problem-how to cram a sophisticated airplane into a tiny container-is
still as vexing today as it was in 1996, when this folding model
of a Mars glider was design' at NASA's Ames Research Center.
The currently envisioned Mars Airplane will have to fit in an
aeroshell no larger than a cooking wok. The space constraint
is imposed by the Ariane 5 rocket, which will give the 40-pound
airplane its ride to Earth orbit.
to build the first extraterrestrial airplane.
by Oliver Morton
permission from Air&Space
magazine December 1999/January 2000 Volume 14 Number 5
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.
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."
The baby eventually
was restored to one piece, and NASA's Langley Research Center in
Hampton, Virginia, was given the go-ahead to start working toward
a November 2002 launch. Project managers even got as far as inviting
industry to propose ideas for how the airplane should be built.
But the goal of meeting the Wright brothers anniversary proved too
much, too fast, and by early November the money managers at NASA
headquarters admitted defeat. Langley's Mars Airplane project is
now on indefinite hold, with launch planned for no sooner than 2004.
the first extraterrestrial airplane flight has been 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 ex- plore a world. "If we do our work prop-
erly," 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 ef- fort 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 thou- sandths 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 per-
cent of Earth's sea level pressure. There is simply not much aero
for an aero- nautical 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.
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 MiniSniffer, 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.
thus solved two the problems facing potential Mars a planes. It
worked in very thin air (thou 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 would 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
The fact that
the Martian atmosphere is mostly carbon dioxide was also, 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, thou, 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.
"We all got
pretty excited," recalls Reed. He and colleagues at JPL and in industry
worked on various Marsplane designs based on Mini-Sniffers and sailplanes.
The grandest came from an aeronautical engineer named Abe Kerem.
It weighed about 1,200 pounds, had a wingspan of about 70 feet,
and used a distinctive inverted-V tail. "He likes that inverted
V-tail," says Reed. In fact, Kerem's innovation later was incorporated
into military unmanned aerial vehicles (UAVs) that evolved into
today's Predator, a medium-altitude, long-endurance reconnaissance
aircraft that Reed sees as "an outgrowth of this Mars airplane."
The Mars vehicles
differed in shape and engine, but they all shared the unusual feature
of starting from the top of the atmosphere, not the bottom. During
the long voyage through interplanetary space they would be folded
up inside an aeroshell like the one that contained the Viking landers.
Upon entering the Martian atmosphere the aeroshell would be slowed
by a parachute, then would peel away. The still falling aircraft
would deploy itself, its wings and tail unfolding as it fell. (In
the case of a sailplane-size aircraft, that's a lot of unfolding.)
The engine would fire, and powered flight would begin.
Bonestell Space Art. Space artist Chesley Bonestell included
an airplane in his classic 1956 painting "The Exploration
of Mars, " based on the ideas of Wernher von Braun
The other end
of the flight seemed as though it would be simpler-at first. "Originally
we were just proposing crashing at the end of the mission," recalls
Reed. "But then we got the scientists on board and they said, 'Oh
no, we don't want to crash. We'd like to use the airplane after
it lands.' "
So Reed found
a way to convert a sailplane to vertical flight and land it with
a rocket. "I took a Schweizer sailplane and rigged the sailplane
so it would pop up," he recounts. "We towed it up to 10,000 feet
and pulled the lever, and [it] came down almost perfectly at a 70-degree
angle. The wing goes into a deep stall at a high angle of attack,
and it stays controllable-it comes down like a parachute." A Mars
aircraft could do the same, switching on hydrazine thrusters in
its belly at the last minute to make a rocket-cushioned soft landing
just as the Viking probes had done. When it was time to lift off
again, the same rockets would kick it back into the sky. Reed's
scheme allowed a Mars airplane to fly to a selected landing site,
stay there while the scientists back home went over its data, then
go on to a second site newly identified as interesting.
1998 MAGE proposal foresaw the PR value of a Wright brothers
anniversary flight over Mars but was rejected by NASA as too
The final mission
concept that evolved from this work in 1978 was deeply ambitious.
