按键盘上方向键 ← 或 → 可快速上下翻页，按键盘上的 Enter 键可回到本书目录页，按键盘上方向键 ↑ 可回到本页顶部！
————未阅读完？加入书签已便下次继续阅读！
d everywhere as a genius ofthe first order； and tombaugh was largely forgotten； except among planetary astronomers；who tend to revere him。
a few astronomers continue to think there may be a planet x out there鈥攁 real whopper；perhaps as much as ten times the size of jupiter； but so far out as to be invisible to us。 （itwould receive so little sunlight that it would have almost none to reflect。） the idea is that itwouldn鈥檛 be a conventional planet like jupiter or saturn鈥攊t鈥檚 much too far away for that；we鈥檙e talking perhaps 4。5 trillion miles鈥攂ut more like a sun that never quite made it。 moststar systems in the cosmos are binary （double…starred）； which makes our solitary sun a slightoddity。
as for pluto itself； nobody is quite sure how big it is； or what it is made of； what kind ofatmosphere it has； or even what it really is。 a lot of astronomers believe it isn鈥檛 a planet at all；but merely the largest object so far found in a zone of galactic debris known as the kuiperbelt。 the kuiper belt was actually theorized by an astronomer named f。 c。 leonard in 1930；but the name honors gerard kuiper； a dutch native working in america； who expanded theidea。 the kuiper belt is the source of what are known as short…period ets鈥攖hose thate past pretty regularly鈥攐f which the most famous is halley鈥檚 et。 the more reclusivelong…period ets （among them the recent visitors hale…bopp and hyakutake） e fromthe much more distant oort cloud； about which more presently。
it is certainly true that pluto doesn鈥檛 act much like the other planets。 not only is it runty andobscure； but it is so variable in its motions that no one can tell you exactly where pluto will bea century hence。 whereas the other planets orbit on more or less the same plane； pluto鈥檚orbital path is tipped （as it were） out of alignment at an angle of seventeen degrees； like thebrim of a hat tilted rakishly on someone鈥檚 head。 its orbit is so irregular that for substantialperiods on each of its lonely circuits around the sun it is closer to us than neptune is。 for most of the 1980s and 1990s； neptune was in fact the solar system鈥檚 most far…flung planet。
only on february 11； 1999； did pluto return to the outside lane； there to remain for the next228 years。
so if pluto really is a planet； it is certainly an odd one。 it is very tiny： just one…quarter of 1percent as massive as earth。 if you set it down on top of the united states； it would cover notquite half the lower forty…eight states。 this alone makes it extremely anomalous； it means thatour planetary system consists of four rocky inner planets； four gassy outer giants； and a tiny；solitary iceball。 moreover； there is every reason to suppose that we may soon begin to findother even larger icy spheres in the same portion of space。 then we will have problems。 afterchristy spotted pluto鈥檚 moon； astronomers began to regard that section of the cosmos moreattentively and as of early december 2002 had found over six hundred additional trans…neptunian objects； or plutinos as they are alternatively called。 one； dubbed varuna； is nearlyas big as pluto鈥檚 moon。 astronomers now think there may be billions of these objects。 thedifficulty is that many of them are awfully dark。 typically they have an albedo； orreflectiveness； of just 4 percent； about the same as a lump of charcoal鈥攁nd of course theselumps of charcoal are about four billion miles away。
and how far is that exactly？ it鈥檚 almost beyond imagining。 space； you see； is justenormous鈥攋ust enormous。 let鈥檚 imagine； for purposes of edification and entertainment； thatwe are about to go on a journey by rocketship。 we won鈥檛 go terribly far鈥攋ust to the edge ofour own solar system鈥攂ut we need to get a fix on how big a place space is and what a smallpart of it we occupy。
now the bad news； i鈥檓 afraid； is that we won鈥檛 be home for supper。 even at the speed oflight； it would take seven hours to get to pluto。 but of course we can鈥檛 travel at anything likethat speed。 we鈥檒l 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 鈥済ravity assist鈥潯echnique inwhich the craft were successively flung from one gassy giant to the next in a kind of cosmicversion of 鈥渃rack the whip。鈥潯ven 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鈥檚 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鈥攖hough getting home again will take rather longer； i鈥檓 afraid。 at allevents； it鈥檚 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鈥攖he sun； the planets and their moons； the billion or so tumblingrocks of the asteroid belt； ets； and other miscellaneous drifting detritus鈥攆ills 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鈥攖he outer giants actually castshadows over each other in many illustrations鈥攂ut this is a necessary deceit to get them all on the same piece of paper。 neptune in reality isn鈥檛 just a little bit beyond jupiter； it鈥檚 waybeyond jupiter鈥攆ive 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鈥檛 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鈥檛 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鈥檛 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鈥攐ur dear； warm； skin…tanning； life…giving sun鈥攈as 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鈥攑luto鈥檚 moon； for example鈥攈ave 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 鈥渁t least ninety；鈥潯bout 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鈥檛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 pastpluto。 if you check your itinerary； you will see that this is a trip to the edge of our solarsystem； and i鈥檓 afraid we鈥檙e not there yet。 pluto may be the last object marked onschoolroom charts； but the system doesn鈥檛 end there。 in fact； it isn鈥檛 even close to endingthere。 we won鈥檛 get to the solar system鈥檚 edge until we have passed through the oort cloud； avast celestial realm of drifting ets； and we won鈥檛 reach the oort cloud for another鈥攊鈥檓 sosorry about this鈥攖en 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鈥攅ver。 it is just too far。
as it is； even with the hubble telescope； we can鈥檛 see even into the oort cloud； so we don鈥檛actually know that it is there。 its existence is probable but entirely