“We wondered if we hadn’t pulverized the asteroid! » The Dart mission told by Patrick Michel

On the night of September 26 to 27, 2022, the Dart probe voluntarily deflected the asteroid Dimorphos, as part of a planetary defense program. Futura gathered on this subject the words of one of the fathers of the mission, the astrophysicist Patrick Michel, who followed the collision live from the applied physics laboratory where the operations center is located in the United States.

It’s done, NASA has deflected an asteroid! Dimorphos was hit by the Dart probe on the night of September 26 to 27, when it had been advancing towards it for 10 months already. A mission planned for almost a decade, and whose success was very recently announced! But what does she portend for the future? Futura interviewed Patrick Michel, CNRS research director at the Côte d’Azur Observatory and pioneer in asteroid deflection, who works on the Dart and Hera missions, for which he is the scientific manager, on this subject.

“We wondered if we hadn’t sprayed Dimorphos”

Present since the beginning of the Dart project, and even one of its initiating fathers, Patrick Michel was in the operating room when the first images of the collision arrived. “As we witnessed the first images from the ground of Dimorphos, the dust ejected was such that we wondered if we had not pulverized the asteroid! » The following images finally revealed many ejecta, which also formed two tails of comets, announced by NASA on October 20. “The two ‘comet tails’ show that the impact was major. They are not surprising, but nevertheless constitute an important part of the result”, explains Patrick Michel.

But what matters most is the deviation of Dimorphos: its orbit has been shortened by 32 minutes. An exciting result, knowing that “73 seconds was the minimum for the deviation to work: this is the value obtained if only the momentum of the Dart probe was transmitted to Dimorphos, linked to its mass and its speedhe adds. Our simulations predicted between 10 and 15 minutes on average: 32 minutes is all the more a success! This shows that a large amount of material was ejected and added thrust”.

The mission is also reassuring, because “This is the first time that we have sent a probe at very high speed to impact at 23,000 km/h a body whose size we only know. The fact that the impact took place is already an achievement. » Indeed, the path to Dimorphos was “far from being a long calm river: the probe could have suffered a lot of damage, internal or external, before its arrival”he adds.

“With an asteroid alone, it would have taken months to observe any deviation”

But why this choice of Dimorphos? First, because it needed one observable from Earth: “This asteroid is a near-Earth object, but that’s not just why it was chosen: we wanted an asteroid close enough to be able to make measurements from the ground”, teaches us Patrick Michel. But also because Dimorphos, 160 meters in size, is in fact a moon from another body 800 meters in diameter, Didymos! Thus, its orbit, instead of being around the Sun, is around Didymos, so it lasts much less time, allowing almost live observations! In effect, “with an asteroid alone, it would have taken months to observe any deviation”, he adds. Didymos was thus chosen with its moon Dimorphos in 2011, knowing that it was passing near Earth at the end of September 2022, leaving eleven years to prepare for the impact.

As a result of this impact, much information remains to be deciphered, especially on the characteristics of Dimorphos, such as its structure and composition. They should not arrive before four years, because “The Hera mission will be launched in October 2024, and will arrive in Didymos in December 2026”regrets the CNRS research director at the Côte d’Azur Observatory, “because if the ESA delegations had approved the mission in time, it planned to arrive a little before Dart on Didymos, in order to make the measurements in the months following the impact”.

But fortunately, “even if we will be four years late there, the information collected at that time will have the same value and will be faithful to the result of the impact”. The Hera mission, of which Patrick Michel is the scientific manager, should have been launched earlier to follow the collision between Dart and the asteroid live, but will thus make it possible to collect valuable information, even four years later.

Other opportunities will arise to study near-Earth objects. Especially in 2029, when the asteroid Apophis comes! “In 2029, it will pass less than 30,000 km from Earth, and will even be visible to the naked eye! », concludes Patrick Michel. For the occasion, this asteroid of 325 meters and more than 40 million tons will be scrutinized from Earth and space, with the aim of studying the consequences of a passage so close to the Planet, in particular by the forces of tide. Finally, “the Hayabusa 2 mission will make a rendezvous with a small near-Earth object 60 m in diameter which rotates in 10 minutes on itself in 2031, making it possible to characterize an object of the size of the one at the origin of the explosion at over the Tunguska on June 30, 1908.”

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