Two days ago, Jim Batterson, an old friend and college classmate who worked for NASA for many years, wrote a precis of the DART mission designed to knock a small asteroid out of its orbit around a bigger asteroid. He and I were two of the many who watched the “near live” impact of the spacecraft on Dimorphos, and we were both thrilled. Here’s the last 18 seconds before impact:
Here, from Facebook, is a telescopic view from Earth of the moment the spacecraft hit Dimorphos (credit: Atlas Project):
After I mentioned to Jim that this was the first time we tried to perturb an orbit with a spacecraft, he sent me his thoughts on the mission and reminded me of a few instances of other impacts or landings on celestial bodies, though these didn’t have the same goal as DART. With his permission I submit Jim’s thoughts for your approval.
Some Post-DART Mission Thoughts
Jim Batterson
Now that the excitement and really spectacular success of the DART asteroid rendezvous and impact is in the rearview mirror, I want to remind readers of some earlier space missions that were akin to DART in an engineering sense.
Much of what NASA does in space missions is “engineering in the service of science”. That is, NASA scientist and engineers, in consultation with scientists from around the world, are responsible for managing and assuring the design of appropriate missions, building (often unique) scientific instruments, and developing the rockets and spacecraft to fly those instruments to a point in space (sometimes on a planet or simply in a planetary atmosphere) where collected data is returned for analysis to scientists on Earth. Comet rendezvous missions are traditionally “pure science” missions to gather unique data on the composition of comets, as it’s thought that a better understanding of comets will lead to a better understanding the formation and early years of our solar system.
DART was a different type of mission, not only in its focus on an asteroid rather than a comet, but also in that it kept its engineering mission for planetary defense pretty clean and did not try to add in a large suite of scientific instruments to gather data. That said, there have been several comet flybys over years and two comet impacts
The impacts were these:
Deep Impact by NASA/Jet Propulsion Lab (JPL). Upon rendezvousing with the Comet Temple-1 on July 4, 2005, the Deep Impact spacecraft launched an impactor craft that smashed into the comet’s nucleus. The debris cloud and crater were then photographed and analyzed by instruments on the mother ship. Here’s a NASA photo with this caption:
This spectacular image of comet Tempel 1 was taken 67 seconds after it obliterated Deep Impact’s impactor spacecraft. The image was taken by the high-resolution camera on the mission’s flyby craft. Scattered light from the collision saturated the camera’s detector, creating the bright splash seen here. Linear spokes of light radiate away from the impact site, while reflected sunlight illuminates most of the comet surface. The image reveals topographic features, including ridges, scalloped edges and possibly impact craters formed long ago.
The Rosetta/Philae mission was carried out by the European Space Agency (ESA) in 2014. Upon rendezvous with Comet 67P after an almost eleven-year journey through space, the Rosetta spacecraft launched a lander (Philae) whose purpose was to soft-land on the comet and drill into its surface, gathering data on the comet’s interior. ESA was partly successful as Philae did make it to the surface. But some of its propulsion equipment failed to operate properly to slow it down completely, so it suffered a hard landing and bounced, leaving Philae in a crevice. This prevented it from accessing sunlight needed to recharge its batteries. Sadly, the subsurface data couldn’t be collected.
However, it did survive to take some incredible photos on the actual surface of a comet nucleus. I think that this was still an extraordinary accomplishment. You can see the photos here.
[JAC: I’ll present one example, a fantastic selfie taken by Philae as it sat in the crevice. The caption is
“The Philae lander of the European Space Agency’s Rosetta mission is safely on the surface of Comet 67P/Churyumov-Gerasimenko, as these first two images from the lander’s CIVA camera confirm. One of the lander’s three feet can be seen in the foreground. The view is a two-image mosaic taken on Nov. 12, 2014.”]
DART didn’t even pretend to be about collecting basic science data, but rather was about using engineering expertise developed over our spacefaring years to help the protect our planet. This was an important proof of a “concept mission” in which the concept was that, if an asteroid were given a proper nudge, its trajectory could be changed enough to move it from a collision course with Earth to a harmless close encounter. This was a test or experiment to gather engineering data. It is a high-school physics problem to show that it works in theory, but would it work in the real world?
