Commentary | The Second-best Plan


Recently, an advisory committee assembled by the National Research Council (NRC) published a report titled “Pathways to Exploration” calling for redirection of NASA’s human spaceflight program. The report is written in a somewhat obscure form of bureaucratese, so as a public service I will provide a brief summary in English.

The NRC report translates as follows: NASA should build a lunar base.

The NRC committee authors never present this as their conclusion. Rather, they attempt to induce the reader to draw it for himself or herself, via the following subtle logic:

1. NASA needs a definite and inspirational goal for its human spaceflight program, and that goal should be the human exploration of Mars.

2. There are three paths to get humans to Mars:

(a) Perform the Asteroid Redirect Mission (ARM). Then send humans to Phobos. Then send humans to Mars.

(b) Build a space station at Lagrange point L2. Then send astronauts to the Moon. Then send astronauts to near-Earth asteroids. Then send astronauts to Phobos. Then send humans to Mars.

(c) Build a Moon base. Then send humans to Mars.

3. Options (a) and (b) make no sense. So choose one of the remaining options.

I must admit that presented with this set of alternatives, the Moon base path seems very attractive.

I am going to criticize this report, but before I do, I would like to point out some parts where it really got things right.

In the first place, point 1 — that NASA needs a definite and inspirational goal for its human spaceflight program, and that goal should be the human exploration of Mars — is correct. Furthermore, the NRC authors gave valuable service in making clear that such a goal-directed program is incompatible with NASA’s current methodology of developing technologies based on bureaucratic constituency support, and then selecting missions based on their value in providing putative rationales for such irrationally chosen technology programs. They also imply, correctly if a bit too politely for clarity, that the ARM mission was chosen on this basis, as its primary purpose is to provide a customer for an otherwise unnecessary high-power electric propulsion system.

They also do a fair job of showing that both option (a), which is unfortunately NASA’s current program of record, and option (b), a sometimes offered alternative, are highly defective as potential pathways to Mars, as they both involve extensive mission operations and technology developments that have nothing to do with the final goal. They fail to offer cogent criticism of Phobos as a way station to Mars, but I can correct that omission here. The idea of using Phobos as a base station for Mars exploration operations is absurd because Phobos is in an equatorial circular orbit that limits access to Mars’ equatorial regions, and even there, involving it places a heavy logistics load on missions to the martian surface. Specifically, stopping at Phobos on the way down to Mars adds a delta-V of 2.14 kilometers per second to the mission, while stopping at Phobos on the way back to Earth from Mars adds another 1.67 kilometers per second, for a devastating total delta-V addition of 3.81 kilometers per second to the mission propulsion requirements. But this omitted data actually greatly strengthen the authors’ main point, that one should not invent pointless missions for the purpose of trying to justify previous irrational decisions — as Phobos-basing now is being pushed to try to concoct Mars relevancy for the ARM mission.

The NRC report does further public service in dismissing out of hand the false claims of some electric propulsion zealots that their pet project offers an enabling technology for quick trips to Mars. Furthermore, the authors offer a useful principle for a goal-driven human spaceflight program, to wit, that in our real universe of finite NASA funding, one should not engage in major human spaceflight programs that have nothing to do with the goal. Unfortunately, however, they don’t follow this solid logic themselves, and thus fail to call for early redirection of international space station funding to their desired lunar exploration program — despite the ISS’s complete irrelevancy to lunar missions — thereby abandoning their paper Moon base to the universe of make believe.

But the most serious problem with the NRC report is the way that the authors avoid competent discussion of the real alternative. If our goal is to send humans to Mars, and we understand that engaging in diversionary activities is counterproductive to that achievement, then instead of engaging in diversionary activities, we should send humans to Mars. The authors however, actually want to build and operate a Moon base, and so to buy (several decades of) time for such an effort, they raise several challenges that supposedly must be solved before a humans to Mars program can be initiated. These are:

  • Entry, descent and landing on Mars.
  • Advanced in-space propulsion and power.
  • Radiation safety.

