The Dart mission (dart, in English) took off in November from California. After ten months of travel, the spacecraft is due to collide with the asteroid Dimorphos at 23:14 GMT on Monday (1:14 am on Tuesday in France), at a speed of more than 20,000 km/h. The ship is no bigger than a car and its target is about 160 meters in diameter (half the height of the Eiffel Tower). Do not panic, Dimorphos does not pose a threat to Earth in any way: its orbit around the Sun passes only seven million kilometers from us at its closest point.
But “it’s important to complete the mission before we discover a real need,” said Andrea Riley, NASA’s mission manager. The moment of impact promises to be spectacular. It is not about destroying the asteroid but about pushing it slightly. The technique is called kinetic impact. Dimorphos is actually the satellite of a larger asteroid, Didymos (780 meters in diameter), which orbits in 11 hours and 55 minutes. The goal is to reduce Dimorphos’s orbit around Didymos in about ten minutes.
This change can be measured with telescopes on Earth. The goal may seem modest, but this demonstration is crucial for the future. The goal is to better understand how Dimorphos, representative of a population of fairly common asteroids, whose exact composition is not known, will react. The effect of the impact will depend largely on its porosity, that is, on whether it is more or less compact.
one frame per second
To hit such a small target, the spacecraft will steer autonomously for the last four hours, like a self-guided missile. Its camera, called Draco, will take at the last moment the first images of the asteroid, whose shape is still unknown (round, oblong, etc.). At a rate of one frame per second, viewable live on Earth with a delay of only about 45 seconds.
“It will start as a small point of light, until it fills the entire frame,” said Nancy Chabot of the Johns Hopkins University Applied Physics Laboratory (APL), where the control center is located. “These images will keep coming, until they stop,” she added, referring to the moment of the explosion. Three minutes later, a shoebox-sized satellite, called LICIACube and launched by the spacecraft a few days ago, will pass within about 55 km of the asteroid to capture images of the ejection. They will be sent back to Earth in the coming weeks and months.
The event will also be observed by the Hubble and James Webb Space Telescopes, which should be able to detect a bright cloud of dust. Then, the European probe Hera, which will take off in 2024, will closely observe Dimorphos in 2026 to assess the consequences of the impact and calculate, for the first time, the mass of the asteroid.
Very few of the known asteroids are considered potentially dangerous, and none will be in the next 100 years. But “I guarantee that if you wait long enough, there will be an object,” said NASA Chief Scientist Thomas Zurbuchen. About 30,000 asteroids of all sizes have been cataloged in the vicinity of Earth (they are called near-Earth objects, that is, their orbit crosses that of our planet). Every year around 3,000 new ones are found.