On August 5, 2012, the world’s attention was captured by the Mars Science Laboratory (MSL) landing. One of the key components in the multifaceted landing of the Curiosity rover safely on Mars was the Thermal Protection System (TPS), or heat shield, on the spacecraft carrying the rover.
The thing people remember most about the heat shield is when it popped off the spacecraft and flung like a frisbee across the Martian landscape, landing with a plume of dust. Measuring nearly 15 ft (4.5 m) in diameter, the MSL heat shield was the largest to ever travel to another planet. That may sound impressive, when it comes to entering an atmosphere bigger is not necessarily better. While more resistance can act as a natural braking system the trade off is enormous heat build up on the spacecraft. And we’re talking serious heat here, 3360º F (1850º C), almost twice as hot as molten lava.
So, what kind of material can protect against that kind of heat? Enter Phenolic Impregnated Carbon Ablator, or PICA (pronounced pee-ka). The material is lightweight and slightly more dense than balsa wood. It was developed at the NASA Ames Research Center in Moffett Field, California which is where the TPS was developed for Apollo and the Space Shuttle. PICA debuted on the Stardust return capsule which brought back comet particles and interstellar dust samples to Earth in January 2006 and holds the record for the fastest Earth reentry speed of any human-made object, 28,860 mph (12.9 km/second).
Due to the size of the MSL heat shield, a tiled version of PICA was developed. Once the tiles were cut they were placed on a molded carbon-composite carrier structure. The heat shield also had two suites of sensors installed on it. The first suite recorded atmospheric data and TPS performance (a TPS report, if you will) and the second were pressure sensors that enabled a reconstruction of the flight trajectory as the spacecraft descended towards the surface of Mars. These sensors did not exist prior to the MSL mission and NASA will use this data to plan future missions to Mars.
In between the 2006 Stardust and the 2012 MLS landings, PICA has been used on another spacecraft, the SpaceX Dragon. The heat shield on the Dragon is slightly smaller than the MSL measuring 13 ft (4 m) in diameter. In February 2009, SpaceX announced successful arc jet testing of three variations of the original NASA PICA material. PICA-X, the SpaceX variant of PICA, was developed with NASA’s help at their Ames Research Center.
One improvement that PICA-X represents is greater ease of manufacturing. Like many things at SpaceX, PICA-X is made in-house and at a fraction of the cost of NASA’s PICA. Produced in large pieces and then cut into tiles about the size of a cafeteria tray they are placed, in a similar fashion to the MLS shield, on to a carbon-composite mold. The tiles are about 3 in (8 cm) thick, and weigh about 2 lbs (1 kg).
That’s right, the heat shield on the Dragon is only three inches thick which hardly sounds thick enough to keep a wheel of cheese from melting. The shield is robust enough that only about half an inch, less than 1 centimeter, burns off during reentry allowing a comfortable safety margin for both cargo and astronauts.
Another noted improvement of PICA-X is it’s potential for reusability, which is the key focus at SpaceX. In a April 2011 SpaceX update it is noted that the PICA-X heat shield has the potential to be used dozens of times for reentry to Earth’s atmosphere. It is unclear if PICA-X is still in a state of development or if it just depends on how much burns off on each reentry. At the point that the Grasshopper project is in full swing, PICA-X will cover the second stage of the Falcon 9 rocket.
Article update: I originally wrote the first and second stage of the reusable version of the Falcon 9 would be covered with PICA-X. It is actually just the second stage. The article has been corrected to reflect that accuracy.