At the end of last month, NASA’s spacecraft OSIRIS-REx entered into a record-breaking orbital insertion, just 1.75 kilometers (1.09 miles) from the center of the asteroid Bennu, which is itself just 492 meters (1,614 feet) across. This is the closest orbit ever achieve around a celestial body, as well being the smallest object every orbited.
During this phase, known as Orbital A, the systems are not actually collecting any scientific data but the NavCam 1 is. This allows the team behind the mission to track the path that the spacecraft is taking around the asteroid, and it allows us to get the best views of Bennu’s southern hemisphere yet.
This approach is called Optical Navigation and the two “OpNav” images were taken on January 17, each with an exposure time of 1.4 milliseconds. The images have been adjusted for contrast making details of the clumpy surface more visible. The particularly striking boulder seen in the two images is 50 meters (165 feet) across.
OSIRIS-REx’s goal is not just observational though. In summer 2020, the spacecraft will get extremely close to the asteroid and stretch out a collection arm to grab a sample of regolith, the loose soil on the surface of Bennu. To achieve this near-touchdown, researchers need to know everything there is to know about the asteroid.
For this reason, OSIRIS-REx is continuing to map the surface of the asteroid in extreme detail as well as studying its composition to find the best spot for sample-collecting. But the orbital phase will also refine measurements related to the mass and gravity of Bennu. The asteroid’s gravity is extremely weak (roughly 5-millionths of Earth’s own) so even weak forces like thermal pressure from Bennu or solar radiation can push the spacecraft around, which means the team needs to keep an eye on the movements of OSIRIS-REx and be ready to make small adjustments.
The probe is currently 91 million kilometers (less than 57 million miles) from Earth and its detailed survey of Bennu will begin in slightly less than a month. OSIRIS-REx is scheduled to fly back to Earth once the sample has been collected, arriving back in 2023.