NASA mission catches near-Earth asteroid ejecting material into space

James Marshall
December 7, 2019

Shortly afterward, the asteroid began launching small particles into space. Asteroids, though diverse in terms of their composition, aren't as exciting from a visual perspective, but a subset known as active asteroids feature dynamic surfaces.

What's next: OSIRIS-REx will continue to survey Bennu, and eventually attempt to collect samples from the surface.

This artist's concept shows the Origins Spectral Interpretation Resource Identification Security-Regolith Explorer (OSIRIS-REx) spacecraft contacting the asteroid Bennu with the Touch-And-Go Sample Arm Mechanism or TAGSAM.

It's a "rubble pile" asteroid, a grouping of rocks held together by gravity rather than a single object.

The main objective of OSIRIS-REx is to collect a sample from the surface of Bennu in 2020 and return it to Earth for analysis in 2023, and the spacecraft has been analyzing the asteroid since the beginning of this year to find an ideal site to sample.

The findings suggest that many asteroids may be similarly active and reveal that rocks blasting off of asteroids may be a new way for meteorites to reach Earth, the scientists wrote in a new study. That said, astronomers have previously documented the shedding of materials from active asteroids using ground-based telescopes, but they're quite rare, with only a handful having been documented since the 1990s, including the active asteroid (6478) Gault. Each resulted in hundreds of centimeter-size particles being thrown off its surface. Some fall back onto the asteroid's surface, while others go into orbit around it, like small satellites. The events occur pretty frequently, with the largest ones detected twice in January and once in February this year, where up to 200 bits of debris were released. All three events occurred in different, specific places on the asteroid. But they hadn't seen any eruptions from Bennu until now. None of them posed a risk to the spacecraft.

Researchers call these episodes particle ejection events.

They also spied persistent particles orbiting the asteroid, six of which they will target with future observations.

"Among Bennu's many surprises, the particle ejections sparked our curiosity, and we've spent the last several months investigating this mystery", principal investigator Dante Lauretta explains.

Active asteroids can typically lose mass and eject particles through disintegration caused by unstable rapid rotations or ice sublimation - but that wasn't the case here. They eventually decided that OSIRIS-REx was safe and no countermeasures were necessary, allowing OSIRIS-REx to keep making up-close observations. And OSIRIS-REx's instruments have confirmed that hydrated minerals, including magnetite, are abundant and widespread on the asteroid. This finding suggests that heating may play a role in these outbursts, as surface temperatures on Bennu can vary by 180 degrees Fahrenheit (100 degrees Celsius) every 4.3 hours, the scientists said. When two of the events were observed occurring at the mid-latitudes of the asteroid, the temperature varied from 250 Kelvin to 400 Kelvin.

What could cause these events?

Some of the particles that escape the asteroid, only to fall back on it, could find their way to Earth. "This is the first view of an active asteroid this close up", study co-author Harold Connolly, a cosmochemist and chair of geology at Rowan University in Glassboro, New Jersey, and a sample scientist for the OSIRIS-REx mission, told Space.com. The mission aims to return a sample of Bennu's surface coating to Earth for study as well as return detailed information about the asteroid and its trajectory. Asteroids could even serve as fuel stations for robotic and human missions if we can unlock the hydrogen and oxygen inside them, NASA said. OSIRIS-REx is the third mission in NASA's New Frontiers Program, which is managed by NASA's Marshall Space Flight Center in Huntsville, Alabama, for the agency's Science Mission Directorate in Washington. Bennu probably broke off a larger asteroid in the belt between Mars and Jupiter a couple billion years ago.

"The probability of a Bennu impact maybe 1 in 2,700 today, but that will nearly certainly change, for better or worse - as we gather more data about its orbit". A direct hit is unlikely, but the data gathered during this mission can help determine the best ways to deflect near-Earth asteroids.

The asteroid could pass closer to Earth than the moon in 2135, with even closer approaches possible in 2175 and 2195.

Other reports by Click Lancashire

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