Asteroid · Near-Earth asteroid (Apollo) · orbits Sun
Bennu is a half-kilometre rubble pile - a loose heap of boulders barely held together by its own feeble gravity, on an orbit that brings it near Earth every six years. NASA’s OSIRIS-REx collected a sample from it in 2020 and dropped the capsule into the Utah desert in 2023, delivering pristine material as old as the Solar System itself.
OSIRIS-REx returned 121.6 g of Bennu to Earth in September 2023.
It has about a 1-in-2,700 chance of hitting Earth on 24 September 2182 - meaning it almost certainly misses.
Its surface is so weak that the sampling arm sank half a metre in, as if into a ball pit.
A flying rubble pile
Bennu is not a solid rock. When OSIRIS-REx pressed its sampling arm against the surface, the ground gave way like a ball pit and the arm sank half a metre - the asteroid is a floating heap of 4.6-billion-year-old debris, reassembled from an older shattered body. Most small asteroids are probably built this way, which matters enormously for any future attempt to deflect one.
Physical characteristics
Diameter
0.49 km
Mass
7.330e+10 kg
Density
1.19 g/cm³
Rotation period
-4.3 h (retrograde)
Geometric albedo
0.04
Atmosphere and composition
Atmosphere
None.
Composition
B-type carbonaceous rubble: water-bearing clays and carbon compounds, with much of its volume empty space.
OSIRIS-REx collected a generous sample from the rubble-pile asteroid Bennu and delivered 121.6 g to the Utah desert in September 2023 - NASA’s first asteroid sample return. The spacecraft flew on as OSIRIS-APEX, bound for the asteroid Apophis in 2029.