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.