dently scampered when it did not。 four of them； alas；didn’t move quite fast enough and were killed。 every june now manson has a weeklong eventcalled crater days； which was dreamed up as a way of helping people forget that unhappyanniversary。 it doesn’t really have anything to do with the crater。 nobody’s figured out a wayto capitalize on an impact site that isn’t visible。
“very 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；” says anna schlapkohl； the town’sfriendly librarian。 “then they go away kind of disappointed。” however； 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’t 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’s craters were indeed formed byimpacts—in itself quite a radical notion for the time—but that the earth’s were not。 mostscientists refused to go even that far。 to them； the moon’s 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—he couldn’t： they don’t exist—but 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—anomalous fine silicas and magnetites principally—that 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’s orbit。
“at the time we started； only slightly more than a dozen of these things had ever beendiscovered in the entire course of astronomical observation；” shoemaker recalled some yearslater in a television interview。 “astronomers in the twentieth century essentially abandonedthe solar system；” he added。 “their attention was turned to the stars； the galaxies。”
what shoemaker and his colleagues found was that there was more risk out there—a greatdeal more—than 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—and keeps—them from coalescing。
when asteroids were first detected in the 1800s—the very first was discovered on the firstday of the century by a sicilian named giuseppi piazzi—they 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—latin for “starlike”—which 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—anobject 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—half 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’t 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’t 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’s orbit as a kind of freeway on which we are the only vehicle； but whichis crossed regularly by pedestrians who don’t know enough to look before stepping off thecurb。 at least 90 percent of these pedestrians are quite unknown to us。 we don’t 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；“suppose 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。” in short； you would see not a couple of thousand distant twinkling stars； but millionsupon millions upon millions of nearer； randomly moving objects—“all 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。” well； be unnerved because it isthere。 we just can’t see it。
altogether it is thought—though it is really only a guess； based on extrapolating fromcratering rates on the moon—that some two thousand asteroids big enough to imperilcivilized existence regularly cross our orbit。 but even a small asteroid—the size of a house；say—could 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’t 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—in cosmic termsthe equivalent of a bullet passing through one’s sleeve without touching the arm。 two yearslater； another； somewhat larger asteroid missed us by just 90；000 miles—the 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