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now 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 even now the number of people searching for such objects ismodest。 the arresting analogy that is always made is that the number of people in the worldwho are actively searching for asteroids is fewer than the staff of a typical mcdonald鈥檚restaurant。 （it is actually somewhat higher now。 but not much。）while gene shoemaker was trying to get people galvanized about the potential dangers ofthe inner solar system； another development鈥攚holly unrelated on the face of it鈥攚as quietlyunfolding in italy with the work of a young geologist from the lamont doherty laboratory atcolumbia university。 in the early 1970s； walter alvarez was doing fieldwork in a elydefile known as the bottaccione gorge； near the umbrian hill town of gubbio； when he grewcurious about a thin band of reddish clay that divided two ancient layers of limestone鈥攐nefrom the cretaceous period； the other from the tertiary。 this is a point known to geology asthe kt boundary；1and it marks the time； sixty…five million years ago； when the dinosaurs androughly half the world鈥檚 other species of animals abruptly vanish from the fossil record。
alvarez wondered what it was about a thin lamina of clay； barely a quarter of an inch thick；that could account for such a dramatic moment in earth鈥檚 history。
at the time the conventional wisdom about the dinosaur extinction was the same as it hadbeen in charles lyell鈥檚 day a century earlier鈥攏amely that the dinosaurs had died out overmillions of years。 but the thinness of the clay layer clearly suggested that in umbria； if1it is kt rather than ct because c had already been appropriated for cambrian。 depending on which sourceyou credit； the k es either from the greek kreta or german kreide。 both conveniently mean 鈥渃halk；鈥潯hichis also what cretaceous means。
nowhere else； something rather more abrupt had happened。 unfortunately in the 1970s notests existed for determining how long such a deposit might have taken to accumulate。
in the normal course of things； alvarez almost certainly would have had to leave theproblem at that； but luckily he had an impeccable connection to someone outside hisdiscipline who could help鈥攈is father； luis。 luis alvarez was an eminent nuclear physicist；he had won the nobel prize for physics the previous decade。 he had always been mildlyscornful of his son鈥檚 attachment to rocks； but this problem intrigued him。 it occurred to himthat the answer might lie in dust from space。
every year the earth accumulates some thirty thousand metric tons of 鈥渃osmicspherules鈥濃�攕pace dust in plainer language鈥攚hich would be quite a lot if you swept it intoone pile； but is infinitesimal when spread across the globe。 scattered through this thin dustingare exotic elements not normally much found on earth。 among these is the element iridium；which is a thousand times more abundant in space than in the earth鈥檚 crust （because； it isthought； most of the iridium on earth sank to the core when the planet was young）。
alvarez knew that a colleague of his at the lawrence berkeley laboratory in california；frank asaro； had developed a technique for measuring very precisely the chemicalposition of clays using a process called neutron activation analysis。 this involvedbombarding samples with neutrons in a small nuclear reactor and carefully counting thegamma rays that were emitted； it was extremely finicky work。 previously asaro had used thetechnique to analyze pieces of pottery； but alvarez reasoned that if they measured the amountof one of the exotic elements in his son鈥檚 soil samples and pared that with its annual rateof deposition； they would know how long it had taken the samples to form。 on an octoberafternoon in 1977； luis and walter alvarez dropped in on asaro and asked him if he wouldrun the necessary tests for them。
it was really quite a presumptuous request。 they were asking asaro to devote months tomaking the most painstaking measurements of geological samples merely to confirm whatseemed entirely self…evident to begin with鈥攖hat the thin layer of clay had been formed asquickly as it