In English, dart means dart. And, Tuesday, September 27, at 1:14 a.m. (Paris time), NASA’s dart hit its target: the DART (Double Asteroid Redirection Test) suicide mission crashed into the small asteroid Dimorphos, 160 meters in diameter, itself a satellite of a larger asteroid, Didymos. All at 11 million kilometers from Earth.
During the hour preceding the collision, we were able to follow the approach of the double asteroid on the NASA channel thanks to Draco, the camera fitted to DART. Although the film was more like a slideshow, since only one frame per second was transmitted, you could almost feel transported to a spaceship. Fifty minutes before impact, when Dimorphos was still a pale speck, it was announced that the target was “locked”just like a fighter plane targeted by a missile.
Two trajectory corrections later, made by the autonomous navigation software, we could clearly distinguish Didymos and its diamond shape. It took until the final minutes for the oval of Dimorphos, whose shape scientists did not know before that, to begin to take shape on the screen. In the final seconds, when there was no longer any doubt that DART would hit its target, Draco sent more and more detail from a rocky surface. It occupied the entire field of vision, then nothing. By dying, the probe signed the success of its mission.
Next step, the Hera mission
The objective of DART, which struck Dimorphos at a speed of 22,000 kilometers per hour, is to slightly modify the trajectory of its target, in order to learn how to deflect an asteroid potentially dangerous for Earthlings. An army of telescopes is aimed at the Didymos-Dimorphos couple in order to measure how long the period of revolution of the second around the first will be modified. The results of these observations are not expected for several days or even weeks. It is also the time that will take the retransmission of the images taken by LiciaCube, an Italian nanosatellite accompanying DART, which, keeping itself cautiously at a distance so as not to risk being destroyed by rocks ejected during the impact, filmed the scene and its result.
That said, DART’s suicide mission is only Act I of the story. In 2027, the European Space Agency’s Hera mission (which will take off in October 2024) will scrutinize Dimorphos from every angle to fully analyze the consequences of the collision. As Patrick Michel, CNRS research director at the Côte d’Azur Observatory and scientific manager of Hera, explains, the latter “will give two sets of crucial information: the final result of the impact and all the internal properties of the target thanks to a radar. We will know if it is an aggregate or a monolith, its mineralogy, its geological properties, its mass.. All this information is crucial for modeling what happened on Tuesday and helping to understand how effective the impact of a craft is in deflecting an asteroid from its trajectory. In order to prepare the right dart for the day when the threat of a hypothetical big rock killer becomes real.