Momentum transfer from the DART mission kinetic impact on asteroid Dimorphos

Abstract

The NASA Double Asteroid Redirection Test (DART) mission performed a kinetic impact on asteroid Dimorphos, the satellite of the binary asteroid (65803) Didymos, at 23:14 UTC on 26 September 2022 as a planetary defence test1. DART was the first hypervelocity impact experiment on an asteroid at size and velocity scales relevant to planetary defence, intended to validate kinetic impact as a means of asteroid deflection. Here we report a determination of the momentum transferred to an asteroid by kinetic impact. On the basis of the change in the binary orbit period2, we find an instantaneous reduction in Dimorphos’s along-track orbital velocity component of 2.70 ± 0.10 mm s−1, indicating enhanced momentum transfer due to recoil from ejecta streams produced by the impact3,4. For a Dimorphos bulk density range of 1,500 to 3,300 kg m−3, we find that the expected value of the momentum enhancement factor, β, ranges between 2.2 and 4.9, depending on the mass of Dimorphos. If Dimorphos and Didymos are assumed to have equal densities of 2,400 kg m−3, $${\beta =3.61}_{-0.25}^{+0.19}(1\sigma )$$ β = 3.61 − 0.25 + 0.19 ( 1 σ ) . These β values indicate that substantially more momentum was transferred to Dimorphos from the escaping impact ejecta than was incident with DART. Therefore, the DART kinetic impact was highly effective in deflecting the asteroid Dimorphos.

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

“…Initial analysis of observations of the system reveals a reduction in the secondary's tangential (along-track) component of orbital velocity by about 2.7 mm s −1 (Cheng et al 2023). This corresponds to a momentum enhancement factor in the range of 2.2-4.9, depending on the unmeasured mass of Dimorphos, indicating the ejecta launched by the impact had a larger contribution to the change in momentum than the actual DART impact itself (Cheng et al 2023). The analysis by Cheng et al (2023) serves as a good first look into the post-impact dynamics of Didymos, which will be measured accurately in detail by the ESA Hera mission in 2027 (Michel et al 2022).…”

“…The impact altered the trajectory of Dimorphos around Didymos, reducing the orbit period by around 33 minutes (Thomas et al 2023). Initial analysis of observations of the system reveals a reduction in the secondary's tangential (along-track) component of orbital velocity by about 2.7 mm s −1 (Cheng et al 2023). This corresponds to a momentum enhancement factor in the range of 2.2-4.9, depending on the unmeasured mass of Dimorphos, indicating the ejecta launched by the impact had a larger contribution to the change in momentum than the actual DART impact itself (Cheng et al 2023).…”

“…However, Cheng et al (2023) only provides a high-level analysis of the impact-induced change in velocity and does not document fully the post-impact orbit dynamics. In this work, we expand on their analysis to characterize in detail the postimpact orbit by calculating the resulting changes to the system's mutual semimajor axis and eccentricity.…”

The Dimorphos Boulder Swarm

“…When viewed 90°from the axis and spacecraft direction (as at the time of impact in 2022 September), the edges of the cone appear as two "wings" separated by a large angle (e.g., panel (c) of Figure 3 in Li et al 2023, where this angle is reported to be 125°± 10°). (We note that the empirical cone axis differed from the impact direction by ∼21°, as mentioned in Cheng et al 2023, but this difference does not materially affect the discussion here.) Viewed along the axis of the cone, the ring would appear as a circular annulus while, for any other viewing direction, it would appear as an ellipse.…”

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.…”