criticism is that Lunar Orbit is a destination where we should put a station after we have a station on the surface of the moon, and that it will be nothing but a toll station, an unnecessary stop on the way to the Moon’s surface. Better yet, let’s put up an orbital station after we have proven In Situ Resource Utilization, if at all. Having a station in lunar orbit and a spaceship that can go there is far more technologically mature than having a surface station. We have had stations in Earth orbit since Salyut 1 in 1971. By now we know what it takes to run one, though how it will function and how astronauts will function semi-detached from the earth is an unknown. Beyond the short surface forays with the Lunar Modules during Apollo, we do not have any experience operating on the surface of another body. On top of that, the lunar surface is a rather harsh place, with 14 (earth) day days and nights with wild temperature swings and an ever present and harsh dust. Also having a space station in lunar orbit is creating infrastructure that helps make lunar exploration more sustainable, as was emphasized to the NASA Advisory Council. Furthermore it is much easier to resupply from Earth a lunar orbital station than a surface station that would require soft landing of the goods sent. What is certain in any case is that the orbital station at the moon has matured enough as a plan that there is a real notional assembly plan expected to be done by 2026. Surface base plans are far more notional, mostly pursued by space advocates.
Surface
The most developed idea to colonize the Lunar Surface is ESA director Johann-Dietrich Wörner’s Lunar Village. The idea is that all the world nations will come together and build a joint settlement on the moon where one building will be from ESA, another from NASA, another from Roscosmos and so on. Wörner has used his authority at ESA and earlier at DLR (German Aerospace Research Center) to fund a few conceptual architectural studies on how a building from regolith would look. There has not been though real development on a plan with milestones and dates to build the village, unlike the LOP-G plan. While NASA has put up tenders for the Gateway with technical descriptions for the modules and there is a preliminary agreement on what the modules should be, there is no such planning for the lunar surface. Russia has proposed a lander from the Gateway to the surface, and so has Lockheed Martin, using a version of its Mars Lander proposal from the Journey to Mars but without a heat shield.
China intends to directly land on the moon at the end of the Chang’e program, possibly in the 2030s. It was announced in the press conference for the first lunar night of Chang’e 4 that Chang’e 8 “will test key technologies to lay the groundwork for the construction of a science and research base on the Moon”. What this means practically is unknown, Chang’e 4 also tested a key technology to construct a base by having an experiment to grow plants and insects on the moon. Apollo plans for longer lunar stays were for a modified version of the Lunar Module without an ascent stage to be used as a habitat for a lunar day while the astronauts would use the Lunar Module on which they arrive only for descent and ascent to the surface. Later the Apollo Application Program envisioned a soft horizontal landing of the upper stage of the Saturn V with the Oxygen tank converted into a long term habitat, per the wet workshop concept.
Post Apollo plans for the most part envision a core module, similar to a space station module, landed on the lunar surface and then covered with regolith for radiation protection using a bulldozer, before longer term construction begins. Specific Chinese base plans have not been announced. The characteristics of the Chinese space program are that it is highly focused but maintains a glacial pace. When the Chinese sent the first person to space in 2003 the Greek magazine Ptisi kai Diastima (Flight and Space) had a comprehensive review of what was known and what had been announced about the Chinese manned space program showing that the effort to send a person in space had begun around 1980. If the Chinese have indeed decided to send a person to the Moon and build a base there, it will happen, eventually. However the decision making process in space, as in many other domains in the People’s Republic of China is quite opaque.
The American (and European) system is quite open, but less focused. The US has vacillated between a return to the Moon and forward to Mars as a destination for human spaceflight for decades, depending on the presidential administration. At least for robotic spaceflight, the US scientific community has created the Decadal Surveys process to create a consensus among the various scientific interests on what destinations and investigations are to be prioritized. China works with 5 year plans, but there is no publicly known long term roadmap in either human or robotic exploration. The Chang’e 4 press conference announcements, which also mentioned a Chinese Mars rover for 2020 have so far been the most public long term plan announced.
Venus and Mercury
Venus and Mercury are grouped together in this post because both will mostly remain the purview of robotic spacecraft. The Apollo Applications Program seriously considered a Manned Venus Flyby for 1974, but in the end the only part of AAP that came to fruition was the least risky one: Skylab. Several Mars plans have included a test at Venus, usually as a dress rehearsal to a Mars mission. Venus is closer and requires less energy to visit than Mars, so before sending a spacecraft on a 15 month mission to Mars, we can send it on a six month mission to and from Venus. Still, the surface of Venus is a sulfuric hellhole, very hard to visit by robot, and there are very few unique things that require human participation to do in orbit, especially considering that the planet does not have any satellites. Recently a Venus airship was proposed as part of NASA’s Advanced Innovative Concept program. Balloons have been dropped on Venus, most recently two French balloons that were part of the Soviet VEGA mission to Halley’s Comet, but this is to my knowledge the first proposal for a crewed dirigible. NASA AICs are at best decades from fruition and most are never implemented.
Venus has proven to be the greatest success of the Soviet space program. Russia has plans to send another probe in the future, Venera D, which will have US participation. Currently the Japanese probe Akatsuki is orbiting Venus. Earlier, ESA’s Venus Express was doing so, but the US has not sent a mission since Magellan in the early 1990s. There have been proposals both in the Discovery and New Frontiers programs but they were not selected. For this reason NASA’s planetary division is investigating a Venus Bridge mission, something that will cost around $200 million and can do more science than Magellan. Also a surface mission is being contemplated. The state of technology is such that a cooling shield at Venus can work for some 5 hours before it fails, and then we can try to have a hot mission as long as the robot lasts. There is some computer hardware out there able to withstand Venusian conditions such as a CPU, but not an entire computer, e.g. no high temperature RAM.
