When the NASA DART mission slammed into a small asteroid, we knew with great precision how much the spacecraft weighed and how fast it was traveling. If you combine that with our estimates of the motion and mass of its target asteroid, Dimorphos, you can easily calculate and estimate how much momentum will be lost by the asteroid and what that will mean for its orbit. That bit of math suggests that Dimorphos’ orbit should end up about seven minutes shorter.
Instead, the track was shortened by half an hour – over four times as much.
Today’s issue of Nature contains five papers 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.
It’s easy to forget the role software played in DART’s success. We did not send anything out to the Didymos/Dimorphos binary system to reconnoitre the site first; instead, NASA just launched DART with one camera and some navigation software that had a rough idea of what to expect. When the software took over, the camera couldn’t even resolve the tiny Dimorphos, and instead simply kept DART moving toward Didymos for about 25 minutes until the smaller asteroid came into view.
Despite Dimorphos being about 150 meters across on its longest axis, we know where DART hit it to within 68 centimeters – and that this location is close to the ideal location to maximize the transfer of momentum from the spacecraft to the asteroid. The onboard camera also provided detailed images of the impact site up to seconds before it was destroyed. Combined with knowledge of the spacecraft’s orientation, this provides a wonderfully detailed description of the impact:
The spacecraft approached the asteroid with its solar arrays tilted slightly toward the surface. The leading edge of the +Y solar array contacted the surface of boulder 1, and this solar array hit boulder 1 directly. Almost immediately afterwards, the -Y solar array grazed boulder 2, with the leading edge of the -Y array contacting the surface near the base of boulder 2 in a downward direction. Finally, the spacecraft bus hit between boulders 1 and 2.
This shows that we currently have the technology needed to run an intercept on a small asteroid without requiring elaborate reconnaissance beforehand. And, as we’ve known for some time, the impact of the spacecraft could significantly change the trajectory of the asteroid. So from a planetary defense perspective, DART was an important confirmation.
Most of the remaining new information focuses on why the orbital shift was so much larger than a simple calculation might suggest.
Potential impact models had already indicated that there was an additional way that DART could affect the orbital momentum of Dimorphos. Because the asteroid is likely to be a “rock pile” of material loosely held together by gravity, any impact was likely to send some of the material shooting off the surface of the asteroid. And all this material would carry its own momentum, directed away from the impact site – which was located on the surface facing the direction of Dimorphos’ trajectory. So, the equal and opposite reaction to the ejection would be a slowing of the asteroid’s trajectory, which would be added to the effect of DART’s impact.
The maximum expected change in orbital period in these models was 40 minutes. Since the trajectory changed by 30 minutes, this suggests that the amount of material sent by DART’s impact was at the upper end of potential scenarios.
However, it is complicated to determine exactly what happened with this ejection. We don’t have an accurate measurement of the density of Dimorphos or Didymos, so we can’t put a number on the lost momentum. This means that we cannot know exactly how much material was ejected. What we can do instead is put some constraints on different figures and build a model that allows us to put upper and lower numbers on some of those numbers. So much of the new work focuses on observations of the ejected material to get some of the constraints needed.
Some of that data came from the Hubble Space Telescope, which tracked the debris plume for the first eight hours after the impact and revisited the material several times afterward. It captured a diffuse cloud of dust in the immediate aftermath, which later resolved into a cone-shaped cloud. Originally, this cloud was composed of fast-moving material escaping the gravity of the Didymos/Dimorphos system. But within a couple of days there was a transition to slower moving particles that bent under the influence of gravity.
This resolved itself into two main streams of material. Due to the orientation of the asteroids, one of these streams was directed almost directly at the Sun, which exerts radiation pressure on the particles in the stream. Within a few days this pressure completely reversed the direction of this stream. The second stream also appeared to have a more fan-shaped profile, although no one is sure why this occurred.
Another important source of data came from users of a commercial digital telescope – one of the papers credits dozens of authors as “Unistellar Citizen Scientist.” These citizen scientists included five individuals who were well positioned to capture the first aftermath of the impact. Together, these amateur astronomers tracked the debris for over three weeks until it fell back to pre-impact levels and together provided additional limits on the total mass of the ejecta. Estimates from these images suggest that the fast-moving ejecta carried away about a third of the energy from DART’s collision.
Where we are
An added bonus of this work is that the characteristics of the ejecta are very similar to the “tails” of what have been called active asteroids. These were thought to be produced by collisions, but no one has ever seen a collision occur. Now that we have it, we are more confident that the active asteroids are the product of a similar process.
For more details on Dimorphos, we will probably have to wait for the arrival of the European Space Agency’s HERA mission, which is expected to launch next year and then rendezvous with the asteroids to provide a more detailed understanding of the system. Meanwhile, we have a much stronger sense that if a small asteroid is discovered that poses a threat to collide with Earth, we can do something about it. The big remaining question is whether the kind of enhanced redirection caused by this debris plume should be expected as a general feature of debris pile asteroids or whether there might be significant differences in the ejection depending on which asteroid is hit.
Nature, 2023. DOI: 10.1038/s41586-023-05810-5 and four associated articles. (About DOIs).