Bennu ejecting particles from its surface on Jan. 6, 2019.

Lauretta et al./Science (2019)

While chasing the potentially hazardous asteroid 101955 Bennu around the solar system for the past year, sometimes from just a mile away, NASA’s Osiris-Rex spacecraft spotted something unexpected. During afternoon on the asteroid in January, the spinning-top-shaped Bennu began spewing particles and debris from its surface out into space. For icy comets approaching the sun, explosive outbursts aren’t uncommon, but for asteroids — huge chunks of rock — the cosmic tantrum is much more rare.

The Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (Osiris-Rex) spacecraft has been tailing Bennu since arriving in its orbit on Dec. 3, 2018. The NASA-built robotic explorer is fitted with a three-camera suite known as OCAMS that detected particles being ejected from the asteroid on Jan. 6, 2019. 

A new study, published in the journal Science, reports “repeated instances” of particle ejection from Bennu and determines properties of the material it spewed out, in addition to their trajectories. The detailed examination is the first look at an “active” asteroid from close up, with Osiris-Rex getting within a mile of the surface. Active asteroids aren’t unknown to science, but they are relatively rare, making the close-up an extremely exciting prospect for astronomers.

The researchers examined numerous events through January and February 2019. Ejected particles were tiny, reaching only up to the centimeter-scale. In the biggest ejection, 200 particles were vomited up by Bennu and the team were able to determine the speed of 117 of them, with the fastest moving at 3.3 meters per second. A further 52 were moving too slowly to escape Bennu’s gravity and fell back to the rock. 

What could cause these events? The team believe the losses may be caused by micrometeorite impacts or even stress fracturing of boulders due to heating. The three largest and most well characterized events all took place in Bennu’s late afternoon, with the biggest occurring just before the asteroid reached perihelion, or the point it’s closest to the sun.  

Observations will go on for the next four years, with Osiris-Rex continuing to examine Bennu from afar. Additional ejection events could help nail down how asteroids evolve and how they contribute dust, debris and other material to space or even other heavenly bodies. Osiris-Rex is scheduled to scoop up rock from the asteroid and the team suggests it may even be possible it samples some of the ejected debris, which will help characterize active asteroid processes even further.

The past year has been good for asteroid-hunting spacecraft. Osiris-Rex has made a number of discoveries at Bennu, finding water on the asteroid the week it arrived and snapping images from half a mile above the surface. Once it finds a landing spot, Osiris-Rex will sample bits of Bennu in 2020. Similarly, the Japanese Space Agency, JAXA, tailed asteroid Ryugu for about a year-and-a-half with their Hayabusa2 spacecraft, making a smash-and-grab sampling of its own. Those samples are expected to return to Earth in December 2020.