A new study portrays the paths of asteroids in the inner solar system as a vast Los Angeles-style traffic system crisscrossed with superhighways along which are hurtling huge, rocky projectiles. And in the middle of the highway network, on a possible collision path, is the planet Earth.

The study estimates that an armada of asteroids, 900 strong, all a kilometer in diameter or larger, present a potential hazard to life on Earth. Some pass within a few moon distances of Earth every year. "Sometime in the future, one of these objects could conceivably run into the Earth," warns astronomy researcher William Bottke at Cornell University. "One kilometer (about .6 of a mile) in size is thought to be a magic number, because it has been estimated that these asteroids are capable of wreaking global devastation if they hit the Earth."

Bottke is lead researcher on a U.S.- French team that has discovered the spatial and size distribution of a large group of asteroids called NEAs (for near-Earth asteroids), a vast system of orbiting rocks in inner space, ranging in size from mere specks to more than 64 kilometers (40 miles) in diameter. The astronomers believe the results of their observational and computer-based study will better quantify the likelihood of future catastrophic collisions with Earth. The survey also is expected to help observational astronomers in improving their search for hard-to-find asteroids that might pose a threat to the planet.

The team's report, "Understanding the Distribution of Near-Earth Asteroids," appears in the latest edition (June 23) of the journal Science. The authors, besides Cornell's Bottke, are astronomers with the Spacewatch group at the University of Arizona's Lunar and Planetary Laboratory and at the Observatoire de la Côte d'Azur in Nice, France.

Calculating which, if any, of the 900 asteroids identified in the study could hit the Earth is tricky, says Bottke. "The problem is that fewer than half of these Earth-threatening asteroids have been discovered so far. Of those we have found, we can accurately predict whether they will strike the Earth over the next hundred years or so, but we can't project out several thousands of years. So it's possible some of these asteroids eventually will move onto an Earth-collision trajectory. It's a dangerous place out there."

The new predictions for the distribution of NEAs in the inner solar system, say the astronomers, imply that 40 percent of the kilometer-or-larger asteroids near Earth already have been discovered. The remaining 60 percent, however, might be more difficult to find, says Bottke. "Most of these asteroids are too far from Earth to be easily detected or are located in regions of the sky that are challenging for astronomers to survey."

The study's authors refer to their survey as a "NEA treasure map" indicating in which orbits most NEAs spend their time. The researchers say the new estimate of the number of large asteroids is about half of that predicted by similar types of analyses reported in the past decade and is slightly larger than an estimate published recently in the journal Nature.

For many decades there has been good evidence that most of the small chunks of rocky or iron material that slam into the Earth's atmosphere daily are chips off old blocks of asteroids. Most of the asteroids in the solar system revolve around the sun on independent orbits, corralled between Mars and Jupiter in a formation known as the main belt. Occasionally, two of these asteroids -- some of them hundreds of miles in diameter -- slam into each other at great speed, causing chunks of all sizes to be blasted off the surfaces.

Most of this material continues to orbit the sun in the main belt. But sometimes the newly formed asteroids migrate to unstable regions of the asteroid belt known as resonances, areas where the tiny gravitational kicks produced by nearby planets such as Mars, Jupiter or Saturn can significantly change asteroid orbits. In some cases, these changes are enough to swing asteroids into a possible future collision path with the Earth.

To find the location of these potentially threatening and hard-to-find projectiles, the researchers used the results of the Spacewatch group's 10-year search for asteroids in the solar system during which it has discovered about 100 NEAs. The problem is that this tally is only a small fraction of the predicted number of NEAs. Using a statistical technique to compensate for the big gaps, Spacewatch astronomers were able to calculate the total number of NEAs but not their approximate location. To obtain the orbits of the undetected NEAs, Spacewatch astronomers combined their NEA population estimates with theoretical models, produced by the Cornell and Nice researchers, which show how asteroids in the main belt are transported to the near-Earth environment.

Other authors of the study were Robert Jedicke of the University of Arizona and Alessandro Morbidelli, Jean-Marc Petit and Brett Gladman of the Observatoire de la Côte d'Azur. The study was funded by NASA and the European Space Agency.

Related World Wide Web sites: The following sites provide additional information on this news release. Some might not be part of the Cornell University community, and Cornell has no control over their content or availability.

Outdoorsman Jim Brook and scientists at The University of Western Ontario (UWO) and the University of Calgary (U of C) have recovered the largest meteorite fall in Canadian history. Analysis shows the meteorite is composed of a very rare material, making it among the most scientifically significant meteorite finds worldwide.

The meteorites fell on the morning of January 18, 2000 in a remote area between Atlin, British Columbia and Carcross, Yukon Territory. A week later on January 25th, a nearby resident, Jim Brook, found the first meteorite fragments while driving homewards on the ice of Taku Arm in Tagish Lake.

Jim Brook describes his discovery, "I was watching closely for meteorites and suspected their identity as soon as I saw them, although I had been fooled several times by wolf droppings. It was obvious what they were as soon as I picked one up, because rocks aren't found on the ice, and I could see the outer melted crust. I was very happy and excited." Darkness soon ended additional meteorite hunting that day, but Jim was back the next morning, collecting several dozen of the space rocks.

Since that find, U of C and UWO researchers, working with the National Aeronautics and Space Administration (NASA), have made several trips to the area to collect samples of the very fragile meteorites and to map the fall area. To date, 500 fragments have been found and hundreds have been recovered from the site - many still encased in ice.