Mercury has the disadvantage of being in a location where the energy necessary to reach it is similar to that needed to reach Jupiter. So far we have had Mariner 10 do three flybys, MESSENGER orbit the planet and ESA’s BepiColombo is on its way there. The US National Academy of Science suggests that the next mission there ought to be a lander. Considering how sporadic missions to Mercury have been and that no Mercury lander has been proposed yet, it will take quite some time until such a mission starts.
Mars
The Red Planet has been the object of popular fascination throughout the telescope era. As David Portree has noted in his book Humans to Mars, over 100 mission architectures have been proposed for a human landing there since Von Braun’s Der Marsprojekt of 1948, but none has yet moved to building the necessary equipment. On the other hand, while Mars has very appropriately been described as the graveyard of space probes, it has also been the setting of spectacularly successful robotic missions. The next big step in robotic missions to Mars is Sample Return. So far the only time that a mission proposal for Sample Return reached the definite launch date phase, at least per Ulivi and Harland’s book series Robotic Exploration of the Solar System, has been the JPL 2003 proposal, which was cancelled after the twin failures of the 1999 missions revealed that “Faster, Cheaper, Better” also meant too much risk. However NASA has persisted and the current sample return architecture looks as such: First a rover will land and cache samples on the Martian surface. That rover is the Mars 2020 rover which is under construction at the Jet Propulsion Laboratory. Then another mission will land that will have a fetch rover which will pick up the samples and bring them back to the return launcher, which in turn will lift them to Mars orbit. There another spacecraft will collect them and return them to earth. So far these two missions have not been authorized, but there are plans. Now that we do know what the weight of the sample cache is, the older plans for a solid rocket liftoff rocket had to be revised: it would be too heavy. Thus a hybrid rocket architecture was selected, and currently there is active development to mature this technology so that it can be used. The notional timeline is that the fetch mission will be launched in 2024 and ESA has signed a preliminary agreement to provide the return spacecraft. However there is also a far more ambitious plan for 2024: Elon Musk intends to land his Starship on Mars.
The SpaceX Architecture
Image: Starship taking off from Mars. Credit: SpaceX.
If someone wanders the internet looking for articles on space, he is bound to run into the idea that the way to move forward in space exploration is to give all problems to SpaceX to solve, give all the money to SpaceX, everything is superior if it is made by SpaceX and that every other effort to solve any other issue in space is doomed at best, a waste of resources at worst, only SpaceX can solve everything. The triumphalism you find online about SpaceX gets quite annoying. My first reaction when I ran into this kind of articles and comments on internet boards was that these might be paid trolls. Closer scrutiny has shown that this is not the case, the posters are often true believers, and indeed inside SpaceX many truly believe that they are the salt of the Earth, or I should say the salt of space. My next reaction though would be, did they not do the Iliad in school in order to learn its lesson of hubris, ate, nemesis? Didn’t they do Herodotus in school, or the tragedies of Aeschylus, Sophocles and Euripides? Has classical education dropped so low in the West?
Then I ran into a Wired article entitled “Dr Elon & Mister Musk: Life inside Tesla’s production hell” [https://www.wired.com/story/elon-musk-tesla-life-inside-gigafactory/]. After he became a millionaire Elon Musk founded SpaceX, Tesla Motors and SolarCity with a utopian vision to change the world for the better. The foundational aspect of the companies is that the current methods in each industry are hopelessly outdated and that through disruptive innovation you can change the world for the better. Work hard enough (i.e. no 8 hour days; have a job, not a life) and the impossible becomes possible unless it violates the laws of physics. This is a general mentality prevalent in Silicon Valley: it is the place that believes that through hard work you can pursue the dream, and a place where hubris is actually welcome. Since Tesla achieved what was though impossible in the past, they think that they will keep on doing it regularly in the future. However the article is an inadvertent story of hubris, ate, nemesis: Tesla designs a great mass market electric car and decides to build it in a highly automated robotic factory to minimize the number of workers. Elon moves the production date six months forward from the original plan (hubris). When he runs into trouble setting up the production lines, and gets in a foul mood after his breakup with Amber Heard, he fires people on a whim when he runs into them if they are not able to answer his questions on the spot to his liking (ate). In the end he realizes that conventional car production companies are not the outdated behemoths he thinks: They have made a real effort to automate more and are already working at the edge of the possible. He manages to deliver his production goal, but 6 months after his original planned date and 12 months after the target date he set, $10,000 more expensive than the cost target and with his company a few weeks from bankruptcy (nemesis). The main thing going for the Tesla Model 3 is that it is a better electric car than the competition.
The relevant question here is, has he been chastened enough by the experience, or will he repeat the same hubris with his Mars plans? Incidentally, Elon’s treatment of his employees would never fly in Europe: if his firings are as arbitrary as described, his ex-employees have a right to sue and force him to hire them back at Tesla Motors. Also the idea of more than 8 hours work on a permanent basis and a general dislike of the idea of people taking vacation is reason enough for a European government to force an ultimatum: either respect the sacrosanctity of the 8 hour work and vacation for your employees or we will shut you down and fine you into personal bankruptcy. California simply does not have strong labor protections by European standards.
