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obody鈥檚 figured out a wayto capitalize on an impact site that isn鈥檛 visible。
鈥渧ery occasionally we get people ing in and asking where they should go to see thecrater and we have to tell them that there is nothing to see；鈥潯ays anna schlapkohl； the town鈥檚friendly librarian。 鈥渢hen they go away kind of disappointed。鈥潯owever； most people；including most iowans； have never heard of the manson crater。 even for geologists it barelyrates a footnote。 but for one brief period in the 1980s； manson was the most geologicallyexciting place on earth。
the story begins in the early 1950s when a bright young geologist named eugeneshoemaker paid a visit to meteor crater in arizona。 today meteor crater is the most famousimpact site on earth and a popular tourist attraction。 in those days； however； it didn鈥檛 receivemany visitors and was still often referred to as barringer crater； after a wealthy miningengineer named daniel m。 barringer who had staked a claim on it in 1903。 barringer believedthat the crater had been formed by a ten…million…ton meteor； heavily freighted with iron andnickel； and it was his confident expectation that he would make a fortune digging it out。
unaware that the meteor and everything in it would have been vaporized on impact； hewasted a fortune； and the next twenty…six years； cutting tunnels that yielded nothing。
by the standards of today； crater research in the early 1900s was a trifle unsophisticated； tosay the least。 the leading early investigator； g。 k。 gilbert of columbia university； modeledthe effects of impacts by flinging marbles into pans of oatmeal。 （for reasons i cannot supply；gilbert conducted these experiments not in a laboratory at columbia but in a hotel room。）somehow from this gilbert concluded that the moon鈥檚 craters were indeed formed byimpacts鈥攊n itself quite a radical notion for the time鈥攂ut that the earth鈥檚 were not。 mostscientists refused to go even that far。 to them； the moon鈥檚 craters were evidence of ancientvolcanoes and nothing more。 the few craters that remained evident on earth （most had beeneroded away） were generally attributed to other causes or treated as fluky rarities。
by the time shoemaker came along； a mon view was that meteor crater had beenformed by an underground steam explosion。 shoemaker knew nothing about undergroundsteam explosions鈥攈e couldn鈥檛： they don鈥檛 exist鈥攂ut he did know all about blast zones。 oneof his first jobs out of college was to study explosion rings at the yucca flats nuclear test sitein nevada。 he concluded； as barringer had before him； that there was nothing at meteorcrater to suggest volcanic activity； but that there were huge distributions of other stuff鈥攁nomalous fine silicas and magnetites principally鈥攖hat suggested an impact from space。
intrigued； he began to study the subject in his spare time。
working first with his colleague eleanor helin and later with his wife； carolyn； andassociate david levy； shoemaker began a systematic survey of the inner solar system。 theyspent one week each month at the palomar observatory in california looking for objects；asteroids primarily； whose trajectories carried them across earth鈥檚 orbit。
鈥渁t the time we started； only slightly more than a dozen of these things had ever beendiscovered in the entire course of astronomical observation；鈥潯hoemaker recalled some yearslater in a television interview。 鈥渁stronomers in the twentieth century essentially abandonedthe solar system；鈥潯e added。 鈥渢heir attention was turned to the stars； the galaxies。鈥�
what shoemaker and his colleagues found was that there was more risk out there鈥攁 greatdeal more鈥攖han anyone had ever imagined。
asteroids； as most people know； are rocky objects orbiting in loose formation in a beltbetween mars and jupiter。 in illustrations they are always shown as existing in a jumble； butin fact the solar system is quite a roomy place and the average asteroid actually will be abouta million miles from its nearest neighbor。 nobody knows even approximately how manyasteroids there are tumbling through space； but the number is thought to be probably not lessthan a billion。 they are presumed to be planets that never quite made it； owing to theunsettling gravitational pull of jupiter； which kept鈥攁nd keeps鈥攖hem from coalescing。
when asteroids were first detected in the 1800s鈥攖he very first was discovered on the firstday of the century by a sicilian named giuseppi piazzi鈥攖hey were thought to be planets； andthe first two were named ceres and pallas。 it took some inspired deductions by theastronomer william herschel to work out that they were nowhere near planet sized but muchsmaller。 he called them asteroids鈥攍atin for 鈥渟tarlike鈥濃�攚hich was slightly unfortunate asthey are not like stars at all。 sometimes now they are more accurately called planetoids。
