Other astronomers had more interest (and fewer viruses). Working from the observations made by Jim Scotti of the University of Arizona, who had first spied 1997 XF 11 through the 36-inch telescope atop Kitt Peak on Dec. 6, two Japanese amateurs drew a bead on its trajectory: it swings around the sun once every 21 months, and in 2028 would come within 500,000 miles of Earth. That was close enough to land it on the list of potential doomsday rocks–the 108 PHAs. Peter Shelus of the McDonald Observatory in Texas then got into the game: his observations in early March gave astronomers what they call an 88-day arc, which is that many days’ worth of the asteroid’s path. The new itinerary had XF 11, at 1:30 in the afternoon, Eastern daylight time, on Oct. 26, 2028, within 26,000 miles of Earth’s surface. Or closer.
When Helin got the news last week via a ““minor planet electronic circular’’–e-mail–she noticed something else. Brian Marsden of the Smithsonian Astrophysical Observatory, who runs this early-warning service, added that XF 11 had swung by Earth in 1990, and if anyone had a picture of that it might be very useful in refining calculations of its orbit. That was all Helin needed: she had run an asteroid survey at the Palomar Observatory from 1973 to 1995. ““I bet we have’’ this, she told her NEATsters. The next morning Ken Lawrence, the archive buff, hit the file cabinets. Using the data from Marsden, he knew exactly which photographic plate to pull and where on the plate to look for XF 11. Bingo: a faint blur hovered near the bottom of one of the images. Lawrence quickly digitized the images so NEAT’s computer could calculate the exact position of XF 11 in the 1990 shot, then sent the results back to Cambridge and waited while Marsden’s colleague Gareth Williams did the math. JPL’s Donald Yeomans and Paul Chodas were also processing the new numbers. The new ““miss distance’’: 600,000 miles.
For 24 hours last week television stations ran terrifying simulations of an asteroid slamming into Earth. Hollywood studios behind the two soon-to-be-released asteroid-and-comet movies went into publicity hyperdrive at the thought of having reality whip up interest in ““Armageddon’’ (from Disney) and ““Deep Impact’’ (Paramount), and office patter turned to the really important question: what might XF 11 do to mutual-fund prices? But just because the immediate threat evaporated literally overnight–inspiring such newspaper headlines as THE END IS NOT NEAR and KISS YOUR ASTEROID GOODBYE–doesn’t mean Earth is out of the cross hairs forever. Projects like Arizona’s Spacewatch and JPL’s NEAT show that space is one big pinball machine, in which any wildly racing orb could make Earth go tilt–or worse. The ubiquitous cratering of planets and the moon, which shows that worlds do indeed collide, ““is one of the most significant discoveries of the space age,’’ says astronomer Clark Chapman of the Southwest Research Institute in Boulder, Colo. Earth itself has been slammed at least 139 times. And that just counts the obvious impact craters, not those hidden by the seas or erased by erosion. As detection systems get better, warnings of impending doom could come as often from science as from psychics.
But should they? Calling XF 11 a major threat ““was crying wolf,’’ charged JPL’s Yeomans. ““A simple e-mail would have accomplished the same thing: “Would you please look for some observations?’’’ he says. Marsden, who issued the bulletin that XF 11 would pass perilously close to Earth, accepted the 600,000-mile figure. But he maintained that XF 11 could still be nudged onto a deadly path if it passed close enough to another asteroid, whose gravity would alter its orbit. He defended his announcement as necessary to get as many astronomers as possible–pros and amateurs alike–to track XF 11 before it disappeared temporarily this summer. That way its precise orbit would be learned. ““If we hadn’t brought it up,’’ says Marsden, ““nobody would have done anything, and we’d be waiting on tenterhooks for two years.’’ Still, the risk of on-again/off-again doomsday is that Earthlings won’t take any future impact warning seriously.