The SpaceX plan depends on the development of a monster rocket, larger in size and more complex than any other rocket produced in history, currently called the Super Heavy. The first stage will have 31 Raptor engines and launch the upper stage, currently called “Starship”, into earth orbit before returning for a landing and eventual reuse. The Starship, which will be able to transport 100 people to Mars and 100 tons of cargo, will be refueled in orbit by a tanker which will be a specialized version of the Starship that only carries fuel, and then launch to Mars. The trip is estimated to last between 80 and 150 days each way. On Mars SpaceX will help build an entire city. The timeline is pretty aggressive: In 2019 a Grasshopper like test vehicle for the Super Heavy will be completed; indeed Elon Musk has shared its images as it is being built from his Texas site. Then in 2020 the first full configuration Super Heavy will be launched to space. In 2022, as part of the #dearMoon project, Japanese billionaire Yusaku Maezawa and 6 to 8 artists will be launched on a weeklong trip around the Moon. In 2024 the first expedition to Mars, composed of two Starships, will be sent to land there and begin the process of colonization.
SpaceX has a history of running late, missing its goals and being secretive with the status of its programs. While the development states and the mishaps of the Space Launch System are quite public, with regular reports for example appearing on the site of the NASA Advisory Council (https://www.nasa.gov/offices/nac/home/index.html), it is quite hard to track the development and the milestones of the Super Heavy. It took an extended effort by the contributors and forum members of NASASpaceflight.com to produce this article (https://www.nasaspaceflight.com/2018/08/evolution-big-falcon-rocket/) with all the redesigns the mission has gone through based on the official presentations and the scraps that get dropped. Tracking the status of the Super Heavy has the feeling of Kremlinology. Elon Musk himself admitted during the first launch of the Falcon Heavy rocket that he almost cancelled that rocket three times, as he has already cancelled the Red Dragon mission to Mars.
Building the Super Heavy is an unprecedented endeavor. So far the only rocket that had so many rocket engines on its first stage was the Soviet N-1, which failed in all four attempts to launch. Now the first stage of the Falcon Heavy contains 27 engines, so SpaceX does have experience in properly starting that many rockets without destroying the vehicle. Raptor development, partially funded by the United States Air Force, seems to be proceeding normally, though again we do not really know. The Air Force funded the Raptor as an upgrade to the upper stage of the Falcon 9 rocket, which is considered underpowered. Elon Musk, though, has stated that its first space flight will be in the Super Heavy, so it will not see earlier activity that would help work out its kinks. Some risk will be retired by having it power the Grasshopper like prototype, but having an unprecedented number of unproven engines is major risk factor. NASA’s contractors have run into unexpected problems making the gigantic tanks and stages of the SLS because no one had built them to that size before with modern methods. What problems will SpaceX run when making its even bigger tanks?
My main issue, though, is with the Starship. SpaceX’s experience with crewed spaceships is limited to the Crew Dragon. SpaceX has never built a space station module or something similar. The Crew Dragon is to carry 7 people and is not intended to provide life support for months on end. For that matter, the most people that have been in space at one time was 13, when STS-131 visited Expedition 23 on the ISS.
Image: STS-131 visiting Expedition 23. Credit: NASA.
No one has ever flown 100 people in space, and this on its own creates its own set of technological problems to solve. When Elon Musk unveiled what was then called the Interplanetary Transport System in the International Astronautics Council of 2016 in Guadalajara, per the reporters the most cringe worthy question was what he will do about sanitation. Elon brushed it off, but it is a very valid concern. The average American consumes a little under one ton of food per year. Now this is considered large and wasteful, so let’s assume that the average astronaut on the trip will consume 500 kg/per year. At 100 people and for three months that means 12.5 tons of food. I do not know how much human waste that will produce; as an agronomist, my know-how has been on animal waste, but even with a low residue diet similar to what the Apollo astronauts ate, it still means several tons. Current ISS technology is to boil the urine, recover some 75% of the water as vapor and throw away the brine containing the other 25%. Feces are dried and rejected overboard.
While this kind of technology will work on earth orbit, and is planned for the LOP-G, it can be quite problematic for a protected destination such as Mars. If you are to reject something overboard while on a Trans Mars Injection orbit, it will simply follow you on the way to Mars. Would we really want to drop a hygienic bomb weighing several tons on to Mars with the colonists’ untreated waste? On cruise ships on earth waste treatment is similar to what is used on land before the treated effluent gets rejected overboard. First there comes primary treatment, using mechanical methods such as gravity-based separation and filtering to remove any debris from the wastewater. Then comes secondary treatment, with the microbes on the waste allowed to grow and consume all the nutrients in the wastewater, turning into what is called activated sludge or biosolids, which are then removed. Then in tertiary treatment methods such as UV radiation and chlorination are used to reduce the bacterial load, up until the treated effluent is often of superior quality to tap water. Needless to say nothing of this sort has been tried in microgravity. On top of everything, what is currently recognized by the National Academy of Science as the hardest technical problem of the Mars trip, Entry Descent and Landing (EDL) on Mars, is just hand-waived.
It is generally recognized that the biggest hurdle that SpaceX will face is raising the money. The more modest NASA plan to Mars was estimated in 2017 to cost $221 billion by the NASA Office of Inspector General, and cost estimates at this stage of a project tend to be lower than the final cost. Estimates of the development cost of the Super Heavy, coming from outside SpaceX, are in the order of $5 to $10 billion. The cost of the NASA Space Launch System alone is $8.9 billion until the first launch, and SLS does not include the development of a new engine. The total cost of SLS, Orion and ground architecture, along with earlier Constellation costs for NASA, are at $26 billion until EM-1. SpaceX has a proven ability to develop and manufacture rockets at a cheaper price than traditional manufacturers, but if the Tesla Model 3 proved anything, it is that there are limits.
