Do We Need to Go to the Moon to Get to Mars?
There’s a palpable longing to return to the moon. But does it make sense?
This is part of IEEE Spectrum’s Special Report: Why Mars? Why Now?
It’s an irrational thing, the pull of the moon. From time immemorial, the White Goddess has been held responsible for menstrual cycles, moods, and madness; she’s the mythic governess of our dreams and emotions.
In 1969, Neil Armstrong’s small step for man electrified people around the world, and in the United States it provided a momentary respite from social upheaval. Work done by Armstrong and his successors transformed our understanding of the moon, setting in motion research that continues to this day.
Of course, nobody pretends that the United States went to the moon mainly for science, and if people return to the moon now, it won’t be all for science, either. In the 1960s, the point was to win a race with the Soviet Union. Today the supposed point is to use the moon as a stepping-stone to Mars.
Of the nearly 7 billion people on Earth today, four out of five were not alive when the first lunar landing took place. Without a doubt, a great many of them would love to see people back on the moon again. But does it make sense to spend the US $50 billion it might cost to get them there? Do we need a base on the moon to get to Mars? And if not, should we bother going to the moon at all?
Playing perhaps more to our passions than our reason, in January 2004 President George W. Bush promulgated a program to return to the moon by 2020 and make it a staging area for a mission to Mars, perhaps two decades later. His father, President George H.W. Bush, had suggested essentially the same plan in 1989, but because of the enormous expense and conflicting U.S. commitments in space, it was dead on arrival. The second time around the vision fared better, eventually winning the endorsement—at least on paper—of all the world’s space powers.
But you didn’t have to scratch very hard to discover that such support was often only skin deep, even in the United States itself. Bush never actually mentioned his vision again, and the U.S. Congress promptly excised funding for the Mars part, instructing NASA to focus strictly on the moon. The effect was to radically disconnect the moon from Mars planning, even though going to Mars was supposedly the main rationale for returning to the moon.
Recognizing the vulnerability of the moon-Mars enterprise, Bush’s NASA administrator, the hard-driving if abrasive Michael Griffin, made it his business to push ahead with the program so fast that ultimately it could not be reversed without incurring unacceptable economic and political costs. He may have succeeded. Even though he would fall out with President Barack Obama’s transition team—informing team members bluntly that it was his job, not theirs, to “look under the hood” at NASA—by the time Griffin stepped down in January, development work on key moon-mission elements was far advanced. The feeling was that the plan would proceed, whether or not it really made sense.
The moon mission, dubbed Constellation, will use an Ares I rocket to transport crew and cargo as well as exploration and lunar-landing vehicles to low Earth orbit and eventually to the moon. The heavy-lift Ares V will take robotic machinery to the moon and perhaps beyond. The lifters derive conceptually from Wernher von Braun’s Redstone and Saturn rockets, which put the first Americans into orbit and then onto the moon, and the design approach closely follows von Braun’s trademark conservative philosophy.
“We’re capitalizing on the nation’s prior investments in space technology wherever possible,” Griffin said in a talk in January 2008. After decades of embarrassing delays and setbacks with the U.S. shuttle program and the International Space Station (ISS), Griffin wanted to be safe, not sorry.
But stripped down and conservative as it was, the Constellation program was open to complaints that its most exciting elements—those pertaining to Mars—had been jettisoned and that the surviving elements were underfunded. “Each year since 2004,” former astronaut Kathryn C. Thornton told the U.S. Congress last spring, “the NASA budget has fallen short of that required to achieve the mandated exploration goals and milestones.”
A month before Thornton’s testimony, a conference held in Tempe, Ariz., brought home how stultifying a redo of the Apollo program threatened to be. When aging Apollo engineers and astronauts were talking, the mood was electric: Participants were on the edge of their seats listening, for example, to accounts of how mission commanders had trained for lunar landings in an incredibly weird simulator designed by veterans of the X-15 suborbital jet program. But when the talk turned to subjects like the selection of landing sites for a new mission, avoiding dust and assuring visibility, and landing techniques, the feeling set in that young engineers were being asked to re-solve problems that had already been solved brilliantly with less sophisticated technology 40 years ago.
