NASA will try to deflect the trajectory of an asteroid

Much more than large potentially dangerous pebbles, asteroids are small varied geological worlds that tell the origins of the Solar System, but they are still little known: it is to explore this “terra incognita” that the HERA probe will take off, in the wake of DART.

During the night of Monday to Tuesday, NASA’s DART mission will attempt to deflect the trajectory of an asteroid by colliding with Dimorphos, a small “moon” which orbits a larger asteroid, Didymos, located 11 million kilometers away. of the earth. This life-size experiment aims to reduce the duration of the orbit of the small asteroid around the larger one, to find out if humanity is capable of voluntarily modifying the trajectory of an asteroid which threatens our planet.

“Such a system of two asteroids is a perfect testbed for a planetary defense experiment, but it is also a completely new environment,” said Ian Carnelli, HERA mission manager for the European Space Agency (ESA).

Named HERA in homage to the Greek goddess of marriage, the European probe is due to take off in October 2024, for an arrival in 2026 on Dimorphos. Objective: return to the “scene of the crime” to assess the consequences of the impact of DART.

The deviation test will thus be fully documented, thanks to the information collected by HERA instruments (cameras, laser, high-resolution imagers, radar, etc.). Which will allow planetary defense experts to reliably feed models to extrapolate impact scenarios.

“A new world”

“We need to know the nature and composition of asteroids because, depending on the texture of the rock, they do not represent the same danger”, underlined Bhavya Lal, associate administrator of NASA, during the International Congress of astronautics this week in Paris.

Scientists expect to be surprised by the results of the investigations. Because “we ignore almost everything” about these celestial bodies, says Patrick Michel, principal investigator of HERA. “It’s a new world that we are going to discover”.

For this astrophysicist, asteroids “are not just boring pebbles in space, but small fascinating and complex geological worlds, with craters, basins, rock fields, particle ejections…”

But science struggles to understand these territories because on their surface, gravity is very weak compared to that of the Earth: the behavior of matter there is “totally counter-intuitive, we cannot base ourselves on images to know how asteroids behave, you also have to touch them”, explains Patrick Michel.

An example? A small explosion caused near the surface of the asteroid Ryugu (discovered in 1999) formed a crater of 15 meters, much larger than what the simulations predicted. And while the rock was supposed to be solid, “the surface behaved like a fluid upon impact, isn’t that surprising?”.

Go back in time

Binary systems like Didymos and its satellite Dimorphos represent about 15% of known asteroids and have so far not been explored.

With its 160 meters in diameter (the size of the Great Pyramid of Giza), Dimorphos will also be the smallest asteroid ever studied.

Shape, mass, chemical composition, internal structure, impact resistance, shape of the crater caused by DART: HERA’s instruments should reveal the secrets of Dimorphos. At the end of the mission, a micro-satellite will even land on its surface to measure how it bounces.

This unpublished documentation will also help astrophysicists to go back in time, asteroids being excellent “tracers of the history of the Solar System”, says Patrick Michel. These small rocky bodies have indeed kept in them the memory of the composition of the system and its planets, which were formed by collisions.

“Today, we are in an era where all solid surfaces in the Solar System have craters. To find the original scenario, we must understand what happens when two bodies collide”. Not in the laboratory, but on a real scale thanks to the DART-HERA couple, the scientists hope.

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