Let us be generous and assume that the whole Super Heavy, Starship and ground infrastructure all the way to the first mission to Mars will cost only $20 billion, which is generously low considering the technology development required. Where will this money come from? The space launch market is simply not large enough. SpaceX believes that since they will have the largest and most economical rocket in history, every commercial payload will gravitate towards them. The Saturn V was the most economical rocket in cost per weight launched, yet it never launched any payload outside the Apollo program. The Energia rocket was cheaper per kg than any other rocket at its time and was made available for commercial uses, but there were no takers. Granted, it did not have the payload adapters of Ariane 5, which is the first rocket designed from the start to launch multiple satellites in one launch. If you follow the news, Arianespace has trouble synchronizing the delivery and integration of two payloads. Very often delays on one payload mean that the other which is ready has to delay its launch to wait for it, or at best Ariane switches around payloads with a future mission if the delays get out of hand. SpaceX will somehow be able to get 10 large commercial payloads from different manufacturers and clients synchronized for a single launch. Ignoring for a moment political demands even for commercial operators to use national launchers. Satellite owners will wish to put all their eggs in one basket, considering that they do fear a launch monopoly.
The other potential use of the Falcon Heavy is in very fast long range travel. The USAF has been quite interested in having the ability to transport troops across the world in a ballistic missile since the 1960s and has had talks with SpaceX. If we are talking about commercial passengers, it is quite debatable how realistic this is. Per SpaceX’s own presentation the mission will be from some specialized terminal out at sea near city A to another specialized terminal out at sea near city B, with the sea commute taking some 45 minutes to and from cities A and B. Left unsaid is the bureaucratic hassle on entering countries unfriendly to foreign visitors: an international visitor entering the US through JFK airport on a busy afternoon might take two hours just to cross immigration even when said visitor has all the valid paperwork. Somehow a ballistic trip lasting less than an hour between London and New York looks less appealing if you need to spend 3 hours just to enter the US.
While SpaceX is a private company and does not publish its financials, leaked financial stated show a company that was profitable in years they did not have a launch mishap, and loss-making in years that they did. They have recently raised some $500 million from private equity which has allowed the construction of the model Super Heavy Grasshopper, but is also supposed to build the Starlink communication cubesat constellation. Two prototypes have already been launched to orbit, though none of them managed to reach their proper orbit after release. The leaked SpaceX plan expected that the profits from Starlink will be such that they will be financing Mars colonization. Be aware though that Starlink is not the only communication cubesat constellation planned; there already are other competitors with prototypes in space, and there is no shortage of satellite bandwidth in the current market. As an article noted, Elon will be competing not only with satellite manufacturers but also with established ground telecommunication companies such as Verizon. Elon will need to keep on doing six impossible things before breakfast for quite some time in order to succeed in his plans, though in his defense, he has achieved things once thought impossible.
NASA and other plans
In my opinion, the most mature Mars exploration plan ever proposed was the Soviet plan from 1989. RKK Energia proposed assembling a transfer vehicle based on Mir technology and sending it off to Mars using the Energia super heavy rocket. Considering that Mir derived modules form what is currently the Russian segment of the ISS and have proven reliable and that Energia flew twice in the 1980s, it was more realistic than many plans that wanted a new mega rocket on newly built modules. It should be noted, though, that the Soviet Union never had a fully successful Mars mission. Exomars, which is a joint ESA/Roscosmos mission is the closest thing to a fully successful mission the Soviet Union and its successor state Russia has had yet. NASA on the other hand has been sending successful robotic missions to Mars since 1965. This however has not translated to human missions yet. In the run up to the Moon landing Congress had a session in 1968 where NASA proposed that the next step after the Moon should be Mars. Congress turned down the offer. It was President George H W Bush that first put a Mars Landing as a national goal in his Space Exploration Initiative, along with the Space Station and a return to the Moon. Very little came out of SEI: While the Space Station was built, though with Russian participation for proliferation reasons, out of the Mars section all that came out was the Mars Program Office at NASA that has mostly coordinated robotic missions and the Design Reference Mission 1. As a reaction to DRM 1 came out Robert Zubrin’s Mars Direct plan, elements of which were eventually integrated in NASA planning. The next administration to pursue Mars as a target for human exploration was the George W Bush administration with its Vision for Space Exploration and its Constellation program, though only after the return to the Moon. Under Barack Obama the Journey to Mars was the main goal of the space program, though that administration never waged political capital to keep it well funded. The current Trump administration per Space Policy Directive 1 wants to go first to the Moon and pretty soon. SpaceX might simply win a Mars race by default.
The current Design Reference Mission to Mars is version 5, delivered in 2010 a little before the cancellation of the Constellation program. A Shuttle derived mega rocket, then the Ares V and currently the Space Launch System, will launch several times with the segments of a transfer habitat, which will be assembled in low earth orbit. A space tug powered by high power electric propulsion will send the habitat to Mars orbit, where it will rendezvous with a prepositioned ascent descent vehicle. Astronauts will land at the surface near the prepositioned rocket In Situ Resource Utilization fuel factory that will be creating methane fuel and oxygen from the Martian atmosphere and explore the region. The first mission will explore the surface for 30 days, but eventually the plan is for 200 day stay opposition missions. The ascent vehicle will refuel from the fuel factory and return the crew to the transfer habitat, which the tug will then send back to earth. The Orion deep space vehicle attached to the habitat will return the astronauts to the earth’s surface.
