Washington (AFP)

Mission almost accomplished: the American probe Osiris-Rex succeeded in securing in an airtight capsule the soil sample of the asteroid Bennu that it had collected last week, NASA announced on Tuesday.

His return to Earth is scheduled for 2023.

“We have successfully completed the operation,” said Rich Burns, project manager for the NASA side, in a conference call.

The collector arm of the probe had collected a large volume of dust and fragments during a contact of a few seconds last Tuesday with Bennu, but three days later, we learned that the valve of the collection compartment was unable to close. , and fragments escaped from into space, endangering the entire mission, launched in 2016.

In an emergency, the arm this week transferred its cargo in a slow operation (around 36 hours) into the capsule, attached to the center of the probe, and the capsule lid successfully closed, according to images from high quality transmitted by Osiris-Rex.

At that distance, each step took two hours, according to Sandra Freund of Lockheed Martin, as each message took 18.5 minutes to reach Earth, and the team wanted to verify each step before ordering the next.

Osiris-Rex will leave from around Bennu in March 2021, with a landing of the capsule scheduled for September 24, 2023 in the Utah desert in the United States.

The unanticipated valve forced NASA to cancel a weighing operation for the sample.

So scientists don't know exactly how many Bennu particles the probe collected, but they can guess.

Dante Lauretta, head of the mission, estimates that "tens of grams" were lost because of the leak.

But according to him, there remains at least 400 grams secure, according to the images, and probably a lot more.

"We are probably over a kilogram," added Lauretta.

He said the sampling found that the asteroid was covered with a layer of several meters of "cohesive" particles, due to the very low gravity, comparable to a ball pool.

Osiris-Rex probably sank 48 cm into the ground upon contact, without encountering any resistance, and if it hadn't turned on its thrusters to go back the other way, "we probably would have gone to through the asteroid, ”said Dante Lauretta.

“There is almost no force that holds the grains together,” he explained.

If an astronaut tried to step on Bennu, "he would sink to his knees or even deeper," the scientist said, fascinated, adding that the data would allow all the asteroid geology models to be recalibrated.

© 2020 AFP