The Science of Discworld Revised Edition

we’re not so concerned with those. And comets are the main (but not the only) source of cosmic junk in Earth’s backyard.
    About a thousand meteoroids bigger than a football hit Earth’s atmosphere each day, together with countless millions of smaller ones. And as time passes, we receive some big and some bigger, with the occasional dinosaur-killer. How often do we expect to see such a big one? About once every hundred million years.
    There is much more of this kind of junk in the Solar system than we used to think, and it rains down on our planet constantly. Every year, we sweep up about 80,000 tones (tonnes) of it. Nearly all of the debris falling on Earth is little bits, mostly somewhat dried-out icy dirt from the tails of comets. Debris of this kind follows the comet’s orbit, marking it out like a gravel path. When the Earth’s orbit takes it through this cometary junkheap, some of the gravel burns up in the atmosphere, and we see spectacular light shows: meteor showers. These arrive on particular dates each year as the Earth passes through that debris. For example the Leonids can be seen in November, and the Perseids in August.
    There is a bit of a mystery about the Geminid meteor showers, which come in December, though. They seem to be associated with a (defunct) comet whose perihelion, closest approach to the Sun, is out by Pluto’s orbit. And that brings us to another source of impactors: the Kuiper Belt, which is the bit of the Oort Cloud not very far outside Pluto. In fact, Pluto and its satellite Charon are now thought not to be a ‘real’ planet-and-moon, but only the biggest lump in the Kuiper Belt. These lumps travel in genuine quasi-elliptical orbits, and may be the source of some of the regular comets with shorter orbital periods – like Halley’s comet, which returns every 76 years or so.
    As well as comets, the asteroids also send rocks our way. Jupiter’s gravitational field is strong enough to disturb the asteroids, especially those in certain ‘resonant’ orbits, with periods that are a simple fraction of Jupiter’s – one third or two fifths, say. Of the 8000 or so known asteroids, about one in twenty has an orbit that comes close to that of the Earth, or even crosses it. All of those that cross are potential impactors. Asteroids whose orbits approach the Sun to within a distance of 1.3 times the radius of the Earth’s orbit are said to be Earth-approaching asteroids, or
Amors
. The best known of these is Eros. Asteroids whose orbits overlap the Earth’s are called
Apollos
. More than 400 Amors and Apollos are known. More worrying are
Atens
, which are Amors too small to be detected easily but still big enough to cause tremendous damage. Most of these probably started out in the main asteroid belt, but were disturbed by Jupiter so that they crossed the orbit of Mars, and were then further disturbed by Mars.
    This leaves us with two opposite ways to view Jupiter – perhaps complementary ways. This largest planet has been proposed as the saviour of Earth’s life forms on countless occasions, its enormous gravity picking up nearly all of the in-falling rocks and icy lumps – as it did comet Shoemaker-Levy 9 in 1994. But it has also been shown to shake the Asteroid Belt about, possibly causing that dinosaur-killer (if it was actually an asteroid) to hit the Earth.
    The message is that a basketball left on a billiard table has a fairly interesting life. Velikovsky, who proposed a wild theory in the fifties that made the Solar System look very much like a snooker table in Biblical times, with Mars moving substantially closer to the Earth and Venus turning from a comet into a planet, wasn’t very wrong in principle.
    Only in every single detail.
    Here’s something else to worry about. Out there in the Milky Way galaxy there’s a lot of stars. Occasionally one goes nova, rarely supernova, as they explode. There is a sphere of very active radiation leaving such stars. If one went off in our vicinity, up to twenty light years away, say, all higher forms on Earth would be sterilised, at least. The bacteria, especially those deep in the Earth’s crust, would survive. They probably wouldn’t notice a thing. Wait a few billion years … higher lifeforms could exist in abundance once more.
    More worrying still are gamma-ray bursters. Gamma-rays are very short wavelength electromagnetic radiation, such as x-rays. When astronomers managed to develop instruments that could detect such