This particular architecture, while differing in details, has been the standard since the mid-1990s. Critiques have been that this plan does not create significant infrastructure and it will only lead to flags and footprint moments like Apollo with people not returning for decades. These plans suggest that first we should put up things like a fuel depot in an Earth-Sun Lagrange point or set up Aldrin cyclers between Earth and Mars to ensure routine transportation to Mars. Another critique is that this plan uses expendable architecture significantly and that we should be using only reusable rockets like the Falcon 9. Also Orion was not originally designed for Mars, so its heat shield is insufficient for several return orbits, and that instead the SpaceX Dragon should be used. Before Elon Musk made his big architecture announcement in Guadalajara there was a plan making the rounds in the literature, including papers by people working for NASA, that the preferred plan to Mars should be as such: Falcon Heavies and Falcon 9s should launch a transfer habitat similar to the Bigelow Expandable Aerospace Module with some specialized materials needed for the trip and Dragons, instead of SLS launching conventional habitats and an Orion. I never quite understood how the ascent/descent vehicle would get to Mars, most likely they would use a Crewed Dragon similar to the unmanned Red Dragon concept, but it would land near an ISRU rocket fuel factory, be refueled there and return to the inflated habitat. I also never understood how the astronauts would ascend from Mars, but landing on Earth would be by Dragon, whose heatshield can withstand Mars reentry. After Bigelow announced that they had an agreement with ULA to design an adaptor for their inflatable module on the Atlas V and its replacement the Vulcan, and the announcement of the Interplanetary Transfer System, which became the Big Falcon Rocket and the Super Heavy, the BEAM/Dragon plan has faded from public view. Lockheed Martin, though, has proposed a variation to DRM 5, the Mars Base Camp. Instead of astronauts just using an ascent/descent vehicle parked in Mars orbit, there will be a fully-fledged space station, a Mars Base Camp, that will facilitate surface landing and allow telepresence operations at the Mars surface with low latency robots.
The Orion spacecraft and the Space Launch System are likely not the optimal solution to footsteps on Mars. They are, though, what Congress wants and what it is funding. Democracy is not a regime that produces the optimum solution but the consensual solution that most people can live with. As Plato noted, the best possible regime is an absolute kingdom ruled by a perfect monarch, and then in the next sentence he says that such a king does not exist in the real world. The Byzantine emperors proclaimed after adopting the Greek title basileus (king) in the 5th century that they were the perfect king of Plato’s description, but no one with an elementary knowledge of history would really proclaim that any of them was a perfect ruler. King Elon would likely be able to mobilize the national resources of the United States better than the current plan of Congress and create something superior, but by his choices to situate his company’s infrastructure and his policy not to subcontract but rather build parts in-house he has shown that he does not play well the political game.
The main selling point about the current NASA plan is that it mobilizes the army of contractors and their workers sufficiently to eventually produce the desired outcome. The Space Launch System uses mature Space Shuttle technology and can trace its lineage in Shuttle Derived Launch Vehicles all the way back to the 1980s. The Orion spacecraft may have been advertised as a modernization of the Apollo capsule, but I would say that its lineage is closer to the Space Station lifeboat, which was intended for cases when the Shuttle was grounded. In turn, the Space Shuttle is also a cautionary note showing how combining kit with a long heritage and high technological maturity can still lead to something with lengthy and expensive overruns when these are drawn into unprecedented combinations. The transfer vehicle will be, as mentioned earlier, an improved variation of the Lunar Orbital Platform Gateway, and considering that it uses highly efficient electric propulsion rather than chemical propulsion of the Spaceship it is in some respects more advanced. Getting the NASA plan accomplished, though, in the end requires a renewed focus on Mars, which is something currently not in vogue in either the White House or Congress. As for other national space programs, while there are paper plans similar to the DRM 5 architecture, I am not aware of concrete plans to actually bring them to fruition.
Asteroids
This section should perhaps be better entitled Small Bodies. Asteroids, whether Near Earth, in the Main Belt and more recently the Jupiter Trojans, have been a target of robotic exploration since Galileo encountered 951 Gaspa in 1991. Other small bodies, though, were visited earlier, with the International Haley Armada of 1986 and Phobos and Deimos visited in 1965 by Mariner 4. The various space agencies will keep on sending missions, with the Jupiter Trojans that are to be visited by the Lucy mission being the next new category to receive a mission. Human plans were active until the recent cancellation of the Asteroid Redirect Mission. When the Obama administration cancelled the Constellation program, the next step in space chosen was to be a visit to a Near Earth Asteroid, which did not require a lander to be developed. After sky searches failed to find an object in a proper orbit to be visited by astronauts on an Orion before it ran out of supplies, the Asteroid Redirect Mission was devised. A robotic mission would go to a small asteroid, bag it, and carry it to a distant retrograde orbit around the Moon. Since no appropriate sized asteroids were found, it was modified so that a boulder would be carried from an asteroid to Moon orbit so that astronauts could visit it. This mission received a lot of derision from both the scientific community and space enthusiast circles. Other people pointed out that NASA does not even have an appropriate spacesuit for visiting the boulder. For now it seems that the first small bodies that people will visit will be Phobos and Deimos in an Apollo 8-like test for a human mission to Mars. Space planners would like to test first-long term survival of a transfer habitat, and visiting Phobos and Deimos does not entail the risk of Entry, Descent and Landing on Mars. A lander to Phobos or Deimos can be adapted from a lunar lander, though the landing will be more like a docking. Spaceships that visit Mars can be adapted to visit asteroid destinations, but so far no one has proposed such a trip to the best of my knowledge.
The Giant Planets
Image credit: SpaceX
In his 2017 Guadalajara presentation, Elon Musk included at the end graphics of his spaceship passing Jupiter’s Great Red Spot, on the surface of Europa, along the rings of Saturn and at the surface of Enceladus. A Hohmann Transfer orbit to Jupiter requires three times the energy and 3.4 times the time required to get to Mars, but if the Starship is as capable as Elon intends it to be, a Jupiter trip is within its abilities. So far this is the most concrete plan for a human mission to the outer planets: a series of graphics in a presentation of a spaceship that has been called by its detractors a fantasy rocket. Saturn is twice the distance to Jupiter, so the realism drops. There are, though, quite robust plans for robots to the giant planets.
