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 radiation, and put them into satellites, they discovered that two or three times per day the Earth is illuminated by an intense burst of gamma-rays coming from somewhere out in space. These gamma ray bursts seem to be extremely energetic: there is good evidence that the source of one of them was 12 billion light years away. Even a supernova would not be visible from that distance, so gamma-ray bursts have to be caused by something really serious.
What? That’s a mystery – perhaps the biggest mystery in today’s astronomy. The best bet is a collision between neutron stars. Imagine a binary star – two stars, orbiting their common centre of mass. Suppose they are both neutron stars. As time passes, they lose energy and fall in towards each other. If you wait long enough, they will come so close together that they collide. This, by the way, is likely to be a very messy business, not at all as simple as two tennis balls sticking together and rounding off. They probably break up and reform. So far, all the gamma-ray bursters we’ve seen are a long, long distance away. But one could light up anywhere. If a pair of neutron stars collapsed on to each other within a hundred light years of Earth, life might survive in the deep seas and the deepest rocks, but the rest of our planet would be dead.
And we wouldn’t even see it coming.
Asteroids and comets give you a bit of notice. We have the capability, given a year’s run-up time, to tackle small Earth-crossing asteroids now. We can see them coming and plot their arrival. But gamma-rays are electromagnetic: they travel at the speed of light. They could be on their way now: we couldn’t know. As soon as we did know, we and our technology would be dead.
Even our own Sun is not trustworthy. The nuclear reactions that make stars burn also make them change, as elements are created or used up, or just reach some critical level that triggers new kinds of reactions. Most stars follow the same series of changes, called the main sequence.
When the Sun first arrives on the main sequence, it is just like our Sun, with a surface temperature of about 6,000 degrees Kelvin, a light output of about 400 septillion watts, and a composition of 73% hydrogen, 25% helium, and 2% everything else. It stays on the main sequence for ten billion years, until nearly all of its hydrogen has been fused into helium. At that point, its core starts to contract, and becomes degenerate – consisting of closely packed neutrons. Outside the core there remains a shell of hydrogen, which continues to undergo nuclear reactions, which cause the outer layers of the star to expand and cool. The star becomes a red giant, between 10 and 100 times as big.
The radius of the Sun now is roughly 450,000 miles (700,000 km). At this stage its surface will probably be somewhere between the orbits of Mercury and Venus, and the Earth will already be in serious trouble. But there is more to come. As the core heats, it ignites a nuclear reaction that turns helium into carbon – the very reaction that allegedly is responsible for the existence of carbon-based lifeforms like us. This ‘helium flash’ happens very quickly, on astronomical timescales, and it destroys the degeneracy of the core. Now the core can once more sustain nuclear reactions, but now it burns helium. The outer layers of the star shrink, and become hotter.
When the helium in the core is used up, the star again burns its nuclear material in two shells: an inner one that burns helium to make carbon, and an outer one that converts hydrogen to helium. The outer layers expand again, and the star becomes a red giant for the second time. Now the outer layers start to blow away, exposing the hot core. The star loses layer upon layer of its material, and shrinks. Finally, the outer layers are all gone, and the core once more becomes degenerate. The star has become a white dwarf.
Our Sun has about 5.7 billion years left on the main sequence; then: kerblooie! Red Giant, and Earth becomes a cinder, or even gets gobbled up completely. But don’t lose any sleep over it. The typical lifetime of a species is 5 million years. We’ll be long gone.
Planets are not comfortable. Even when life has made its own bed (nice oxygenated atmosphere with an ozone layer to keep out
the nasty ultra-violet, nice ooze in the bottom of the oceans, nice long relaxation times for the thermal atmospheric oscillators), there are still plenty of things the Universe can throw at a planet that can still make it a bit fragile. If not kill it altogether.
Which brings us back to our original question. Is life fragile, and have we been extraordinarily lucky? Or is it robust, and therefore common? Is life so adaptable that it can handle virtually anything that the universe sends its way?
