by Bill Bryson
The launch of the Voyager 1 spacecraft in 1977, from the Kennedy Space Center in Florida, was timed to coincide with an alignment of the planets Jupiter, Saturn, Uranus and Neptune that occurs only once every 175 years. (credit 2.6)
Now, the first thing you are likely to realize is that space is extremely well named and rather dismayingly uneventful. Our solar system may be the liveliest thing for trillions of miles, but all the visible stuff in it—the Sun, the planets and their moons, the billion or so tumbling rocks of the asteroid belt, comets and other miscellaneous drifting detritus—fills less than a trillionth of the available space. You also quickly realize that none of the maps you have ever seen of the solar system was drawn remotely to scale. Most schoolroom charts show the planets coming one after the other at neighbourly intervals—the outer giants actually cast shadows over each other in many illustrations—but this is a necessary deceit to get them all on the same piece of paper. Neptune in reality isn’t just a little bit beyond Jupiter, it’s way beyond Jupiter—five times further from Jupiter than Jupiter is from us, so far out that it receives only 3 per cent as much sunlight as Jupiter.
Such are the distances, in fact, that it isn’t possible, in any practical terms, to draw the solar system to scale. Even if you added lots of fold-out pages to your textbooks or used a really long sheet of poster paper, you wouldn’t come close. On a diagram of the solar system to scale, with the Earth reduced to about the diameter of a pea, Jupiter would be over 300 metres away and Pluto would be two and a half kilometres distant (and about the size of a bacterium, so you wouldn’t be able to see it anyway). On the same scale, Proxima Centauri, our nearest star, would be 16,000 kilometres away. Even if you shrank down everything so that Jupiter was as small as the full stop at the end of this sentence, and Pluto was no bigger than a molecule, Pluto would still be over 10 metres away.
So the solar system is really quite enormous. By the time we reach Pluto, we have come so far that the Sun—our dear, warm, skin-tanning, life-giving Sun—has shrunk to the size of a pinhead. It is little more than a bright star. In such a lonely void you can begin to understand how even the most significant objects—Pluto’s moon, for example—have escaped attention. In this respect, Pluto has hardly been alone. Until the Voyager expeditions, Neptune was thought to have two moons; Voyager found six more. When I was a boy, the solar system was thought to contain thirty moons. The total now is at least ninety, about a third of which have been found in just the last ten years. The point to remember, of course, when considering the universe at large is that we don’t actually know what is in our own solar system.
Now, the other thing you will notice as we speed past Pluto is that we are speeding past Pluto. If you check your itinerary, you will see that this is a trip to the edge of our solar system, and I’m afraid we’re not there yet. Pluto may be the last object marked on schoolroom charts, but the system doesn’t end there. In fact, it isn’t even close to ending there. We won’t get to the solar system’s edge until we have passed through the Oort cloud, a vast celestial realm of drifting comets, and we won’t reach the Oort cloud for another—I’m so sorry about this—ten thousand years. Far from marking the outer edge of the solar system, as those schoolroom maps so cavalierly imply, Pluto is barely one-fifty-thousandth of the way.
A conventional rendering of the solar system with the distances between the planets and other objects hopelessly, but inevitably, out of scale. It couldn’t be otherwise in any normal book. Drawn to this scale (with the Earth the diameter of a pea) the illustration would actually have to fold out to a length of over two and a half kilometres to convey Pluto’s orbit accurately—and would need to stretch several kilometres more to capture the majestic sprawl of the outer solar system. (credit 2.7)
Of course we have no prospect of such a journey. A trip of 386,000 kilometres to the Moon still represents a very big undertaking for us. A manned mission to Mars, called for by the first President Bush in a moment of passing giddiness, was quietly dropped when someone worked out that it would cost $450 billion and probably result in the deaths of all the crew (their DNA torn to tatters by high-energy solar particles from which they could not be shielded).