Jupiter
The largest planet in the solar system has so far been visited exclusively by American space probes: Pioneers 10 and 11, Voyagers 1 and 2, Galileo, Cassini, New Horizons and currently Juno. Juno is the first solar powered space probe to Jupiter; the rest have been powered by Radioisotope Thermal Generators. It will be joined in the next decade by two other solar powered probes, ESA’s JUpiter ICy moons Explorer or JUICE and NASA’s Europa Clipper. NASA has plans for another mission to Europa, the Europa Lander. Funding for both Europa missions was found by Republican Texas Representative John Culberson. Since he lost his reelection bid, both missions are in peril. While the Clipper has the support of the broader scientific community, the Lander does not. Both missions currently expect to be launched on the SLS, which is a risk factor per advocates of the space science community since it is not certain that the rocket will not be cancelled and bring down the probes with it, as happened with the Mars Voyager program in the 1970s. While Clipper has a direct ascent trajectory to Jupiter, the Lander will do a tour of the inner solar system, like JUICE, to pick up orbital velocity in order to reach Jupiter. There it will land on Europa for a battery-powered mission that will last a few hours. The scientific community would prefer to launch a lander after the Clipper has sent back information, and follow it up with an ice drilling mission, somewhere in the 2030s. China and Russia also have preliminary plans for missions to the Jupiter system which are not as publicized as NASA’s and ESA’s plans.
Saturn
After the end of the Cassini mission in 2017, the most expensive space science mission in history, Saturn is without a robotic presence. But there have been several proposals in the most recent NASA Discovery and New Frontiers mission proposal requests. One of the finalists in the 12th Discovery competition was the Titan Mare Explorer (TiME), also known colloquially as the Titan boat, to drop a floater in one of the hydrocarbon lakes of Titan. In the current New Frontiers proposal, round one of the two finalists is the Dragonfly mission. It is a concept for a nuclear powered drone in the skies of Titan. The other object of interest at Saturn is Enceladus, which has an underground ocean. The Enceladus Life Finder was proposed in the recent 13th Discovery competition, but was not even shortlisted.
The mission has been reworked into a proposal that might become the first private planetary mission, to be funded by Yuri Milner to the tune of $60 million. A solar powered small probe will fly by the South Pole of Enceladus while it is erupting and collect a sample. While the probe is leaving the Saturn system (it will not enter orbit), it will analyze the sample with modern analytical equipment, rather than the vintage 1990s equipment that Cassini had. Private space probes have been proposed since the start of the space age, but have not come into fruition yet. Most recent was the Sentinel Space Telescope from the B612 foundation that was to scan the skies in an orbit between Earth and Venus so as to catch asteroids coming from the sun side of Earth. While they did raise some $2 million, this was far less than the $500 million they needed according to their budget estimates. SpaceIL has managed to raise $95 million for its lander to the Moon through philanthropy, and Yuri Milner has given to both his exploration initiatives (Breakthrough Starshot and Breakthrough Listen) some $200 million, so the Enceladus mission may happen, assuming it can surpass any problems that arise during development.
Uranus and Neptune
The two ice giants have only been visited by Voyager 2 in 1986 and 1989 respectively. The logical progression is that they are to be visited by orbiters. There have been several proposals both for NASA and ESA over the years and a Uranus orbiter was the third priority flagship mission in the most recent Decadal Survey of the Planetary Science community, after the Mars caching probe which became Mars 2020 and a Europa Orbiter which became the Europa Clipper. In preparation for the next Decadal Survey, a study was performed by the ice giants community that found that there is compelling reason to visit both, and that it is more of an issue of orbital mechanics which one to select. Four options were studied, from a Uranus flyby with an atmospheric probe to orbiters for both planets. The missions assume launches on the SLS. The cheapest option would be a tad less expensive than Cassini/Huygens; the more ambitious missions would dethrone it as the most expensive space probe.
NASA headquarters apparently lacks sufficient manpower to supervise multiple flagship missions, and lack of supervision was one of the main reasons that the James Webb Space Telescope has suffered delays and cost overruns. If Europa Lander continues, we are not likely to see a Uranus or Neptune mission, and even if it is cancelled the situation is quite dicey, because Congress would need to appropriate the funding and the Government Accountability Office grant a New Start. In a recent paper by members of the New Horizons team, they calculated trajectories with flybys of the outer planets that would lead to Kuiper Belt objects. In one of the summer schools at JPL, a student team proposed a Uranus mission that would fit inside a New Frontiers budget, which if proven correct would be something that the scientific community would rally behind. So far, though, a mission to the ice giants remains in the realm of proposals. Perhaps in the next decade one of them might get selected leading, to a mission that, if all goes well, should arrive at one of the ice giants in the middle of this century.
The Kuiper Belt and the Oort Cloud
When I was growing up, the Solar System, which was the only planetary system known, had 9 planets. I got interested in space with the Neptune flyby in 1989 and I was looking forward to a mission to the last planet, Pluto. I had to wait another 26 years until New Horizons flew by Pluto, which at that time had been demoted from planet, a mistake in my personal opinion. New Horizons just encountered 2014 MU69, nicknamed Ultima Thule by the New Horizons team, and it is currently sending back the data it collected. New Horizons scientists hope to fly by another object in the Kuiper Belt, though that would have to be discovered first. When the Voyagers crossed the Kuiper Belt it was still a theoretical concept; the first object other than Pluto was only discovered in 1992. Today some 3,000 objects are known, and some such as Sedna are considered likely to belong to the even further out Oort cloud, the origin of comets. Several comets have been encountered in the inner solar system but Ultima Thule is the first such object visited in its native environment. Scientists have proposed several planetary sized objects in the outer solar system, the most famous being Planet 9, which so far are all theoretical. There have been proposals to visit again this part of the solar system, the most mature of which was New Horizons 2, which would have been built from the spare parts of New Horizons and sent to Eris via a Uranus flyby, but none have moved beyond studies on paper.
