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NASA will attempt to do what science fiction has long imagined: throw an asteroid off course for the first time by crashing a spacecraft onto its surface. A test of “planetary defense”, which should allow humanity to be better protected against a possible future threat.
The test is imminent. The Dart mission (dart, in English) took off in November from California. After ten months of travel, the spacecraft will collide with the asteroid Dimorphos at 23:14 GMT on Monday, September 26 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 accomplish the mission before we discover a real need,” said Andrea Riley, NASA mission manager. The moment of impact promises to be spectacular and can be followed live through the video channel of the American agency.
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, observing the variation in brightness when the small asteroid passes in front of the large one.
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.
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’s going to start with a small point of light, until it fills the entire frame,” said Nancy Chabot of the Applied Physics Laboratory (APL) at Johns Hopkins University in Maryland (East), where the control center is located. .
“These images will keep coming, until they stop,” he 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 55 km of the asteroid to capture images of the ejecta, the fragments generated by the explosion. 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.
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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.
30,000 near-Earth asteroids
Very few known asteroids are considered potentially dangerous, and none will be in the next hundred 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 the Earth (they are called near-Earth objects, that is, their orbit crosses that of our planet). Every year there are about 3,000 new ones.
Those of a kilometer or more have been sighted almost all, according to scientists. But they estimate that they only know about 40% of the asteroids that measure 140 meters or more, those capable of devastating an entire region.
If Dart doesn’t hit its target, the ship should have enough fuel for another try in two years.
And if the mission is successful, it will be a first step toward a real defense capability, according to Nancy Chabot. “Earth has been hit by asteroids for billions of years and it will happen again. As humans, let’s make sure we live in a civilization where we can avoid it.”