Based on what we know now and can reasonably imagine, there is absolutely no prospect that any human being will ever visit the edge of our own solar system—ever. It is just too far. As it is, even with the Hubble telescope we can’t see even into the Oort cloud, so we don’t actually know that it is there. Its existence is probable but entirely hypothetical.1
About all that can be said with confidence about the Oort cloud is that it starts somewhere beyond Pluto and stretches some two light years out into the cosmos. The basic unit of measure in the solar system is the Astronomical Unit, or AU, representing the distance from the Sun to the Earth. Pluto is about 40 AUs from us, the heart of the Oort cloud about fifty thousand. In a word, it is remote.
But let’s pretend again that we have made it to the Oort cloud. The first thing you might notice is how very peaceful it is out here. We’re a long way from anywhere now—so far from our own Sun that it’s not even the brightest star in the sky. It is a remarkable thought that that distant tiny twinkle has enough gravity to hold all these comets in orbit. It’s not a very strong bond, so the comets drift in a stately manner, moving at only about 220 miles an hour. From time to time one of these lonely comets is nudged out of its normal orbit by some slight gravitational perturbation—a passing star, perhaps. Sometimes they are ejected into the emptiness of space, never to be seen again, but sometimes they fall into a long orbit around the Sun. About three or four of these a year, known as long-period comets, pass through the inner solar system. Just occasionally these stray visitors smack into something solid, like Earth. That’s why we’ve come out here now—because the comet we have come to see has just begun a long fall towards the centre of the solar system. It is headed for, of all places, Manson, Iowa. It is going to take a long time to get there—three or four million years at least—so we’ll leave it for now, and return to it much later in the story.
So that’s your solar system. And what else is out there, beyond the solar system? Well, nothing and a great deal, depending on how you look at it.
In the short term, it’s nothing. The most perfect vacuum ever created by humans is not as empty as the emptiness of interstellar space. And there is a great deal of this nothingness until you get to the next bit of something. Our nearest neighbour in the cosmos, Proxima Centauri, which is part of the three-star cluster known as Alpha Centauri, is 4.3 light years away, a sissy skip in galactic terms, but still a hundred million times further than a trip to the Moon. To reach it by spaceship would take at least twenty-five thousand years, and even if you made the trip you still wouldn’t be anywhere except at a lonely clutch of stars in the middle of a vast nowhere. To reach the next landmark of consequence, Sirius, would involve another 4.6 light years of travel. And so it would go if you tried to star-hop your way across the cosmos. Just reaching the centre of our own galaxy would take far longer than we have existed as beings.
Comets develop their distinctive tails when their surface material begins to evaporate as they approach the Sun. These tails can be millions of kilometres long and are presented here in a lithograph from the mid-nineteenth century showing different comets throughout history. (credit 2.8)
Space, let me repeat, is enormous. The average distance between stars out there is over 30 million million kilometres. Even at speeds approaching those of light, these are fantastically challenging distances for any travelling individual. Of course, it is possible that alien beings travel billions of miles to amuse themselves by planting crop circles in Wiltshire or frightening the daylights out of some poor guy in a pickup truck on a lonely road in Arizona (they must have teenagers, after all), but it does seem unlikely.
Still, statistically the probability that there are other thinking beings out there is good. Nobody knows how many stars there are in the Milky Way—estimates range
from a hundred billion or so to perhaps four hundred billion—and the Milky Way is just one of a hundred and forty billion or so other galaxies, many of them even larger than ours. In the 1960s, a professor at Cornell named Frank Drake, excited by such whopping numbers, worked out a famous equation designed to calculate the chances of advanced life existing in the cosmos, based on a series of diminishing probabilities.
A semi-serious attempt, from a popular magazine of the 1930s, to hypothesize the likely appearance of a being from Mars. (It is interesting that Martians were thought to be both larger and more technologically advanced than Earthlings.) Because the universe is so crowded, there may well be millions and millions of other advanced civilizations out there—many in our own Milky Way. But because space is also so vast, the prospect of our shaking hands with a visitor from distant parts is likely to remain for some time the stuff of science fiction. (credit 2.9)
Under Drake’s equation you divide the number of stars in a selected portion of the universe by the number of stars that are likely to have planetary systems; divide that by the number of planetary systems that could theoretically support life; divide that by the number on which life, having arisen, advances to a state of intelligence; and so on. At each such division, the number shrinks colossally—yet even with the most conservative inputs the number of advanced civilizations just in the Milky Way always works out to be somewhere in the millions.
