The final descent through Earth's atmosphere promises to be perilous, but the US space agency hopes it will result in a smooth arrival, around 09:00 local time (15:00 GMT), on a military zone usually used to test missiles.

After liftoff in 2016, the Osiris-Rex probe collected pebbles and dust from asteroid Bennu in 2020.

Some 250 grams of material (+ or - 100 grams), according to NASA's estimate, which should "help us better understand the types of asteroids that could threaten the Earth", and illuminate "the very beginning of the history of our solar system", said the head of the space agency, Bill Nelson.

"The return of this sample is truly historic," NASA scientist Amy Simon told AFP. "This will be the largest sample we bring back from the moon rocks" of the Apollo program, concluded in 1972.

But before accessing the precious cargo, the maneuver to be executed is "dangerous", she acknowledges.

The Osiris-Rex probe must release the capsule containing the sample four hours before landing, more than 100,000 km from Earth.

The OSIRIS-REx mission returns to earth © Jonathan JACOBSEN, Philippe MOUCHE / AFP

During the last 13 minutes, this capsule will pass through the atmosphere: it will enter at more than 44,000 km/h, with a temperature rising to 2,700 ° C.

The fall, observed by army sensors, will be slowed by two successive parachutes, which it is essential that they deploy correctly to avoid a "hard landing".

It could be decided at the last moment not to release the capsule if it appears that the target area (58 by 14 km) will be missed. The probe would then go around the Sun, before trying its luck again in 2025. But if its delivery goes well, it will set off for another asteroid.

Two Japanese samples

Once the capsule is on the ground, a team equipped with gloves and masks will check its condition, before placing it in a net, then lifted by a helicopter and carried to a temporary "clean room".

The capsule should be exposed to the sand of the American desert for as short a time as possible, in order to avoid contamination of the sample that could disferee subsequent analyses.

The laboratory where samples of asteroid Bennu collected by the Osiris-Rex probe will be analyzed, July 24, 2023 in Houston, Texas © Mark Felix / AFP / Archives

On Monday, it will be flown to the Johnson Space Center in Houston, Texas. This is where the box will be opened, in another airtight room. The process will take days.

NASA is planning a press conference on October 11 to unveil initial results.

The majority of the sample will be retained for study by future generations. About 25% will be immediately used for experiments, and a small portion will be shared with partner Japan and Canada.

Japan had even given NASA some grains from the asteroid Ryugu, of which it had brought back 5.4 grams in 2020, during the Hayabusa-2 mission. In 2010, he reported a microscopic amount of another asteroid.

This time, Bennu's sample is "much bigger, so we're going to be able to do a lot more analysis," Simon said.

History of our origin

Asteroids are composed of the original materials of the solar system, 4.5 billion years ago. Unlike Earth, they remained intact.

Image taken by NASA's Osiris-Rex spacecraft, December 2, 2018, showing asteroid Bennu © HO / NASA / Goddard / University of Arizona / AFP / Archives

So they hold "clues about how the solar system formed and evolved," Melissa Morris, head of NASA's Osiris-Rex program, told a news conference. "This is the story of our own origin."

By hitting our planet, "we think that asteroids and comets brought organic matter, potentially water, that helped life develop on Earth," explained Amy Simon.

Scientists believe that Bennu (500 meters in diameter) is rich in carbon, and contains water molecules enclosed in minerals.

The asteroid also surprised scientists: its surface turned out to be less dense than expected during the collection of the sample. The probe arm had sunk, much like in a ball pool.

Image from a NASA video showing the robotic arm of the Osiris-Rex spacecraft making contact with asteroid Bennu to collect samples, October 21, 2020 © Handout / NASA TV/AFP/Archives

A better understanding of its composition could prove useful in the future.

There is a small risk (1 in 2,700) that Bennu will hit Earth in 2182, a collision that would be catastrophic. But NASA managed last year to deflect the trajectory of an asteroid by hitting it.

© 2023 AFP