We’ve had several posts on September 26th’s DART mission—the one in which NASA crashed a small spacecraft into the asteroid Dimorphos at 14,000 miles per hour. The object was to perturb Dimorphos’s orbit around a larger asteroid, Didymos. (DART stands for “Double Asteroid Redirection Test”.) The perturbation was effected by transferring momentum from the DART spacecraft (which crashed in a satisfying cloud of dust) to Dimorphos.
The ultimate goal of this program is to see if we can deflect a comet or asteroid heading towards Earth, staving off the immense destruction that a collision could cause. And, judging by DART, it’s at least possible.
As usual, my old friend and former NASA employee Jim Batterson gives us the details:
Earth Global Defense Test Results (DART Experiment)
Jim “Bat” Batterson
When the NASA/APL (Applied Physics Lab) spacecraft successfully impacted the small asteroid, Dimorphos on September 26, some WEIT readers wanted to know when we would know if it achieved its full mission – an actual perturbation of Dimorphos’ orbit.
In the post-impact press conference later that day, the mission engineering leadership estimated that the answer would come in a couple of weeks or so as Earth-based telescopes took careful measurements of Dimorphus’ orbital path around its larger companion asteroid, Didymos. They were right! Yesterday afternoon, NASA held a press conference at NASA Headquarters in which mission leaders gave us the answer: the orbit of Dimorphos around Didymos changed significantly – from 11hrs 55min to 11hrs 23min – a 32 minute change.
Here’s the full press conference, an hour long:
The first 30 minutes comprises what I thought was a very informative presentation from three lead project scientists; the final 30 minutes consists of the scientists answering questions from the global press. They explain in pretty good detail how the orbital change was measured and what these results mean.
Here’s one last video that APL [Johns Hopkins Applied Physics Lab, which partnered in the venture] put out on a summary sheet. The 40-second video compresses the final pics from the DART spacecraft and is really exciting to me. The final frame before blackout due to collision is only 51 ft across, which means the bigger boulders are about ten feet across and the visible small rubble is a foot in size or even smaller. Incredible technology.
Click on the screenshot below to go to the summary sheet and video. This is the moment before impact:
Sub
Sub
This success throws a wrench at Bay’s Armageddon.
Yeah, why’d they have to go to all that trouble? But DART wouldn’t have made much of a movie! 🤣
Thanks for the update Jim (and PCC(E))!
Nice presentation- thanks! But they didn’t do a great job of explaining the physics of the orbital period change, twice ignoring a question about the reason for the effect of the ejecta. Presumably it is mainly an “equal and opposite reaction” effect, but continual loss of mass might have an effect too. I have a hard time seeing how the weak force of gravity holds together such a small heap of rubble at all. I also wonder about the origin of the rocks- can a geologist look at them and say something about how or where they might have been formed, or what types of conditions would be required, or even the nature of the rock itself?
The combined momentum of ejecta can increase the momentum imparted on the target object by the impact event as a whole, not just the impactor alone. The effect is quantified by a factor β, having values between ≲1 to ≈5.
Without ejecta, only the impactor’s momentum affects the target orbit. But if ejecta are thrown up and away from a crater, they will “carry away” additional momentum. For a reasonably technical explanation, see: https://www.jhuapl.edu/FeatureStory/200723-predicting-the-unpredictable-DART-kinetic-impact and referenced publications.
The key quote is: “Estimating the beta factor is kind of the holy grail,” and NASA may not be ready yet to quantify the effect of the ejecta, even knowing the change in orbital period.
Note that momentum is still conserved for the initial impact as such. But it has the secondary effect of heating up near-surface rocks such that they subsequently explode. Their matter thrown up acts the same as a rocket engine would that is aimed down into the asteroid, hence, β > 1.
Thanks Michael. Very helpful. I expect that, over time, as they are developed, preliminary analysis results will be published at DART conferences or workshops as well as working papers and journal articles. Also, as pointed out in the press conference, planetary defense made it into the 2022 Decadel Survey which should help frame follow-on work.
Thanks, Michael! So is it fair to say that beta was the real unknown here and that the purpose of this experiment was not to test Newtonian physics but was to see what we should expect beta to be in a typical case?
I think that one of the panelists points may have been that there is no “typical” asteroid structure – at least not the one that they can plan on being the one that will hit Earth – but rather a range of structures that we must be prepared to defend against. He said that this is a single data point that can be used to anchor future simulations and then added to if there are further experiments. If I understood correctly, they would like enough surveillance warning time to be able to send out a pilot ship to determine the asteroid structural parameters and using that information, better custom-design the impactor tactics.
Newtonian physics is “right” at this macro scale and needn’t be tested, but the real-world closeness of these types of collisions to billiard balls (chacterized by the beta parameter if I understood correctly) is being tested. At least that is my understanding of what I heard.
As Jim B. noted, the “typical case” may be elusive, but overall, the key point is indeed to narrow the range of β values that might occur, in order to help us select orbital parameters for an actual deflection mission to be most effective. Thank you for putting it so well.
BTW, any secondary “blow-up” effects leave room for Bruce Willis et al.😉: A subplanted explosive device would not only break up a single large object into less damaging smaller fragments (the movie scenario), but also perturb the orbit of large remnants (actually also in the movie, if I recall). The trick for such a mission is — as it always has been — to not make things worse by inadvertently steering a NEO closer into an earth-bound trajectory keyhole.
Yeah, that assertion needs to be tested. It’s probably true for fairly small impacts – such as the Barringer/ Meteor Crater one. But comparing that to the areas’ plethora of nuclear weapons test craters – all considerably smaller – does challenge the assertion that scattering that body into several dozen impacts over a few % of the Earth’s surface (say, the area of the contiguous USA) would be less damaging than taking one hit.
When you get to bigger impacts – such as the “dinosaur killer”, distributing the impact over the whole facing hemisphere would still have the whole sky glowing at a red heat … which wouldn’t be a “good hair” day for anyone. Getting to larger impacts – such as the impact deposit sequences preserved in the Archean of South Africa, depositing tens of metres of gravel-sized ejecta spherules in a single event from an impact on an unconstrained location – it is even less clear that replacing one localised impact with dozens or thousands of of smaller ones would be less devastating, at least on a “human” scale.
(There was some work a while ago examining the effects of big impacts on the oceans ; it actually turned out that it took a really big impact to actually boil the oceans “dry” – in which case the atmospheric pressure would have gone up by a factor of several to several-10s, raising the boiling point of water at “sea level” up into the mid-150s (°C) … bad news for a humanoid ; not necessarily such bad news for an extremophile bacterium. It’s far from definitive – but it is an interesting point of view.)
Speaking of celestial bodies, here are fascinating new computer simulations of the origin of the Earth’s moon:
The paper: Immediate Origin of the Moon as a Post-impact Satellite: https://iopscience.iop.org/article/10.3847/2041-8213/ac8d96
The videos:
https://www.youtube.com/watch?v=AQmeomxvokM