What an interesting and exciting thought. We may be only one of millions of advanced civilizations. Unfortunately, space being spacious, the average distance between any two of these civilizations is reckoned to be at least two hundred light years, which is a great deal more than merely saying it makes it sound. It means, for a start, that even if these beings know we are here and are somehow able to see us in their telescopes, they’re watching light that left Earth two hundred years ago. So they’re not seeing you and me. They’re watching the French Revolution and Thomas Jefferson and people in silk stockings and powdered wigs—people who don’t know what an atom is, or a gene, and who make their electricity by rubbing a rod of amber with a piece of fur and think that’s quite a trick. Any message we receive from these observers is likely to begin “Dear Sire,” and congratulate us on the handsomeness of our horses and our mastery of whale oil. Two hundred light years is a distance so far beyond us as to be, well, just beyond us.
So even if we are not really alone, in all practical terms we are. Carl Sagan calculated the probable number of planets in the universe at as many as ten billion trillion—a number vastly beyond imagining. But what is equally beyond imagining is the amount of space through which they are lightly scattered. “If we were randomly inserted into the universe,” Sagan wrote, “the chances that you would be on or near a planet would be less than one in a billion trillion trillion.” (That’s 1033, or 1 followed by 33 zeros.) “Worlds are precious.”
Which is why perhaps it is good news that in February 1999 the International Astronomical Union ruled officially that Pluto is a planet. The universe is a big and lonely place. We can do with all the neighbours we can get.
1Properly called the Öpik-Oort cloud, it is named for the Estonian astronomer Ernst Öpik, who hypothesized its existence in 1932, and for the Dutch astronomer Jan Oort, who refined the calculations eighteen years later.
Time-lapse photograph of the night sky taken in 1979 at the Anglo-Australian Observatory in Siding Spring, Australia, showing the trails of unset stars around the south celestial pole. (credit 3.1)
THE REVEREND EVANS’S UNIVERSE
When the skies are clear and the Moon is not too bright, the Reverend Robert Evans, a quiet and cheerful man, lugs a bulky telescope onto the back sun-deck of his home in the Blue Mountains of Australia, about 80 kilometres west of Sydney, and does an extraordinary thing. He looks deep into the past and finds dying stars.
Looking into the past is, of course, the easy part. Glance at the night sky and what you see is history and lots of it—not the stars as they are now but as they were when their light left them. For all we know, the North Star, our faithful companion, might actually have burned out last January or in 1854 or at any time since the early fourteenth century and news of it just hasn’t reached us yet. The best we can say—can ever say—is that it was still burning on this date 680 years ago. Stars die all the time. What Bob Evans does better than anyone else who has ever tried is spot these moments of celestial farewell.
By day, Evans is a kindly and now semi-retired minister in the Uniting Church in Australia, who does a bit of locum work and researches the history of nineteenth-century religious movements. But by night he is, in his unassuming way, a titan of the skies. He hunts supernovae.
A supernova occurs when a giant star, one much bigger than our own Sun, collapses and then spectacularly explodes, releasing in an instant the energy of a hundred billion suns, burning for a time more brightly than all the stars in its galaxy. “It’s like a trillion hydrogen bombs going off at once,” says Evans. If a supernova explosion happened in our corner of the cosmos, we would be goners, according to Evans—“it would wreck the show,” as he cheerfully puts it. But the universe is vast and supernovae are normally much too far away to harm us. In fact, most are so unimaginably distant that their light reaches us as no more than the faintest twinkle. For the month or so that they are visible, all that distinguishes them from the other stars in the sky is that they occupy a point of space that wasn’t filled before. It is these anomalous, very occasional pricks in the crowded dome of the night sky that the Reverend Evans finds.
To understand what a feat this is, imagine a standard dining-room table covered in a black tablecloth and throwing a handful of salt across it. The scattered grains can be thought of as a galaxy. Now imagine fifteen hundred more tables like the first one—enough to make a single line two miles long—each with a random array of salt across it. Now add one grain of salt to any table and let Bob Evans walk among them. At a glance he will spot it. That grain of salt is the supernova.
