Washington (AFP)

After four years of travel and observation, the American probe Osiris-Rex will descend on October 20 on the surface of the asteroid Bennu to take samples of its soil, in the hope of bringing them back to Earth and possibly of shed light on the formation of the solar system, NASA said Thursday.

The 490-meter-diameter asteroid had been carefully chosen for this mission because scientists believed, based on telescope observations, that it was covered in sand "like a beach," told a conference telephone Dante Lauretta, University of Arizona, scientific officer of the mission.

Such a surface would have guaranteed a safe sampling operation.

But upon arriving near the pebble in late 2018, footage showed it was actually covered in rocks.

"The surface is rough, rugged, rocky," the scientist described.

The team spent 2019 painstakingly mapping the surface to select the site that appears to be the safest: Nightingale Crater.

On October 20, the probe will slowly approach Bennu, then deploy a sampling arm and target an area eight meters in diameter, relatively flat, as wide as four parking spaces.

"Years of preparation and hard work by this team will boil down to this contact with the ground for five to ten seconds," said Mike Moreau, deputy project manager at NASA.

By touching the ground, the robot will blow nitrogen, which will make fly grains which will then be recovered by the arm of Osiris-Rex.

The goal is to collect at least 60 grams.

According to Mike Moreau, there is up to a 30% chance that the arm will not recover enough material, for example if the contact is on a large stone instead of finer grains.

Engineers over 320 million kilometers away cannot guarantee absolute precision.

A possible second attempt could take place in January in another crater.

In March 2021, Osiris-Rex will begin its long journey back to Earth.

She will eventually release the container containing the samples for a landing in the Utah desert, slowed down by a parachute, on September 24, 2023.

Before that, the Terrans are preparing to analyze samples from another asteroid, Ryugu, which the Japanese probe Hayabusa 2 visited last year.

The return to Earth of this dust is scheduled for December 6.

© 2020 AFP