Most of the orbital change came from the momentum carried away by debris. //

When the NASA DART mission slammed into a small asteroid, we knew with great precision how much the spacecraft weighed and how fast it traveled. If you combine that with our estimates of the motion and mass of its target asteroid, Dimorphos, then you could easily do the math and estimate how much momentum would be lost by the asteroid and what that would mean for its orbit. That bit of math would suggest that Dimorphos' orbit should end up roughly seven minutes shorter.

Instead, the orbit was shortened by a half hour—over four times that number. //

Today's issue of Nature contains five articles that collectively reconstruct the impact and its aftermath to explain how DART's collision had an outsized effect. And, in the process, the articles indicate that impactors like DART could be a viable means of protecting the planet from small asteroids. //

This shows that we currently have the technology needed to run an interception on a small asteroid without requiring elaborate reconnaissance in advance. And, as we've known for some time, the impact of the spacecraft can significantly shift the orbit of the asteroid. So, from the planetary-defense perspective, DART was a major validation.

Most of the remaining new information focuses on why the orbital shift was so much larger than a simple calculation might suggest. //

Eject!
Potential impact models had already indicated that there was an additional way that DART could influence the orbital momentum of Dimorphos. Because the asteroid is likely to be a "rubble pile" of material loosely held together by gravity, any impact was likely to send some of that material shooting off the surface of the asteroid. And all of that material would carry momentum of its own, directed away from the site of impact—which was located on the surface that faced toward Dimorphos' direction of orbit. So, the equal and opposite reaction to the ejecta would be a slowing down of the asteroid's orbit, which would be added to the effect of DART's impact.

The maximum expected change in the orbital period in these models was 40 minutes. Since the orbit changed by 30 minutes, this suggests that the amount of material sent off by DART's impact was on the high side of potential scenarios.