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.

Fig. 1. eVscope observations of the impact, ejecta and tail.

a–f, The initial ejecta plume from DART’s impact into Dimorphos as observed from L’Étang-Salé, Réunion. a, Didymos before the DART impact. Scale bar, 100′′. b–f, Didymos after the DART impact, where b is roughly 6 min, c is 9 min, d is 14 min, e is 19 min, and f is 23 min after panel a. Each panel a–f is comprised of a stack of 11 4-s exposures. The compass and image scale in a applies to b–f as well. The fast-moving ejecta plume moves eastwards on the plane of the sky and dissipates over time, from a–f. g,h, Two tails (solar and antisolar directions) developed from the ejecta produced by the DART spacecraft roughly 113.7 h after impact into Dimorphos. The image in h is a zoomed-in version of the image in g. This image is a median stack of 1,205 4-s exposures as observed from Nagahama, Japan. The two light-blue arrows mark the two tails visible to an eVscope as visual aids. Scale bar, 70′′.

Fig. 2. Apparent magnitude of Didymos before, during and after impact.

The light curve of the Didymos binary system on 26 September 2022 during the DART spacecraft’s impact into Dimorphos as observed by three citizen astronomers located on Réunion Island using eVscopes. The dotted lines are the measured apparent magnitude before impact (bottom) and after the fast-moving ejecta dissipated after the impact (top). The shaded regions represent the standard deviation on the value of the dotted lines, and error bars represent sky background noise. Source data

Fig. 3. Fading of Didymos after impact.

The absolute magnitude of the Didymos system faded over time after the brightening due to DART’s impact into Dimorphos. The solid blue line is a weighted, linear fit to magnitudes measured from just after impact on 26 September to 25 October 2022, after which measurements were consistent with the resting absolute magnitude. Some measurements between 15 and 25 October overlap the resting magnitude at the roughly 1σ level but others remain above it. Therefore, we consider the fading time may range between around 18 and 28 days after impact, with our best-fit model providing a fading time of 23.7 ± 0.7 days. The value of the resting absolute magnitude is calculated from the pre-impact average apparent magnitude plotted in Fig. 2. The error bars and shaded region represent the 1σ measurement uncertainties. Before 4 October 2022, there are two outlying observations that resulted in measurements that were too faint due to poor weather conditions, and we therefore do not include these points in the fitted line. Source data

Fig. 4. Colours of Didymos before, during and after impact.

a–c, Measured B-G (a), G-R (b) and B-R (c) colours of Didymos over time as observed from L’Étang-Salé, Réunion. Error bars represent the noise introduced by the sky background magnitude. It is apparent in a and c that the Didymos binary system became redder directly after the DART spacecraft struck Dimorphos and that when the fast ejecta dissipated, the colours returned to their original colours. The G-R colours in b show no significant change in colour during the time of impact. Source data