The creation of entry, descent and landing systems that can place large payloads on Mars, and nuclear power systems that can provide 30 to 100 kWe (kilowatts electric, or 1,000 watts of electric capacity) on the martian surface, are important developments that need to be done, but they are not fundamental technology issues. They would be done in the course of a human Mars exploration development program, and not done outside of it. The claim that advanced propulsion is a fundamental challenge for the Mars program is curious, because, as noted above, the authors correctly dismiss electric propulsion as a means of achieving much of interest. They therefore implicitly endorse nuclear thermal propulsion. However, while a useful enhancing technology, nuclear thermal propulsion would best serve to cut launch mass in half rather than substantially cut trip time, and thus its proper relevance is as a potential way of reducing downstream costs of a continuing Mars exploration program, rather than an upstream tall pole perpetually cited as a unfulfilled requirement to block the initiation of such a program.

But citing “radiation safety” as a technology challenge that must be mastered before we can start a Mars program truly confuses the issue. Regardless of whether it is using chemical or nuclear thermal propulsion, a properly designed Mars mission would use a six-month outbound transit because that is the two-year free return orbit, and so in addition to cutting payload capacity and thus critical system redundancy, attempting to go faster would further compromise mission safety by forfeiting this important abort option. So the cumulative radiation dose that Mars exploration crews would get from galactic cosmic rays is well known, irreducible (solar flare doses in transit can be protected against using provisions to make a storm shelter), and — since dose rates in low Earth orbit are 50 percent those of interplanetary space, have already been experienced by half a dozen cosmonauts and astronauts who have done extended stays on either the ISS or Mir, without any radiological casualties. This is to be expected, since the total galactic cosmic rays round-trip mission dose of 0.6 to 0.8 sieverts represents a probabilistic casualty rate of about 1 percent. Furthermore, since a Mars mission spends only about 40 percent of its total duration in transit (with infinite mass available for shielding while on the martian surface), and has a comparable crew size to the ISS, the total dose, measured in person-sieverts, that the continuously occupied ISS program will impose on its crews over the next 10 years will be the same as that which would be imposed on the personnel of a humans to Mars program, assuming five missions, each flying at every biannual opportunity over the same period.

So the argument that we should defer human Mars exploration for a couple of decades while scientists engage in unproductive “radiation safety” research that will produce nothing not already discovered by the previous seven decades of radiation safety research is a snow job — advanced with all the sincerity of a 10-year-old making the case that a light overnight snowfall, must, for the safety of all children, entail the cancellation of school. The primary difference being that with a human spaceflight program running a tab on the order of $10 billion per year, a two-decade-long NASA snow day will hit the public with something like $200 billion in extra costs before Mars can be reached.

The NRC authors’ lunar initiative should be recognized for what it is. It is the second-best plan. It is greatly superior to NASA’s current scatterbrained program of record, which would have our human spaceflight program accomplish nothing of significance for the next decade or more. A Moon base is not a critical step on the way to Mars, but it would be a real accomplishment, and it would certainly be better for NASA to accomplish something than nothing. 

They are also right in saying that NASA needs to choose a goal and stick with it. The George W. Bush-Mike Griffin Constellation program can readily be criticized, but if it had been held to, we would be looking at initiating operations on the Moon five years from now employing our astronauts — as they should be — as explorers, instead of a future of endless experiments using astronauts stuck in low Earth orbit as human subjects for pointless research on zero gravity medical effects that could all be avoided by rotating a spacecraft. 

But if we want to get to Mars, we need to aim for Mars.


Dr. Robert Zubrin is president of Pioneer Astronautics and the Mars Society and the author of “The Case for Mars: The Plan to Settle the Red Planet and Why We Must.” The paperback edition of his latest book, “Merchants of Despair: Radical Environmentalists, Criminal Pseudo-Scientists, and the Fatal Cult of Antihumanism,” has just been published by Encounter Books.