I think that one of big engineering challenges for the DART test was navigating and guiding using the light from the two-asteroid system and then, a few minutes before scheduled impact, to autonomously focus on and keep the smaller, dimmer asteroid as its target. This was especially challenging because the engineers had no real knowledge of the reflective characteristics or shape of either body. It turned out, as we all saw in the final couple of minutes of approach, that both bodies were non-spherical with lots of surface irregularities, which scatter sunlight in all directions and cast shadows. But the software had been designed with enough robustness to deal with these non-ideal bodies.
For folks who want to learn more about Near-Earth Objects, I recommend Donald Yeomans’ 2013 book, Near-Earth Objects: FindingThem Before They Find Us (Princeton) as an excellent summary the general reader.
Another great writeup.
I am still wondering how the velocities are known or calculated – is it as straightforward as I assume? They take a picture at t=0 and again at t=x and divide the distance traveled? Then add the vectors for the combined impact force?
I am sorry but I do not understand the question. Can you ask it a lttle differently and i will give it a try. Thanks
I appreciate the effort – I’ll try to just get real basic :
How was DART’s velocity measured?
How was the asteroid’s velocity measured?
And perhaps what the error is.
I apologize – asking in person is way easier!
Yeah written bandwidth is awful! I do not know how these guys and gals get velocities they use for their models in these simulations, but they certainly know the velocity of the Didymos system at any instant from very well documented orbital (around the sun) characteristics. I expect they know the spacecraft velocity from calculations of thrust, gravitational forces and velocities since launch (Newton’s Laws). I am a bit confused about the use of an ion engine onboard a couple of times but did not seem to change the 7:14 impact time the other night. So I will try to find someone at apl to ask exactly what they do for these missions. Thanks for the tweak!
I appreciate the reply! This mission is so intriguing, I’m glad it was given special attention here.
There is a full and informative write-up on the Rosetta mission in the Wikipedia at https://en.wikipedia.org/wiki/Rosetta_(spacecraft)
I read an interview with Bobby Braun, now a senior technical manager at the Applied Physics Lab and formerly, as a grad student in the late 1980’s, worked in my organization at NASA Langley Research Center. Bobby spent two decades on missions designed to carefully land instrumentation packages on the surface of Mars. He said that the DART mission was very strange to him in that success was measured in part by the destruction of the spacecraft. He also pointed out that because of our lack of a priori knowledge regarding the target asteroid’s shape, if it turned out to be shaped like a doughnut, then a perfectly guided impactor would fly right through the asteroid and continue on out into space failing to generate any path change result or data. Cute!
All very interesting! So what’s next? Of course the effects here will be analyzed, but shouldn’t the next major project in this area involve developing special impactor vehicles, ready for deployment should the day come?
But what if, some day, we are confronted by a much larger asteroid headed for Earth! Seems the next thing would be to try this again on a much larger asteroid. I’m assuming the mass of the spacecraft might not be sufficient. might need a small nuke – although there would be a danger of splitting the rock without accomplishing any useful push.
Or an asteroid of approximately the same mass, which may or may not have similar internal structure (in detail and in bulk properties such as overall stiffness). So we get a handle on the variation of properties of asteroids in general. Because until you melt them (which only happens above 100-odd km in diameter, which would be a planet-sterilising impact on Earth), there’s no particularly good reason to expect consistent properties from one asteroid to another. Ditto comets, but with possibly more variation, since comets have had fewer warm-cold cycles than a moderately eccentric asteroid.
Which if it were to be tried, to avoid the splitting problem you mention, would be work by vapourising some of the surface material on the “hemisphere” facing the bomb, to provide a reaction force to move the asteroid.
Which depends on the surface chemistry, averaged, of the body under consideration.
Astronomers still don’t agree on the number of different surface chemistry classes of asteroids, as determined by reflection spectroscopy. Let alone what those spectroscopic classes actually mean in terms of surface chemistry.
