Spinning Slower: How the DART Impact Changed Asteroid Didymos
The Double Asteroid Redirection Test (DART) made history in 2022 as the first mission to deliberately crash a spacecraft into an asteroid. While the main goal was to measure how the impact changed the orbit of the small moonlet Dimorphos, Ďurech and collaborators set out to explore a subtler question: did the impact also affect the rotation of the larger primary asteroid, Didymos? Their study uses two decades of brightness measurements, called light curves, to show that Didymos’s rotation actually slowed down by a tiny but measurable amount after the collision.
Background and Motivation
The authors begin by explaining why this investigation matters. Earlier work had focused on Dimorphos because its orbital period change directly reveals how efficiently the impact transferred momentum. But several theoretical studies predicted that the primary asteroid, Didymos, might also be affected. Because Didymos spins rapidly (once every ~2.26 hours), even small changes to its shape or balance could alter its rotation. Detecting such a change required careful separation of Didymos’s brightness variations from those of its companion, along with a clean dataset, particularly avoiding the dusty weeks immediately after the impact when ejecta contaminated the light curves.
Data and Methods
Ďurech et al. compiled light curves from six observing campaigns between 2003 and 2023, selecting only portions taken outside “mutual events,” when the two bodies eclipse or occult each other. They removed the small constant light contribution from Dimorphos and applied a technique known as light curve inversion, which uses changing brightness over time to reconstruct the asteroid’s shape and rotation period. By modeling Didymos both before and after impact, and even generating thousands of “bootstrap” versions of the dataset to test reliability, the team determined two independent rotation periods: a pre-impact period of 2.2603891 ± 0.0000002 hours and a post-impact period of 2.260440 ± 0.000008 hours. The difference corresponds to a slowdown of 0.18 ± 0.03 seconds, small enough to be invisible to the eye but statistically significant at the 6σ level.
Testing for Systematic Effects
To ensure this result wasn’t caused by modeling limitations, the authors tested for multiple sources of systematic error: uncertainties in Didymos’s spin axis direction, limitations of the convex shape model, contributions from Dimorphos’s irregular post-impact rotation, and possible long-term changes in spin rate. None of these factors reproduced a fake period change or erased the observed difference. The detailed plots (for example, Fig. 4 on page 6) highlight real mismatches between constant-rotation models and the data, supporting the conclusion that Didymos’s spin genuinely slowed.
Physical Interpretation
Finally, Ďurech and colleagues consider what physical process might account for the measured slowdown. The most plausible explanation is reshaping of Didymos itself. Because Didymos was already rotating near its structural stability limit, even slight disturbances, such as low-speed ejecta from the DART impact falling back onto its surface, could trigger small landslides or shifts in material. These would make the asteroid slightly more oblate, increasing its moment of inertia and therefore causing it to rotate more slowly. Their calculations show that the observed 0.18-second slowdown could result from just a 13-millimeter shortening of the spin axis and a 9-millimeter expansion of the equatorial radius, a subtle effect, but consistent with theoretical predictions.
Conclusion
In summary, the paper presents compelling evidence that Didymos’s rotation period increased shortly after the DART impact, likely due to minor reshaping of its surface. As more post-impact observations accumulate, astronomers will be able to refine this measurement and test whether Didymos continues to evolve. When ESA’s Hera mission arrives at the system in 2026–2027, it may even spot the tiny landslides or displaced material responsible for this delicate but scientifically rich change.
Source: Ďurech
