Scientists reconstruct mission that deflected asteroid – 01/03/2023 – Science

In a batch of five scientific articles published in this week’s edition of the journal Nature, researchers around the world have just analyzed and reconstructed what happened when NASA’s Dart spacecraft collided with an asteroid in order to divert it from its natural course. —in the first practical test of planetary defense, in case one day we actually need to deflect one of these celestial bolides.

To begin with, and as previously announced by the American space agency, Dart (English acronym for Double Asteroid Redirection Test) had a much greater effect than the minimum expected when, on September 26 last year, it collided with Dimorfo , the small moon of Didymus (we are talking about an asteroid revolving around another, hence the probe’s name).

If all of the momentum brought by Dart were perfectly transferred to Dimorpho upon impact in what scientists call a perfectly inelastic collision (ie, where no one is deformed in the process), its orbital period after the collision would be changed by 7 minutes. .

What Cristina Thomas of the University of Northern Arizona (USA) and her colleagues reported in the first of the papers, however, was a much larger change: 33 minutes (with a 1-minute margin of error and 3-sigma confidence, the equivalent to 99.7%).

The result matches the preliminary calculations presented by NASA in October, which indicated an alteration of about 32 minutes.

The calculation reflects two different methods: monitoring successive “eclipses” of Dimorphus passing under Didymus’ shadow (which reflects a reduction in brightness, as it is not possible from Earth to distinguish the two objects with telescopes) and radar observation. (which allows you to reasonably monitor the relative position of the two). The result was the same in both cases.

And that, of course, is good news. The even larger-than-expected effect confirmed the broad viability of this kinetic impact strategy for deflecting an asteroid—simply hitting it to deflect it off course. And, more than that, it allowed scientists to understand exactly how the process would take place for the first time.

The second article on the list is led by Ronald Terik Daly, from APL-JHU (Laboratory of Applied Physics at Johns Hopkins University), the institute responsible for managing the mission, and reconstructs the step by step of the maneuvers that led to the collision and the exact location in which the probe crashed into Dimorpho—which could not even be known before it got close and could precisely self-guide for impact, sending photos of its target right up to the moment of the steamy encounter.

Dart was only able to “see” Dimorphus as a separate object 73 minutes before the collision, and the maneuvers defined by the on-board computer for the successful impact only began to be implemented 50 minutes before. With two-and-a-half minutes to go, it coasted on, sending full images up to 1.8 seconds before the end (developing 5.5 cm detail per pixel). A final partial photo was taken 0.855 seconds before impact, revealing surface detail at 2.6 cm per pixel.

The results allowed estimating the size of the asteroid at 151 meters (plus or minus 5). The impact occurred right “in the middle” of it, between two large rocks, at an angle of 73 degrees (plus or minus 7) with respect to the local horizon, at a speed of 6.14 km/s (translating to a more typical unit day to day, 22,100 km/h). It wasn’t a three finger kick; it was a spike.

When the collision occurred, it was anything but “inelastic”. A huge amount of debris was ejected away from the asteroid, as one would expect from an object that is not very cohesive, made of several pebbles held together weakly by the weak gravity of the star. And the plume of debris produced, transforming the star into an “active asteroid”, was the main topic of the third published study, led by Jian-Yang Li, from the Institute of Planetary Science, in Tucson, Arizona (USA).

Using the Hubble Space Telescope, the team analyzed the “coma” formed by the impact, at first dominated by the system’s gravity, but then influenced by the pressure of solar radiation to form a “tail” of dust, similar to that of comets. The results are valuable for understanding other asteroids that are going through a similar situation, derived from impacts not with spacecraft, but with other asteroids.

With a similar spirit, the group led by Ariel Graykowski, from the Seti Institute, in Mountain View, California, brought results obtained by amateur telescopes around the world, among them three on Réunion Island (located in the Indian Ocean and owned by France) and one in Nairobi, Kenya, who captured the flash of the exact moment of impact. The data made it possible to estimate the mass and energy of the dust raised by the impact, as well as its evolution, which will help to understand the outcome of other impact missions.

Finally, an article led by Andrew Cheng, also from APL-JHU, analyzed the dynamic effects of the impact, which reduced Dimorph’s orbital velocity by 2.7 mm/s (producing the measured period change), much more than than what would be expected from the impact alone, indicating that the ricochet of material ejected by the impact was responsible for a good part of the change in velocity and trajectory, which amplified the result between 2 and 5 times. “Therefore, Dart’s kinetic impact was highly effective in deflecting the Dimorph asteroid.”

All in all, just great news in case we ever need to “look up” when an asteroid is discovered on a collision course with us. The ones that are a kilometer or more (the ones that could end civilization) have pretty much all been discovered already, and none have our name on them, so to speak. But there are still many smaller objects, similar in size to the Dimorph, that remain unknown and would be capable of immense damage. It is up to astronomers to continue looking for them to remove once and for all the risk of being victims of this natural catastrophe.