How to build the first extraterrestrial airplane.

Artist’s conception of the Mars Airplane in flight. (NASA)
Air & Space Magazine

As a monument to the aerospace century, the idea could hardly be bettered. A hundred years to the day after the Wright brothers took off from Kitty Hawk, the first aircraft built for another planet would fly through the light pink skies of Mars. A technology that had girdled and transformed one world—Antarctica is the only continent without contrails regularly dissecting its sky—would begin afresh on a second.

Right from the start, the idea stirred powerful emotions. NASA Administrator Dan Goldin announced in early 1999 his agency’s plan to send a little robot aircraft, one of a proposed series of Martian “micromissions,” to arrive at Mars on December 17, 2003. More than just an adventure, it would have the benefit of bringing together the normally separate aeronautics and space sides of NASA’s house. Unfortunately, what seemed an appealing interdisciplinarity ended up making the project’s infancy more troubled, with different factions in NASA fighting over who should do what. At one point responsibility was divided between two research centers on opposite coasts. “It was like the wisdom of Solomon,” says one observer, “except that they actually did cut the baby in half.”

Now the baby is back in one piece. The Mars Airplane program is being run by NASA’s Langley Research Center in Hampton, Virginia, which by the time you read this should have selected an industry-led team to build the first extraterrestrial aircraft. Even with no further bureaucratic infighting, it will be a challenge. Launch in November 2002 makes the development schedule tight, the budget (though not yet fixed) will be low, and no one has ever done such a thing before. Success will be a triumphant 20 minutes of data. Failure will be a far-off, unheard crash.

Even if this particular invention never reaches Mars, or is postponed to some less historic date, the Kitty Hawk anniversary has already served to focus attention on the argument that someday, the exploration of Mars will require flight. If a human expedition ever gets under way (“And you and I know that it will,” says Joel Levine, the Mars Airplane project scientist, with commendable faith), powered flight could vastly increase its scope. If unable to fly, Martian pioneers will be able to explore the vicinity of their landing site using rovers that cover perhaps a state’s worth of territory. With airplanes, they will be able to explore a world. “If we do our work properly,” says Marsplane pioneer Dale Reed of NASA’s Dryden Flight Research Center in California, “we should have a two-seater airplane available when the astronauts get there 15 to 20 years from now. That’s what this whole effort should be leading to.”

 The idea is not new. Nearly half a century ago, Wernher von Braun described Mars landings using hypersonic gliders—Chesley Bonestell painted one sitting on the dusty Martian plain like a silver arrow. Von Braun might not have bothered, though, if he had known what we know now about the Martian atmosphere. Before the Space Age, it was understood to be thin. Just how thin wasn’t appreciated until the first spacecraft flybys in the 1960s. The pressure at the planet’s “datum”—the notional surface that serves as a sea level on sealess Mars—turns out to be only about six millibars, or six thousandths of the atmospheric pressure at Earth’s sea level. Even well below the datum, in the heart of the vast canyon system known as Valles Marineris or in the depths of the Hellas basin, it never climbs much above one percent of Earth’s sea level pressure. There is simply not much aero for an aeronautical engineer to work with.

But if the planet’s atmosphere was disappointingly thin, the fascinating surface revealed by NASA’s three Mars orbiters of the 1970s—Mariner 9 and Vikings 1 and 2—more than made up for the letdown. Some of the landscapes are astonishing: volcanoes the size of countries, canyons that could stretch across continents, flood channels through which a sea could drain in a matter of days. This was clearly a place worth exploring.

After the Viking landings in 1976, aircraft came to be seen as an exciting way of carrying the exploration forward. The pioneers of the Space Age had most admirably solved the problem of reaching other planets, but hadn’t been able to move around once they got there. The Viking program, for example, dispatched extremely sophisticated machines to a world millions of miles away, where they inspected only a few square yards of the surface. The attraction of spacecraft that could investigate larger areas at higher resolutions than you could achieve from orbit was obvious. So engineers at the Jet Propulsion Laboratory in Pasadena, California, the center that handles most of NASA’s planetary science, began to think about airplanes. Their thinking soon led them to Dale Reed.

While NASA’s planetary probes were opening up the solar system, Reed was concentrating on a completely different, if also rather futuristic, problem—the development of supersonic airliners on Earth. One worry, then and now, was that these high fliers might do all sorts of damage to the stratosphere. NASA therefore started a program to measure the environmental impact of supersonic flight by sampling the wake of an SR-71 traveling through the stratosphere at Mach 3. That required another aircraft that could get up to 70,000 feet and take the samples. To meet the requirements, Reed designed the Mini-Sniffer, a small, remote-controlled vehicle powered by a unique hydrazine engine. Hydrazine blows itself apart in the presence of the right catalyst, a trait that has long made it a popular fuel for spacecraft thrusters. Reed’s design used heat given off by this reaction to run a little steam engine; that engine in turn drove a propeller.

The Mini-Sniffer thus solved two of the problems facing potential Mars airplanes. It worked in very thin air (though not as thin as that on Mars) and it generated all its power with onboard fuel. This mattered because the Martian atmosphere, such as it is, is composed almost entirely of carbon dioxide. Jets and internal combustion engines wouldn’t work there, but Reed’s hydrazine steam engine would do just fine. What’s more, it could use a fuel that any Mars-bound spacecraft would likely carry anyway.

The fact that the Martian atmosphere is mostly carbon dioxide was also, in a small way, a bonus. At any given pressure, carbon dioxide is denser than the air on Earth, which would increase a wing’s lift. The biggest plus of all, though, was the low gravity on Mars, which reduces the wing loading on an aircraft, allowing it to get by with less lift. All these factors suggested to Reed and to the NASA engineers who approached him in 1978 that Mars flight might indeed be feasible.

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