Until we can explore other worlds and see what kinds of life, if any, are present, anything we say here has to be speculative. The big difficulty is an ‘anthropic principle’ point. Suppose that life is incredibly rare, and that on most worlds it never really gets going, or doesn’t last long, because of all the disasters lying in wait. Nonetheless, there’s a lot of galaxies out there, each having billions or even trillions of stars. Even if the chances of survival are very, very small, occasionally one planet will get lucky. Some proportion of planets must get lucky, that’s how probability works.
Because life on this world has survived, we are therefore one of the lucky ones. It then becomes completely irrelevant how small our chances were. We are not representative. The probability that we survived is certainty, because we did. So we cannot reason, from our existence, that the chance of survival has to be fairly large. Whether it is large or small, we are here. So this is a case where the anthropist can legitimately frighten us. Perhaps all planets do get life and, if they’re allowed enough time; even extelligent life on a few. But we really could be the only one who’s survived to ask the question.
On the other hand … The very diversity of nasty things that the universe has up its sleeve argues for the adaptability and versatility of life. Life on Earth does not look like a bunch of lucky survivors. It looks like a bunch of tough guys who have overcome every obstacle put in their way. Sure, they took casualties, sometimes severe. But as long as a few survive the battle, pretty soon the planet is covered with life again, because life reproduces – fast. Whatever the disaster, it bounces back.
So far, anyway.
THIRTY-THREE
THE FUTURE IS NEWT
HEX WAS THINKING hard again. Running the little universe was taking much less time than it had expected. It more or less ran itself now, in fact. The gravity operated without much attention, rainclouds formed with no major interference and rained every day. Balls went around one another.
HEX didn’t think it was a shame about the crabs going. HEX hadn’t thought it was marvellous that the crabs had turned up. HEX thought about the crabs as something that had happened. But it had been interesting to eavesdrop on Crabbity – the way the crabs named themselves, thought about the universe (in terms of crabs), had legends of the Great Crab clearly visible in the Moon, passed on in curious marks the thoughts of great crabs, and wrote down poetry about the nobility and frailty of crab life, being totally accurate, as it turned out, on this last point.
HEX wondered: if you have life, then intelligence will arise somewhere. If you have intelligence, then extelligence will arise somewhere. If it doesn’t, intelligence hasn’t got much to be intelligent about. It was the difference between one little oceanic crustacean and an entire wall of chalk.
The machine also wondered if it should pass on these insights to the wizards, especially since they actually lived in one of the world’s more interesting outcrops of extelligence. But HEX knew that its creators were infinitely cleverer than it was. And great masters of disguise, obviously.
The Lecturer in Recent Runes had designed a creature.
‘Really, all we need is a basic limpet or whelk, to begin with,’ he said, as they looked at the blackboard. ‘We bring it back here where proper magic works, try a few growth spells, and then let Nature take its course. And, since these extinctions seem to be wiping out everything, it’ll gradually become the dominant feature.’
‘What’s the scale again?’ said Ridcully, critically.
‘About two miles to the tip of the cone,’ said the Lecturer. ‘About four miles across the base.’
‘Not very mobile, then,’ said the Dean.
‘The weight of the shell will certainly hamper it, but I imagine it should be able to move its own length in a year, perhaps two.’
‘What’ll it eat, then?’
‘Everything else.’
‘Such as …’
‘Everything. I’d advise suction holes around the base here so that it can filter seawater for useful things like plankton.’
‘Plankton being –?’
‘Oh, whales, shoals of fish and so on.’
The wizards looked long and hard at the huge cone-shaped object.
‘Intelligence?’ said Ridcully.
‘What for?’ said the Lecturer in Recent Runes.
‘Ah.’
‘It will withstand anything except a direct hit with a comet, and I estimate it’ll have a lifespan of about 500,000 years.’
‘And then it’ll die?’ said Ridcully.
‘Yes. I estimate it will, by then, take it twenty-four hours and one second to absorb enough food to last it for twenty-four hours.’
‘So after that it will be dead?’
‘Yes.’
‘Will it know?’
‘Probably not.’
‘Back to the drawing board, Senior Lecturer.’
Ponder sighed.
‘It’s no good ducking,’ he said. ‘That won’t help. We’re paying special attention to comets. We’ll let you know in plenty of time.’
