Balloons provide a unique vantage point for scientific observation.
Balloons can fly one hundred times closer to the surface of Mars than
orbiters and can travel a thousand times further than rovers in a
comparable period, thus providing views of much broader areas
of the surface.
Balloons have been flying for decades in Earth's stratosphere, which
has an atmosphere as thin as that on the surface of Mars. Conventional
stratospheric balloons have lifetimes limited to a few days because of
the daily heating and cooling of the balloon. Helium super-pressure
balloons, currently under development for the Ultra Long Duration
Balloon (ULDB), will fly more than 100 days and perhaps as long as a
year here on Earth. Smaller super-pressure balloons carrying payloads
of only a few pounds have already flown for as long as a year.
Using this technology, a future Mars balloon would deploy soon
after a spacecraft entered the Mars atmosphere and would then rapidly
inflate from a helium tank as it descended beneath a parachute. After
inflation, the parachute and tanks would detach and the balloon would
fly at a nearly constant altitude both day and night. The balloon's
internal pressure would be higher during the day than at night,
although the balloon volume would remain the same. Strong,
lightweight, leak-proof materials are under development to permit large
tool kits of science instruments to be flown on such a balloon. Tests of
balloon deployment in the Earth's atmosphere are currently underway
as well.
Another kind of lightweight balloon that might be useful on Mars
is called a solar Montgolfiere balloon, named after the French brothers who
flew the first hot air balloon. It does not have to be inflated with a light
gas such as helium. Instead, the balloon would deploy upon entering the
Martian atmosphere. An opening at the bottom of the balloon would fill
up with Martian "air" while falling to the surface. The balloon
would then be quickly heated by the sun, which provides buoyancy.
Montgolfieres are attractive because they are not vulnerable to leaks, as
any leaking martian "air" would be quickly replaced and
re-heated by the sun. However, the balloon lifetime is limited to a few
hours, because it is only buoyant until the sun goes down. However,
the Montgolfiere balloon can play two important roles in exploration:
provide a soft,
slow landing for small craft on potentially hazardous terrain, with greater
control than a parachute-assisted or rocket landing system.
go back up into
the atmosphere after dropping off the landed craft so that it could image
the surface further and gather more science data before nightfall
