During the historic collection event, the sampling head of the OSIRIS-REx spacecraft sank 0.5 meters into the asteroid’s surface. Apparently, Bennu’s exterior is made up of loosely packed particles that aren’t very tightly bound together based on what happened when the spacecraft collected a sample. If the spaceship Had he not fired his thruster to retreat after the rapid accumulation of dust and rocks, he might have sunk straight into the asteroid.
Bennu is a debris and asteroid shaped like a spinning top, made up of rocks held together by gravity. It is about 500 meters wide.
“If Bennu were fully packed, that would mean almost solid rock, but we found a lot of void space in the surface,” said study co-author Kevin Walsh, a member of the OSIRIS-REx science team at Southwest Research Institute in Boulder , Colorado , in a statement.
So what would have happened if the spacecraft’s thrusters hadn’t ignited immediately?
“It may be that OSIRIS-REx would have penetrated deeper into the asteroid, which is both intriguing and frightening,” said study co-author Patrick Michel, an OSIRIS-REx scientist and research director at the Center National de la Recherche Scientifique on the Côte d’ Azur Observatory in Nice, France.
Bennu defies expectations
When the spacecraft arrived in Bennu in December 2018, the OSIRIS-Rex team was surprised to find that the asteroid’s surface was covered in boulders. Previous observations had prepared them for a sandy, beach-like terrain.
The scientists also observed particles from the asteroid being released into space.
“Our expectations of the asteroid’s surface were completely wrong,” study author Dante Lauretta, principal investigator of OSIRIS-REx at the University of Arizona, Tucson, said in a statement.
The spacecraft took pictures of where it had taken a sample of Bennu, further confusing the team. Although OSIRIS-REx touched the asteroid very gently, it kicked up a huge amount of rocky debris and left a crater 8 meters wide.
“What we saw was a huge wall of debris radiating out from the sample site,” said Lauretta, Regents Professor of Planetary Sciences and Cosmochemistry at the University of Arizona’s Lunar and Planetary Laboratory. “We said, ‘Holy Cow!’ Every time we’ve tested the sampling procedure in the lab, we’ve made very little difference.”
Before and after photos of the landing site show the dramatic difference. The images show what appears to be a depression in the surface, with several large boulders at its base. The sampling event itself likely caused this sunken landform. The asteroid’s dark surface also has more reflective dust near the collection point, showing where rocks were moved during the event. These changes can be seen in the slider below.
Analyzing the spacecraft’s acceleration data, the team found that they encountered very little resistance, about as much as someone might feel when pushing the plunger of a French coffee maker.
Understanding more about Bennu’s composition can help scientists study other asteroids, whether the goal is to plan missions like OSIRIS-REx or to protect Earth from possible collisions with space rocks.
An asteroid like Bennu, barely holding itself together, could break apart in Earth’s atmosphere, which could pose other risks even if it’s not a direct hit.
“We need to continue to physically interact with these bodies, as this is the only way to truly determine their mechanical properties and their response to external events,” Michel said. “Images are crucial, but they don’t give us an answer as to whether they are weak or strong.”
OSIRIS-REx – which stands for Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer – was the first NASA mission to be sent to a near-Earth asteroid, and once there performed the closest orbit of a planetary body through a spacecraft that you give Bennu is the smallest object ever orbited by a spacecraft.
The Bennu spacecraft souvenir is the largest sample collected by a NASA mission since Apollo astronauts brought back moon rocks.
As soon as OSIRIS-REx approaches Earth in 2023, it will eject the capsule containing the sample, which will shoot through Earth’s atmosphere and parachute into the Utah desert.
If OSIRIS-REx is still in good health after dropping the sample, it will embark on a new expedition to study other asteroids.