It is a
difficult business to design, build, successfully launch and then operate
spacecraft on other worlds. Accomplishing the mission is particularly difficult
for planetary probes, which go through significant atmospheres, collecting data
on their way to the surface and, due to the challenges of the extreme
environments associated with such destinations, often only survive for
relatively brief periods once they have landed. Planetary probe missions go to
places like Venus where the surface temperatures are 900° F, the pressures are
90 times that of Earth, and the largely carbon dioxide atmosphere has a
significant component of sulfuric acid. Others attempt to reach giant gas balls
like Saturn (only the Saturn moon Titan has been the target of a probe) or
Jupiter. Jupiter's gaseous outer layer was successfully entered by the Galileo
probe up to the point where the pressures were 22 times that of Earth. Such
entries, unlike entering the atmosphere and landing, are more like blasting
into the very stuff of the planet itself, dodging lightning strikes on the
way.
NASA lost
some early probe missions like Mariner 1, their first mission to Venus, and the
international community has an even spottier record than the US. These missions require years of
development and expensive, specialized advanced technologies like pressure
chambers and thermal protection systems, not to mention specialized
instrumentation. There have been a few notable successes such as the Pioneer
Venus multi-probe mission, the Galileo
probe and the recent European Huygens
probe to Titan, which was part of the Saturn Cassini Mission. These
missions, though, were either a while ago, hugely expensive, or both. The
challenge for the next generation of missions is to take advantage of new
technological developments, but no one wants to put unproven technologies on
missions where close to $1 billion may be a stake. Yet, we need to consider
some of the tasks at hand. How do you get there from here?
Consider
the thermal protection system or TPS. Spacecraft traveling at 40,000 to 50,000
mph, which is required to reach outer planets like Jupiter and Saturn, arrive
at their targets carrying a tremendous amount of energy that must be shed if
they are to attempt entry into the target's atmosphere. In other words, they
have to slow down. In the relative vacuum of space, the high speeds cause no
problems, but once a spacecraft encounters an atmosphere with lots of molecules
of gases, things start to heat up pretty quickly. The faster the probe goes,
the hotter it gets. The Galileo probe, admittedly the most difficult
atmospheric entry ever attempted, experienced
temperatures twice as hot as the Sun's surface temperature and deceleration
forces up to 230 g's (230 times the acceleration of gravity at Earth's surface)
as it approached Jupiter's atmosphere. Such extreme conditions are survived by
the use of a "heat shield" which is carefully designed, carefully
tested and protected by ultra-specialized materials such as carbon phenolic composites. The material has to be thick enough so
that a significant fraction of it can burn off in the process of slowing and
yet leave enough to protect the sensitive spacecraft. It's a bit like wrapping
a spacecraft in charcoal briquettes – the outside burns off but leaves an
insulating layer that continues to take the heat. Of course, the more weight in
TPS a probe has to carry, the less weight it can carry in science instruments.
During the
years following the launch of the Galileo Mission in 1989, some new materials
have been developed that are better at thermal protection as well as being
lighter weight. Getting any new technology "flight qualified" is a
challenge, however. One material, invented at NASA
Ames Research
Center in Silicon
Valley, is called PICA (Phenolic
Impregnated Carbon Ablator). This clever stuff is extremely lightweight,
relatively easy to manufacture and even easier to custom fit to the specific
shapes required by heat shields, representing a major advance in space probe
technology. The Stardust Mission used PICA for its heat shield, the first
flight of this material. It was launched on February 7, 1999, during NASA's era
of "Faster, Better, Cheaper" (FBC) missions. The FBC approach resulted
in some spectacular failures due to extreme cost cutting (Mars Polar Lander and
Mars Climate Orbiter) and NASA has since moved away from this philosophy.
Still, FBC had at least one interesting positive outcome. The philosophy
included a commitment to allow some measure of risk, based on the notion that
if a mission is smaller and cheaper, one can afford to be a bit riskier since
the loss will be less catastrophic and there may be something to be gained by
attempting new things. The Stardust
Mission returned comet samples to Earth on January 15, 2006, a stunning
success that proved PICA performed beautifully.
It's
difficult and expensive to send probes to planets and their satellites. They
are all so varied in their technical requirements that the international probe
community now gets together once a year to share ideas on new technologies and
compare notes on which targets hold the most promise for great science. The
most recent meeting was the 5th International Planetary Probe Workshop, in Bordeaux, France
in late June 2007. The technologies discussed ranged from blimp type vehicles
(actually, "ballutes," a kind of
combination balloon/parachute) that could float above nasty surfaces, to ever
better TPS systems and advanced super lightweight instruments using
micro-fluidics and nanotechnologies. The issue with these new technologies is,
as always, flight qualification. With a costly probe mission, you can't fly
something unless it's been flown already. It's a bit like getting a bank loan.
The bank won't give you the money unless you can prove that you don't need it.
Enticing
targets for future probes range from Venus and Mercury, which could help us
understand how the solar system has evolved and why those planets are now such
inhospitable places, to some of the moons of Saturn and Jupiter, such as Europa with its liquid ocean under a layer of ice sheets
and its potential to harbor some form of living organism. Many believe that Europa has all the basic requirements: liquid water, an
energy source, and nutrients. The only way to find out, though, is to go there
with the right spacecraft and the right instrumentation. None of the trade-offs
and decisions are easy. There isn't enough money to do everything that everyone
would like and even with money, these places are tough destinations. In fact,
it's rocket science.
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