But just how did the OSIRIS-REx team choose Bennu from the over 500,000 known asteroids in the Solar System?
Proximity to Earth
The closest asteroids to Earth are called Near-Earth Objects (NEOs). As the name suggests, NEOs are objects that orbit within 1.3 AU of the Sun. (1 AU = the distance between Earth and the Sun, or ~93 million miles) For a sample return mission like OSIRIS-REx, the most accessible asteroids for a spacecraft to reach are located between 1.6 AU and 0.8 AU. The ideal asteroid has an Earth-like orbit with low eccentricity and inclination. At the time of the mission’s asteroid selection in 2008, there were over 7,000 known NEOs, but only 192 had orbits that met these criteria.
Size
Asteroids with small diameters rotate more rapidly than those with large diameters. With a diameter less than 200 meters, an asteroid spins so rapidly that the loose material on its surface (regolith) can be ejected from it. The ideal asteroid has a diameter larger than 200 m so that a spacecraft can safely come into contact with it and collect a sufficient regolith sample. This size requirement reduced the number of candidate asteroids from 192 to 26.
Composition
Asteroids are divided into different types based on their chemical composition. The most primitive asteroids are carbon-rich and have not significantly changed since they formed nearly 4 billion years ago. These asteroids contain organic molecules, volatiles, and amino acids that may have been the precursors to life on Earth. Of the 26 asteroids left on the list, only 12 had a known composition, and only 5 were primitive and carbon-rich.
From these 5 asteroids, Bennu was selected. Bennu is a B-type asteroid with a ~500 meter diameter. It completes an orbit around the Sun every 436.604 days (1.2 years) and every 6 years comes very close to Earth, within 0.002 AU. These close encounters give Bennu a high probability of impacting Earth in the late 22nd century. Bennu’s size, primitive composition, and potentially hazardous orbit make it one of the most fascinating and accessible NEOs … and the ideal OSIRIS-REx target asteroid.