小惑星の軌道、形状はDART衝突後に変化した(NASA Study: Asteroid’s Orbit, Shape Changed After DART Impact)

2024-03-19 NASA

＜関連情報＞

• https://www.nasa.gov/missions/dart/nasa-study-asteroids-orbit-shape-changed-after-dart-impact/
• https://iopscience.iop.org/article/10.3847/PSJ/ad26e7

DART衝突後の小惑星ディモルフォスの軌道と物理的特性評価 Orbital and Physical Characterization of Asteroid Dimorphos Following the DART Impact

Shantanu P. Naidu, Steven R. Chesley, Nicholas Moskovitz, Cristina Thomas, Alex J. Meyer, Petr Pravec, Peter Scheirich, Davide Farnocchia, Daniel J. Scheeres, Marina Brozovic,＋
The Planetary Science Journal  Published:2024 March 19
DOI:10.3847/PSJ/ad26e7

Abstract

The Double Asteroid Redirection Test (DART) mission impacted Dimorphos, the satellite of binary near-Earth asteroid (65803) Didymos, on 2022 September 26 UTC. We estimate the changes in the orbital and physical properties of the system due to the impact using ground-based photometric and radar observations, as well as DART camera observations. Under the assumption that Didymos is an oblate spheroid, we estimate that its equatorial and polar radii are 394 ± 11 m and 290 ± 16 m, respectively. We estimate that the DART impact instantaneously changed the along-track velocity of Dimorphos by −2.63 ± 0.06 mm s⁻¹. Initially, after the impact, Dimorphos’s orbital period had changed by −32.7 minutes ± 16 s to 11.377 ± 0.004 hr. We find that over the subsequent several weeks the orbital period changed by an additional 34 ± 15 s, eventually stabilizing at 11.3674 ± 0.0004 hr. The total change in the orbital period was −33.25 minutes ±1.5 s. The postimpact orbit exhibits an apsidal precession rate of 6.7 ± 02 day⁻¹. Under our model, this rate is driven by the oblateness parameter of Didymos, J₂, as well as the spherical harmonics coefficients, C₂₀ and C₂₂, of Dimorphos’s gravity. Under the assumption that Dimorphos is a triaxial ellipsoid with a uniform density, its C₂₀ and C₂₂ estimates imply axial ratios, a/b and a/c, of about 1.3 and 1.6, respectively. Preimpact images from DART indicate Dimorphos’s shape was close to that of an oblate spheroid, and thus our results indicate that the DART impact significantly altered the shape of Dimorphos.