Of course we have no prospect of such a journey. A trip of 240,000 miles 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.[2]
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 forty 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 some of these lonely comets are nudged out of their 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 toward the center 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 neighbor 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 that is still a hundred million times farther 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 center of our own galaxy would take far longer than we have existed as beings.
Space, let me repeat, is enormous. The average distance between stars out there is 20 million million miles. Even at speeds approaching those of light, these are fantastically challenging distances for any traveling individual. Of course, it is
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 100 billion or so to perhaps 400 billion-and the Milky Way is just one of 140 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 in the cosmos based on a series of diminishing probabilities.
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 them 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 number of probable planets in the universe at large at 10 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 a one followed by thirty-three zeroes.) “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 neighbors we can get.
3 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 deck of his home in the Blue Mountains of Australia, about fifty miles 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-the stars not 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 semiretired minister in the Uniting Church in Australia, who does a bit of freelance 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.
Supernovae occur 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 brighter 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 within five hundred light-years of us, 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 someone throwing a handful of salt across it. The scattered grains can be thought of as a galaxy. Now imagine