Griffin himself was highly sensitive to the dilemma. In his January 2008 talk, he conceded that one of the most common criticisms he heard about Constellation was that “it looks too much like Apollo.” He was also touchy on the subject of the moon-Mars connection. Even as he took note of unequivocal congressional restrictions on Mars-oriented R&D, Griffin deemed it “not credible…that we will return to the moon and then start with a ‘clean sheet of paper’ to design a system for Mars.” In a September 2007 talk, he specified that the Ares V launcher, with no more than half a dozen launches, would be able to lift the 500 metric tons (into low Earth or lunar orbit) considered necessary for a Mars mission. “By the 2020s, we will be well positioned to begin the Mars effort in earnest,” he said.
Without knowing, however, what propulsion technology will actually take people to Mars [see “Rockets for the Red Planet,” in this issue], how is it possible to specify how much mass we will need to loft? And how can we know whether or how the moon figures in a Mars staging operation? “If the ultimate goal is to organize a mission to Mars within the next several decades, then that goal needs to be clearly articulated now,” concludes a report written late last year under the direction of David A. Mindell, a professor of engineering history at MIT [see Mindell's essay, "The End of the Cult of the Astronaut"]. The report pointedly observes that the space architecture currently envisioned “for trips to the moon…is not extensible to missions to Mars.”
Do we need to return to the moon to get to Mars? Intuitively, it feels like the obvious way to go. In fact, it’s not obvious at all.
Take propulsion dynamics. If we were to lift everything needed for a Mars mission first to the moon—above all, the propellant—that would mean having to take enough propellant to escape not only the gravitational field of Earth but also that of the moon. And although the moon’s gravity is only a sixth that of Earth’s, the penalty is not trivial. Payload-to-propellant ratios are poor in any chemically fueled deep-space mission and at the margin of what’s tolerable in a Mars mission; tacking on gratuitous trips to the moon makes the ratio even worse.
Rocket scientists frame these questions in terms of delta-v, the change in velocity needed to transfer a space vehicle from one path to another. Leaving Earth is one delta-v; leaving low Earth orbit adds another. Going from that orbit to the moon needs a third, and getting from the moon to Mars requires a fourth. Add them up and you end up with a larger total than you’d get by just going directly to Mars or by mounting a Mars mission from, say, a Lagrangian point where the gravitational fields of the sun and Earth cancel.
Donald Rapp, the author of a useful textbook that lays out the enabling technologies needed for human Mars missions, has little use for the notion that the moon is a stepping-stone to the Red Planet. The moon is so much closer to Earth that there’s an exponential difference between moon and Mars missions, he points out: “Saying we have to go to the moon to get to Mars is kind of like saying that in order to get to Europe from New York City, we need to go to Montauk Point on eastern Long Island first.”
The one thing that could redeem the moon as a stepping-stone, Rapp and others argue, is if you could produce propellant on the moon to use in the rocket that went to Mars. But Rapp points out that the prospects for extracting oxygen on the moon are not promising. [For a radically opposing view, see William Stone's essay, "Mining the Moon".] One approach is to mine oxygen near the moon’s equator from regolith, the fluffy, silicate-rich material that covers most of the lunar surface. But the silicates, which are about 30 or 40 percent oxygen, would have to be heated to 2600 °C, too hot for any known container. Or you could mine iron oxide from the regolith and then make water by reacting it with hydrogen carried from Earth, which can be done at 1200 °C. (Rapp would prefer to just ship oxygen from Earth in the first place.) Alternatively, it might be possible to extract hydrogen and oxygen from water at the poles, if such water exists and proves accessible. But even that would be no mean trick. At 40 kelvin, ice is very hard; processing it would be power intensive, and the work would have to be done in total darkness, in difficult, rocky terrain.
Taking all those considerations into account, Rapp concludes caustically that for Martian voyagers, at least, “the one thing we know for sure, it makes no sense to go back to the moon.”
But even if the moon-Mars vision makes no sense in terms of propulsion dynamics, a case can still be made for a lunar relanding. The moon may yet teach us things we’ll need to know once we get to Mars. Learning how to mine, process, and store materials on the moon could be useful, even though the resources and procedures are somewhat different from those that would ultimately be used on Mars. Terrestrial help is just three days away on the moon but at least six months away on Mars. In either case, you wouldn’t be able to just go down the street to the corner hardware store whenever an unexpected problem arose. Both situations would demand the practicality and inventiveness of the true pioneer.
