After one o’clock in the morning next Tuesday, Spanish peninsular time, and around 6:00 p.m. on Monday in Mexico City, 11 million kilometers from Earth, a space probe the size of a refrigerator is going to collide head-on and at full speed with an asteroid 160 meters in diameter. It will be the first time that humanity tries to change the movement of a celestial body, a dress rehearsal to avoid future impacts that could wipe an entire city off the map.
Tom Statler, chief scientist for this NASA-funded mission, sums up his goal with a phrase from a movie: “We can’t let a rock from space plunge us back into the Stone Age.”
So far more than 95% of all asteroids larger than one kilometer in diameter are known; capable of causing a planetary catastrophe similar to the one that caused the extinction of the dinosaurs 66 million years ago. More worrying are the bodies of 140 meters or more, similar in destruction to a nuclear bomb and that would open a crater two kilometers in diameter. Of these, only 40% are known, which implies that there are thousands of unknown projectiles whose trajectory could cross our planet at some point. “The probability that you will live through an impact of an asteroid of this size throughout your life is about the same as the probability that your house will burn down,” explains Statler, an astrophysicist at NASA’s Planetary Protection Office, in a telephone interview. “If we have home insurance to avoid being homeless, why not be better prepared in case an asteroid comes?” The threat is so significant that in 2017 NASA approved spending 324 million dollars to make this mission a reality.
DART stands for dart in English and stands for Double Asteroid Redirection Test. It was launched in November 2021 and is currently heading to a perfect pair of asteroids to rehearse its plan. It is about Dídimus —twin in Greek—, a rock 780 meters in diameter, and its small moon Dimorpho —with two forms—, which completes one revolution around it every 11.9 hours.
The ship is equipped with an autonomous navigation system that will make it collide with the center of Dimorfo at about 22,000 kilometers per hour. DART weighs 570 kilos, about 10 million times less than Dimorph. But if all goes well, the impact will slow down the asteroid and make it fall slightly towards Didymus, so that its orbit will be shortened by a few minutes or, what is the same, it will deviate by about 15 meters, according to Statler. It may not sound like much, but it’s quite an achievement considering that no one on this mission knows what the asteroid looks like or what it’s made of or how hard it is because it’s so far away. To ground-based telescopes, this pair is just a tiny point of indecipherable light.
The DART camera is expected to reveal Dimorph’s appearance three-quarters of an hour before impact. At first it will be a mere pixel of light, but for the last 30 minutes it will grow larger and larger with each second, the rate at which the probe takes images and sends them back to Earth. In its last moments of life, the ship will show Dimorph’s surface in enough detail to distinguish stones the size of an orange. And then nothing. The first proof that DART has hit its first target—hitting the asteroid—will be signal loss. The silence will be captured by the control center, located at the Johns Hopkins University Applied Physics Laboratory (USA), and confirmed by NASA’s deep space antenna array. The impact is expected for Monday afternoon in America, early Tuesday in Europe.
Three minutes behind DART — about a thousand kilometers behind — is LICIACube, a small satellite developed by the Italian Space Agency. This toaster-sized device disengaged from the main probe a few days ago. After the crash, it will make a pass 55 kilometers from the surface of Dimorfo and then follow it for a few minutes. It’s unclear when the first images it takes of the crater and the cloud kicked up by the collision will arrive. “We hope to have them one or two days later at most,” explains Stavro Ivanovski, astrophysicist on the mission.
The trail of dust and dirt kicked up by DART will act as a booster rocket to help slow down the asteroid. The James Webb Space Telescope, Hubble and many ground-based observatories will point their lenses at the pair of astroids to try to capture the debris from the impact. The two bodies work like a high-precision clock. When Dimorfo passes in front of Dídimo he produces a small eclipse that happens with a perfect cadence. In the hours, days, or at most weeks after the collision, the telescopes will be able to confirm whether DART has succeeded in its second goal: changing the asteroid’s orbit.
At the Centro de Astrobiología (CAB) near Madrid, the suicide crash of DART has been simulated by shooting a two-centimeter plastic ball at a sand target. The speed and power are equivalent to a shot from a 44 Magnum, the revolver used by Clint Eastwood in Dirty Harry, explains Jens Ormö, head of the CAB’s Experimental Impact Laboratory. “We use these types of projectiles because they disintegrate completely after impact, which is what we think will happen to the probe if, as we suspect, Dimorpho is a fluffy asteroid and not very dense,” he details. “If this body were completely solid and collided with the Earth, it would cause a huge explosion in the atmosphere that could destroy the entire Community of Madrid,” warns the scientist.
Current models suggest that the most feasible way to avoid an impact from a medium-sized asteroid like this is to fire a probe at it. The characteristics of the ship would depend on the time available and the type of rock in question. All possible parameters have been modeled with computers. The data from the DART mission “will help to show whether these models are correct, so that we can adapt them if an asteroid really threatens the Earth in the future,” summarizes Isabel Herreros, CAB researcher.
Even if the mission is a success, it will take years or decades of technological development to be able to deal with this threat. An important milestone will be marked by the launch of Hera, a mission of the European Space Agency financed with 130 million euros. The probe will take off in 2024 and, two years later, it will reach Dídimo and what is left of Dimorfo to follow them for months. This will be the first spacecraft capable of accurately determining the mass, composition and internal structure of these two asteroids and will perform a detailed three-dimensional reconstruction of the crater left by DART. “We need tons of data to validate this technique for deflecting asteroids by impact, and Hera will be the mission that allows us to develop this new technology,” explains Ian Carnelli, head of the project.
The idea is that a system like this could be ready in about 30 years. If there were a threat, one or several impact probes could be designed in two years, “a very short time for the rhythms of the space industry,” says Carnelli. All of this would work if the asteroid came from the outer parts of the solar system. If it comes from the opposite side, in the direction of the Sun, we may not be able to detect it until it is already too late. This already happened in the winter of 2013, when a body fell on Siberia (Russia) causing more than 1,000 injuries due to shattered windows. It was barely 17 meters in diameter.
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