BEFORE they had rockets to fire their imaginations, science fiction writers
were devoted to the balloon. It was a sort of proto-spacecraft, a piece of
technology that represented humankind鈥檚 transcendence over nature, that allowed
new views of the world and promised travel to the most unlikely places.
Unfortunately, balloons鈥� practical limitations鈥攖hey really do need
atmospheres鈥攈ave meant that for most of this century, the exploration of
space, in fiction and in fact, has been a heavier-than-air affair. But if
engineers at NASA get their way, balloons could soon be roving alien skies,
providing insights that no orbiting spacecraft could ever achieve.
So far, only Venus has played host to balloons from Earth鈥攁 couple of
sturdy specimens were released into the Venusian atmosphere by the
Franco-Russian Vega missions in 1985. Floating 50 kilometres above the surface,
they survived for nearly two days and were carried a third of the way round the
planet.
With a thick, stable atmosphere, Venus makes a pretty good place for
ballooning, and some enthusiasts are eager to return and probe the planet鈥檚
strange winds. But they will have to wait a while; there鈥檚 a 鈥渂een there, done
that鈥� feeling about Venus among those who plan planetary missions. The planet
with scientific sex appeal鈥攁nd thus the promise of funding鈥攊s Mars,
and so that鈥檚 where the balloonists are now headed.
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The Red Planet is hardly a friendly place for ballooning. The Martian
atmosphere is more than a hundred times thinner than Earth鈥檚, with extreme
variations in pressure and temperature, not to mention the vast volcanoes that
jut into it here and there. Indeed, according to James Cutts, who is in charge
of developing a prototype Mars balloon at the Jet Propulsion Laboratory (JPL)
near Los Angeles, the Martian skies may be less suited to ballooning than any
other atmosphere in the Solar System. But Mars is interesting and relatively
easy to reach. And the challenges mean that if balloons can succeed there,
they鈥檒l succeed anywhere. If they prove themselves on Mars, balloons鈥攐r
aerobots, as lighter-than-air craft are known in the space business鈥攃ould
be sent to Venus, to Saturn鈥檚 moon Titan and beyond.
The principles of ballooning are the same whatever planet you鈥檙e on. You need
a bag of gas that is lighter than the air outside it. The gas can be lighter
either because it is hotter or because it is made up of lighter molecules. Both
approaches are used on Earth and both are under discussion for other
planets.
All alone
Although the principles of deep space ballooning are familiar, the
practicalities are not. On Earth, balloons are usually launched from the ground,
with a fair amount of human help. What鈥檚 more, the Earth鈥檚 atmosphere is quite
dense, which makes generating lift easy. Elsewhere, balloons will fall into the
atmospheres from space at high speeds. They will have a relatively short time to
inflate, and no humans on hand to assist.
One prototype Mars balloon from JPL has already shown that it can do this,
more or less. It is a solar montgolfier鈥攁 hot-air balloon with no burner
that looks like a big black sock. The sun heats the sock, the sock heats the air
inside and the montgolfier soars. Earlier this year, a folded up sock like this
was dropped from a more traditional balloon at a height of 32 kilometres over
Tillamook, Oregon, an altitude at which the Earth鈥檚 atmosphere is about as thick
as the Martian atmosphere at ground level.
A thin atmosphere is something of a problem for most balloons. The thinner
the atmosphere, the more of it you have to displace for a given amount of lift,
which makes it important to use balloon material that is as light and thin as
possible. But for solar montgolfiers, a thin atmosphere is ideal because it can
be heated up more quickly by the Sun-warmed envelope. This means you get lift
rapidly. And that鈥檚 more or less how it happened in the stratosphere over
Oregon. The sock fell open end first, filled up with air and stabilised at about
20 kilometres.
Solar montgolfiers like this could make a big difference to the way Mars
missions are put together. At the moment the parachutes, retrorockets and
associated structural toughness that get a Mars lander from outer space to the
surface in one piece represent a fair bit of the mission鈥檚 mass. According to
Jack Jones at JPL, replacing the retrorockets and their fuel with a solar
montgolfier deployed during the descent could increase the fraction of total
mass delivered to the ground, especially for small missions, and put it there
more gently. People planning future missions are already taking note.
Night flying
Solar montgolfiers, though, are by their nature one-day wonders鈥攃ome
nightfall, they descend to the surface. For a longer mission, you need another
approach, which is why Cutts and his team are working on super-pressurised
helium aerobots. Rather than using the air on the planet, these balloons would
be sealed and inflated with helium to above atmospheric pressure, so the balloon
is rigid and will continue to float day and night. The disadvantage is that they
need a source of pressurised helium.