Larry Lemke, who has worked on Mars aircraft designs at NASA's Ames
Research Center near San Francisco, remembers that the plan called
for three spacecraft to enter Martian orbit at around the same time,
each carrying four Mars airplanes in Viking- style aeroshells. Each
spacecraft would require a space shuttle to deliver it to Earth
orbit, and the three launches would have to take place within a
period of less than a month to make use of the limited window of
opportunity for Mars missions. (Back then, NASA still aimed to fly
a shuttle every week or so.) The squadron of twelve aircraft, which
might carry a variety of scientific instruments, would fly down
to the surface one by one, some revisiting sites of interest spotted
by earlier missions. With each aircraft capable of flying perhaps
3,500 miles before landing (1,800 if it used fuel to land, take
off, and land again), the mission had the potential to completely
circle Mars and explore at least a dozen sites close up, investigating
the planet in more detail than ever would have been possible before.
By the time
the shuttle actually started to fly-once every few months, if NASA
was lucky-it was clear such a grandiose and expensive project would
never get off the ground. Mars aircraft were taken off the agenda.
But it turned out you didn't have to be thinking about Mars to do
useful work on the problem. The key issue involved, in aeronautical
terms, is a low Reynolds number. Essentially, this describes the
way in which a fluid (air, in this case) flows, and depends on the
density of the fluid, the speed of the airflow, and the chord of
the aircraft wing. Flying at high altitudes, at slow speeds, or
with small wings all translate to low Reynolds numbers. Work on
high-altitude research aircraft and on human-powered airplanes like
Paul MacCready's Gossamer Albatross taught engineers new tricks
that could be applied on Mars. In fact, at least one existing aircraft-the
very-high altitude, solar-powered Pathfinder UAV built by MacCready's
company AeroVironment-would in principle be nearly capable of flying
on Mars, if you could get it and its support team there.
advances in Marsplane design sometimes come from solving other
problems. Dale Reed's Mini-Sniffer (below) flew in the thin
air of the stratosphere, sampling the wake of an SR-71. Later
Reed showed that a Schweizer sailplane (bottom) could be rigged
to make a vertical landing.
these advances in other fields, by the time NASA's planetary exploration
program started to pick up again in the early l990s, it could draw
on more expertise relevant to Mars aircraft than ever before. In
1992, at the first workshop devoted to NASA's Discovery program
of low-cost planetary missions, a Mars aircraft proposal was put
on the table by John Langford, whose company, Aurora Flight Sciences,
had developed high-altitude aircraft for NASA to use in programs
much like the environmental impact study that had led Dale Reed
to design the Mini-Sniffer. Langford had also managed the Daedalus
project to build the human-powered aircraft that a cyclist flew
74 miles across the Sea of Crete in 1988.
under contract to NASA's Jet Propulsion Laboratory, engineers
at AeroVironment, Inc. built and flight tested a full-scale
(five-foot wingspan) model of their Mars glider, Otto, in California's
Red Rock Canyon last spring.
As the Discovery
program developed, more ideas for Mars airplanes surfaced. Larry
Lemke's team at the Ames center came up with a craft that was basically
a scaled-down version of the Reed Marsplane of the 1970s. Weighing
about 400 pounds, it would fly for six hours or so, land, study
the surface, then take off a month later for more cruising. The
Ames people even had a target in mind: Gusev Crater, which, evidence
suggests, may have once been a lakebed. Water inside the crater
might have been warmed by a large volcano more than 100 miles to
the north. Many researchers-especially at Ames, where the crater
has a particularly passionate set of advocates-think Gusev could
hold traces of past Martian life.
proposal, done in cooperation with planetary scientist Mike Malin's
small company (see "Getting the Picture," Aug./Sept. 1999), was
MAGE, a mission that used a graceful flying wing with a pusher propeller
to carry a suite of geophysical instruments over the Martian canyons.
At the same time, a team involving AeroVironment, JPL, and others
suggested an even simpler mission, which flew a series of small
gliders rather than a single powered aircraft. "We ended up in a
situation where we more or less had to choose between carrying a
propulsion system and carrying a scientific payload," says Carlos
Miralles of AeroVironment. Flying several vehicles instead of one
added resilience. "You can tolerate failures, you can target them
independently, you can cover a larger total range and get more diversity
than if you are stuck with one airplane trying to fly for a long
period of time," he says. Six gliders would have been popped down
at different sites in Valles Marineris. Although each would have
flown for at most 60 miles, together they might have provided data
on the whole length of the canyon system.