The staggering distance between us and the stars means that what we see when we look at the night sky is the stars not as they are now, but as they were dozens or hundreds or even thousands of years ago when the light now reaching us left them. (credit 3.2)
Evans’s is a talent so exceptional that Oliver Sacks, in An Anthropologist on Mars, devotes a passage to him in a chapter on autistic savants—quickly adding that “there is no suggestion that he is autistic.” Evans, who has not met Sacks, laughs at the suggestion that he might be either autistic or a savant, but he is powerless to explain quite where his talent comes from.
“I just seem to have a knack for memorizing star fields,” he told me, with a frankly apologetic look, when I visited him and his wife, Elaine, in their picture-book bungalow on a tranquil edge of the village of Hazelbrook, out where Sydney finally ends and the boundless Australian bush begins. “I’m not particularly good at other things,” he added. “I don’t remember names well.”
“Or where he’s put things,” called Elaine from the kitchen.
He nodded frankly again and grinned, then asked me if I’d like to see his telescope. I had imagined that Evans would have a proper observatory in his back yard—a scaled-down version of a Mount Wilson or Palomar, with a sliding domed roof and a mechanized chair that would be a pleasure to manoeuvre. In fact, he led me not outside but to a crowded storeroom off the kitchen where he keeps his books and papers and where his telescope—a white cylinder that is about the size and shape of a household hot-water tank—rests in a home-made, swivelling plywood mount. When he wishes to observe, he carries them, in two trips, to a small sun-deck off the kitchen. Between the overhang of the roof and the feathery tops of eucalyptus trees growing up from the slope below, he has only a letterbox view of the sky, but he says it is more than good enough for his purposes. And there, when the skies are clear and the Moon is not too bright, he finds his supernovae.
The term supernova was coined in the 1930s by a memorably odd as
trophysicist named Fritz Zwicky. Born in Bulgaria and raised in Switzerland, Zwicky came to the California Institute of Technology in the 1920s and there at once distinguished himself by his abrasive personality and erratic talents. He didn’t seem to be outstandingly bright, and many of his colleagues considered him little more than “an irritating buffoon.” A fitness fanatic, he would often drop to the floor of the Caltech dining hall or some other public area and do one-armed push-ups to demonstrate his virility to anyone who seemed inclined to doubt it. He was notoriously aggressive, his manner eventually becoming so intimidating that his closest collaborator, a gentle man named Walter Baade, refused to be left alone with him. Among other things, Zwicky accused Baade, who was German, of being a Nazi, which he was not. On at least one occasion Zwicky threatened to kill Baade, who worked up the hill at the Mount Wilson Observatory, if he saw him on the Caltech campus.
A supernova occurs when a giant star, one much bigger than our Sun, collapses and then spectacularly explodes, releasing in an instant the energy of a hundred billion stars.
If a nearby star were to explode in a supernova, such as the one photographed here in 1987, the passing blast could easily wipe out life on Earth. Fortunately, supernovae are fairly rare and—so far—safely distant. (credit 3.3)
But Zwicky was also capable of insights of the most startling brilliance. In the early 1930s he turned his attention to a question that had long troubled astronomers: the appearance in the sky of occasional unexplained points of light, new stars. Improbably, he wondered if the neutron—the subatomic particle that had just been discovered in England by James Chadwick, and was thus both novel and rather fashionable—might be at the heart of things. It occurred to him that if a star collapsed to the sort of densities found in the core of atoms, the result would be an unimaginably compacted core. Atoms would literally be crushed together, their electrons forced into the nucleus, forming neutrons. You would have a neutron star. Imagine a million really weighty cannonballs squeezed down to the size of a marble and—well, you’re still not even close. The core of a neutron star is so dense that a single spoonful of matter from it would weigh 90 billion kilograms. A spoonful! But there was more. Zwicky realized that after the collapse of such a star there would be a huge amount of energy left over—enough to make the biggest bang in the universe. He called these resultant explosions supernovae. They would be—they are—the biggest events in creation.