It’s an Armageddon-like mission that NASA launched ten months ago. After traveling 11 million kilometers, the small Dart probe arrives at its destination and is about to make a destructive dive. At 1:14 am (Paris time) on the night of Monday to Tuesday, it will crash at a speed of 26,000 km / h on Dimorphos, a small asteroid 160 m long which would largely take the place of Concorde in Paris.
A life-size test of planetary protection
Dimorphos is actually the small moon of another body, Didymos, about a kilometer away. This double object does not threaten the earth, but it is the target chosen in 2010 for this first so-called planetary defense test. In fact, it passed close enough to Earth to be accessible to a spacecraft. Patrick Michel, astrophysicist at the Observatory of the Côte d’Azur, wants to be reassuring: Dimorphos does not threaten our planet. “What we are looking to do is deflect the trajectory of a small moon orbiting its main asteroid and not deflect the trajectory of an asteroid orbiting the sun“, he specifies. Clearly, it is a test to validate a concept and prepare future generations in the event that, one day, our planet would be really threatened. A major challenge as there are so many unknowns.
Like aiming for a coin from 10,000 kilometers away
It’s a journey into the unknown that Dart is about to finish. First, because no one has ever seen the asteroid in question. Astronomers have a vague idea of its size, but neither its precise shape, nor its mass, nor its composition are known. This “new world”, scientists and the general public will discover together and simultaneously. Indeed, Dart, only equipped with a camera and navigation software, will send its images as it approaches its objective. One hour before impact, the probe will automatically correct its trajectory, “heading by sight one might say”, specifies Aurélie Moussi, of the National Center for Space Studies. The images taken by the camera will be used for trajectory corrections. And if three seconds before the time of impact, the black images continue to reach us on the NASA TV site, it means that the probe is missing its target.
Simple deformation or destruction of the asteroid?
The shock should cause a crater 20 meters in diameter, according to Alain Herique, head of the Planeto team at the Institute of Planetology and Astrophysics in Grenoble. “There is a priori no global deformation of the expected body“, he explains while remaining cautious. “There are so many times when once you arrive there, you don’t understand anything you see… However, we believe that Didymos is the result of the collision between much larger parent bodies (about a hundred kilometres) and the debris from this collision re-accreted to form what are called aggregates, made up of blocks of several hundreds of meters down to grains of sand. It cannot be excluded that it is a monolith “.
Patrick Michel is also modest in his prediction, because two recent missions have led astrophysicists to completely review the physics of the collision. Depending on the level of resistance encountered, the Dart dive will eject more or less material. But not like on Earth. The low gravity of small asteroids does not cause a violent shock as shown by Hollywood, but rather a soft phenomenon, like a film in slow motion. The trajectory of Dimorphos will be modified anyway. “Two months after the impact, terrestrial telescopes will be able to measure this deviation“, specifies Alain Herique. But for Patrick Michel, the global deformation is not excluded. The deviation will in fact consist of a slowing down of the rotation of Dimorphos around Didymos. Instead of going around the large asteroid in 12 hours , the moon could after this impact take 12 hours and 1 minute and a half!
It is only in 2027 that Patrick Michel will have the details of the consequences of the collision thanks to Hera, the European mission in which he participates and which completes the work of Dart. Funded by the European Space Agency, this probe is equipped, among other things, with a radar which must reveal the structure of Dimorphos and visualize on the spot the crater and the modifications undergone by the asteroid. If the test is validated, it will not prevent other missions in the future. Because according to Aurélie Moussi, HERA project manager, “there are many other ideas: braking by solar veil, use of the thermal effect for example, but deflecting an asteroid by an impact indeed seems the simplest“to guard against it.