Clever as they
were, these ideas were slightly ahead of their time. In November
1998, after the Discovery review panels worried about the risk involved
in using unproven technologies, NASA turned down both MAGE and the
fleet of gliders. But the December 2003 Wright centennial, which
happens to coincide with a favorable launch opportunity for reaching
Mars, had already begun to generate a buzz for Mars airplanes. Both
proposals had used the name Kitty Hawk-MAGE viewgraphs even had
the word proudly emblazoned on the wing.
The folding, 10-pound vehicle got its name from German aviation
pioneer Otto Lilienthal
Below: Otto's first flight, documented by a tail-mounted camera,
lasted about 40 minutes. The first Mars Airplane flight won't
be that long, and radio communication will be possible for
only 20 minutes. That should still be long enough to send
at least one real-time picture back to Earth.
a plasma physicist at Princeton University, had also noticed that
the Wright anniversary coincided with a Mars launch window, and
mentioned it to Norm Augustine, the former Lockheed Martin CEO who
had moved to Princeton. Augustine became an enthusiastic proponent
of the idea, talking it up to Dan Goldin and others in the space
agency, who saw its potential as a Mars Pathfinder-like source of
And so it was
that the NASA budget came to include a bold, if modestly funded,
new project: the Mars Airplane.
To the people
at Ames, JPL, Langley, Dryden, Aurora, AeroVironment, and other
places who had been thinking about Mars aircraft, the most striking
thing about the proposal was how small the vehicle was. The Mars
micromissions are parasites lifted to Earth orbit by a European
Ariane 5 rocket. While going about its everyday business of launching
communication satellites two at a time, the Ariane 5 has enough
oomph left over to put very small payloads into highly elliptical
orbits around Earth. From there, with the help of a couple of lunar
swing-bye to pump up their velocity, such spacecraft can go on to
Mars. French and American researchers have all sorts of tentative
plans for little orbiters, penetrators, and communications relay
satellites that could travel to Mars this way, all based on or carried
by a standard "microspacecraft" that the Mars Airplane will be the
first to use.
the Ariane constraints, the aeroshell will be at most 30 inches
across, meaning that the first Mars aircraft has to fit inside a
container the size of a large wok. Even though the airplane will
be small, weighing maybe 40 pounds, fitting it in such a tight space
will require some clever origami. The designs that Langley worked
on last summer had five separate folds. The outer segments of the
wings fold in across the body. The vertical rudder flattens itself
down onto the sailplane. And the two booms that attach the body
to the tail assembly have to bend too (the booms fit on either side
of the casing for the parachute, which sits in the small of the
job is made even more difficult by the Martian atmosphere. On Earth,
aerodynamic pressure is used to bend a folded aircraft into shape.
On Mars, according to AeroVironment's Miralles, who was part of
a team bidding this fall to build the Mars Airplane, you have to
use springs. A test model that AeroVironment developed while working
on the glider project was able to spring itself into shape in only
a second while falling. But springs add mass, and mass is one of
the things the Mars Airplane is short of. Ease of folding was one
reason why the MAGE design used a flying wing with no tail. It's
unlikely that any of the teams bidding to build NASA's Marsplane
will offer a flying wing, however, because there's an associated
lift penalty that makes it infeasible for a very small aircraft.
The ultimate solution to the unfolding problem therefore remains
request for proposals, released in September, listed the things
that the aircraft should be able to do-demonstrate powered flight
and certain maneuvers, carry a small instrument package including
some cameras, and so on-but didn't say how they should be done.
Contractors who bid on the Mars Airplane were left to decide for
themselves what shape would be best, what sort of power source to
use, and other specifics.
mean, however, that the Langley team didn't have its own ideas.
