It was a breathtaking maneuver - which ended with a small moment of shock.

On October 20, 2020, NASA's Osiris-Rex probe descended to the surface of the approximately 500-meter-wide asteroid Bennu, which it had previously orbited for almost two years.

Using a robotic arm, she whirled up surface material with the help of a jet of nitrogen and caught it in a ring-shaped container.

However, subsequent camera recordings showed that some rocks blocked the lid of the container so that it could not close completely.

Parts of the sample then escaped into space.

Sibylle Anderl

Editor in the feuilleton, responsible for the "Nature and Science" department.

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NASA therefore canceled a rotation maneuver of the spacecraft, which should have served to determine the mass of the collected material.

The moments of inertia of the probe with an empty and full container would have been compared with each other.

The sample was instead stowed directly in a robust return capsule, which is scheduled to be dropped from Osiris-Rex to Earth later next year, before the probe itself will move on to the next asteroid, 99942 Apophis, under the new mission name Osiris-Apex.

So some patience is needed before the sample can be analyzed on Earth.

However, the scientists have already been able to derive a few clues about the contents of the capsule, as they currently report in "Science".

First, they analyzed the images of the escaped particles to estimate their size and speed - at least 55 grams of the sample was lost before stowage.

The escaped particles were on average just under a centimeter in size and weighed 30 milligrams.

dust on the optics

On the other hand, the scientists examined material that had stuck to the metal surfaces of the red upper arm.

In particular, the dust that had settled on the probe's optics could be evaluated with regard to its spectral properties.

These were similar to those of Bennu's surface material.

In particular, the dust also showed signs of strong chemical modification of the carbonaceous material by water - albeit in the distant past.

Previous studies had indicated that the bodies from which Bennu was formed may have contained liquid water.

Finally, the researchers were able to make up a mass estimate of the sample by analyzing the momentum transfer during movements of the robotic arm on the probe before and after sampling.

The result: Osiris-Rex was able to store between 150 and 350 grams of rock in the capsule.

The original goal of collecting at least 60 grams has thus been clearly exceeded.

left a crater eight meters wide

The traces left on the surface by the sampling observed during the probe's final flyby of the site of descent in April 2021 also revealed new insights into the nature of the asteroid.

The arm's contact with the surface for a few seconds and the engines firing to return to orbit left a crater around eight meters wide.

With the help of model calculations, the evaluation of the images of the crater showed that the rubble lay very loosely without any significant adhesive strength and had a significantly lower density than the asteroid as a whole.

According to this, around 6000 kilograms of rock were set in motion by the sampling - significantly more than had been expected.

The scientists suspect that the sample should not only contain material from the surface, but also material that was originally lower down and is therefore hardly affected by cosmic weathering.

In other words, the anticipation of receiving and examining the sample on Earth has continued to grow.