Asteroid Bennu continues to surprise scientists. NASA’s spacecraft OSIRIS-REx reached this tiny world on December 3, 2018, and since then our perspective of it has radically changed. The asteroid’s surface has more boulders than expected, it's releasing plumes of material, and it has water-rich minerals.
If all of this data from the last three months isn't enough for you, there are also indications that the asteroid might be spinning itself to death. The findings are reported in three papers (here, here, and here) published in the journal Nature Astronomy.
The data collected so far by OSIRIS-REx are being used by NASA to plan the rest of the mission. The spacecraft will hover above the asteroid's surface and scoop up material with an extendable arm. This sample will then be brought back to Earth in 2023.
To collect material safely, the team is mapping the asteroid's surface to avoid the many boulders on it, as well as monitoring the asteroid’s movements and gravitational field. Bennu is only 492 meters (1,614 feet) across and weighs 73 billion kilograms (160 billion pounds), so weak forces like solar radiation can have an effect on its dynamic properties. And that’s how the researchers discovered that the ground at its equator is outside its Roche lobe.
This lobe is the region where gravity is the dominant force. For material at Bennu’s equator, this is no longer the case. And given that radiation from the Sun is increasing the asteroid’s rotation on its axis, Bennu might break apart in the future.
“As that Roche lobe narrows further and further around the equator, it becomes easier and easier for this asteroid to lose material,” lead author of one of the papers, Daniel Scheeres from the University of Colorado Boulder, said in a statement. “So far, that material has been trapped by gravity, but at some point, if the asteroid keeps spinning faster, then you fall off the cliff.”
Scientists are not just interested in the future of the asteroid. They are extremely curious about its past too, as it could tell us something about the formation of the Solar System. Using infrared observations, the researchers were able to establish similarities between Bennu’s composition and certain meteorites that have landed on Earth.
“Scientists are interested in the composition of Bennu because similar objects may have seeded the Earth with water and organic materials,” the Southwest Research Institute's Dr Victoria Hamilton, a mission co-investigator and lead author, stated. “OSIRIS-REx data confirm previous ground-based observations pointing to aqueously altered, hydrated minerals on the surface of the asteroid.”
OSIRIS-REx is in a record-breaking orbit around Bennu, 1.75 kilometers (1.09 miles) from the center of the asteroid. This close orbit allows for some incredible observations. The scientists have witnessed boulders break up due to heating and cooling from the Sun and have also spotted particles erupting from Bennu's surface.
“The discovery of plumes is one of the biggest surprises of my scientific career,” explained Dante Lauretta, OSIRIS-REx principal investigator at the University of Arizona, Tucson. “And the rugged terrain went against all of our predictions. Bennu is already surprising us, and our exciting journey there is just getting started.”