You're listening to the London Review of Books podcast. I'm Thomas Jones. I'm joined today by Chris Lintot,

Professor of Astrophysics at Oxford and one of the presenters of the BBC's The Sky at Night.

He has a piece in the latest issue of the LRB on NASA's Dart mission, the recent and we now know successful

attempt to alter the path of an asteroid by crashing a spaceship into it, and the technique could

theoretically be used to divert any large objects from hitting the Earth.

Hello, Chris, and thank you very much for talking to me today.

My pleasure. It's nice to be here.

So to begin, perhaps you could tell us what, or I don't know, should I say where, is an asteroid?

So you can think of asteroids as rubble left over from the time that the solar system was

forming. So these are the small bodies of the

solar system, which have proved to be much more numerous and much more varied than I think

people would have expected. We've known about them for a while. The first asteroids were discovered

back in the early 19th century, when there was much excitement that new planets were being added

to the solar systems catalogue. In particular, there's a thing called Bode's Law, which is this nice geometrical relationship between the orbits of the planets. There's a nice pattern between the size of the orbit of Mercury, the size of the orbit of Venus, the size of the orbit of Earth, the size of the orbit of Mars, and then there's this gap, and then Jupiter and Saturn pick up the pattern.

And so it was expected for a long while that there must be a missing planet.

And when the first asteroids, particularly something called series, which is the largest of them, was found, that fitted in the gap nicely.

And then they kept finding more asteroids.

And we now know of hundreds of thousands of these things, a catalogue so large that most of them just have numbers rather than names. And essentially, what happens when you fail to form a planet?

So those in the main belt, which sits between Mars and Jupiter, we think that Jupiter's

influence, the gravity of Jupiter stopped a large planet assembling in that gap,

and so we're left with all of these bits. Now, most of them sit happily in the bout.

There are plenty of asteroids in the outer solar system as well. Pluto has its retinue of objects

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