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NASA will attempt to do what science fiction has long imagined: redirect an asteroid’s orbit for the first time by crashing a ship onto its surface. A test of “planetary defense” that would 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 a ten-month journey, the spacecraft will hit the asteroid Dimorphos at 23:14 GMT on Monday 26 September at a speed of over 20,000 km/h.
The ship is no bigger than a car and its dimensions are about 160 meters in diameter, half the height of the Eiffel Tower.
Don’t panic, Dimorphos in no way represents a threat to Earth: its orbit around the Sun only passes seven million kilometers from us at its closest.
But the mission “is important to get to before we see a real need,” said Andrea Riley, who is in charge of the mission at NASA. The moment of impact promises to be spectacular and can be followed live on the American agency’s video channel.
It’s not a matter of destroying the asteroid, but of pushing it a little. The technique is called kinetic impact.
Dimorphos is actually the satellite of a larger asteroid, Didymos (780 meters in diameter), which it orbits in 11 hours and 55 minutes. The goal is to reduce Dimorphos’ orbit around Didymos by about ten minutes.
This change can be measured with telescopes from Earth, by observing the variation in brightness as 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 largely depend on its porosity, i.e. whether it is more or less compact.
To hit such a small target, the ship will steer autonomously for the last four hours, like a self-guided missile.
His camera, called Draco, will take the very first last-minute pictures of the asteroid, whose shape is not yet known (round, oblong, etc.). At a speed of one frame per second, visible live on Earth with a delay of only about 45 seconds.
“It will 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.
“These pictures will keep coming until they don’t,” she added, referring to the moment of the explosion.
Three minutes later, a shoebox-sized satellite called LiciaCube and released by the craft a few days ago will pass about 55 km from the asteroid to capture images of the ejecta, the fragments generated by the explosion. They will be sent back to Earth in the following 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 dust cloud.
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Then, the European Hera probe, due to take off in 2024, will closely observe Dimorphos in 2026 to assess the consequences of the impact and calculate the asteroid’s mass for the first time.
30,000 near-Earth asteroids
Very few known asteroids are considered potentially hazardous, and none will be within the next hundred years.
But “I guarantee that if you wait long enough, there will be an object,” said NASA’s chief scientist Thomas Zurbuchen.
Almost 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). There are around 3,000 new ones every year.
Those of a kilometer or more have almost all been seen, according to the researchers. But they estimate that they only know about 40% of asteroids that measure 140 meters or more—those capable of destroying an entire region.
If Dart misses its target, the ship should have enough fuel for another attempt in two years.
And if the mission succeeds, according to Nancy Chabot, it will be a first step towards a real defense capability. “Earth has been hit by asteroids for billions of years and it will happen again. Let us as humans make sure we live in a civilization where we can avoid it.”