More details have been revealed about the X-37B spaceplane's upcoming OTV-4 mission. When it launches on May 20 from Cape Canaveral Air Force Station in Florida, the (not entirely) secret X-37B will carry a NASA experiment called Materials Exposure and Technology Innovation in Space (METIS) designed to test new materials for use in future spacecraft.
Space is an exceedingly harsh environment with extremes of heat and cold, hard radiation, and equally hard vacuum that does common materials no good at all. For this reason exotic materials are often used in spacecraft, but some of these have properties that are not fully understood.
Even the best laboratory testing is limited, so to avoid unpleasant surprises, engineers have been keen on testing space materials on site ever since the first Sputnik launches. Back in 1969, the Apollo 12 astronauts recovered bits of the Surveyor 3 lander, which was sent to the Moon two years before. From 1984 to 1990, NASA orbited its Long Duration Exposure Facility (LDE) satellite containing a battery of samples open to space, which was followed by the Mir Environmental Effects Payload (MEEP), which flew on the Russian Mir space station from 1996 to 1997. Then there was the Materials International Space Station Experiment (MISSE) that carried 4,000 samples on the International Space Station (ISS) from 2001 to 2013.
METIS builds directly on these experiments with about 100 different materials samples the size of a US quarter including polymers, composites, and coatings. Some have already flown on MISSE and the idea is to verify results by exposure to a different orbital environment. Open to space for over 200 days, NASA says that the goal is that some test materials could replace obsolescent ones on future spacecraft and constructions.
"By exposing materials to space and returning the samples to Earth, we gain valuable data about how the materials hold up in the environment in which they will have to operate," says Miria Finckenor, the co-investigator on the MISSE experiment and principal investigator for METIS at NASA’s Marshall Space Flight Center in Huntsville, Alabama. "Spacecraft designers can use this information to choose the best material for specific applications, such as thermal protection or antennas or any other space hardware."
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