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Late one evening last year, an 87-year-old lady I know was out for a walk in the suburb where I live. As she waited to cross a busy road, a speeding car turned towards her, its headlights blinding her. She stepped back hurriedly. We don’t know quite what happened next, but she did fall on her back. When I reached the spot, she had been helped to her feet and was standing there in a daze. There was a wound on the back of her head, and small wounds on her shins. The surmise, thus, is that the car did hit her, at least gently.

We, who know her, were of course alarmed and worried. Luckily, she wasn’t badly hurt. But for days, I couldn’t get out of my mind the image of that car and its bright lights rounding on her, probably braking hard, her stumbling backward, the car knocking her over ... Was there anything she could have done to prevent the accident? Aside from staying permanently indoors, probably not. These things happen.

This episode, of all things, returned to my mind when I read recent news from, of all places, Nasa. That’s because Nasa is making plans to prevent an accident. Though of course, what they are looking at is a collision on a far greater scale, and one that will likely be predicted years in advance. If it happens, it will cause enormous damage and suffering, possibly even to the extent of wiping out life altogether on this planet. That awful spectre makes prevention conceivable and worthwhile. And on Tuesday, a Nasa mission will leave the Earth to test this idea of prevention.

This is, you may have guessed, about an asteroid colliding with Earth. The last time that happened, it was an asteroid about 10km in diameter. It left the enormous Chicxulub crater, half on and half off the coast of the Yucatan Peninsula in Mexico. It also wiped out the dinosaurs and about three-fourths of all plant and animal species then on the planet. That was about 66 million years ago. No, it’s not that another collision is due sometime this week or the next or even next year. But today we can observe asteroids and predict their paths precisely. We have not yet found one that will collide with the Earth anytime soon, but it’s good to be prepared just in case one comes hurtling from the darkness.

And that’s the spirit behind this Nasa mission. Can we intercept an asteroid that’s going to collide with the Earth? It’s unlikely that we can destroy it—think of what it would take to blow up a rock that’s 10km in diameter. But is it possible to deflect it? If we intercept it far enough out, even a tiny change in the asteroid’s path should be enough to send it whizzing past the Earth rather than slamming into us.

And that’s what this Nasa mission is designed to test: our ability to nudge an asteroid just that tiny bit off course.

How will that happen? In the final months of World War II, in a last-ditch attempt to turn the tide of the war, Japanese fighter pilots launched kamikaze, or suicide, attacks on the US Navy. They deliberately flew their Mitsubishi Zeros into the enemy ships. They did sink a few, but in the end, it was just a blip in the inexorable American march to victory. Still, Nasa’s plans are reminiscent of nothing quite as much as those kamikaze flights.

That is, Nasa will fly the spacecraft that launches on Tuesday into an asteroid. This is an unmanned craft, of course—so this is not a suicidal mission in the same way as Japanese pilots flew in the 1940s. But that apart, definitely kamikaze. The mission is called DART (Double Asteroid Redirection Test). After it is launched, it will head for a pair of asteroids that circle around each other, Dimorphos and Didymos. They are about 10 million km away, and DART will reach there in about 10 months. That’s a long flight, and neither asteroid is projected to collide with the Earth. So why go there at all? Remember that this is just a test. We can actually observe this pair of asteroids using telescopes on Earth; and in any case, about 10 days before getting there, DART will also deploy a small satellite carrying cameras.

So we can both see and measure what happens next September, when DART collides with the smaller of the two asteroids, Dimorphos.

That’s exactly the plan. Flying at about 25,000 km/h, DART will crash head-on into Dimorphos. Now the asteroid is over 150m in diameter—not the monster that destroyed the dinosaurs 66 million years ago, but this is still one enormous rock. So Nasa doesn’t expect DART, which weighs about 550kg, to destroy Dimorphos. Instead, the collision should measurably deflect the asteroid from its path. Today, it orbits Didymos once in 11 hours and 55 minutes. After the collision, DART’s designers believe that orbital period will reduce by just over 1%, to perhaps 11 hours and 45 minutes.

No doubt that strikes you as an almost trivial reduction, almost insignificant. But remember the distances we are dealing with. They serve to magnify such a change to an extent that makes an significant difference to us on Earth.

To understand this, imagine that Dimorphos was actually on a collision course with our planet. If the collision actually happened, it would be a disaster on a scale humankind has never known. So it’s vital to try to prevent it from happening—and one possibility is to crash a spacecraft into it. For the sake of this bit of reasoning, let’s say that if the run-in with DART is able to turn Dimorphos 90 degrees one way or the other, we’ll call that a 100% deflection from its path. Presumably we’d be entirely safe from it then.

But what if DART causes only a 1% deflection—meaning, less than a degree’s deflection? A little trigonometry tells us that a nudge like that would make it miss the Earth by over 150,000km. Dimorphos would sail harmlessly past the Earth, at its closest about halfway between us and our Moon. We’d be saved a fate something like the extinction of the dinosaurs. Suddenly, you see, 1% doesn’t seem quite that tiny. That’s all we need to save the planet.

To be sure, spacecraft kamikaze is just one possible way to defend our planet from collisions with asteroids. There are other possibilities. For example, Philip Lubin, a professor of physics at the University of California, Santa Barbara, has a research group that has proposed a different scheme for dealing with threatening asteroids. They call it “PI", for “Pulverize It". PI suggests that maybe we won’t need to predict asteroid paths far in advance and send spacecraft on suicide missions.

Of that and other alternatives, another time. But whatever we choose, the interesting thing is that we are at a moment in human history when we can both conceive of such planetary defence mechanisms, and deploy the technology to implement them. As Lubin remarked: “Humanity could finally control its fate and prevent a future mass extinction like that of the previous tenants of the Earth who did not bother with planetary defence, the dinosaurs."

All I want now is a mechanism that will prevent cars running into pedestrians. Then again, we could also take steps in that direction by driving with a little more consideration for others on the road.

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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