‘You’ve got no idea what it was like!’ said Rincewind, creeping along the beach. ‘And the noise!’
‘Have you seen the Luggage?’
‘It certainly made my ears ring, I can tell you!’
‘And the Luggage?’
‘What? Oh … gone. Have you looked at that side of the planet? There’s a whole new set of mountain ranges!’
The wizards had let time run forward for a while after the strike. It made such a depressing mess of everything. Now, drawing on its bottomless reserves of bloodimindium, life was returning in strength. Crabs were already back although, here, at least, they didn’t seem inclined to make even simple structures. Perhaps something in their souls told them it’d be a waste of time in the long run.
Rincewind mentally crossed them off the list. Look for signs of intelligence, the Archchancellor had said. As far as Rincewind was concerned, anything really intelligent would be keeping out of the way of the wizards. If you saw a wizard looking at you, Rincewind would advise, then you should walk into a tree or say ‘dur?’.
All along the beaches, and out below the surf, everything was acting with commendable stupidity.
A soft sound made him look down. He’d almost stepped on a fish.
It was some way from the water line, and squirming across the mud towards a pool of brackish water.
A kind man by nature, Rincewind picked it up gingerly and carried it back to the sea. It flopped around in the shallows for a while and then, to his amazement, inched its way back on to the mud.
He put it back again, in deeper water this time.
Thirty seconds later, it was back on the beach.
Rincewind crouched down, as the thing wiggled determinedly onwards.
‘Would it help to talk to someone?’ he said. ‘I mean, you’ve got a good life out there in the sea, no sense in throwing it all away, is there? There’s always a silver lining if you know where to look. Okay, okay, life’s a beach. And you’re a pretty ugly fish. But, you know, beauty is only sk– scale deep, and –’
‘What’s happening?’ said Ponder’s voice in his ear.
‘I was talking to this fish,’ said Rincewind.
‘Why?’
‘It keeps coming out of the water. It seems to want to go for whatever is the opposite of a paddle.’
‘Well?’
‘You told me to keep a look out for anything interesting.’
‘The consensus here is that fish aren’
t interesting,’ said Ponder. ‘Fish are dull.’
‘I can see bigger fish in the shallows,’ said Rincewind. ‘Perhaps it’s trying to keep away from them?’
‘Rincewind, fish are designed for living in water. That’s why they’re fish. Go and find some crabs. And put the poor freakish thing back in the sea, for goodness’ sake.’
‘Perhaps a rethink is in order here,’ said Ridcully.
‘About the newts,’ said Ponder.
‘Newts is going far too far,’ said the Dean. ‘I’ve seen more shapely things in the privy.’
‘I want the person who put the newts on this continent to own up right now,’ said Ridcully.
‘No one could,’ said the Senior Wrangler. ‘No one’s seen the Luggage since the last comet. We couldn’t get anything in there.’
‘I know, because I had a tank of thaumically treated whelks all ready to go,’ said the Lecturer in Recent Runes. ‘And what, pray, am I supposed to do with them?’
‘Some sort of chowder would appear to be in order,’ said the Dean.
‘Evolution makes things better,’ said Ridcully. ‘It can’t make them different. All right, some rather dull amphibians seem to have turned up. But, and this is important, those fish Rincewind reported are still around. Now, if they were going to turn into things with legs, why are they still here?’
‘Tadpoles are fish,’ said the Bursar.
‘But a tadpole knows it’s going to be a frog,’ said Ridcully patiently. ‘There’s no narrativium on this world. That fish couldn’t be saying to itself “Ah, a new life beckons on dry land, walking around on things I haven’t yet got a name for.’” No, either the planet is somehow generating new life, or we’re back to the old “hidden gods” theory.’
‘It’s all gone wrong, you know,’ said the Dean. ‘It’s the bloodimindium. Even gods couldn’t control this place. Once there’s life, there’s complete and utter chaos. Remember that book the Librarian brought back? It’s a complete fantasy! Nothing seems to happen like that at all! Everything just does what it likes!’
The Science of Discworld Revised Edition Page 28