It is a historic success for NASA: For the first time, mankind has changed the course of a celestial body. About two weeks after the impact of the darts-A probe on the asteroid moon Dimorphos, analyzes using ground-based telescopes have revealed that its orbit around its central body Didymos has shortened significantly, much more than expected. Before hitting the darts-Probe took Dimorphos eleven hours and 55 minutes for one round. According to NASA, it is now only eleven hours and 23 minutes, more than half an hour less.
Before the approximately $330 million asteroid defense mission, it was difficult to predict how big the impact would be because you didn’t know the composition and consistency of the target. If Dimorphos – which has never posed a threat to Earth either before or after the impact – was a maximally hard boulder, the probe could only have transmitted its own momentum upon impact. However, even with the high impact speed of more than six kilometers per second, a closet-sized probe alone has only a limited effect on an asteroid that is about the size of a large Egyptian pyramid – a frontal impact without a crater would have the orbital period of only around 73 seconds shortened.
But Dimorphos appears to be a lot less like a solid block of rock than expected. Before the impact, NASA experts had assumed that the probe would leave a decent crater. According to the calculations, the material thrown out should distract Dimorphos far more than the probe alone, with an orbital time reduced by a few minutes.
The first pictures had shown a huge cloud of dust
But shortly after the impact, the images of the accompanying probe, which was also part of the Dart mission Liciacube showed that the impact left enormous traces: A huge cloud of dust was created. “It was surprising that so much material was apparently thrown out by the impact,” says Stephan Ulamec from the German Aerospace Center (DLR).
That was probably one of the reasons why it took so long to precisely measure the change in the orbital period. In principle, this is relatively easy with ground-based telescopes, because each time Dimorphos passes by, the sunlight reflected by Didymos darkens somewhat, so that the periodic fluctuation can be used to read the orbital period. But because of the large cloud of dust, it was obviously a little harder to see than usual.
The fact that one orbit now takes a good half hour less than before impact is extremely exciting for the researchers involved. “Dimorphos appears to be very loosely bound, more like a pile of rubble than a homogeneous boulder,” says Ulamec. Although it is not known whether this is the case with all smaller asteroids, Dimorphos may be an isolated case. “But asteroids that have already been visited by probes, such as Bennu or Ryugu, look similar. One can assume that deflecting their orbit through an impact would also work better for them than initially assumed.” This is basically good news for protecting Earth from an asteroid on a collision course.
Presumably, says Ulamec, a correspondingly large crater was also formed. However, we will only know for sure when the European Hera mission, which is scheduled to start in 2024, reaches the asteroid system in 2026 and can take new pictures of Dimorphos.
“This mission shows that NASA is trying to be prepared for whatever the universe throws at us. NASA has proven that we take defending the planet seriously,” said Space Agency chief Bill Nelson. He called the mission’s success a “watershed moment” in protecting humanity from an asteroid impact.
Currently, among the approximately 10,000 known near-Earth asteroids with a diameter of more than 140 meters, there is none that could hurtle directly towards Earth in the foreseeable future. But there are countless others that only came with new telescopes like the Vera Rubin Observatory in Chile or the Neo Surveyor should be detected in space. It is all the more relevant to know what to do in the event of an emergency.