Probing Planets: Can You Get There From Here?

It is adifficult business to design, build, successfully launch and then operatespacecraft on other worlds. Accomplishing the mission is particularly difficultfor planetary probes, which go through significant atmospheres, collecting dataon their way to the surface and, due to the challenges of the extremeenvironments associated with such destinations, often only survive forrelatively brief periods once they have landed. Planetary probe missions go toplaces like Venus where the surface temperatures are 900° F, the pressures are90 times that of Earth, and the largely carbon dioxide atmosphere has asignificant component of sulfuric acid. Others attempt to reach giant gas ballslike Saturn (only the Saturn moon Titan has been the target of a probe) orJupiter. Jupiter's gaseous outer layer was successfully entered by the Galileoprobe up to the point where the pressures were 22 times that of Earth. Suchentries, unlike entering the atmosphere and landing, are more like blastinginto the very stuff of the planet itself, dodging lightning strikes on theway.

NASA lostsome early probe missions like Mariner 1, their first mission to Venus, and theinternational community has an even spottier record than the US. These missions require years ofdevelopment and expensive, specialized advanced technologies like pressurechambers and thermal protection systems, not to mention specializedinstrumentation. There have been a few notable successes such as the PioneerVenus multi-probe mission, the Galileoprobe and the recent European Huygensprobe to Titan, which was part of the Saturn Cassini Mission. Thesemissions, though, were either a while ago, hugely expensive, or both. Thechallenge for the next generation of missions is to take advantage of newtechnological developments, but no one wants to put unproven technologies onmissions where close to $1 billion may be a stake. Yet, we need to considersome of the tasks at hand. How do you get there from here?

Considerthe thermal protection system or TPS. Spacecraft traveling at 40,000 to 50,000mph, which is required to reach outer planets like Jupiter and Saturn, arriveat their targets carrying a tremendous amount of energy that must be shed ifthey are to attempt entry into the target's atmosphere. In other words, theyhave to slow down. In the relative vacuum of space, the high speeds cause noproblems, but once a spacecraft encounters an atmosphere with lots of moleculesof gases, things start to heat up pretty quickly. The faster the probe goes,the hotter it gets. The Galileo probe, admittedly the most difficultatmospheric entry ever attempted, experiencedtemperatures twice as hot as the Sun's surface temperature and decelerationforces 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 bythe use of a "heat shield" which is carefully designed, carefullytested and protected by ultra-specialized materials such as carbon phenolic composites. The material has to be thick enough sothat a significant fraction of it can burn off in the process of slowing andyet leave enough to protect the sensitive spacecraft. It's a bit like wrappinga spacecraft in charcoal briquettes – the outside burns off but leaves aninsulating layer that continues to take the heat. Of course, the more weight inTPS a probe has to carry, the less weight it can carry in science instruments.

During theyears following the launch of the Galileo Mission in 1989, some new materialshave been developed that are better at thermal protection as well as beinglighter weight. Getting any new technology "flight qualified" is achallenge, however. One material, invented at NASAAmes ResearchCenter in Silicon Valley, is called PICA (PhenolicImpregnated Carbon Ablator). This clever stuff is extremely lightweight,relatively easy to manufacture and even easier to custom fit to the specificshapes required by heat shields, representing a major advance in space probetechnology. The Stardust Mission used PICA for its heat shield, the firstflight of this material. It was launched on February 7, 1999, during NASA's eraof "Faster, Better, Cheaper" (FBC) missions. The FBC approach resultedin some spectacular failures due to extreme cost cutting (Mars Polar Lander andMars Climate Orbiter) and NASA has since moved away from this philosophy.Still, FBC had at least one interesting positive outcome. The philosophyincluded a commitment to allow some measure of risk, based on the notion thatif a mission is smaller and cheaper, one can afford to be a bit riskier sincethe loss will be less catastrophic and there may be something to be gained byattempting new things. The StardustMission returned comet samples to Earth on January 15, 2006, a stunningsuccess that proved PICA performed beautifully.

It'sdifficult and expensive to send probes to planets and their satellites. Theyare all so varied in their technical requirements that the international probecommunity now gets together once a year to share ideas on new technologies andcompare notes on which targets hold the most promise for great science. Themost recent meeting was the 5th International Planetary Probe Workshop, in Bordeaux, Francein late June 2007. The technologies discussed ranged from blimp type vehicles(actually, "ballutes," a kind ofcombination balloon/parachute) that could float above nasty surfaces, to everbetter TPS systems and advanced super lightweight instruments usingmicro-fluidics and nanotechnologies. The issue with these new technologies is,as always, flight qualification. With a costly probe mission, you can't flysomething 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.

Enticingtargets for future probes range from Venus and Mercury, which could help usunderstand how the solar system has evolved and why those planets are now suchinhospitable places, to some of the moons of Saturn and Jupiter, such as Europa with its liquid ocean under a layer of ice sheetsand its potential to harbor some form of living organism. Many believe that Europa has all the basic requirements: liquid water, anenergy source, and nutrients. The only way to find out, though, is to go therewith the right spacecraft and the right instrumentation. None of the trade-offsand decisions are easy. There isn't enough money to do everything that everyonewould like and even with money, these places are tough destinations. In fact,it's rocket science.




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