“More work is needed”, a lot more work, before we’re anywhere near having a plan. This mission is a step forward. One of many necessary steps.
Whatever they do, in the meantime, they should make sure to find all the potential impactors we possibly can. There are several efforts underway on terra firma and in space, and though the really big ones are (mostly) known, there are a great deal more left to find, especially objects of the size they just smacked into. Let’s hope we aren’t surprised.
I really have no idea but I did notice in the post-impact presser, the NASA moderator jumped in fairly quickly to grab a similar question of “whats next” before the engineers could opine. He directed the questioner to the NASA press office I believe….I guess it is a sensitive political (read funding) policy issue. Also, IMO, the recently stood up U.S. Space Force may take these planetary defense missions under its mandate now that NASA has done a first effort at proof of concept.
This heterogeneity of the bodies – which we’ve already seen on other asteroids and comets, and is expected to be generally the case for “small bodies”. When an impactor hits them, some of the impactors momentum will be transferred to the body as a whole, but some will be transferred to debris that is ejected – possibly including material spalled off the opposite side to the impact.
(Every few months I hear essentially the same idea coming from people who think that the Deccan (lava //Large Igneous) Province in India was “triggered” by the Chixulub impact in the Gulf of Mexico, approximately at the Deccan’s antipode at the time ; this idea is somewhat hampered by the Deccan “LIP” having started half a million to a million years before the Chixulub impact. But the idea has been out there for 40-odd years, and the dating information is only a decade and a bit old, so the “Chixulub yields Deccan” idea will be resurfacing long after I’m dead.)
From the motion of the 150-odd metre diameter asteroid moon Dimorphos before (we have an astrometric record of several decades) versus after the impact, we get a fairly good value for the change in momentum of the body ; we know (fairly accurately) the momentum of the impacting spacecraft ; so we can calculate the efficiency of momentum transfer into a “rubble pile” body as opposed to the collision’s ejecta. Which gives us some error bars on how much momentum we’d have to apply to a putative “primate killer” impactor in order to move it off an Earth- colliding orbit in 1 year, 10 years … however far out it is when it is spotted, and it’s orbit calculated with sufficient precision to be sure of it impacting.
Whether we get 1 year, 10 years … whatever of warming is a rather different question. That there is “one out there”, “with our name on it” is pretty much guaranteed by the impact record. Read http://www.passc.net/EarthImpactDatabase/New%20website_05-2018/Diametersort.html for craters bigger than about 10km – which would be devastate a hemisphere, and probably agriculture-ending for the other hemisphere ; sort the same list by age, and you’ll see that it’s unlikely, but not impossible, to happen tomorrow.
Flying an instrument to a point in space is unfortunately reductive and colonialist. Do we not expect NASA to adopt a more progressive outlook? When that arrives, NASA technicians will be evaluated primarily on their Diversity Statements. Missions will be framed through a lens of Diversity, Equity, and Inclusion. We can thus look forward to a NASA devoted to making space a more welcoming and inclusive, uhhh, space.
I watched it too. I don’t think I can wait weeks to find out whether it was successful.
When will we know how much the orbit was perturbed?
I just tried to check at approx 2100 EDT Wednesday and still see nothing. I think Elena Adams, one of the lead project engineers was thinking a week or two to get some consensus from the various ground-based measurement groups and significantly longer for a good final answer. I do not know why we haven’t seen pics from the cubesat Italian experiment…unless it is so small that it needs to relay data through a data relay satellite for amplification for filtering andimage processing.
Thanks, Jim – informative as always!
Thanks for the contribution, Jim. I always enjoy your comments as well.
best,
D.A.
NYC
David and Jez: glad it was helpful. I am always happy to chip in what I can, as do most readers, as our host provides so many important issues and opportunities for the WEIT audience.
Philae did probe the 67P/Churyumov-Gerasimenko interior in concert with Rosetta through radio [“Rosetta’s Philae Lander Was Alive on the Surface of 67P for 63 Hours, Trying to Communicate”, Universe Today 2020] and found some complex molecules in the dust that was sampled after the impacts [“Rosetta and Philae”, NASA].