HomeScienceAstronomers learn from first asteroid deflection test

    Astronomers learn from first asteroid deflection test

    It was five months ago, on the night of September 26, 2022. At 11 million kilometers from Earth, NASA’s Double Asteroid Redirection Test (DART) was traveling at over 22,000 km/h and intentionally hit Dimorphos, a small asteroid satellite of a larger one, Didymus. The goal of this experiment was to see if a relatively light (580 kilos) object could deflect an object weighing several million tons from its trajectory. The bigger idea was to learn whether we could protect our planet from a future “killer asteroid.” The answer, which was affirmative, came two weeks later. But astronomers have obviously gone further than that simple “yes” and have drawn lessons from DART in a flurry of five papers published by Nature on Wednesday, March 1.

    The choice of Dimorphos was not random. Not only is this object small (about 150 meters in diameter) and accessible, but astronomers have been observing the pair it forms with Didymos for about 20 years. They therefore knew exactly how long it took for Dimorphos to orbit its neighbor before the encounter with DART: 11 hours and 55 minutes. After the impact, this period was reduced by 33 minutes, which tells us that the event brought the asteroid closer to Didymos.

    Compared to the preliminary results revealed in October 2022, there are several additional pieces of information in the five articles in Nature, said Patrick Michel, who co-authored three of them. “First of all, we give an estimate of the amount of motion that was transmitted to Dimorphos, which really measures the effectiveness of the maneuver,” said the asteroid specialist, director of research at the French National Center for Scientific Research Côte d’Azur Observatory.

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    The researchers, who expected a reduction in the period of revolution of about seven minutes, were surprised to find that the impact was very effective. When it hit Dimorphos, DART did not sink into it like clay. The impact ejected a lot of material into space. Because of the action-reaction principle, the objects expelled in one direction pushed Dimorphos in the other direction, which amplified the deflection by a factor of at least 2. This multiplier effect could be even larger but the researchers, not knowing the exact density and mass of Dimorphos, remain cautious.

    “The second interesting aspect,” said Michel, “is that we came up with a model for the shape of Dimorphos, based on the 2D images that DART transmitted during its approach. Dimorphos has a flattened spherical shape, whereas we expected it to be elongated, a bit like a rugby ball.” Finally, the analysis of the photographs confirms the great geological diversity of Dimorphos, whose surface seems to be entirely covered with rocks of all sizes, more or less buried. While this is different from what the Japanese probe Hayabusa saw in 2005 on the asteroid Itokawa, where one could distinguish smooth areas made of gravel, the surface of Dimorphos is very similar to those of Ryugu and Benou, asteroids recently visited by the Japanese and American probes Hayabusa-2 and Osiris-Rex. This suggests an internal structure that is in no way monolithic, but rather one which is a very heterogeneous aggregate.

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