Fast or Slow to Mars?
27 September 2016
Now that Elon Musk has delivered his highly anticipated talk “Making Humans a Multiplanetary Species,” providing an overview of his plan for a Martian settlement sufficiently large to be self-sustaining (he mentioned a million persons moving to Mars in a fleet of 1,000 spacecraft leaving Earth en masse), the detailed analysis of this mission architecture can begin. Musk said in his talk that he thought it was a good idea that there should be many different approaches, so he clearly was not making any claim that his plan was the one and only workable mission architecture.
As both public space agencies and private space companies go beyond the talking phase and begin the design, testing, and construction of a Mars mission (or missions), these designs will embody assumptions about the best way to get to Mars with contemporary technology (there are many ways to do this). The assumptions, as usual, aren’t often explicitly discussed, because assumptions are foundational, and you have to have a community of individuals who share the same or similar assumptions even to begin designing something as complex as a human mission to Mars. Foundational assumptions may be challenged in initial “brainstorming” sessions, but once we get to sketches and calculations, the assumptions are already built into the design.
One of the most important assumptions about Mars mission design is whether that mission should be slow or fast. In this context. “slow” means following one of the well-established gravitational transfer trajectories (Hohmann Transfer Orbits) that many uncrewed missions to Mars have followed, which requires a minimum of fuel use and little or no braking upon arrival, but instead requires time.
A Hohmann transfer orbit to Mars would require many months (six months or more; cf. Flight to Mars: How Long? Along what Path?, which gives a figure of 8.5 months), the window to make the journey only occurs every 25 months, and during a long voyage such as this the crew would have to be maintained in good health, protected from radiation, and have enough space onboard to keep from going stir crazy. A Mars cycler configuration would involve travel times on the order of years. This is definitely a “slow” option, but also an option that minimizes propellant use.
The Mars Design Reference Mission (which I recently quoted in A Distinctive Signature of an Early Spacefaring Civilization), a design document produced by NASA in July 2009 (the full title is Human Exploration of Mars: Design Reference Architecture 5.0), characterizes their mission architecture as “fast” (the document repeatedly cites “fast transit trajectory”), but involves a one-way transit time of 6 to 7.5 months:
“…the flight crew would be injected on the appropriate fast-transit trajectory towards Mars. The length of this outbound transfer to Mars is dependent on the mission date, and ranges from 175 to 225 days.”
A “slow” mission to Mars such as this (which NASA calls a “fast” mission) ought to be designed about a large, rotating habitat that can simulate gravity (this has featured in films, such as The Martian). No one wants to spend six months in a “capsule.” An additional benefit of a large and slow Mars mission is that the rotating habitat sent to Mars could be maintained in Mars orbit as a Martian space station (such as I wrote about in A Martian Space Station and A Passage to Mars) and subsequent missions could add to this Martian space station.
Alternatively, instead of a large and comfortable habitat in which to travel, a slow mission to Mars might involve induced torpor in the crew (effectively, human hibernation), and while this would require far less food and water for the journey, this option, too, might be best achieved with simulated gravity. Human bodies evolved in a gravity field, and don’t do well outside that gravity field (cf. Hibernation for Long-term Manned Space Exploration by Shen Ge, which includes many links to resources on induced torpor).
A “fast” mission to Mars I will identify as anything faster that the six months or so required for a Hohmann transfer orbit. Fast journeys could be anything from a gentle ion thrust, using very little propellant and only cutting a little time off the trip, to powering half way to Mars (preferably at 1 g acceleration in order to again simulate gravity) and then decelerating for the second half of the trip. Musk’s mission design as presented in his IAC talk called for initial transfer times “as low as” 80 days (i.e., less than three months; his graphic for this section of the talk showed transit durations from 80-150 days), perhaps improving to as little as 30 days further in the future, but little detail was offered on this part of the mission architecture.
The quickest “fast” trips to Mars contemplated with contemporary technology would be about two weeks. A nuclear-powered ion engine might make the trip in three months, which is a lot better than six months, and might be considered “fast,” but Musk’s 30-80 day transit times are all designed around well-known chemical rocket technology, which makes the effort much closer to being practical in the near term. If you have enough rocket engines, big enough engines, and enough fuel, you can make the trip to Mars more quickly with chemical rockets than is usually contemplated, and that seems to be the SpaceX approach; much of the talk was taken up with concerns of propellant, fuel transfer in Earth orbit, and producing fuel on Mars.
It is important to point out that most of the technologies I have mentioned above — rotating spacecraft, induced torpor, nuclear rockets, and so on — have been the object of much study, but little practical experience. (An early version of the Nerva nuclear rocket was built and tested, but it wasn’t flown into space; cf. Secrecy and the STEM Cycle.) However, we have a pretty good grasp of the science involved in these technologies, so building actual spacecraft incorporating them is primarily an engineering challenge, not a science challenge (except in so far as there is a science of technology design and engineering application; cf. Testing Technology as a Scientific Research Program: A Practical Exercise in the Philosophy of Technology). In other words, we don’t need any scientific breakthroughs for a mission to Mars, but we need a lot of technological development and engineering solutions.
Hearing a presentation such as Elon Musk gave today is exciting, and definitely communicates that this project can be done, and even that it can be done on a grand scale. This is invigorating, and stokes what Keynes called our “animal spirits” for a voyage to Mars. If the momentum can be maintained, the development of a spacefaring civilization can be a practical reality within decades rather then centuries. Musk discussed the “forcing function” of having a settlement on Mars, and he is correct that this human outpost away from Earth would entail continual improvements in space transportation, and moreover it would extend human consciousness to include Mars as a human concern.
Once humanity begins to make itself a home on Mars, and human beings can call themselves “Martians” (perhaps even with a certain sense of pride) and adopt a genuinely Martian standpoint, humanity will be a multiplanetary species, a multiplanetary human civilization will begin to emerge, and this multiplanetary civilization will be distinct from our planetary civilization of today. Mars, in this scenario, would be a point of bifurcation, the origin of a new kind of civilization, localized in the same way that the industrial revolution can be localized to England.
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