Some things might be much easier on the moon, of course, but sometimes the opposite will be true, because Mars resembles Earth more than the moon does in some ways. Take landing: It was devilishly difficult to anticipate the feel of putting down on the moon, where the gravitational tug is weak and there is virtually no atmosphere to provide lift and drag. (Armstrong, who had crashed a landing simulator on Earth, landed the Apollo vehicle on the moon much more actively than generally appreciated; to avoid unexpected boulders, he searched for a suitable site until the last possible second and almost ran out of gas.) The feel of landing a vehicle on Mars would be more familiar. And of course, many related aspects—having vehicles meet in orbit, descend, and reascend—could be practiced in lunar orbit for the Martian analogues.
But a strongly opposing school of thought sees little or no merit in such speculation. We could just as well practice orbital maneuvers in Earth’s vicinity, the argument goes, and the conditions would be more Martian than those found near the moon. And if you want to see how small groups of people get along during long periods of isolation, there’s no need to go to the moon; just go to the ISS, which after all cost tens of billions to build and has a projected lifetime cost of about $100 billion, according to the European Space Agency (ESA).
Arguably, we should exploit the station to the hilt, not only because it’s up and running but also because it’s an international collaboration. The United States will need all the help it can get to mount something as costly as a manned mission to Mars. Yet, strangely, it has given the station short shrift, most recently in its decision to stop shuttle flights next year. That will leave the station dependent on the aging Russian Soyuz spacecraft.
If you ask European astronautics experts about the U.S. vision, they tend to echo a refrain—that we should focus on testing technologies on the moon that someday will be relevant to Mars. But the more you ask about just what’s on that list of technologies, the shorter it seems to get.
Britain’s Surrey Satellite Technology has proposed a MoonLite mission, calling for swarms of artillery-like penetrators to be launched at the moon’s surface from small orbiters; a similar kind of sensing could be done on Mars from dirigibles, suggests Sir Martin Sweeting, Surrey’s founding chairman. Sweeting also talks about supporting lunar or Martian operations with orbit-to-surface telecommunications, a field in which the United Kingdom feels it has a comparative advantage. ESA, having just sent Europe’s first cargo vessel successfully to the space station, now has issued a “request for information” to develop a cargo lander for a moon mission between 2017 and 2020, to complement NASA’s human landing.
Europeans have not, however, proposed to contribute to any of the major elements of the moon return—the development of the big launchers, the crew exploration vehicle, or the astronaut lander—nor, say some, have they even been asked. Their preference for robotic exploration and their skepticism about the moon venture are obvious to all.
Even so, Europeans have been reluctant to come right out and reject a moon return. After all, they aren’t paying for it, and what harm can it do to talk about going to Mars via the moon so long as somebody else is footing the bill?
Two years ago, the European space powers, plus Australia, Canada, China, India, Japan, Russia, South Korea, and Ukraine, joined with the United States in issuing a report that seemingly endorsed the U.S. space vision. Their global exploration strategy [PDF], released in May 2007, called the moon “our nearest and first goal” and Mars “also a prime target.” Spelling out the lunar rationale, the report said that “to sustain human presence beyond Earth, we must learn from science ’on the moon’ how to live and work on other celestial bodies.” (Note the anomalous quotation marks, which are in the original.) The moon, said the report, “is the ideal place for humanity to develop the capability to journey to Mars and beyond.” That’s because the moon “has a strong place in the culture of many peoples and it instinctively appeals to the human imagination.”
Fine words, but what do they mean? They certainly don’t imply that any of those partner countries are getting ready to pump tens of billions of dollars into human exploration of the moon. On the contrary, the Europeans are deeply suspicious of any big international space venture. In the last such endeavor, the ISS, the Europeans were burned and burned badly by the severe delays that were mainly associated with problems in the U.S. shuttle program. Now they are having to contemplate the additional inconveniences connected with the early retirement of the shuttle.
At the 2008 International Astronautical Congress, in Glasgow, ESA Director General Jean-Jacques Dordain complained that by the time the ISS opened shop—more than a decade behind schedule—its original clients had lost interest and moved on. The payoff also came too late, he said, for the younger generation. For the future, therefore, “we have to define milestones that are challenging enough” to engage youth’s interest and keep it engaged.
You don’t have to think too hard to wonder whether a repeat mission to the moon can inspire the younger generation or whether the Mars prospect is just too distant to turn anybody on. Will a 15-year-old girl get excited about putting a man on Mars if it won’t happen until she’s in her 40s or 50s?
