Mankind can defend itself against dangerous asteroids. This is the most important result of NASA's "Dart" mission: On September 26 last year, the 570-kilogram probe hit the asteroid moon Dimorphos - and changed its orbit as hoped. And much stronger than expected, as the US space agency Nasa announced a few days later. The research teams involved have now published detailed observations and analyzes of the "dart" impact in the journal "Nature". In particular, they provide an explanation for the surprisingly large change in Dimorphos' orbit.
The abbreviation "Dart" stands for "Double Asteroid Redirection Test" - on November 24, 2021, it made its way to the 780-meter asteroid Didymos. The small celestial body orbits the sun close to the earth's orbit and is therefore easily accessible for space probes.
More importantly, however, Didymus has a 160 meter moon with Dimorphos. And the orbit of Dimorphos around Didymos could be precisely measured both before and after the impact of "Dart". For a single asteroid, it would not have been possible to determine the effect of the collision with such great accuracy.
On September 26, 2022, the time had come: "Dart" hit the asteroid moon as planned at a speed of almost 24,000 kilometers per hour. "The dart impact was surprisingly effective in deflecting the trajectory of Dimorphos," write Andrew Cheng of Johns Hopkins University in College Park, Maryland, and his colleagues. The speed of the small moon has decreased by about ten meters per hour as a result of the impact.
That sounds like little, but it resulted in a significant change in orbit, bringing the moon closer to Didymos. This shortened its orbital period of almost twelve hours by 33 minutes. The researchers had expected a maximum change of ten minutes, even under optimistic conditions.
Using numerous telescopes on Earth and in space, the scientists closely observed not only the orbit of Dimorphos, but also the ejection of a debris cloud triggered by the collision, which quickly spread into space. In addition to the Hubble space telescope, many hobby astronomers around the world took part in these measurements. Overall, the team led by Ariel Graykowski from the Seti Institute in Mountain View, California, estimates based on this data that Dimorphos has ejected at least 0.3 to 0.5 percent of its mass into space - that's about 12,000 tons of rock.
In addition to these observations, Terik Daly of Johns Hopkins University and his team are providing an accurate reconstruction of the impact using the images back-to-earth from Dart. The impact occurred between two large boulders on the surface, one of which the probe touched immediately before impact.
Taken together, the data and analysis show that it was the recoil of the matter thrown into space by the impact that caused the astonishingly large change in orbit. "This ejection contributes significantly more to the change in orbit than the actual impact," summarize Cheng and his colleagues. And this knowledge is of considerable importance should an asteroid actually be on a collision course with Earth.
However, the effect of a targeted impact also depends on what a celestial body consists of and how it is structured. Therefore, the European space agency Esa, in cooperation with NASA, is planning to send another space probe into the Didymos Dimorphos system. The mission called "Hera" is scheduled to start in October 2024 and take a close look at the asteroid moon and in particular the impact crater of "Dart" with numerous instruments and two small nanosatellites.
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