Interstellar Space
Image: Concept poster of the JHUAPL interstellar probe. Credit: Johns Hopkins University.
This is a blog on interstellar travel, so many mission concepts using various engine technologies have been published here. I am limiting myself here to missions that use more mature technologies. In 2012 Voyager 1 became the first spacecraft to pass the heliopause and enter interstellar space. Its twin Voyager 2 followed it in 2018. New Horizons will follow them sometime in the future, if it does not run out of power earlier. New Horizons is headed towards the IBEX ribbon, which the Voyagers missed because they are headed far from the ecliptic. Missions to the heliopause and beyond have been proposed by the heliophysics community since the start of the space age, but were not approved due to their high cost and risk.
There is currently a proposal for an interstellar probe being created by JHUAPL to be part of the next Heliophysics Decadal survey. The most accessible online summary of the proposal is at the FISO Telecon Archive at http://fiso.spiritastro.net/telecon/McNutt_9-5-18/ from a September 5 2018 presentation. There were also updates for the proposal at the 69th International Astronautical Congress in Bremen in October 2018 [http://iafastro.directory/iac/paper/id/44169/summary/] and at the 2018 Fall AGU meeting in Washington in December 2018. The Bremen presentation led to a Space.com article [https://www.space.com/42935-nasa-interstellar-probe-mission-idea.html]. There were two sessions at AGU dedicated to this proposal according to the online program, one of oral presentations and one poster session. Some of principal investigator’s Ralph McNutt’s slides have appeared online and someone did tweet with the hashtag #InterstellarProbe about one of the oral presentations at the AGU about dust in the Kuiper Belt. In the same session there was also talk of a Chinese interstellar probe, but I was not in Washington DC for AGU and I am not an AGU member in general so I do not know anything about that beyond the AGU summaries. I will try to summarize my understanding of the proposal based on what has been posted online.
The proposal as mentioned is for the next Heliophysics Decadal Survey and is to be delivered to NASA in February 2019. The scientific case for the mission is good; the Voyagers have whetted our appetites rather than solve the major questions about interstellar space. The problem with the Voyagers and New Horizons is that they were optimized for the planets; for that matter New Horizons does not even have a magnetometer because they would need amagnetic versions of the rest of its instruments, which would have doubled their cost. The goal is to have a probe at a distance of 1,000 AU in the time of 50 years. For this they would have to have it travelling at least twice the speed of Voyager 1. The probe will weigh in the same range as New Horizons (478 kg) or the Parker Solar Probe (685 kg), both of which are JHUAPL probes. Three options are being studied: an Oberth maneuver around the sun with a solid rocket stage to be carried at 4 solar radii from the surface (closest Parker approach is 8 solar radii); a powered flyby of Jupiter; and a New Horizons-style solid-fuel earth departure stage with Jupiter gravity assist. The launch vehicle so far is the SLS Block 2. Since they will be visiting the region they will also encounter a Kuiper Belt Object such as Quaoar, Ixion or Haumea. The fastest option currently, at 12 AU per year, would be a CASTOR-30XL upper stage using an Oberth maneuver around the sun, but at 50 years it would only get them to 600 AU or so.
We will not have to wait long for the report if it is released. Is NASA Heliophysics willing to go forward with this mission? So far, after the Parker Solar Probe and US participation in ESA’s Solar Orbiter, NASA Heliophysics does not have any flagships coming up. The longevity of the Voyagers, though, has shown several problems that arise in long lived probes. The Voyager team is hoping to push both probes to be active 50 year after their launch in 2027. Due to the decay of their RTGs they will have to start turning off instruments soon because there will not be enough power anymore for all of them. The Voyagers are the only mission that still mostly communicates in the S band with the Deep Space Network, so NASA is keeping old equipment just for them. Both are so far out that the minimum speed of their tape playback is higher than the maximum communication speed, so data is lost when played back to earth from the tape. On top of everything, with both Voyagers being built with 1970s technology that means that they are using computational tools of that time. When a website reported inaccurately that they were looking for a programmer that knew their archaic version of FORTRAN because one of their coders was retiring, the people that came forward on the web were also septuagenarians because of the obsolescence of that language. Which of the current programming tools will still be used some 50 years from now? As someone who constantly deals with unsupported computer systems as part of my day job, this is something that I relate to. Voyager 1 is currently at 145 AU and travelling at 3.6 AU/year. In 2027 it will be at 175 AU, a distance record that it will definitely hold for decades. The interstellar probe is intended to reach over 5 times that, which will require upgrades of the DSN.
Conclusion
There are many technical issues that arise in the process of space exploration, but so far I think that the biggest problem is money and interest. A spaceship to Mars can open the solar system from Mercury to Jupiter for human exploration. The NASA Office of Inspector General estimate for its Mars plans was at $221 billion for two missions to Mars, while the estimate of the National Academies of Science was at $300 to $500 billion. SpaceX believes that they can do this mission for a fraction of the cost with a rocket and a spaceship that will be more capable than anything ever built. They also believe that they can raise the money through their activities. On the robotic side, there are many interesting destinations, far more than what the budgets and oversight capacity can buy. It would be interesting to reread this article in a few years and see what has actually come to fruition from all these plans.
by Paul Gilster | Apr 6, 2018 | Advanced Rocketry |
Looking at John Brophy’s Phase II NIAC award reminds us how useful the two-step process can be at clarifying and re-configuring deep space concepts. Brophy (Jet Propulsion Laboratory) had gone to work in Phase I with a study called “A Breakthrough Propulsion Architecture for Interstellar Precursor Missions.” The work studied a lithium-fueled ion thruster with a specific impulse of 58,000 seconds. If that didn’t get your attention, consider that the Dawn spacecraft’s ISP is 3,000 seconds, and think about what we might be able to do with that higher figure.
