Home >Opinion >Columns >And yet she pirouettes, there above Bennu

As a diversion from corona news and endless political posturing, it’s hard to top this: just a few hours ago as I type these words — meaning early morning India time on Wednesday, 21 October —a NASA spacecraft reached out and touched an asteroid.

This is absolutely true. I mean, our species has landed spacecraft on other celestial objects before—the moon, Mars, Jupiter and the comet Philae come to mind. Some of those spacecraft even contained humans. But this craft didn’t land. It only touched.

This was no accident, you understand. The whole purpose of NASA’s OSIRIS-REx (a complicated acronym I will leave for you to discover) mission is to pick up a sample of material from the surface of this asteroid, Bennu, and return it to Earth. Four years since it was launched, two years since it reached Bennu, OSIRIS-REx finally fulfilled the first half of that mission. Over the last two years, it has been orbiting Bennu, getting a better sense of what its surface looks like, identifying a target zone to do this sampling, working out how best to close in, make contact and then back off. And on Wednesday morning, it pulled off that manoeuvre.

There’s so much about this mission that is delicate and intricate. I’m going to run through some of that and, finally, pick one particular operation to explore in a little more detail. For in some ways, that one captures the whole complex ingenuity of this experiment and yet is utterly simple, almost charming, in its design and intent.

What does it take to pick up a sample of material from a distant object? I’m not sure how NASA zeroed in on Bennu in particular. But earlier observations had shown it is not very large — about the size of the Empire State Building, as NASA has pointed out more than once. This makes it large enough to be a reasonable target, but not so large that it will pose its own gravitational challenges to a friendly approach.

Bennu is also pretty far away. Right now, it is some 330 million kilometres from us, inside the orbit of Mars on the other side of the solar system. (Of course there are times when their respective orbits bring Bennu and Earth closer). But getting there wasn’t simply a matter of drawing a straight line from Earth to Bennu and sending OSIRIS-REx scudding along that line. Given that both objects are moving through space on their own orbits around the Sun, drawing this straight line doesn’t make sense anyway. For tomorrow it will look different, and a month from now even more different.

Instead, the craft used a long loopy path that’s typical of spaceflight. After launching in September 2016, it actually flew towards the Sun, nearly reaching the orbit of Venus, before swinging back out. A year later, it swung past the Earth, picking up a “gravity assist" from its home planet that sent it soaring deeper into the solar system towards Bennu. In December 2018, it “reached" Bennu — it managed to position itself about 20km from the asteroid and matched its orbit and speed around the Sun. By the end of that month, OSIRIS-REx started orbiting Bennu, at a distance averaging 1km, taking 62 hours to go round the asteroid once. That’s where it has been since. Bennu may be 330 million km away as you read this, but its little visitor from Earth has travelled over 300 billion km so far. Ten times longer than a meaningless straight line.

And what’s OSIRIS-REx been doing through the hundreds of orbits it has made around Bennu in the last two years? Its mission, remember, is to collect a sample of whatever makes up Bennu. Towards that end, the first job is to identify an appropriate spot on the asteroid to make contact and pick up whatever’s there. So, OSIRIS-REx has been extensively photographing Bennu’s surface. Once, it made a reconnaissance sweep that took it to just 250 metres above the surface, to look more closely at one promising target area that NASA called “Nightingale".

NASA put together a mosaic of the images from that sweep — high-resolution images, with each pixel representing just 4mm. The result is a stunning black-and-white view of Nightingale. There are a few large rocks — possibly as large as a car — smaller stones and, in the middle, a patch of what looks like sand. Or, as one report had it, “relatively fresh and fine-grained material that hasn’t been exposed to the harsh deep-space environment for long."

The patch is what clinched it. NASA began preparations for OSIRIS-REx to rendezvous with Bennu at Nightingale.

No easy task, though. With its 3.5m long robotic arm extended for the contact, the craft would need to negotiate past at least three enormous boulders and several smaller ones, to this patch that is only about 8m wide. OSIRIS-REx is the size of a large van, so you can imagine the complexities involved here. Then it would need to gently place the large cup at the end of the arm down on the sand, ensuring the lip wasn’t resting on a stone. If satisfied with the cup’s placement, the craft would explode a small bottle of nitrogen. The grains and small stones the nitrogen stirred up would be swallowed by the cup. About 10 seconds after contact, OSIRIS-REx would slowly withdraw and rise back to its orbit.

Remember all this is happening 330 million km from the Earth. This means any NASA instructions take 18 minutes to reach OSIRIS-REx, so navigating in real time is impossible. Instead, NASA programmed a set of instructions that the craft would follow, designed not just for Bennu but specifically for the features of Nightingale. Included was the freedom to abort the attempt if necessary. But it didn’t need to abort. The whole intricate effort worked like a dream and OSIRIS-REx did pick up some Bennu material.

Think of the calculations and analysis needed to accomplish all that at a 300 million km remove. Takes my breath away — but there’s more.

Over the next few days, OSIRIS-REx will figure out how much material it grabbed off Bennu — meaning, what’s its weight? But how will it do this? Here’s a hint: at some point before descending, OSIRIS-REx stuck out its arm and did a pirouette. NASA researchers calculated how much force was needed to ramp up to a given speed of rotation.

Puzzled? Well, with stuff from Bennu now in the craft’s grasp at the end of that arm, OSIRIS-REx will do the same pirouette again, and the researchers will do the same calculation again. Here’s one of those times that obscure concepts from classes in your youth come leaping off the page. The “moment of inertia" of an object tells us the object’s resistance to being rotated. As you might expect, the heavier the object, the greater its moment of inertia. Imagine tying a string to a pebble and whirling it around your head; then tie the string to a boulder and try doing the same.

In the same way, now that it has a fistful of pebbles and sand, OSIRIS-REx will naturally be more resistant to rotation than when it was empty-handed. So, the extra force needed to make it rotate can tell us, very precisely, the weight of that fistful of material. That’s how we’ll know.

My college math and physics ideas, taking real shape somewhere across our solar system in a pirouetting spacecraft. What’s not to like?

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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