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now the bad news； i’m afraid； is that we won’t be home for supper。 even at the speed oflight； it would take seven hours to get to pluto。 but of course we can’t travel at anything likethat speed。 we’ll have to go at the speed of a spaceship； and these are rather more lumbering。
the best speeds yet achieved by any human object are those of the voyager 1 and2 spacecraft；which are now flying away from us at about thirty…five thousand miles an hour。
the reason the voyager craft were launched when they were （in august and september1977） was that jupiter； saturn； uranus； and neptune were aligned in a way that happens onlyonce every 175 years。 this enabled the two voyagers to use a “gravity assist” technique inwhich the craft were successively flung from one gassy giant to the next in a kind of cosmicversion of “crack the whip。” even so； it took them nine years to reach uranus and a dozen tocross the orbit of pluto。 the good news is that if we wait until january 2006 （which is whennasa’s new horizons spacecraft is tentatively scheduled 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