Last week, after an unprecedented 718 days in orbit, the U.S. Air Force’s robotic X-37B space plane has returned to Earth, touching down safely at the Kennedy Center Space in Cape Canaveral, Fla.

The robotic vehicle, which resembles NASA’s famous space shuttle, is 29 feet long with a wingspan of 15 feet. It was built in tandem by Boeing’s Phantom Works division as well as Lockheed Martin. The plane was made with composite structures instead of aluminum. The X-37B’s payload bay (the area in which the cargo is packed) measures 7 feet long by 4 feet wide — about the size of a pickup truck bed.

Two different X-37B vehicles have flown a total of four missions, which are known as OTV-1 (short for Orbital Test Vehicle), OTV-2, OTV-3 and OTV-4 (the latest mission). All four flights have reached space with the help of United Launch Alliance’s Atlas 5 rocket, which is made of carbon fiber composites.

While the Air Force was very secretive about what was on the space plane, it did reveal details about an advanced materials investigation during the two-year orbit. Known as the Materials Exposure and Technology Innovation in Space (METIS), the experiment exposed nearly 100 different quarter-sized samples of polymers, composites and coatings to the harshness of space. METIS builds on previous experiments conducted by the Materials on International Space Station Experiment (MISSE), which observed over 4,000 composite samples in space from 2001-2013.

“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,” said Miria Finckenor, the principal investigator for METIS at NASA’s Marshall Space Flight Center in Huntsville, Ala.

To see the space plane’s historic landing, watch the video below: