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Scientists publish their results from the impact of Dimorphos by DART

Seconds after impact
Seconds after impact. Click for movie, taken by amateur
astronomer Bruno Payet from the Réunion Island.

Scientists today published five papers outlining their results from the impact of Dimorphos by DART, summed up as follows:

  • Dimorphos’s density is about half that of Earth’s, illustrating its rubble pile nature.
  • The orbital period around the larger asteroid Didymos was changed by 33 minutes.
  • The ejection of material from Dimorphos during the impact had a greater effect on the asteroid’s momentum than the impact itself
  • The mass ejected was only 0.3 to 0.5% of Dimorphos’s mass, showing that the asteroid was not destroyed by the impact.
  • The impact turned Dimorphos into an active asteroid, with a tail like a comet.

The data not only tells us a great deal about this asteroid binary itself, it suggests that this impact method might be of use in defending the Earth from an asteroid impact. There are caveats however. First, the orbital change was not to the system’s solar orbit, the path that would matter should an asteroid threaten the Earth, but to Dimorphos’s orbit around its companion asteroid. We don’t yet know the effect on the solar orbit. Second, the impact did not destroy this small rubble pile asteroid, which means such an asteroid might still be a threat to the Earth even after impact. Third, in order for an impact to be the right choice for planetary defense, detailed information about the target asteroid has to be obtained. Without it such an impact mission might be a complete waste of time.

The irony to all this is that we knew all this before the mission. DART in the context of planetary defense taught us nothing, so NASA’s claim that this mission was to learn more about planetary defense was always utter bunkum. The mission’s real purpose was the study of asteroids, but selling it that way was hard. The sizzle of planetary defense however was a better lobbying technique, and it worked, even if it was dishonest.

That the press was also fooled by it, and continues to be fooled by it, is a subject for a different essay.

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  • Jeff Wright

    Now for a nuclear test.

  • john hare

    I disagree in the strongest terms that this test taught nothing about planetary defense. This was a sub scale test that measured a change in velocity around a parent body that would map to a solar orbit to some degree. It can be reasonably calculated what an impact on a solid body would accomplish. It is now realized that a rubble pile might be more easily deflected than a solid body.

    One of the suggestions i made at least a decade back was to map as many asteroids as possible with an eye to diverting a smaller one into a hazardous large one. This test indicated that it might be feasible with a smaller impactor than I expected.

  • Richard M

    I wouldn’t say we learned *nothing* in regards to use of kinetic impactors for planetary defense. But I think you could argue that this aspect of the mission was somewhat oversold.

    Clearly, though, there is much, much more to be done. And there is some urgency in doing it.

  • Michael Keehn

    >>>The irony to all this is that we knew all this before the mission. DART in the context of planetary defense taught us nothing, so NASA’s claim that this mission was to learn more about planetary defense was always utter bunkum

    I’m going to have to disagree. Planetary defense technology (being able to optically lock onto and guide a potential
    weapon to target using on board guidance) was a resounding success. That was a huge unknown prior to the mission. The impact itself, while it did not demonstrate anything *unexpected* really, does give us a path forward for future Earth defense technologies with practical confirmations of what we only *thought* we knew.

  • Michael Keehn: You will have to explain to me how this controlled impact was any different than any other of the innumerable planetary mission course corrections leading to precise targets, including the Deep Impact mission that hit a comet in 2005. And if it simply used different systems for achieving that impact, I will remain unconvinced. Asteroids have very predictable paths, even though there is some chaos involved in their orbits. We had the methods and technology for doing this decades ago (consider for example the Ranger lunar impacts in the 1960s). DART didn’t demonstrate anything new in doing it again.

    I am not unhappy that DART worked. Nor was I ever against the mission itself. I just dislike dishonesty. Its prime and real purpose was always asteroid research, not planetary defense, and just saying the latter over and over and over again will never make me believe it was true.

  • Richard M

    I do think the difference with Temple 1 was that there was never any prospect of an impact (of an impactor the size of Deep Impact, at any rate), altering the comet’s trajectory in any measurable way, given that Temple 1 is . . . 3.7 miles in diameter (7.2×1013 to 7.9×1013 kg in mass). Nor was there was any attempt to try measure any such change, even so. Whereas this was certainly in prospect with Dimorphos, given its very small mass. Granted, the results were within expected parameters . . . but this should still permit refinement of impact models.

    I suppose what I am really hoping for is that the actual, physical results of DART gets Congress to fund more ambitious asteroid deflection missions, and to fund NASA’s Planetary Defense Coordination Office at a higher level. NASA sort of dragged its feet on NEO Surveyor even despite Congress throwing more money at it for FY2023 (worse, its longer development timeline is increasing the cost even more!): we can’t deflect these things if we don’t know about ’em. Some more priority and urgency pressure on NASA HQ would not be amiss, either.

  • Star Bird

    Meteor Crater Arizona still the largests perfectly preserved meteor crater on Earth

  • Mark

    Does anyone know how long before we know what the solar orbit variance of this system is, if any? This would seem to be a rather important data point from a planetary defense standpoint.

  • Edward

    John hare,
    I agree with your first point, although not “in the strongest terms.” We now have a data point for impacting asteroids that have a large amount of rubble on the surface. We didn’t learn “nothing,” as Robert suggests, but I don’t think we learned a great deal. We now have a data point that does not give good information for precise knowledge of the affects of impacting other rubble asteroids, as they may be composed differently.

    I somewhat disagree with your second point, however. If smaller asteroids are not solid, then we do not know well enough how an impact would affect its orbit, so we could not easily navigate it into impacting a larger one. We just cannot rely upon the same momentum-change multiplier at another asteroid. A way to attack one asteroid could be to hit it with another. How feasible this is we will have to determine in the future.

    Robert has a point. The news media has not done its research into what we really learned from this impact and whether it can be applied to an actual killer asteroid. For the scientists to get a probe by selling it as something that it isn’t wasn’t just dishonest, it was using the philosophy of the ends justifying the means. The intentions may be good, but that road leads to a bad place.

  • John hare

    Much gentler means of changing the orbit of the smaller to impact the larger Would be necessary for the degree of control needed for the required accuracy. Gravity tractor, tow, solar sail etc. Beanstalk on the the small one use it its’ own mass and rotation for thrust. And so on.

  • Jeff Wright

    I have always had a dim view of gravity tractors—-they have to having looping Rosetta type matching trajectories—and that just eats into a spacecraft’s service life.

    A nuke allow a lot of mass/energy to be delivered to target in a fly-by trajectory. The Orion type atomic pulse units would replace the copper impactor disk in something like the Deep Impact bus.

    You want something like this at the ready:

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