finding asteroids became a popular activity in the 1800s； and by the end of the centuryabout a thousand were known。 the problem was that no one was systematically recordingthem。 by the early 1900s； it had often bee impossible to know whether an asteroid thatpopped into view was new or simply one that had been noted earlier and then lost track of。 bythis time； too； astrophysics had moved on so much that few astronomers wanted to devotetheir lives to anything as mundane as rocky planetoids。 only a few astronomers； notablygerard kuiper； the dutch…born astronomer for whom the kuiper belt of ets is named；took any interest in the solar system at all。 thanks to his work at the mcdonald observatoryin texas； followed later by work done by others at the minor planet center in cincinnati andthe spacewatch project in arizona； a long list of lost asteroids was gradually whittled downuntil by the close of the twentieth century only one known asteroid was unaccounted for鈥攁nobject called 719 albert。 last seen in october 1911； it was finally tracked down in 2000 afterbeing missing for eighty…nine years。
so from the point of view of asteroid research the twentieth century was essentially just along exercise in bookkeeping。 it is really only in the last few years that astronomers havebegun to count and keep an eye on the rest of the asteroid munity。 as of july 2001；twenty…six thousand asteroids had been named and identified鈥攈alf in just the previous twoyears。 with up to a billion to identify； the count obviously has barely begun。
in a sense it hardly matters。 identifying an asteroid doesn鈥檛 make it safe。 even if everyasteroid in the solar system had a name and known orbit； no one could say what perturbationsmight send any of them hurtling toward us。 we can鈥檛 forecast rock disturbances on our ownsurface。 put them adrift in space and what they might do is beyond guessing。 any asteroid outthere that has our name on it is very likely to have no other。
think of the earth鈥檚 orbit as a kind of freeway on which we are the only vehicle； but whichis crossed regularly by pedestrians who don鈥檛 know enough to look before stepping off thecurb。 at least 90 percent of these pedestrians are quite unknown to us。 we don鈥檛 know wherethey live； what sort of hours they keep； how often they e our way。 all we know is that atsome point； at uncertain intervals； they trundle across the road down which we are cruising atsixty…six thousand miles an hour。 as steven ostro of the jet propulsion laboratory has put it；鈥渟uppose that there was a button you could push and you could light up all the earth…crossing asteroids larger than about ten meters； there would be over 100 million of these objects in thesky。鈥潯n short； you would see not a couple of thousand distant twinkling stars； but millionsupon millions upon millions of nearer； randomly moving objects鈥斺�渁ll of which are capableof colliding with the earth and all of which are moving on slightly different courses throughthe sky at different rates。 it would be deeply unnerving。鈥潯ell； be unnerved because it isthere。 we just can鈥檛 see it。
altogether it is thought鈥攖hough it is really only a guess； based on extrapolating fromcratering rates on the moon鈥攖hat some two thousand asteroids big enough to imperilcivilized existence regularly cross our orbit。 but even a small asteroid鈥攖he size of a house；say鈥攃ould destroy a city。 the number of these relative tiddlers in earth…crossing orbits isalmost certainly in the hundreds of thousands and possibly in the millions； and they are nearlyimpossible to track。
the first one wasn鈥檛 spotted until 1991； and that was after it had already gone by。 named1991 ba； it was noticed as it sailed past us at a distance of 106；000 miles鈥攊n cosmic termsthe equivalent of a bullet passing through one鈥檚 sleeve without touching the arm。 two yearslater； another； somewhat larger asteroid missed us by just 90；000 miles鈥攖he closest pass yetrecorded。 it； too； was not seen until it had passed and would have arrived without warning。
according to timothy ferris； writing in the new yorker； such near misses probably happentwo or three times a week and go unnoticed。
an object a hundred yards across couldn鈥檛 be picked up by any earth…based telescope untilit was within just a few days of us； and that is only if a telescope happened to be trained on it；which is unlikely because e