“Planetary defense” mission: Dart, the NASA missile probe to attack an asteroid


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At the end of September 2022, somewhere in space, 11 million kilometers from Earth. A probe of the American Space Agency (Nasa), crashes at nearly 24,000 km / h on Dymorphos, a 160-meter-wide asteroid orbiting Didymos, another 780-meter asteroid. This is the mission of Dart (Double Asteroid Redirection Test), which must take off from the Vandenberg Air Force Base, California, United States, on November 24, 2021 at 7:21 a.m. KST.

“The impact will be monitored from Earth by radars and telescopes, which will determine whether the orbital period of Dymorphos around Didymos has changed, and therefore whether the probe has struck the asteroid and with what consequences”, explains Patrick Michel, CNRS research director at the Côte d’Azur observatory and member of the Dart team. In principle, the crash should not change the orbital speed of the asteroid by more than 1%, or a few millimeters per second. “But we don’t know what will happen exactly: if Dymorphos is very crumbly and hollow, it could be completely deformed!”, Confides the specialist. This is why the small 550 kg probe will drop a small satellite from the Italian space agency, LICIACube, just before impact. Trained by the speed of Dart, he will have only a few seconds to pass in front of the two asteroids and scan any debris in order to specify the nature of the impact. Do not panic, however, even if the experience turns sour, the trajectory of the main asteroid – Didymos – will not be changed. He will never threaten the Earth. Or not for thousands of years.

Defend the Earth, learn to shoot and mine in space

Dart’s objective is multiple. The first, worthy of a science fiction film, is stamped “planetary defense”. This is indeed the first mission to test our ability to deviate from its trajectory an asteroid at risk of crashing into Earth. The first technique, that tested with Dart, consists in sending a “kinetic impactor” on a target spotted years in advance. Thus, even a very slight deviation would allow the danger to be sufficiently removed over time. The other technique involves detonating a nuclear bomb near an asteroid so that the shock wave changes its trajectory. But the likelihood of pulverizing it into scarcely less dangerous pieces makes it an option of last resort, if it is discovered at the last moment or if the star is too big – more than a kilometer wide – for a kinetic projectile. .

La mission DART percutera le satellite de l'astéroïde (65803) Didymos. Les observations post-impact effectuées par des télescopes optiques terrestres et des radars planétaires permettront de mesurer le changement d'orbite du satellite autour du corps parent.

The DART mission will strike the asteroid (65803) Didymos satellite. Post-impact observations made by terrestrial optical telescopes and planetary radars will measure the change in the satellite’s orbit around the parent body.

NASA/Johns Hopkins Applied Physics Lab

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The risk of a huge asteroid colliding with Earth is low, but the consequences could be dramatic. The eradication of dinosaurs from the face of the Earth is proof of this. “The impact of a devastating asteroid on Earth is a bit like a pandemic: the probability is low, but the consequences are enormous: you have to prepare for it well before you need it”, underlines Patrick Michel . It is for this reason that NASA has undertaken, since 1998, to list all the near-Earth asteroids, that is to say those whose orbit intersects that of the Earth and which could, one day, strike us. In a little over 20 years, the American agency has already detected almost all of those more than 10 kilometers wide, the impact of which would cause a cataclysm from which humanity would not recover. Good news, none will threaten Earth for millions of years. The American agency has also detected more than 90% of those more than 1 km wide, which would cause a planetary catastrophe. Again, no danger for at least 100 years. But what about “little pebbles”? “In June 2005, the American Congress asked NASA to detect 90% of NEOs over 140 meters by 2020 … We are not there at all, underlines Francis Rocard, astrophysicist at the National Center for space studies. The figure is rather around 40%, as for those less than 140 meters wide, we detected only 0.15% “.

However, an asteroid of a few hundred meters could, depending on its speed and its composition – metal or rock – raze a city, a region or even a country. The explosion of an asteroid 15 to 20 meters in diameter over Chelyabinsk, Russia on February 15, 2013, triggered a meteor shower and a shock wave equivalent to 30 bombs from Hiroshima , which left more than a thousand injured. And on April 15, 2018, the asteroid 2018 GE3, whose estimated size was three to six times larger than that of Techliabinsk, passed 192,000 kilometers from Earth, half the distance that separates us from the Moon. . The American Space Agency (NASA) spotted it less than twenty-four hours before its passage, but this time would have been insufficient to organize the evacuation of the area concerned if its trajectory had crossed that of the Earth. If he had fallen on Paris, he would have razed the whole of the capital as well as its suburbs. “Things should nevertheless change significantly with the commissioning of the LLST telescope in November 2022 in Chile. It will have the particularity of having a gigantic focal plane of 3.2 billion pixels which will allow it to scan the entire sky. visible every three days, says Francis Rocard. It should detonate the detection of NEOs of all sizes – we expect it to find five million more – which is a small revolution in this area ” . In addition, NASA will launch, in 2025, the NEOSM (Near Earth Object Surveyor Mission), which will also have the mission of identifying all NEOS. Within 10 years, no more potentially dangerous asteroids should escape us.

Understanding the history of the solar system

It is also difficult not to see Dart as an industrial-military objective. After all, the probe takes off from an Air Force base and will behave almost like a missile. “All the automatic navigation of the probe is based on land-based military missile technologies, but applied to a planetary defense mission,” admits Francis Rocard. Enough to confirm their effectiveness in space, which will be useful in the event of an “Armageddon” scenario, but also in the event that, one day, humanity decides to mine space resources. However, the comparison stops there. The 300 million dollars that the mission costs are supported by the budget of the “planetary defense” office of NASA, and not by the budget of the ministry of defense. And neither the camera nor the algorithm used for navigation is classified defense secret, specifies Patrick Michel.

The other main focus of Dart is purely scientific. Indeed, the physics of the impacts of bodies is at the heart of the entire history of the solar system. “The planets were formed thanks to a phenomenon of accretion by collision, this phase was followed by giant impacts of which the Moon is probably one of the offspring, and finally of asteroid impacts, continues Patrick Michel. models retracing the collisional history of the Solar System are based on parameters attempting to answer the question: What happens when a collision occurs? However, for the moment our understanding of the phenomenon is based on numerical simulations validated by laboratory experiments on centimeter-sized targets that do not tell us if they remain valid at the scale of an asteroid “. And since Dart will not be able to answer everything either, the European Space Agency has planned another mission called Hera. This probe, equipped with two small satellites, will take off in October 2024 and will position itself around Didymos and Dymorphos at the end of December 2026. Originally, Dart was to be accompanied by the European probe AIM (Asteroid Impact Mission), responsible for filming operations live. But its funding was canceled in 2016.

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“We will arrive later, it’s a shame, but the scientific component of Hera will still be extremely rich”, positive Francis Rocard. Despite this 4-year delay, the European probe and its two small onboard satellites will be able to precisely measure the size of the crater, determine the physical properties of the two celestial pebbles and even probe the inside of the crater. “Dart is crime, Hera the detective. The whole is essential to bring our models to the right scale”, rejoices Patrick Michel, who is also the scientific manager of Hera and the coordinator of the Hera-Dart collaboration. . In other words, the data collected by the two probes will allow us to protect ourselves against a future threat by evaluating the necessary deflection forces according to the size of the cars, but also to better understand the history of the creation of our Solar System. .


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Frédéric Filloux is a columnist for L'Express and editor of the Monday Note.Frédéric Filloux

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CREDIT: LAURA ACQUAVIVAChristophe Donner

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Robin Rivaton, essayist, member of the scientific and evaluation council of the Foundation for political innovation (Fondapol).By Robin Rivaton

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