Home / Opinion / Columns /  Celebration of an 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 10 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) 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 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 toward 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 dizzying 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, and 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.

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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