They should. The relevant question about the next big asteroid impact is not ““if’’ but ““when.’’ The first Earth-crossing asteroids–those whose paths around the sun intersect Earth’s–were discovered in 1932. Traveling at 45,000 miles per hour, an XF 11-size asteroid aimed at Earth would slam into the atmosphere at 100 times the speed of a bullet and, less than a second later, strike the planet with an explosive force of 500,000 megatons of TNT, calculates astronomer Henry Melosh of UA. (The atomic bomb that hit Hiroshima packed an estimated .015 megaton.) ““It would drill a tunnel through the atmosphere [five miles] deep,’’ he says. The shock wave from the impact would level every parking meter, tree and building within miles and launch a plume of vaporized and pulverized stone, which would blast through the hole in the atmosphere blown out by the asteroid’s entry. The plume would then fall back to Earth, raining pebbles, dust and even boulders on survivors: the impact of an asteroid in what is now Germany millions of years ago lofted building-size blocks of rock into Switzerland, says Chapman. The impact would throw tons of sulfur into the air, forming a haze of sulfuric acid shrouding the planet for three to five years, causing crops to fail worldwide. XF 11, about one mile across, happens to be the size that would bring damage ““beyond the experience of human history,’’ as a 1997 study put it.
IF ANYONE THOUGHT THAT only Hollywood wipes out planets with asteroids, they weren’t paying attention in July 1994. Over the course of a week, fragments of Comet Shoemaker-Levy 9 smacked into Jupiter and threw into its atmosphere clouds of debris as big as Earth. Jupiter’s pummeling taught astronomers that a milewide asteroid like XF 11 is ““the likely threshold size for a civilization-threatening ecological disaster,’’ says Chapman.
The extent of the devastation, if an asteroid hit Earth, can be summed up in two words: size matters. Even one 2,000 feet across, less than half XF 11’s size, would produce an explosion powerful enough to ignite fires and level buildings over 10,000 to 100,000 square miles. Earth endured a trial run of such a disaster on a quiet June morning in 1908, when a comet exploded five miles above Tunguska, Siberia. The 20-megaton blast ignited the clothes of a man 60 miles away, felled more than 700 square miles of forests and annihilated reindeer 30 miles away. If the Tunguska object had exploded over New York or Moscow, it would have destroyed either.
An ocean landing wouldn’t be much better. ““The impact would form a crater,’’ says physicist Jack Hills of Los Alamos National Laboratory. ““The rim itself would propagate outward, water would rush in to fill it, overshoot, come back again and form a deep-water wave’’ speeding away from the point of impact. When the wave smacked up against the continental shelf, its speed would fall and its height would rise. ““An impact anywhere in the Atlantic by an asteroid [1,200 feet] in diameter would devastate the coasts on both sides of the ocean by tsunamis over [300 feet] high,’’ he says. If XF 11 fell in the Atlantic, its tsunami would cover several thousand miles of shoreline, for tens of miles inland. ““Where cities stood,’’ says Hills, ““there would be only mud flats’’ littered with ruins.
Earth spins through some treacherous neighborhoods. NASA estimates that 1,000 to 4,000 asteroids cross Earth’s orbit and are larger than half a mile across; of those, only a few more than 150 have been identified. Of the 300,000 at least 300 feet across–packing power comparable to the comet that blew up over Tunguska–we know too few to mention. A 1995 NASA commission said that given $50 million spread over 10 years it could mount a project called Spaceguard, finding and calculating orbits for more than 90 percent of the asteroids over a half mile in diameter that could threaten the Earth. It didn’t get the money.
SO WHILE PROGRAMS LIKE Spacewatch and NEAT scrabble for a few million dollars a year to catalog all the irresistible forces out there, scientists elsewhere are trying to figure out what happens after the ““Incoming!’’ Last year a NASA probe rendezvoused with an asteroid moving 56,000 miles an hour, but other probes have missed the mark–Clementine, a joint NASA-Department of Defense mission, died on the way to the asteroid Geographos in 1994. Scientists still aren’t entirely sure what asteroids are made of, or even if they’re one piece or just flying piles of rubble. If they ever have to push one out of an earthbound trajectory, they’ll need to know a lot more about composition, density and topography.
That’s not to say there aren’t plans for planetary defense. Some of the schemes to pull off a world-saving maneuver have the elegant looniness of science fiction, relying on technologies that don’t exist: giant space-based lasers, solar sails or mass drivers that excavate the rock to death en route. But ““the only technology we have is a nuclear explosive,’’ says Johndale Solem, a physicist at Los Alamos. ““It makes my skin crawl, but that’s the fact.’’ Nobody wants a lot of nukes lying around waiting for the off chance that a planet killer might happen by, but everyone agrees the sooner you see the threat, the sooner you can intercept and the easier it is to switch the thing’s course to one that doesn’t include us. That’s why sentinels like Eleanor Helin keep watching the skies.
