All sorts of exciting with OSIRIS-Rex

The arrival of OSIRIS-Rex around asteroid Bennu at the end of 2018 is all sorts of exciting. Well, apart from the rather lame acronym the probe’s been saddled with. But other than that, it’s got plenty of promise.

Bennu was discovered in 1999 and isn’t very large – just 300 metres across. Its interest comes from the fact that it’s a near-Earth asteroid, and that there’s a non-zero chance of it hitting us one day. The scenario is this: it’s in an orbit that’s close to ours, but slightly larger with a year of 438 days, specifically. Every six years or so, it encounters Earth more closely than otherwise. Each encounter introduces slight changes to its orbit, and in 2035 there’s a chance that it’ll be lobbed into an orbit that will hit Earth in 2085.

The chance of that is low; and Bennu isn’t of dino-killer scale. But it could still do a number on an inhabited area. Assuming our civilisation lasts that long, we’d have a chance of doing something about that – there’d be fifty years warning – but exactly what to do depends, among other things, on knowing exactly how Bennu is composed. Is it an ice-ball with dust? If it’s rock, what’s its tensile strength? Is it a cloud of loosely-packed boulders?

One of the things OSIRIS-Rex will do is scoop up a sample of that material and return it to Earth. It will do so (incredibly) from low orbit. That’s possible because orbital velocity around Bennu is so low – and, in theory, the probe can orbit from an altitude of just a metre or so. The idea is that it’ll scoop up a sample as it drifts by a likely looking patch of soil.

A composite image of the Apollo-class asteroid Bennu as seen from the OSIRIS-REx spacecraft from a distance of around 50 miles (80 km) . NASA, Goddard, University of Arizona, public domain.

How cool is that? Very cool, if you ask me, and don’t try this at home folks.

The probe raises the question – if Killer Asteroid No. 1 was discovered hurtling for us, anything from a city-buster to a dino-scale annihilator, what could we do with current or near-future technology? I’m not talking about sending Bruce Willis up on a modified Space Shuttle to nuke the thing. Aside from minor issues such as the fact that in space, linear distance isn’t a matter of travel as it is on Earth and you have to instead match orbits with something hurtling towards you, I mean. But assuming our action-hero astronaut has an Unfeasibly Stupid Mass Ratio Propulsion System or something (maybe Heinlein’s superficially plausible but ultimately hand-waved mass-conversion torch drive), heroically flying out to the incoming rock with a crew of oil-drillers and nuking it still might not avert the crisis. Nor would the more obvious and technically practical method of launching a robot nuclear missile straight out to hit it head-on.

The problem is that, in Hollywood, blowing space objects up causes them to disappear, but reality works a bit differently. Maybe a nuke blast might actually deal to the intruder sufficiently, if it’s a loose pile of ice or rubble. But depending on its nature, there’s a good chance that the object might not shatter and would carry on heading for Earth with most of its original velocity and kinetic energy. The thing is that a large part of the effect of nuclear weapons is because they are in atmosphere and on Earth, where there’s stuff to kick around, and it’s this that gives them their blast and the classic mushroom cloud. In space all you get is a bit of kinetic effect from the bomb and casing itself, coupled with the usual thermal and radiation effects. This was pointed out by Arthur C. Clarke in his 1955 novel Earthlight, wa-a-a-a-y before the US 1962 ‘Fishbowl’ experiment in which the US exploded a nuke some 400 km out in space, over the Pacific.

What this means is that, potentially, nuking an incoming killer-roid would do almost nothing, or it might simply produce an expanding cloud of debris still heading for Earth, with most of the original velocity and net total kinetic energy, this time spread over a slightly wider area. That would reduce the effect somewhat and likely alter the nature of the result, but – depending on the size and relative velocity of the original body – maybe not enough.

The practical and real options, as I understand it, all rely on having plenty of advance warning – as in years or (certainly for the dino-scale killers) decades – and are:

1.       Detonate something on or near it, cleverly placed so the force of the blast shifts the orbit of the object away from Earth. The risk is you’ll shatter the object and not change its orbit, see above. This might work as an emergency measure if you get only a few years’ warning.

2.       Land a ‘space tug’ on it, maybe with an ion motor, and start nudging the thing into a different orbit, micro-gravity by micro-gravity of acceleration. Or deploy a giant solar sail to tug it into a different orbit, same sort of deal for acceleration but no limit on fuel. Although the total time needed for the manoeuvre depends on the scale, vector and velocity of the intruding object, even relatively small problem objects will demand a lot of time. Start early.

3.        Paint it black. Just like the old Stones song said. But only on one side. The point about this bit of interplanetary graffiti is that it will alter the radiative property of the object. As the object re-radiates the heat (energy) it’s absorbed from the sun, it’ll do it asymmetrically. The photons emitted create thrust – known as the Yarkovsky Effect – which is miniscule, but everything adds up over time. After a while, and possibly a very long while, the asteroid’s no longer going to score a direct hit on Earth. Again, start early. Very early.

You can see from this that the idea of heroically launching the rocket you’ve got conveniently on the pad, just hours before impact, won’t work. The energies needed and the realities of orbital mechanics make sure of that. In reality, humanity would likely have to build the necessary gear first, test it, then prepare a rocket to launch it on – we’re talking several years, at least, just for that. Then there’s the time taken to get to the object. It’s not a matter of just flying out to it; you also have to rendezvous, which means matching orbits. But given a bit of time – actually, a lot of time – there’s every chance we could avert the crisis. And, of course, it could all be done with robots. Sorry, Bruce.

Copyright © Matthew Wright 2019