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uled to depart for pluto） we can takeadvantage of favorable jovian positioning； plus some advances in technology； and get there inonly a decade or so—though getting home again will take rather longer； i’m afraid。 at allevents； it’s going to be a long trip。
now the first thing you are likely to realize is that space is extremely well named and ratherdismayingly uneventful。 our solar system may be the liveliest thing for trillions of miles； butall the visible stuff in it—the sun； the planets and their moons； the billion or so tumblingrocks of the asteroid belt； ets； and other miscellaneous drifting detritus—fills less than atrillionth of the available space。 you also quickly realize that none of the maps you have everseen of the solar system were remotely drawn to scale。 most schoolroom charts show theplanets ing one after the other at neighborly intervals—the outer giants actually castshadows 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 waybeyond jupiter—five times farther from jupiter than jupiter is from us； so far out that itreceives only 3 percent as much sunlight as jupiter。
such are the distances； in fact； that it isn’t possible； in any practical terms； to draw the solarsystem to scale。 even if you added lots of fold…out pages to your textbooks or used a reallylong sheet of poster paper； you wouldn’t e close。 on a diagram of the solar system toscale； with earth reduced to about the diameter of a pea； jupiter would be over a thousand feetaway and pluto would be a mile and a half distant （and about the size of a bacterium； so youwouldn’t 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 jupiterwas as small as the period at the end of this sentence； and pluto was no bigger than amolecule； 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 e sofar that the sun—our dear； warm； skin…tanning； life…giving sun—has shrunk to the size of apinhead。 it is little more than a bright star。 in such a lonely void you can begin to understandhow 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 wasthought to have two moons； voyager found six more。 when i was a boy； the solar system wasthought to contain thirty moons。 the total now is “at least ninety；” about a third of which havebeen found in just the last ten years。
the point to remember； of course； is that when considering the universe at large we don’tactually 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 pastpluto。 if you check your itinerary； you will see that this is a trip to the edge of our solarsystem； and i’m afraid we’re not there yet。 pluto may be the last object marked onschoolroom charts； but the system doesn’t end there。 in fact； it isn’t even close to endingthere。 we won’t get to the solar system’s edge until we have passed through the oort cloud； avast celestial realm of drifting ets； and we won’t reach the oort cloud for another—i’m sosorry about this—ten thousand years。 far from marking the outer edge of the solar system； asthose 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 stillrepresents a very big undertaking for us。 a manned mission to mars； called for by the firstpresident bush in a moment of passing giddiness； was quietly dropped when someone workedout that it would cost 450 billion and probably result in the deaths of all the crew （their dnatorn 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 prospectthat 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’tactually 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 somewherebeyond pluto and stretches some two light…years out into the cosmos。 the basic unit ofmeasure in the solar system is the astronomical unit； or au； representing the distance fromproperly called the opik…oort cloud； it is named for the estonian astronomer ernst opik； who hypothesized itsexistence in 1932； and for the dutch astronomer jan oort； who refined the calculations eighteen years later。
the sun to the earth。 pluto is about forty aus from us； the heart of the oort cloud about fiftythousand。 in a word； it is remote。
but let’s pretend again that we have made it to the oort cloud。 the first thing you mightnotice is how very peaceful it is out here。 we’re a long way from anywhere now—so far fromour own sun that it’s not even the brightest star in the sky。 it is a remarkable thought that thatdistant tiny twinkle has enough gravity to hold all these ets in orbit。 it’s not a very strongbond； so the ets drift in a stately manner； moving at only about 220 miles an hour。 fromtime to time some of these lonely ets are nudged out of their normal orbit by some slightgravitational perturbation—a passing star perhaps。 sometimes they are ejected into theemptiness of space； never to be seen again； but sometimes they fall into a long orbit aroundthe sun。 about three or four of these a year； known as long…period ets； pass through theinner solar system。 just occasionally these stray visitors smack into something solid； likeearth。 that’s why we’ve e out here now—because the et we have e to see hasjust 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 atleast—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 asempty as the emptiness of interstellar space。 and there is a great deal of this nothingness untilyou 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 sissyskip 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 madethe trip you still wouldn’t be anywhere except at a lonely clutch of stars in the middle of avast nowhere。 to reach the next landmark of consequence； sirius； would involve another 4。6light…years of travel。 and so it would go if you tried to star…hop your