numbers tell a dart story

I wrote about DART (Double Asteroid Redirect Testing) in my last column here. In this column, let’s talk numbers- some of the numbers behind this mission. Space travel and research are full of eye-popping numbers, true, and the story of Dart is no exception. It is always worth trying to understand what the numbers mean. For me, it’s one way I get a sense of the remarkable feats that we humans are doing. Then I can marvel at them.

First, distance. Somewhere out there is this asteroid pair, Didymos and Dimorphos, and the dart crashes into Dimorphos, which is by far the smaller of the two. This binary asteroid system is about 11 million—11,000,000—km away from us. How far is?

To cover a distance of so many kilometres, you would have to travel around 10,000 times between Mumbai and Delhi. This hints at the reality of space travel: Distances on Earth are of no real help in comparison. So try this instead: You’d have to travel roughly 35 times between Earth and our Moon to cover a distance of 11 million km.

Far away, isn’t it? Yet it is important to maintain some perspective. The larger this distance seems, the smaller it is compared to other parts of space. Want to visit the Sun? That’s about 150 million km from Earth, or better than 13 times the journey it would take to reach Dimorphos by dart. Or do you say you want to reach the nearest star which is not the Sun? That would be a fixed Proxima Centauri, which is 4.25 light-years away, or about 40 trillion km, or about 4 million times farther from Dimorphos.

That is the nearest star. It took ten months for DART to reach Dimorphos. Zooming in at that speed, it would take more than three million years to reach Proxima Centauri. Yet the simple truth of our universe is that other stars and galaxies are still far enough away that they, in turn, make the distance to Proxima Centauri smaller.

But if the numbers themselves aren’t, you probably had some idea of ​​their eye-popping scale. Maybe you’re tired of even the regular gasp at celestial distances. Therefore, consider a few different ways of understanding them, and thus the challenges they present to intrepid scientists who set out to observe celestial objects.

Back to Didymos and Dimorphos. How do we know they’re all out? Because we’ve actually seen them. Not with the naked eye – they are too small and too far away for that – but through powerful telescopes. Imagine a process like this. Point your telescope in a specific direction in the sky and take an image of whatever is in the field of view. You will see stars and galaxies and possibly some other objects. Repeat this after a while—maybe a few minutes, maybe an hour, whatever—and compare the two images. Most of what appears in the frame doesn’t seem to be transferred. Again, stars and galaxies are so far away that even if they are moving really fast, a few minutes – maybe an hour, whatever – is too little time for us on Earth to clearly detect that motion. Is.

But from time to time, comparing two images will show you something that’s really shaken. Ordinarily, it would be a very close object – a planet, an asteroid – scattered across the field of view. Thus we first detected didymos and dimorphos. It’s also how astronomers made a short and dramatic video of the time Dart entered the asteroid. Through a telescope, he took several images of the part of the sky that contained Didymos, then threaded them together. The effect is that we see Didymos as a spot of light hitting the last 10 or 12 other bright spots, and then it suddenly explodes. It becomes a large, bright spot of light and a cloud of hazy dust rises out of it, like a pretended veil.

But wait, Dart got into Dimorphos. So why do we see Didymos explode? There is one more interesting thing about this pair. The two rocks are so far apart that it’s not like we can’t see them with the naked eye. It is also why we cannot separate them as individual rocks even through our most powerful telescopes. We know that there are only two because the light of them diminishes from time to time, regularly. The obvious guess is that it is not one asteroid, but two. As it orbits the larger Didymos, Dimorphos passes in front of Didymos, momentarily reducing the brightness of the larger asteroid.

And what about that cloud? The so-called “radiation pressure” of the Sun – a subject for another time – shaped it into a comet-like tail. A few days after the impact, two astronomers released a spectacular picture of this plume of dust trailing behind Didymos. How long was this feather?

Well, if you could actually see it without binoculars, the angle it makes at your eye is about 3 arc-minutes, or about a twentieth of a degree. It is positively small. But given that the plume is about 11 million km away, simple trigonometry tells us that it is – hold your breath – 10,000 km long, the distance between Mumbai and Sydney. In fact, this plume seems to indicate the kind of impact DART has had over the past week.

And finally, can we get a measure of that effect? Use it We know that the dart weighed about 600 kg and was traveling at a speed of about 22,000 kmph. Dimorphos weighs about 5 trillion kilograms. If it was stationary when the dart hit – which it was not – then the transfer of momentum would have hurt Dimorphos – hold his breath again – at 2.5mph. Yes, meters.

Not a lot to write home about? And yet, that kind of accident, that kind of transfer of momentum, may someday save our planet. Think on that for a while.

Dilip D’Souza, once a computer scientist, now lives in Mumbai and writes for his dinner. His Twitter handle is @DeathEndsFun.