Getting probes to distant celestial bodies is the easy part. Landing the multimillion dollar machines on the surface without going kaplooie is an entirely different matter. To prevent a plummet to the surface, NASA employs hypersonic airbags to cushion Mach 5 impacts.
Known as Inflatable Aerodynamic Decelerators (IADs), these devices are essentially inflatable heat shields. They're designed to take the brunt of the heat and friction caused by atmospheric entry. Development on IADs began in the 1960s, when the Air Force and NASA worked with Goodyear Aerospace to design gear that was considered (but eventually passed on) for the early Viking, Pioneer Venus and Galileo missions. Currently, space agencies are eyeing the technology as a low-cost means of returning samples from the ISS or even Mars — both aboard robotic rovers or as part of a manned mission.
So why go with an inflatable cone rather than a conventional heat shield? Size, primarily. An inflatable shield takes up drastically less weight and space aboard vessel. Also, according to one NASA report, "they increase landed mass, accuracy and altitude in a variety of space applications". The current prototype employs layers of off-the-shelf materials to resist heat up to 704C.
According to a NASA Langley report:
Layups consisted of an outer (heatshield) layer, gas barrier, and inner (insulator) layer of various materials and combinations. The combination heatshield and insulator forms the Thermal Protection System (TPS). Results showed that the heavier outer fabrics had the best durability and an aerogel-based insulator performed the best out of the configurations tested.
Much of the research into IADs is also being conducted at the National Full-Scale Aerodynamics Complex (NFAC) at NASA Ames Research centre. It's a giant, 12m x 24m wind tunnel powered by a six-fan drive system capable of wind speeds up to 300 knots.
August 2009 marked the first successful test of NASA's latest IAD, the Inflatable Reentry Vehicle Experiment II (IRVE-II). The vacuum-packed, 15-inch (38cm) diameter payload "shroud" launched from Wallops Flight Facility in Virginia. Once in orbit, the shield successfully moved through its paces. It inflated to a diameter of 3m using an on-board nitrogen tank, and the layers of silicone-coated fabric proved their thermal tolerance. The device was also tested for stability and structural integrity while entering the atmosphere. From this data, NASA is working to improve the shield design and construction ahead of the next test flight, which is scheduled for 2013. [NASA Aeronautics, NASA HIAD, NASA IRVE, Planetary Probe, NASA Langley]