The Dart probe which struck Dimorphos had a more considerable efficiency than the researchers thought, who were able to study the surface of the star.
by Pierre Barthélémy
It was five months ago, on the night of September 26 to 27, 2022. 11 million kilometers from the earth, the Dart (Double Asteroid Test Redirection) probe of NASA, launched at more than 22,000 kilometers per hour, intentionally struck Dimorphos, a small satellite asteroid of a larger asteroid, Didymos. The aim of this experience: to see if an impactor after all modest (580 kilos) could deviate from its trajectory an object of several million tonnes, with the idea of learning to protect our planet from a future “asteroid killer”. The positive answer fell two weeks later. But the astronomers have obviously gone further than this simple yes and learn from Dart in a burst of five articles published by nature Wednesday 1 er Mars.
The choice of Dimorphos was not left to chance. Not only is this object small (approximately 150 meters in diameter) and accessible, but astronomers have observed the couple they have formed with Didymos for twenty years. They therefore knew with precision the period of Dimorphos revolution around his neighbor before the meeting with Dart: eleven hours and fifty-five minutes. After the impact, this period was reduced by thirty-three minutes, which indicates that the event has compared the asteroid of Didymos.
In relation to the preliminary results revealed in October 2022, several new features appear in the five articles of nature, explains Patrick Michel who co -signed three. “First of all, we give an estimate of the quantity of movement which was transmitted to Dimorphos, which really measures the effectiveness of the maneuver,” insists this asteroid specialist, CNRS research director at the Observatory of the Côte d’Azur.
multiplier effect
The researchers, who expected a reduction in the revolution period of about seven minutes, were surprised to see that the impact had been very effective. While reaching Dimorphos, Dart did not break into it as in plasticine. On the contrary, the shock has ejected a lot of material into space. Due to the action-reaction principle, the elements expelled in one sense have pushed Dimorphos in the other direction, which amplified the deviation of a factor at least equal to two. This multiplier effect could be even more important but the researchers, ignoring the exact density and mass of dimorphos, remain cautious.
“The second interesting aspect, continues Patrick Michel, is that we offer a model of form of dimorphos, from 2D images that Dart transmitted during his approach. Dimorphos has a form of smarties, flattened , while we expected rather an elongated form, a bit like a rugby ball. “Finally, the analysis of the photographs confirms the great geological diversity of Dimorphos, whose surface seems entirely covered with rocks of all sizes, more or less buried. If this is different from what the Japanese probe Hayabusa saw in 2005 on the asteroid Itkawa, where we distinguished smooth areas made of gravel, the surface of Dimorphos recalls many of those of Ryugu and Bénou, asteroid recently visited by the Japanese and American probes Hayabusa-2 and Osiris-Rex. This suggests that the internal structure is not monolithic but would rather similar to a heterogeneous aggregate.
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