Lucy, a mission to study the Trojans

For French schoolchildren, Lucy evokes above all an Australopithecus, discovered in 1974 in Ethiopia, and who revolutionized knowledge about bipedalism. With “his” Lucy, the American space agency hopes for discoveries at least as exceptional, but on the solar system and a particular category of asteroids, the Trojans.

Very special asteroids

The launch of the space probe is scheduled for Saturday, October 16 from Cape Canaveral, United States, for a first meeting with a Trojan in August 2027, if all goes well. ” The Trojans are asteroids which follow the same trajectory as Jupiter but at 60 degrees on either side of the planet », Explains Patrick Michel, CNRS research director.

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If we represent the orbit of Jupiter by a circle, two swarms of asteroids also rotate on this circle, the first in front of the gas giant and the second behind. ” These asteroids never land back to Earth as a meteorite, he continues. Due to the distance (several hundred million kilometers, editor’s note) telescope observations remain very sketchy. You have to go see them. Lucy would have to get up to 400 kilometers from those rocks.

Indicators of a past upheaval

The Trojans do not form two homogeneous groups. Very diverse, they were probably born in different conditions and places and would then have been “brought back” to their current place. The presence of the Trojans signals past upheavals where they are.

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At the start of the solar system, over 4 billion years ago, Jupiter first emerges from the protoplanetary disk. Follow the other giant planets, Saturn, Uranus and Neptune. ” Then, the models indicate that instability must have taken place, describes Patrick Michel. As if the giant planets had started to dance and this dance had shaken the solar system “. In this great rout, small rocks approached, and became the Trojans.

The Lucy probe will fly over at least six, perhaps more if the trajectories allow it or if the mission is extended beyond 2033. Among its objectives: to analyze the surface, the geology and the composition of these asteroids, to verify the hypothesis of the planetary “dance”. ” We’re always looking to turn back time, indicates the head of the “theory and observations in planetology” team. The planets have lost the memory of the solar system because they have evolved. Asteroids, on the other hand, have remained small enough not to have been modified and provide access to this past time. »

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