Light curves and colours of the ejecta from Dimorphos after the DART impact

Abstract

On 26 September 2022, the Double Asteroid Redirection Test (DART) spacecraft struck Dimorphos, a satellite of the asteroid 65803 Didymos1. Because it is a binary system, it is possible to determine how much the orbit of the satellite changed, as part of a test of what is necessary to deflect an asteroid that might threaten Earth with an impact. In nominal cases, pre-impact predictions of the orbital period reduction ranged from roughly 8.8 to 17 min (refs. 2,3). Here we report optical observations of Dimorphos before, during and after the impact, from a network of citizen scientists’ telescopes across the world. We find a maximum brightening of 2.29 ± 0.14 mag on impact. Didymos fades back to its pre-impact brightness over the course of 23.7 ± 0.7 days. We estimate lower limits on the mass contained in the ejecta, which was 0.3–0.5% Dimorphos’s mass depending on the dust size. We also observe a reddening of the ejecta on impact.

The Dimorphos Boulder Swarm

“…The ejected mass is comparable to models of impact into a dense-packed field of 7 m sized boulders (panel (c) from Raducan et al 2022) and of the same order as the nominal 1.5 × 10 6 kg mass loss predicted by Fahnestock et al (2022). A slightly larger mass, 0.3%-0.5% of the mass of Didymos, was inferred from the diffuse material (Graykowski et al 2023) while a wider range, 0.2%-1.2%, was determined from submillimeter wavelength observations (Roth et al 2023). Since the boulders are traveling very slowly, they carry only a small fraction (∼10%) of the recoil-dominated momentum delivered to Dimorphos by the impact, and their contribution to the deflection of Dimorphos awaits consideration of the (currently unknown) angular distribution of the boulder ejection velocities.…”

“…The multiple spikes on the (saturated) image of Didymos-Dimorphos are the telescope diffraction spikes and CCD charge transfer trails of individual images rotated to bring the field of view into alignment. The most prominent feature of the image is the debris trail (see Graykowski et al 2023;Li et al 2023), which extends between the projected antisolar and projected orbit directions, and which is caused by the action of solar radiation pressure on centimeter particles. The 2022 December 19 composite also shows a set of point sources comoving with Dimorphos.…”

“…Dimorphos was impacted by the NASA DART spacecraft on UT 2022 September 26, resulting in the ejection of debris and the formation of a long, comet-like tail swept in the antisolar direction by solar radiation pressure (Graykowski et al 2023;Li et al 2023). The impact is scientifically interesting both as a way to study the mechanical response of a rubble pile to an energetic collision and as a well-characterized analog of certain active asteroids in which mass loss has been shown to result from impact (Jewitt et al 2010;Kim et al 2017).…”

The Perturbed Full Two-body Problem: Application to Post-DART Didymos

“…We center our sampling on the pre-impact value and draw from a half-normal distribution with a standard deviation of 0.1 m, ensuring we do not increase the radius. The standard deviation of the post-impact radius was chosen to approximately match the initial mass-loss estimate of Graykowski et al (2023), around 0.3%-0.5% of the assumed Dimorphos pre-impact mass.…”

Asteroid collision shows how much amateur astronomers have to offer

“…These skywatchers are among the authors of a study in Nature that describes how the asteroid, named Dimorphos, became temporarily brighter and redder as the spacecraft hit it 1 . One of five papers about the impact published in Nature [1][2][3][4][5] , it describes a real-time view of a cosmic collision -similar to that when Comet Shoemaker-Levy 9 slammed into Jupiter in July 1994.…”