The reason the Voyager craft were launched when they were (in August and September 1977) was that Jupiter, Saturn, Uranus, and Neptune were aligned in a way that happens only once every 175 years. This enabled the two Voyagers to use a gravity assist technique in which the craft were successively flung from one gassy giant to the next in a kind of cosmic version of crack the whip. Even so, it took them nine years to reach Uranus and a dozen to cross the orbit of Pluto. The good news is that if we wait until January 2006 (which is when NASAs New Horizons spacecraft is tentatively scheduled to depart for Pluto) we can take advantage of favorable Jovian positioning, plus some advances in technology, and get there in only a decade or so-though getting home again will take rather longer, Im afraid. At all events, its going to be a long trip.
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 were remotely drawn to scale. Most schoolroom charts show the planets coming one after the other at neighborly 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 isnt just a little bit beyond Jupiter, its way beyond Jupiter-five times farther from Jupiter than Jupiter is from us, so far out that it receives only 3 percent as much sunlight as Jupiter.
Such are the distances, in fact, that it isnt 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 wouldnt come close. On a diagram of the solar system to scale, with Earth reduced to about the diameter of a pea, Jupiter would be over a thousand feet away and Pluto would be a mile and a half distant (and about the size of a bacterium, so you wouldnt be able to see it anyway). On the same scale, Proxima Centauri, our nearest star, would be almost ten thousand miles away. Even if you shrank down everything so that Jupiter was as small as the period at the end of this sentence, and Pluto was no bigger than a molecule, Pluto would still be over thirty-five feet 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-Plutos 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, is that when considering the universe at large we dont 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 Im afraid were not there yet. Pluto may be the last object marked on schoolroom charts, but the system doesnt end there. In fact, it isnt even close to ending there. We wont get to the solar systems edge until we have passed through the Oort cloud, a vast celestial realm of drifting comets, and we wont reach the Oort cloud for another-Im 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.
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 cant see even into the Oort cloud, so we dont actually know that it is there. Its existence is probable but entirely hypothetical.
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.