Sep 22

Nearly a year ago, a spacecraft we launched nearly two years ago slammed
into an asteroid. This was the DART mission, hitting the asteroid
Dimorphos. The idea was to see if we can alter the path through space of
such an object. Because who knows, one day we might find that such an
object is barrelling directly towards us on Earth, and this might be the
only way to deflect it.

The test was a success. The collision did cause a measurable difference to
the path of Dimorphos. But what recently struck me about this whole episode
was that a team of high-school students were observing Dimorphos for some
weeks after the crash, and came up with some significant findings.

That's what prompted my Mint column for September 15.

Students watch an orbit shorten,
https://www.livemint.com/opinion/columns/students-watch-an-orbit-shorten-11694716700154.html

cheers,
dilip

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Students watch an orbit shorten


The 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 that's
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://www.livemint.com/opinion/columns/celebration-of-an-obliteration-11664482091649.html).
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 their 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 program 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 ten 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 analysed. 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 minutes. As these things go, that's a significant change - "a
3.5 σ 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.

-- 
My book with Joy Ma: "The Deoliwallahs"
Twitter: @DeathEndsFun
Death Ends Fun: http://dcubed.blogspot.com

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