OSIRIS-REx is a NASA mission to explore near-Earth asteroid Bennu and return a sample to Earth. Prior to arriving at Bennu, mission planners had expected the asteroid’s surface to consist largely of fine particulate material, similar to gravel. When OSIRIS-REx arrived in December 2018, however, it was greeted by a rocky world covered with boulders.
This unexpected ruggedness means that there are few places on Bennu where OSIRIS-REx can safely touch down and collect a sample. After a year of studying the asteroid, the mission announced a primary sample collection site, which they designated “Nightingale,” along with a backup site called “Osprey.” In August 2020, OSIRIS-REx will descend to Bennu and attempt to collect up to four-and-a-half pounds of loose material, for return to Earth in 2023.
In the thermal map above, asteroid Bennu’s surface temperatures dramatically change from the night side to the day side, experiencing swings of 270 degrees Fahrenheit. The time that it takes for an object to heat up or cool down is determined by its thermal inertia — a property that can be useful in the remote study of planetary bodies.
Fine particulate materials like sand or gravel tend to heat up and cool down quickly (low thermal inertia), while solid objects like rocks and boulders do so more slowly (high thermal inertia). By observing how a distant body like Bennu changes temperature over the course of a day, scientists can usually infer the average size of particles on its surface.
Before OSIRIS-REx arrived, scientists had observed that Bennu’s surface heats up and cools down relatively quickly, leading them to predict a mean particle size of about 2-3cm. OSIRIS-REx, however, was greeted by a predominantly rocky asteroid, littered with giant boulders. This discrepancy is one of the major surprises of the OSIRIS-REx mission.