Engineers at the center spent most of last summer working on preliminary
ideas and running tests in wind tunnels. This early work suggested
that a rocket engine would be the best way to go-a hydrazine engine,
already proven in spaceflight and capable of delivering two or three
pounds of thrust. According to Joel Levine of Langley, the chief
scientist on the program, it seems simpler and more certain than
using a more complex motor and a propeller that would have to work
in that terribly thin air. The incorporation of a rocket also gave
one of Langley's preliminary designs a pleasingly otherworldly look-a
flattened teardrop of a body with top-mounted, swept-back wings
and tail booms kinked like a downhill skier's knees to lift the
forward-swept tail out of the rocket's hot exhaust plume. The choice
between conventional and swept wings involves another tricky aerodynamic
tradeoff. At low Reynolds numbers, there is a pronounced "separation
bubble" between the smooth flow of air over the front of the wing
and the turbulent flow at the back. At high speeds-the Mars aircraft
needs to fly at about 250 mph- this bubble stretches, eventually
reaching a point where the flow no longer attaches at all and the
wing stalls. Sweeping the wings may help achieve the desired reattachment,
as might various other tricks, such as putting knobby "turbulators"
to disturb the flow on the upper surface just behind the leading
edge. Split flaps on the trailing edge can also help by generating
lift with out reattachment. But due to its tight fit inside the
aeroshell, sweeping the Mars Airplane's wings has the down side
of decreasing their total area, and area is important when you need
every ounce of lift you can scrape together. "We barely have enough
lift to make this go," says Juan Cruz, the project's chief engineer
Langley Research Center produced this artist's concept of the
Mars Airplane (above) before testing models with raised tail
surfaces in wind tunnels last summer The vehicle's precise shape
and engine are yet to be determined-contractors, not NASA, will
do most of the work.
lift will be particularly important at the beginning of the flight
After its release from the aeroshell, the airplane will be in a
dive. "On Earth," says Cruz, "the same airplane would pull out of
that dive in a few hundred feet. On Mars it's going to take anywhere
from [2 to 5.6 miles]. We just don't have enough extra lift to bring
it around." According to Miralles at AeroVironment, it's this first
dive that will be the moment of truth. The longer it goes on, the
faster the aircraft will be going and the closer it will come to
the speed of sound (in Mars' cold, thin atmosphere, that's only
520 mph). High Mach numbers make the separation problem worse, so
you want to get out of the dive as fast as possible. AeroVironment
learned a few helpful tricks in this regard when designing propellers
for its high-altitude UAVs, but Miralles was reluctant to talk about
them this fall, while the NASA contract was still up for grabs.
of the preliminary designs to come out of the Langley center
had the Mars Airplane's tail mounted on twin booms. The booms
fit on either side of the casing for the parachute that slows
the vehicle's initial descent through the atmosphere. Still
undecided is whether the aircraft will be rocket- or propeller
driven, and whether its wings will be swept or straight.
Once the Mars
Airplane levels out- assuming it does-low lift will still cause
problems, and will make changing direction a chore. Cruz estimates
the turning circle will be more than three miles in radius. So even
though the aircraft will be speedy, there won't be any hot-dogging.
"It will be like flying in an airliner where you sit and watch the
terrain just going by," says Miralles. "You'll be closer to the
terrain, but it will be the same sensation."
it may feel, this will be a purposeful flight. "The reason people
build airplanes is not because an airplane can take you anywhere,
but because an airplane can take you somewhere you want to go,"
Cruz says. "So we want the airplane to demonstrate that it can hold
a heading and then change that direction to some preselected second
heading. And then return to the original heading." AII of this will
have to be done with pre-programmed maneuvers, because Earth will
be too far away to transmit advice, even at the speed of light.
that the airplane can fly straight and level, turn when required,
and ride out whatever turbulence may occur is the project's primary
goal. But the aircraft also has a 4.5-pound science payload consisting
of cameras and-if room allows-a spectrometer for assessing the mineral
content of rocks and a magnetometer (one of the great surprises
to come out of the current Mars Global Surveyor mission is that
some regions on the planet appear to have strong magnetic fields).
