OSIRIS-REx

OSIRIS-REx, American spacecraft that orbited the asteroid Bennu and returned a sample of it to Earth. The roughly 250-gram (9-ounce) sample was the largest extraterrestrial sample returned to Earth since the Apollo missions to the Moon and the first asteroid sample collected by an American spacecraft.

OSIRIS-REx launched on September 8, 2016, from Cape Canaveral, Florida, on an Atlas V rocket. It traveled around the Sun and flew by Earth on September 22, 2017. It entered orbit around Bennu on December 31, 2018.

Bennu was chosen for several reasons. With a diameter of 492 meters (1,614 feet), it is large enough to have a slow rotation period—4.3 hours. Smaller asteroids rotate faster and thus it would be hard for a spacecraft to match their velocity and touch down on them. Moreover, fast-spinning asteroids throw off much of the loose rock and dust called regolith that OSIRIS-REx was designed to collect. Bennu’s material has not been appreciably altered since the beginning of the solar system, and the asteroid is rich in carbon. Thus, it could shed light on the role of asteroids in transporting organic compounds to Earth.

Scientists had expected Bennu to have a relatively smooth surface, but when OSIRIS-REx arrived at Bennu, it found the surface to be so rocky that mission scientists had difficulty selecting a site for the spacecraft to collect a sample. A site was eventually selected, and on October 20, 2020, OSIRIS-REx briefly touched down on Bennu. The spacecraft placed the head of the Touch-and-Go Sample Acquisition Mechanism (TAGSAM) against Bennu’s surface for five seconds. TAGSAM fired a burst of nitrogen gas at the surface, which disturbed the rock and dust and blew about 250 grams (9 ounces) of surface material into TAGSAM’s sample collector. The spacecraft then fired its thrusters to back away from Bennu.

Scientists later learned that the surface layers of Bennu have very little cohesion. When the spacecraft fired its thrusters, it had sunk about 50 cm (20 inches) into Bennu from its gentle initial contact and was sinking still farther into the asteroid. The burst of nitrogen gas, which had been expected to make a small pit on Bennu’s surface, instead made a crater 8 meters (26 feet) in diameter.

OSIRIS-REx left Bennu on May 10, 2021, and began its journey to return its sample to Earth. The spacecraft made two orbits of the Sun and on September 24, 2023, it jettisoned the sample return capsule, which parachuted to the ground at the U.S. Department of Defense’s Utah Test and Training Range. The spacecraft was given a new mission: to orbit the asteroid Apophis after its flyby of Earth in 2029. The mission was renamed OSIRIS-APEX (OSIRIS-Apophis Explorer).