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y 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 week and go unnoticed。
an object a hundred yards across couldn’t 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’srestaurant。 （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—wholly unrelated on the face of it—was 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—onefrom 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’s 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’s history。
at the time the conventional wisdom about the dinosaur extinction was the same as it hadbeen in charles lyell’s day a century earlier—namely 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 “chalk；” whichis 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—his 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’s 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 “cosmicspherules”—space dust in plainer language—which 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’s 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’s 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—that the thin layer of clay had been formed asquickly as its thinness suggested。 certainly no one expected his survey to yield any dramaticbreakthroughs。
“well； they were very charming； very persuasive；” asaro recalled in an interview in 2002。
“and it seemed an interesting challenge； so i agreed to try。 unfortunately； i had a lot of otherwork on； so it was eight months before i could get to it。” he consulted his notes from theperiod。 “on june 21； 1978； at 1：45 p。m。； we put a sample in the detector。 it ran for 224minutes and we could see we were getting interesting results； so we stopped it and had alook。”
the results were so unexpected； in fact； that the three scientists at first thought they had tobe wrong。 the amount of iridium in the alvarez sample was more than three hundred timesnormal levels—far beyond anything they might have predicted。 over the following monthsasaro and his colleague helen michel worked up to thirty hours at a stretch （“once youstarted you couldn’t stop；” asaro explained） analyzing samples； always with the same results。
tests on other samples—from denmark； spain； france； new zealand； antarctica—showedthat the iridium deposit was worldwide and greatly elevated everywhere； sometimes by asmuch as five hundred times normal levels。 clearly something big and abrupt； and probablycataclysmic； had produced this arresting spike。
after much thought； the alvarezes concluded that the most plausible explanation—plausible to them； at any rate—was that the earth had been struck by an asteroid or et。
the idea that the earth might be subjected to devastating impacts from time to time was notquite as new as it is now sometimes presented。 as far back as 1942； a northwesternuniversity astrophysicist named ralph b。 baldwin had suggested such a possibility in anarticle in popular astronomy magazine。 （he published the a