Oct 3

Last week, DART slammed into Dimorphos at a speed of over 20,000 kmph. It
didn't survive. But it sent photographs till the very end, leaving watchers
like me around the globe oohing and aahing in wonder.

Never mind the why and wherefore, you can ooh and aah too. I couldn't
resist writing about the crash for Mint (September 30).

Celebration of an obliteration,
https://www.livemint.com/opinion/columns/celebration-of-an-obliteration-11664482091649.html

cheers,
dilip

---

Celebration of a far-off obliteration


The scene that perhaps best conveys the spirit of it all: a large gathering
of scientists and engineers and administrators, all cheering wildly,
hugging and backslapping and high-fiving, grins of pure delight on every
face. It's some command room deep in the bowels of NASA, and these people
are celebrating ... well, what?  A spacecraft that has just been
obliterated, that's what. A spacecraft they designed, built and guided
through ten months to its destination. Its destiny, even.

Travelling at over 22,000 kmph, their DART craft slammed into a rock
somewhere out in space. It was a large rock by any earthly standards -
perhaps 170m in diameter - but a speck in comparison to the behemoths that
dot the universe every which way you look. Of course DART did not survive.
The team behind its months-long journey through space did not intend it to.
This was an intentional crash, and it happened just as the team wanted, to
within a few dozen feet of the planned point of impact. That's right, they
planned this crash. They absolutely wanted their craft to be destroyed. And
when it happened, of course they high-fived.

DART set out ten months ago to approach and then crash on an asteroid
called Dimorphos. (See my column
https://www.livemint.com/opinion/columns/spacecraft-kamikaze-to-defend-earth-11637265274420.html.)
The crash is a test of an idea that, you could say, was born over 65
million years ago. That's when a massive asteroid smashed into our Earth,
wiping out the dinosaurs. When scientists found the evidence of that
unimaginable catastrophe - the Chixculub crater in Mexico's Yucatan
Peninsula - and when astronomers were able to detect asteroids and predict
their paths, a possibly old question raised its head all over again: what
can we do about an asteroid strike? If we really can predict one in
advance, do we sit around waiting for fiery oblivion? Or do we try somehow
to avert it?

Well, that second question prompted the idea of DART. Suppose we smash a
spacecraft onto the asteroid. Would that deviate it from its path? After
all, if we did this far enough from Earth, even a tiny deviation would be
enough for the asteroid to sail past our planet instead of smashing into
us. There's an idea, and why not test it at this time when there's no
immediate asteroid danger to us?

Thus was DART conceived and designed.

But how do we identify an asteroid to conduct this test on? Consider the
constraints. It can't be very large, because then we'd need to hit it with
something pretty large too, and that becomes a very expensive test. It
can't be too small, because we don't want to shatter the whole thing. It
shouldn't be somewhere near Earth, because what if it does shatter and
sends fragments - maybe large fragments - hurtling towards us? But then if
it's relatively far away, how do we observe it and detect a change in its
path?

For these reasons and more, the ideal candidate asteroid is actually an
asteroid binary: two rocks that circle each other while wandering through
space. If one is substantially larger than the other, the smaller one
effectively orbits the larger, like our Moon does the Earth. We can measure
the smaller rock's orbital period by how often it passes in front of the
larger asteroid (think of a bat circling a nearby streetlamp). We aim for
the smaller one, but of course it must be small enough that the strike can
deflect it noticeably. Afterward, we look for a period change in the binary
system. That will tell us how much we managed to deviate the asteroid.

This is what made the Didymos/Dimorphos pair the best choice for this test.
This is why the mission was called DART, for Double Asteroid Redirection
Test. We have known of Didymos since 1996, Dimorphos since 2003, and have
observed the pair through various telescopes. They are about 11 million km
from the Earth, posing no danger to us. Dimorphos is about a fifth the size
of Didymos, though still a pretty large rock. It takes nearly 12 hours to
orbit Didymos.

So now spare a thought for what DART achieved. It set out last November,
taking 10 months to traverse those 11 million km. As it neared Didymos, it
was travelling at 22,000 kmph, or about 6 km per second. Signals to and
from the craft - instructions, photographs - took nearly 40 seconds in
either direction. We have never seen Didymos and Dimorphos as distinct
celestial bodies - just a single spot of light that sometimes dims. So DART
was equipped with image-processing software that detected Dimorphos as it
got closer and the two asteroids separated in its camera's field of view.
Then it adjusted its path to bypass Didymos and lock onto Dimorphos for
that much-anticipated crash.

Thus did a spacecraft weighing about 500kg aim to move a rock weighing an
estimated 5 billion kg, or 5 million tonnes. That's why it had to crash at
such a high speed. NASA estimates that DART's obliteration will shorten the
orbit of Dimorphos by about 10 minutes. That it reached and hit its tiny
target with such precision is a feat roughly akin to threading a needle
that's 70 km away.

DART sent images back to Earth all along, so plenty of us had a
dizzy-making view of that final approach. The small dot that grew slowly
larger; then a fainter, smaller second dot; then a few boulders on larger
Didymos as it slips past; then Dimorphos quickly fills the field of view,
rubble and boulders oddly like you'd find anywhere on Earth; then the
screen goes nearly, but appropriately, blood-red.

DART had slammed into Dimorphos, transmitting only part of its final image
before it was itself reduced to rubble.

My buddy Ajay, on the other side of the globe, sent me these words: "Oh my
god! Aaaaaaaaaarg!" Spoke for me, he did. And you know, I'd be
back-slapping and high-fiving too.

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

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