In October 2020, a small spacecraft made brief contact to bring a piece of an asteroid back to Earth. About two years after this event, scientists learned that if the contact time of the OSIRIS-REx spacecraft were extended a little longer, it would sink into the asteroid.
The asteroid Bennu did not have the structure that scientists had predicted. Rather than a solid, flying rock, the bennu consisted of small, pebble-like particles that were not strongly bonded together, creating too many voids on its surface. According to a new statement from NASA, the asteroid looked quite similar to a pool of plastic balls.
“Our expectations about the surface of the asteroid were completely wrong,” said Dante Lauretta, principal investigator of OSIRIS-REx and lead author of a recent paper describing the findings in more detail.
OSIRIS-REx arrived at the asteroid in December 2018 on a mission to collect a sample from Bennu and transport it to Earth for analysis. The spacecraft made contact with Bennu in October 2020, extending its robot arm with the aim of retrieving a piece of the asteroid. OSIRIS-REx then immediately started to move away from Bennu, firing its thrusters. The spacecraft’s sampling head touched Bennu’s surface for about 6 seconds before retracting. OSIRIS-REx was able to capture some material while venting dust and gravel on the asteroid.
This brief contact had a huge impact on Bennu, resulting in a chaotic gravel explosion and an 8-metre-wide crater. “Every time we tested the sampling procedure in the lab, it was pretty tough,” said Lauretta. It was only after examining the images from the actual sampling that the scientists learned the cause of the problem.
After analyzing the volume of debris seen in the before-and-after images of the landing site, scientists learned in a statement from NASA that OSIRIS-REx faced as much resistance from touching the asteroid as “one would feel when pressing down on a French-press teapot plunger.” So the spacecraft encountered very little resistance, which was absolutely unexpected from landing on a rocky object.
“If Bennu were completely full, that would mean almost solid rock, but we found a lot of voids at the surface,” said Kevin Walsh, a member of the OSIRIS-REx science team and lead author of a second paper on Bennu’s composition.
When OSIRIS-REx first arrived at the asteroid, close-up images of Bennu showed that its surface was strewn with chunks of rock rather than the predicted smooth sandy surface.
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