NASA’s OSIRIS-REx spacecraft is on an incredible mission to study the asteroid Bennu and send samples back to Earth. The team didn’t expect that OSIRIS-REx would also contribute to our understanding of black holes. No, Bennu doesn’t have anything to do with black holes, but an X-ray instrument on OSIRIS-REx spotted a distant black hole just to the side of Bennu during a routine sweep.
The primary mission of OSIRIS-REx is to pick up a tiny sample of regolith from Bennu and return it to Earth, much the same as Japan’s Hayabusa2 probe. It’s not like we get out to orbit asteroids all the time, so NASA wanted to make the most of the trip by scanning Bennu with various scientific instruments. One of those is the Regolith X-ray Imaging Spectrometer (REXIS), which analyzes how Bennu’s surface reflects X-rays and what that tells us about its composition. Guess what’s very active in the X-ray part of the spectrum? Yep, black holes.
In November 2019, researchers noted a small X-ray flare to the side of Bennu. Upon further examination, researchers confirmed that REXIS had detected a newly active black hole binary sitting about 30,000 light-years away. The object is most likely a binary system consisting of a black hole and a main-sequence star. As matter from the orbiting star falls into the accretion disk, it heats up and releases energy, mainly in the form of X-rays.
The black hole is known as MAXI J0637-430 because it wasn’t discovered by OSIRIS-REx—NASA’s asteroid explorer was a few days late to the party. Japan’s MAXI X-ray telescope aboard the International Space Station (ISS) was the first to detect the X-ray flare from the black hole. Several days later, NASA’s NICER instrument on the ISS confirmed the finding. Just a few days later, the OSIRIS-REx team spotted MAXI J0637-430 with REXIS.
Earth’s atmosphere blocks most interstellar radiation, so the only way to reliably detect objects like MAXI J0637-430 is in space. The ISS and orbital telescopes are key parts of such efforts. However, this is the first time we’ve detected a black hole from interplanetary space.
NASA has identified a location where OSIRIS-REx will touch down in the coming months to collect its sample. The team is taking its time verifying the plan before moving forward. If successful, OSIRIS-REx will bring its sample back to Earth around the middle of 2023.