I think about ideas like this in terms of infrastructure. The relation to interstellar flight is this: While we may well get robotic nano-probes off on interstellar missions (think Breakthrough Starshot) some time this century, the idea of human expansion into the cosmos awaits the growth of our civilization into the rest of the Solar System. Along the way, we will learn the huge lessons of closed-loop life support, means of planetary protection and propulsion technologies to shorten trip times. Building that infrastructure is an early phase of the interstellar project.
What Brophy has been working on with his Phase I study is the question of how to provide power to craft at the outer edge of our planetary system. When I looked at the 2017 NIAC awards, a stumbling block for Brophy seemed to be the need for a 10-kilometer laser array.
Brophy’s idea was to beam power to a lightweight photovoltaic array aboard the actual spacecraft — this is how he generates the power needed to drive those ion thrusters. Closer to home we would use solar power to do the trick today, while looking at nuclear power when far from the Sun. Brophy is talking about ion thruster operation in deep space without the need for a heavy onboard power source. This gets around the problem of increasingly inefficient solar panels as we go further from the Sun, as well as their alternative, a bulky nuclear reactor.
Image: John Brophy (JPL) initiated the NSTAR Project that provided the ion propulsion system for Deep Space 1, delivered the ion propulsion system for the Dawn mission, and co-led the study at Caltech’s Keck Institute that resulted in the Asteroid Redirect Mission. His Phase II NIAC award will be used to investigate beamed power delivery to a powerful ion engine. Credit: JPL.
If we can power up the lithium-ion engine, mission velocities of 100 to 200 km/s would be possible. This is exciting stuff: The Phase I study talked about a 12-year flight-time to 500 AU, which gets us into gravitational lensing territory, while flight time to distant Pluto/Charon could be reduced to 3.6 years. For that matter, imagine a Jupiter mission in roughly a year.
A 10-kilometer array is daunting indeed, but now we’re going into Phase II, which significantly refines the concept based on the results of the Phase I study. Phase I assumed a 100 MW output power at a laser frequency of 1064 nm, feeding a 70 MW electric propulsion spacecraft with a 175-meter photovoltaic array coupled to its lithium-fueled ion thrusters.
What the Phase I study revealed was a better set of parameters: Brophy will begin the Phase II study with a laser array of 2-kilometer diameter aperture with an output power of 400 MW at a laser frequency of 300 nm. Aboard the vehicle, a 110-meter diameter photovoltaic array is now considered. It will power a 10 MW electric propulsion system.
Results of the Phase I work brought the specific impulse down to 40,000 seconds, which is the figure of merit for Phase II, all in the service of a specific mission outcome: A journey to the solar gravity focus at 550 AU. The changes in configuration came from feasibility analysis — could we develop the required photovoltaic arrays with the required areal density (200 g/m2)? Could we achieve photovoltaic cells tuned to the laser frequency with efficiencies greater than 50%? And what about pointing the laser array with the needed accuracy, not to mention stability to supply the reference mission at the gravity focus?
In terms of beamed propulsion, Brophy’s laser array should remind us of Breakthrough Starshot’s own plans for such an array, though Starshot does not plan on building the array in space. Starshot’s array is to be on the ground, avoiding the huge issue of constructing and controlling a gigantic laser outside our planet, while creating a host of other questions, such as atmospheric attenuation of the laser signal. In both instances, however, we deal with a power source separate from the spacecraft, producing real benefits in terms of weight and efficiency, and creating a reusable driver for missions throughout the system.
Image: Graphic depiction of A Breakthrough Propulsion Architecture for Interstellar Precursor Missions. Credit: J. Brophy.
And what about the power developed in the onboard photovoltaic array? The output in the Phase I study was expected to be 12 kV, which goes a long way beyond the best solar arrays available today. Those on the International Space Station, for example, produce 160 volts. One part of Phase II, then, will be to show that the photovoltaic system can be operated at more than 6 kV in the plasma environment produced by the lithium-ion propulsion system.
Have a look at the precis to see what Brophy considers to be the outstanding technical issues, which include modeling the lithium plasma plume created by the engine and demonstrating a small aperture phased array that can become scalable to larger apertures. Depending on the outcome of these investigations, a roadmap should emerge showing how we might develop demonstrator missions along the way to finalizing a workable system architecture.
Let’s put all this into perspective. The challenge of operating beyond 5 AU is the rapid dropoff in solar power available to a spacecraft, despite recent advances in photovoltaic array technology. A laser system like Brophy’s could increase the power density of photons available to a spacecraft bound for the outer Solar System by several orders of magnitude, giving us an ion propulsion system of considerable power. Moreover, this would be a spacecraft that does not need to create its own power but receives it, doing away with power processing hardware.
I come back to infrastructure when evaluating concepts like this. A workable laser array of this magnitude becomes a way to solve power issues far from the Sun that could drive missions throughout the system. Like the combined laser / neutral particle beam concept we’ve looked at over the last two days, it assumes a hybrid meshing of technologies that could go a long way toward enabling robotic and manned missions deep into the Solar System. That makes laser beaming to an ion engine an alternative well worth the continued scrutiny of NIAC Phase II.