The team is currently working on a balloon 10 metres in diameter, which could
carry a payload of about 3 kilograms. This would be small enough to piggyback on
someone else鈥檚 launch vehicle as a cheap way of proving the idea could work. If
it does, a full-size mission, comparable in scope to one of JPL鈥檚 Mars Surveyor
missions, might circumnavigate the planet six times with a payload five times as
large, perhaps including a magnetometer, a radar to peer below the surface and
stereo cameras.
The advantage of an aerobot is that it could take these instruments to within
5 kilometres of the Martian surface. That鈥檚 a lot closer than any orbiting
spacecraft鈥攖he Global Surveyor currently orbiting the Red Planet doesn鈥檛
get within a hundred kilometres of the surface. Pictures of Mars鈥檚 canyons and
mountains sent back by a balloon could be utterly spectacular鈥攁 point
NASA鈥檚 planners, ever keen for good PR, are unlikely to miss.
In previous aerobot designs for Mars, such as the Franco-Russian one that bit
the dust in the mid-1990s along with most of Russia鈥檚 space plans, the balloon
has been inflated from the top. The JPL team has gone for a more straightforward
system of inflating from below. A sort of leaky windsock fixed over the gas
inlet inside the balloon ensures that the gas spreads evenly throughout the bag
as it inflates鈥攚hich it may have to do at a rate of 100 cubic metres a
second.
Trials this summer, beneath helicopters and in a pressure chamber simulating
Martian conditions, have worked well. The team is getting used to working with
the flimsy material of which the balloons are made: Mylar just eight-micrometres
thick. 鈥淭hey look pretty fragile,鈥� says Cutts of his gossamer aerobots, but
experience is teaching the team that the Mylar鈥檚 thinness has advantages, not
least when it comes to squeezing the balloons into small deployment devices.
If the tests on the balloons convince those who allocate NASA鈥檚 funds, then a
small Mars balloon could fly as soon as 2003. Success could see much larger
aerobots being sent to Mars and even farther afield. Saturn鈥檚 moon Titan is a
prime candidate, being a much more clement place for ballooning. Its thick
atmosphere, unlike that of Mars, can be relied on to get gradually warmer as you
go deeper. This stable temperature profile means that Titan is ideal as a target
for 鈥渞eversible fluid鈥� balloons, which control their altitude by exploiting the
fact that gases condense at low temperature.
A Titan aerobot based on the reversible fluid approach might consist of a
primary helium balloon and a secondary argon one. At high altitudes, the argon
would condense and the liquid would be bottled. The aerobot would then descend
gently until it reached the surface. When it wanted to take off again, the argon
would be uncorked and the liquid would vaporise. As the secondary balloon fills
up, the aerobot would rise again. The whole process could be repeated over and
over again.
Marine odyssey
A balloon like this could investigate hundreds of sites on Titan鈥檚 surface,
sampling not just its crust but also its oceans and lakes鈥攕hould they
exist鈥攕omething no wheeled rover could hope to do.
Once they have been tried out in the atmospheres of Mars, Titan and Venus,
the aerobots will be ready for new challenges. One is powered flight.
Super-pressurised balloons do not have to be spheres. They could be made in more
aerodynamic shapes suitable for powered, directional flight. With that in mind,
the JPL team is looking at the new techniques used to shape sails for yachts by
building them out of thin films laid onto three-dimensional forms. This is just
one of the tricks they hope to pick up from the well-funded, hi-tech world of
the America鈥檚 Cup.
Another challenge is to go for the really big atmospheres鈥攖hose of
Jupiter and the other gas giants. That will be tough. The gas giants鈥� outer
atmospheres are made largely of hydrogen, so a helium balloon, for example,
would plummet like a stone. Hot-air balloons are the only option, but the
distance from the Sun and the thickness of the atmospheres means that solar
heating would be slow to work.
In the meantime, Cutts has his eye on the most far out use of a balloon
suggested so far. NASA has plans for a small mission to Pluto, the only planet
in the Solar System that humanity has not yet reached. To be launched in 2004,
it would fly by its target a decade or more later at a speed of about 19
kilometres a second鈥攆ar too fast to stop and look around.
But Pluto has an atmosphere, a very thin one, admittedly, but also a very
extended one because the planet鈥檚 gravity is so weak. You couldn鈥檛 use a normal
parachute, because parachutes need a certain amount of resistance just to
unfurl. But a balloon can fulfil a parachute鈥檚 function. As it passes through
thousands of kilometres of atmosphere, such a 鈥渂allute鈥� could slow down a small
payload to a speed where it could survive a landing.
While a Mylar ballute would melt as it descended, polybenzoxazole, a polymer
used to stiffen high-performance yacht sails, could probably withstand the heat.
What鈥檚 more, deploying a balloon in a vacuum is easy compared to doing it
falling towards the surface of Mars. So humanity鈥檚 farthest footprint may yet be
left by a tiny ballooning robot. Jules Verne would be pleased.
