The Science of Discworld Revised Edition

recent well known impact on Earth was the Tunguska meteorite, which exploded 4 miles (6km) above Siberia in 1908, causing an explosion that felled trees for more than thirty miles (50 km) around. Craters, or other evidence of even more recent impacts have been found. A double-crater in the Saudi Arabian desert may be only a few centuries old.
    Where do all these rocks (and other junk, like ice) come from? Who or what is throwing them at us?
    First, some terminology. When you look into the night sky and see a ‘shooting star’, a glowing streak, that’s a meteor. It’s not a star, of course: it’s a lump of cosmic debris that has hit the Earth’s atmosphere at high speed and is burning up because of friction. The debris itself is called a meteoroid, and any part that remains when it has hit is called a meteorite. For convenience, though, we’ll generally use the word ‘meteorite’ for all of these. But we thought we ought to show you that we could have been pedantic if we’d wanted to.
    Some of these bodies are mostly rock, and some are mostly ice. And some are a bit of both. Wherever they come from, it’s not the Earth. At least, not directly. A few may have been splashed off the Earth by a previous impact and then come back down the next time we ran into them. However they got Up There, that’s clearly where they are coming from. What’s Up There? The rest of the universe. The closest bit is our own Solar System. So that has to be the most likely culprit. And there’s no question that it has a lot of ammunition.
    Earlier, we described the Solar System as being rather chaotic: nine planets and a few moons with some quite interesting real estate. We mentioned that there was quite a lot left over after these larger bodies were accounted for. There were relatively small lumps of actual rock in the asteroid belt, but nearly all the ‘loose change’, after the Sun and planets had been paid for out of the Solar nebula, was lumps of dirty ice.
    The biggest collection of these is the Oort Cloud, a vast, very thinly spread mass that lives outside the Solar System ‘proper’ – that is, further out than Pluto (or Neptune when Pluto gets inside the orbit of Neptune, which can happen). In 1950 Jan Hendrik Oort proposed that the source of most of the comets we see from Earth must be some such cloud, and got it named after him. The main evidence we have for its existence is that comets with very long thin orbits, which are common, must have come from somewhere. The bodies in the Oort cloud range from pebble-sized up to lumps perhaps as big as Pluto.
    These comet materials are the usual source of the meteorites we pick up and put into museums, after most of their substance has burnt up in the atmosphere. We’re beginning to get an idea of how big the Oort Cloud could be. Its mass is about a tenth that of Jupiter, and it extends way outside Pluto’s orbit, perhaps as far as 3 light years – two thirds of the way to the nearest star. This spreads the material though a volume millions of times as great the volume inside Pluto’s orbit, our actual planetary Solar System. So the ‘cloud’ is so rarefied that if you went there, you probably wouldn’t see anything.
    The gravitational pull of the Sun is tiny at those distances, and the dirty ice lumps barely move along their orbital paths, which are probably close to circles. To the extent that the ice lumps
have
orbits, and don’t just drift slowly about, they take millions of years to go once round the Sun. But the universe doesn’t let them keep doing that without interference. Oort called his cloud ‘A garden, gently raked by stellar perturbations’. As nearer stars, and the pull of the whole galaxy, interact with the Sun’s pull, many of these lumps are pulled away from their normal paths.
    It turns out that the disturbances need not be as gentle as Oort supposed. About once every 35 million years a star passes
through
the Oort cloud, and havoc ensues. Since the 1970s another source of big disturbances has been recognised: Giant Molecular Clouds. These are huge accumulations of cold hydrogen, where stars and solar systems are born. Their masses can be a million times that of the Sun. They don’t have to come anywhere near us to shake ice lumps out of their sedate near-circular orbits in the Oort cloud.
    Such disturbances can cause lumps of ice to drift in towards the Solar System. At that point, they become comets. Some probably drift outwards too, but