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As in a video game, the Dart probe hit an asteroid: it is the first planetary defense mission

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As in a video game, the Dart probe hit an asteroid: it is the first planetary defense mission

The dot in the center of the screen grew larger and larger until it showed the pair of small asteroids. Frame by frame has come to fill the full screen of images transmitted by NASA’s Dart probe, and so we have seen everything happen like in a video game, or a movie. Until the happy ending. Hit.

Dart hit the mark: for the first time a man-made object hit a celestial body with the aim of changing its orbit in a measurable way. A successful shooting from millions of kilometers away. It was the goal of the American Double Asteroid Redirection Test (Dart, in fact) mission, which took off on November 24, 2021 and which crashed, in the Italian night, against a small asteroid orbiting the Sun in the vicinity of the Earth. When it was a quarter past one in the morning in our country, Dart threw herself at full speed against Dimorphos, the tiny moon that accompanies a larger asteroid, Didymos. The first planetary defense mission in deep space was successful: it is a test to understand, if and when we will spot an object that threatens to hit the Earth, how to give it a push to deflect it.

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In diretta it meets con the asteroid

NASA had announced it: “We too will see for the first time, together with the public, the images transmitted by Dart from 11 million kilometers away”. And so it was: on the screens of the Mission Control Center at the Laboratory of Applied Physics (APL) of Johns Hopkins University and on those of the American Space Agency, images taken from Dart’s only eye, the Draco camera, appeared simultaneously in live streaming. A dot that about an hour and a half before impact resolved into two objects. Here for the first time Didymos and Dimorphos, from a distance never reached, for the first time they appeared as they are. And then Dart, with its refined autonomous navigation system, it has adjusted its course by itself to aim for the small satellite of the asteroid. Within 3 minutes of impact all the engineers at APL lifted their hands from their keyboards, there was nothing left for them to do but watch Dart meet his fate. He continued to broadcast photos, one every second, showing Dimorphos up close, tiny world, of dimensions of the great Pyramid of Giza, mass of rocks and dust. Now we know how it’s done thanks to those latest images. Dart swooped down like a car-sized half-ton eagle, with the wingspan of the solar panels nearly 20 meters at a speed of 6.6 kilometers per second. Over 23,000 kilometers per hour. A bang equivalent, according to studies, to about 3 tons of TNT. Then everything went dark. The signal was interrupted, further confirmation that everything went well.

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Dart is destroyed, NASA’s Deep Space Network antennas will no longer receive anything from her. But to observe everything was there Liciacube, the Italian witness of this historic mission. Built by the Turin-based Argotec for the Italian Space Agency, Liciacube is a cubesat the size of a microwave, took off together with Dart, a couple of weeks ago it detached from the mother probe to fly with her. A few minutes before impact, she positioned herself in such a way as to shoot everything from a safe distance. In the next few hours we will see if with its two chambers, Luke and Leia, managed to retrieve the cloud of debris that is expected to have been projected by the collision. And so have a first estimate of the impact effect.

Liciacube is the first all-Italian interplanetary probe, projected into deep space and destined to continue traveling in the dark after resuming the consequences of the impact. It orientates itself with an autonomous navigation system capable of recognizing the objectives towards which it must aim. It will make the overflight unaided from Earth, passing about 55 kilometers away. It will do it all by itself. A bit like his sister Argomoon, also out of the Argotec factories, ready in the launch light for act as supporting actor in another historical mission of NASA: Artemis. For the extraordinary event, there were also two special observers among the spectators. The most powerful space telescopes have turned their eyes to Didymos and Dimorphos: Hubble and James Webb. Lucy also turned in that directionthe mission of NASA as it travels to Jupiter and its Moons.

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How will we understand if it worked

Liciacube images are not enough. The Didymos binary system will be a special sight especially in the coming weeks. See both asteroids from Earthin fact, it is not an easy undertaking. They are very small even for the most powerful telescopes. However, they have a peculiar characteristic, which is also the reason why they were chosen: together they constitute a binary system with eclipses. It means that, observing them from Earth, their brightness changes regularly when one of them passes in front of the other. And it is precisely by measuring, with the observations of telescopes, how this period will change, which corresponds to the period of revolution of Dimorphos around Didymos, which astrophysicists will be able to calculate by how much Dimorphos’ orbit has changed. And hence the effects of the kinetic test.

This will be one of the crucial tests for the Dart mission and to evaluate the planetary defense system. In fact, very little is known about Dimorphos. If it were to be a hard, monolithic asteroid, the blow would displace it a certain extent, carving a small crater, bringing it closer to Didymos and increasing its orbital speed. If, on the other hand, it should be what in the jargon is defined as rubble pile, that is a heap of rubble, a heap of rocks, dust and ice that is not very coherent, held together by the very low force of gravity, it would be a much less dense object. The material thrown back by the impact would have a propelling effect, accelerating it and bringing it much closer to Didymos. All this will be seen, first of all, by the frequency with which the eclipses of the binary system increase. To understand how it went, “it will be necessary to wait a few days or weeks – underlined Tom Statler of NASA – because this small difference in the period of the eclipses will become more marked as the days pass”.

And then there is Hera, the European mission that will take off in 2024 to reach Didymos and Dimorphos in 2026. He’ll get things done, but he’ll bring with him many tools to understand, like an investigator, what happened. Starting with the shape and size of the crater left by Dart. Hera will remain in orbit around the pair of asteroids for months, she will have time to study the composition of the small satellite, analyzing with special camera equipped with spectrometers the materials revealed by the crater. It will be information of great scientific value, because asteroids are remnants of the formation of the Solar System. But the main purpose will be to accurately assess how badly Dart did Dimorphos, how hard his shoulder was.

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Not if, but when

There are no known asteroids, large enough to threaten the Earth, on a collision course in the next century. However the history and the scars our planet bears say that sooner or later it will happen again. According to NASA, we know pretty much all those big enough to cause a planetary disaster (to get an idea, the asteroid that exterminated the dinosaurs ago was about 10 kilometers in diameter). But there are others, at least a few hundred meters large, that is, like Dimorphos, which still escape observation and which however, they could cause regional or continental damage. Raising tsunamis as high as mountains, and wiping out millions or billions of lives. We know about two thirds of them. Something could therefore come out of nowhere and make us tremble. That’s why it’s best to be ready.

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