The cameras will be able to spot details on the surface as small
as six inches across. "If Mars has rabbits, we'll see them," jokes
we on Earth won't be able to watch live video of the mission because
the aircraft can't carry a radio powerful enough to send back that
much data. Instead, it will send signals to the spacecraft that
brought it to Mars, which will store them and send them back to
Earth over a period of days. This is not a perfect arrangement,
not least because the carrier spacecraft will be in line-of-sight
communication with the aircraft for only 20 minutes of a flight
that might last longer. But it should allow some images to come
back, with at least one transmitted in real time. There will be
some sort of show for Earthlings after all.
scientists look on at all this unimpressed. They worry that the
Mars Airplane will cost more than currently envisioned-$60 million
was one outside panel's estimate- and that what is basically a technology
mission will start to eat into NASA's science budget. Some would
rather have used the first micromission opportunity for a communications
relay satellite that could benefit other Mars exploration spacecraft
with more ambitious research agendas. And some just don't think
it can be made to work. One scientist cites the Monty Python sketch
about the difficulties involved in teaching sheep to fly: "The thing
is, they don't so much fly as plummet."
But you can
bet that if the Mars Airplane does fly, scientists will soon be
queuing up to make use of its descendants' ability to explore the
Martian landscape. Once landing and takeoff are mastered-this aircraft
will not try either-scientific instruments could routinely be sent
to many sites during the same mission, making the investigations
that much more productive. And aircraft could do things that no
lander (unless extremely lucky) could ever do, like sniff out molecules
given off by things living on or under the surface, if they exist
Because such molecules would be local, scarce, and short-lived,
says Levine, they would probably be undetectable from orbit. But
a search by aircraft (Dale Reed's Mini-Sniffer again) could well
find them. And although the existence of underground life would
likely only be firmly established by drilling holes, a sniffer could
at least show you where to drill.
Mars glider drop tests (above) lasted a few minutes at most.
After separating from the parachute, the Mars Airplane will
unfold and go into a long, nerve-racking dive before leveling
out. NASA's Ames center is among the places where engineers
have long thought about Mars flight. Ames even flight tested
a prototype Martian glider in 1996 (right).
The Mars Airplane
is the first word in Mars aircraft design, not the last. "This is
just the beginning of a generation of airplanes that will fly in
the atmospheres of other planets," says Levine. After all, if you
can fly in the near vacuum of Mars, you can fly more or less anywhere.
"Over the next 30 years we're going to have many planes going to
Mars, planes flying below the cloud layer on Venus to study the
surface for the first time in visible wavelengths; we'll study the
organic haze on Titan; we'll be sending planes to Jupiter and Saturn
and looking under the clouds." He points out that a recent report
from the National Research Council concluded that mobility is not
just important for solar system exploration, it's essential. And
mobility is just what airplanes promise.
that make good on that promise will have all sorts of shapes and
sizes. "There's not one right way to make a Mars airplane, any more
than there's one right way to make an Earth airplane," says Larry
Lemke of NASA's Ames center. Big sailplane-like vehicles may be
good for some types of remote sensing. But if plans to manufacture
rocket fuel from ingredients in the Martian atmosphere pay off,
point-to-point mobility might be achieved with aircraft that use
sheer speed to get around the difficulties of flying through a thin
atmosphere, just as the SR-71 does on Earth. For other purposes,
aerodynamically shaped dirigibles might be the way to go. The relative
density of Mars' carbon dioxide atmosphere makes lighter-than-air
flight attractive; so does the fact that hydrogen, a better working
fluid in every way than the helium used on Earth, will not burn
in carbon dioxide. Give a big arrowhead dirigible a flat top and
cover it with thin-film solar cells to generate power, and it could
fly around Mars forever. Someday. Perhaps.
The Mars Airplane
will bequeath technology to these far-off projects, but that may
not be its major contribution. The Wright brothers changed not just
the way we travel around the world but also the way we see it. Today
all the images we have of Mars, save for those of three rocky landing
sites, come from looking down at the planet. This orbital viewpoint,
while wonderfully revealing, can't help but turn Mars into a scientific
specimen, a data set, a planet to study rather than a world to experience.
The Mars Airplane will let us look out, not down, to distant horizons
and what lies beyond them. It will let us watch our shadow moving
on the rocks below as we fly through the sky. The camera in its
rudder will show us the delicate banking of the aircraft's wings
as it heads off in directions no one has ever followed before. Long
before human pilots fly over the Red Planet, these pictures may
rekindle the romance of a new world in the audience back home.