BackStudents watch an orbit shorten
Something odd is happening with Dimorphos. We know as much because of the efforts of a schoolteacher and some of his students
PremiumThe spacecraft slammed into the asteroid. We got spectacular photos of the crash and the aftermath. Later observations showed that the crash had indeed changed the path of the huge rock. Which, of course, was the idea. Humankind lost a spacecraft, yes. But it wasn’t in vain. We learned that day that we have the ability to change the course of an asteroid speeding through space, and that knowledge far outweighs the loss of the craft. Why? Because what if the asteroid was speeding straight at us, here on earth?
That’s why we sent the DART (Double Asteroid Redirection Test) spacecraft out nearly two years ago. That’s why we engineered its collision with a distant asteroid called Dimorphos, on 26 September last year (https://t.ly/2CF5n). As a demonstration of how we might save our planet from an asteroid strike, it was a smashing success.
Yet nearly a year later, a new research paper suggests that something odd is happening with Dimorphos.
Before I get to that, a short aside on the way astronomy happens. Listening to a podcast about the planet Jupiter a few days ago, I heard an astronomer make a most interesting observation. Professional astronomers interested in Jupiter, he said, would love to use the world’s many powerful telescopes to peer at it every night, to learn all there is to know about that fascinating giant planet. But of course, that’s impossible, for there are far too many other claimants on telescope time.
So, we learn more about Jupiter—and in fact plenty of other celestial bodies—via the efforts of amateur astronomers the world over. These are women and men who set up telescopes in their backyards or on terraces. They go peer at Jupiter—or another object of their interest—every night, and produce astonishingly good data. In a real sense, we owe a large chunk of our astronomical knowledge to amateurs like these. End of aside.
So indeed, something odd is happening with Dimorphos. We know as much because of the efforts of—wait for it—a schoolteacher and some of his students. This is at the Thacher School in Ojai, California. (Aside #2: Thacher is neighbour to the Krishnamurthy Foundation campus there, founded by the philosopher J. Krishnamurthy, and only a few miles from the Foundation’s own Valley School. End of Aside #2.)
This teacher, Jonathan Swift, established a serious, “research-grade" observatory at the school in 2016. As a result, students there do some serious astronomy. As Swift wrote in 2020: “The Thacher Observatory has been fully renovated and outfitted with professional grade equipment in recent years, and a progressive research programme has been established which has...pushed the envelope of what can be accomplished by motivated and dedicated high school students."
But back to Dimorphos. It orbits a much larger parent asteroid, Didymos. Before the collision, Dimorphos took 11 hours and 55 minutes to complete one orbit. DART’s aim was to change that and, if that happened, we’d know the mission was a success. Nasa estimated that the impact would shorten the orbit by about 10 minutes. When astronomers started observing the asteroid several hours after the impact, they found that the orbit had indeed shortened—in fact, Nasa reports a reduction of about 32 minutes, to about 11 hours and 23 minutes. As you might expect, that number is not precise—these rocks are 11 million km away, after all. The “margin of uncertainty", though, is only about two minutes, which itself gives you an idea of how powerful the telescopes used to observe Didymos are.
Also, it’s worth noting that this change in the orbital period is due to two things: first, the collision itself; second, the debris that the collision launched into space. As the debris flies off Dimorphos, it pushes the asteroid in the opposite direction. Admittedly, it’s not as if this debris is streaming off Dimorphos in one direction. More likely, it was ejected all over the place, so its overall effect on Dimorphos is hard to pin down. Still, hold on to that thought about this “ejecta".
Among those watching Dimorphos at the time were some folks at the Thacher School. For 10 nights over several weeks, starting before the collision, Swift and some of his motivated students aimed the Thacher telescope at Dimorphos. Note that this is hardly just a matter of looking through the telescope at distant points of light, as I would do with my telescope. Instead, they collected data that they then analyzed. Here’s a line from their paper, for example: “Seven nights of data were used to perform a Fourier decomposition of the phase folded light curves using 7 terms." (New Post-DART Collision Period for the Didymos System: Evidence for Anomalous Orbital Decay, Jonathan Swift et al., 31 August 2023, https://arxiv.org/pdf/2308.15488.pdf.)
Never mind what that means. What their analysis showed them was that the change in the orbital period was 34.2 minutes, with a margin of uncertainty of just 0.1 minute. As things go, that’s a significant change—“a 3.5 s discrepancy", Swift and students remark, which magnitude statisticians will recognize—from the previously measured 32 minutes. And this was based on observations taken 20-30 days after the collision.
Why, Swift and students wanted to know, is the orbit of Dimorphos continuing to shorten? There was, they note, an observed decay in its motion already, before the collision. But that is “4 orders of magnitude too small to account for the difference we see". What else could account for the ongoing post-collision decay?
Well, could it be that ejecta? Estimates are that the collision expelled into space between 10 and 50 million kg of debris. This is less than 1% of the weight of Dimorphos itself—an estimated 5 billion kg. A tiny fraction. But if some of it lies directly in the path of Dimorphos, could the friction it causes slow the asteroid?
Swift and students actually calculated how much debris there would need to be in the path of Dimorphos to account for the slowdown they observed: about 3 million kg. Certainly, then, there’s enough ejecta. But this still isn’t a likely explanation.
As Swift and students point out, the estimated speeds of the ejecta and the low escape velocity from Dimorphos mean that much of the debris would be quickly swept out of Dimorphos’s vicinity.
So what accounts for the shortening of the orbit? Answer: We don’t know. Maybe there’s a team of amateur astronomers in your vicinity, trying to find out.
Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun.
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