The global commitment to the moon-Mars vision may be thin and fragile, but that doesn’t mean that interest in the moon itself is weak. In fact, her allure is greater than ever: All the world’s aspiring space cavaliers want to visit her as soon as possible. China, India, Russia, and Japan all have major missions in the works. Flaunting national prowess is the name of the game.
In Europe, to be sure, lunar missions are not high on national agendas, although ESA did mount a very successful and innovative moon mission in 2003 powered by ion propulsion. Germany, which hopes to launch its Lunar Exploration Orbiter in 2012, is Europe’s rule-proving exception. The country was barred after World War II from pursuing work on missiles. Yet European propulsion work is now centered at the Bremen quarters of EADS Astrium (a subsidiary of the European Aeronautic Defence and Space Company), not far from the Peenemünde test grounds, where von Braun and his hugely talented associates invented modern rocketry. Why is Germany so singularly interested in a moon mission? The question arose in conversation with a senior space manager at Thales Alenia Space, in Turin, Italy, which has built about half the ISS’s habitable space. With a diffident shrug, he replied quietly, “Power.”
The same logic guides China’s space program, whose official 20-year goal is to “utilize space resources to…enhance overall national power.”
All this is not to suggest that lunar science as such is without interest. On the contrary. Just this February, Japanese researchers reported that images sent back by their Kaguya (Selene) lunar orbiters indicate that the moon’s crust is more rigid than Earth’s and may therefore lack water and other compounds that easily evaporate. In January, researchers at MIT and at the Berkeley Geochronology Center, in California, published an analysis of an ancient lunar rock showing that the early moon must have had a metallic core and a magnetic dynamo. It was Apollo astronaut Harrison H. Schmitt, a Ph.D. geologist and later a U.S. senator, who picked up that little rock; by general consent, it is easily the most interesting thing anybody has ever found on the moon.
People used to regard the moon as a big hunk of dead, inert matter, but Apollo data proved that the early moon consisted of an “ocean magma.” Evidently, the heat generated by the impact of some huge object with Earth was so great, it liquefied the material it hurled into orbit. And so, says David L. Schuster of the Berkeley Geochronology Center, “in what was probably a well-mixed molten mass, the denser materials might have cohered into a core.”
A decade ago, Schuster and MIT’s Ben Weiss examined a Martian meteorite found on Earth to estimate its temperature at the time of its ejection. Donald Bogard, Pratt Johnson, and Robert Pepin made the unlikely discovery in the 1980s and 1990s that certain meteorites on Earth had come from Mars, based on Viking 1’s sampling of the Martian atmosphere in 1976; to date, 31 such meteorites have been identified. Weiss says he was surprised at how much attention his particular work got. Evidently, the public is more interested in planetary science than it generally gets credit for.
The moon has always exercised a profound hold on the human imagination. Surely, millions of people would love to see a return to the moon and perhaps even the establishment of a permanent colony there. What difference does it make, then, whether it makes sense to go back to the moon? If we want to go there, let’s just go!
The lunar enterprise, however, is meant in some sense to be a template for the much bigger mission to Mars. Why, then, is the moon mission almost strictly made in the U.S.A.? This isn’t the Cold War; the United States doesn’t have to go to the moon to win a missile race with the USSR. Yet the U.S. government is proposing to shoulder pretty much the whole cost of returning to the moon, not to mention Mars, which it can’t actually afford. Italy’s space commissioner, Enrico Saggese, speaking to reporters last fall in Glasgow, estimated the cost of returning to the moon at $50 billion and of going to Mars at $500 billion—a lot to spend during a stubborn world recession.
The diplomatic impediments to globalization of the moon mission are not trivial, to be sure. The Europeans are in a slow-burning rage over the ISS. Though Russia’s space experts are highly respected, their national space agency is a skeleton, and nobody trusts their government. China, Germany, India, Japan—they’d all much prefer to strut their stuff in national showcase missions, not join as junior partners in a big, complicated international effort.
If only the problems were mainly technical. But they’re not. Returning to the moon is not all that technically challenging. What’s challenging is to make it an international effort that puts behind past grievances and sets the stage for a truly challenging international mission to Mars.
For more articles, go to Special Report: Why Mars? Why Now?
This article was corrected on 25 June 2009.