Last January, NASA’s OSIRIS-REx broke the record for the smallest object ever orbited by a spacecraft, as it journeyed around asteroid Bennu, which is only 492 meters (1,614 feet) in diameter. It also smashed the record for the closest orbit, reaching just 1.75 kilometers (1.09 miles) from its center. The spacecraft has now moved even closer.
Last week, OSIRIS-REx executed its second orbital insertion maneuver, reaching a height above Bennu's surface of just 690 meters (2,264 feet), or 0.6 kilometers (0.4 miles). One of the three navigation cameras on the spacecraft, NavCam1, snapped the closest picture yet of the asteroid, detailing its now notorious and prominent boulder in the southern hemisphere.
While the entire asteroid is an aggregation of gravel and rocks loosely held together by gravity, the boulder is much larger, estimated between 10-20 meters (33 to 66 feet) across. Another important characteristic is the presence of an equatorial ridge. Researchers believe that due to its fast rotation and low gravity loose soil, otherwise known as the regolith, accumulates there.
The regolith is the mission's target, as next summer OSIRIS-REx will fly down to the surface, scoop up a sample, and take it back to Earth. Before it can do that, researchers need to have a precise map of the asteroid to find the most promising site to collect material from.
Bennu’s gravity is extremely weak, about five-millionths of what we experience on Earth, so the OSIRIS-REx team has to be on the ball when it comes to the probe's orbital parameter. They constantly perform small maneuvers to correct for effects such as the pressure from sunlight and even the heat emitted by the asteroid. And if something unexpected happens, OSIRIS-REx is programmed to fly away towards the Sun (although nobody expects that to happen).
The constant vigilance surrounding the spacecraft's orbit is not just for safety. The team is using it to work out both the gravity and the mass of the asteroid, which is why the orbital phase is crucial for the mission. During these next few months, the team will be selecting a place for the probe's touch-and-go landing, so detailed knowledge of the object’s gravity is crucial. OSIRIS-REx is expected to arrive back home with its sample in 2023.