"This is the find of a lifetime," says Peter Brown, meteor scientist in the Department of Physics and Astronomy at The University of Western Ontario and co-leader of the meteorite recovery investigation. "The size of the initial object, the extreme rarity and organic richness of the meteorites combined with the number we have uncovered make this a truly unique event."

"Of all the times I dreamed of finding meteorites, I never thought of finding them like this," says Alan Hildebrand, planetary scientist in the Department of Geology and Geophysics at the University of Calgary and the other investigation co-leader. "One day while I was picking pieces of meteorite out of porous ice I thought that the experience must be a bit like sampling on the surface of a comet. We believe these to be the most fragile meteorites ever recovered."

Initial analysis by Michael Zolensky, a meteoriticist at NASA's Johnson Space Center showed the meteorites were a type of carbonaceous chondrite - a rare, organically rich, charcoal-like class of meteorites. Zolensky says that his work and that of colleagues "provides indications that the meteorites are unique carbonaceous chondrites with hints of relation to the CI chondrites." Carbonaceous chondrite meteorites make up about three per cent of meteorite finds. The possible chemical class of this fall constitutes less than 0.1 per cent of all meteorites recovered to date, and represents the most primordial samples known from the early solar system. While the possibilities have researchers very excited, the meteorites' true significance remains to be fully understood. However, Jim Brook's careful collection of pristine meteorites from the icebox of a Canadian winter and subsequent frozen storage has opened brand new doors for meteorite researchers around the world.

The Nomenclature Committee of the Meteoritical Society has officially designated the name Tagish Lake Meteorite for the fall specimens.

Using eyewitness and photographic data gathered during the field investigations, and observations from two US Department of Defense satellite systems, the trajectory and velocity of the fireball were determined. The ability to calculate this is a relatively new development in meteorite science - essentially allowing researchers to determine a meteorite's pre-fall size, orbit and origin in space.

"There have only been four previous meteorites for which accurate orbits are known and no orbits for a carbonaceous chondrite have ever been secured," says Brown. "The entire process of recovery of the material and determination of where it comes from makes this the scientific equivalent of an actual sample-return space mission - at a thousandth of the cost."

"The Tagish Lake fall is the largest ever recorded over land by the satellite systems," notes Hildebrand. "The recovery of hundreds of meteorites allows studies which will precisely constrain the meteorite's size when it entered the Earth's atmosphere. Calibrating the satellite observations for such a large object will help us understand all the fireballs that the satellites record around the globe, in effect creating a global fireball camera system. These observations will increase our knowledge of both the hazards and opportunities created by the Earth-crossing asteroids and comets."

In the same spirit with which hundreds of eyewitnesses described their observations and donated photographs and videos to the investigation, and the U.S. Department of Defense quickly supplied satellite data, the two universities and Jim Brook have agreed to immediately make available some of the rare meteorite to researchers. Forty grams of once water-soaked (but now dried) meteorite fragments are now available on a proposal basis to interested researchers. Work descriptions and sample requirements (to a maximum length of one page) should be sent to hildebra@geo.ucalgary.ca for consideration before June 30, 2000. Material for analysis will be provided to all successful proposals within 30 days barring unanticipated circumstances.

Backgrounder

January 18, 2000 - The fireball - A spectacular meteor crosses the Yukon Territory into northern British Columbia at 08:43 PST. Eyewitnesses reported a brilliant, multicolored fireball that lit up the countryside. Sizzling sounds and peculiar smells that remain to be adequately explained accompanied the fireball. Ground shaking detonations followed a few minutes after the meteor's passage when its sound arrived at the land surface. The fireball and its explosions were so stunning that local residents were concerned about the safety of their children and friends. The fireball was also observed by satellites in Earth orbit, maintained by the U.S. Department of Defense (D of D). These observations established an asteroid weighing 200 tonnes and approximately five metres across had impacted the Earth's atmosphere. Data from D of D satellites were available within hours of the event, the quickest any such data have been released after a bolide event by the D of D.

January 19, 2000 - Airbourne sampling - From information gathered via email and press reports, Peter Brown, meteor scientist at The University of Western Ontario, discusses with Dr. Michael Zolensky of NASA's Johnson Space Center (JSC) the possibility of arranging an ER-2 aircraft sampling flight over the area to attempt to recover small airborne particles. A series of two flights is approved, but technical problems ground a first flight and only one air photo/air sampling flight is performed on January 21, 2000. Analysis of particles from this air-sampling mission are ongoing.

January 26, 2000 - Meteorites discovered - While Jim Brook was driving south on the ice of Taku Arm, Tagish Lake, British Columbia, he noticed small dark rocks on the ice. He suspected that these were meteorites from the fireball. He carefully collected the rocks, covering his fingers with clean plastic and placing the meteorites in plastic bags. Brook uncovered almost one kilogram of this material during a total of only a few hours of searching on the lake ice late on January 25 and early on January 26. Snow blankets the area on January 27 ending recovery opportunities.

February 7, 2000 - Rare meteorite type confirmed - Zolensky receives two samples from Brook with transportation arranged by the Geological Survey of Canada. He confirms their suspected identification as carbonaceous chondrites. Counting of short-lived cosmogenic nuclides begins immediately at JSC.

February 16 - 28, 2000 - Fireball investigation and field search - An initial field investigation is led by Brown and Hildebrand to the fall area. Eyewitnesses of the fireball across the Yukon and northern B.C. are interviewed, and video and photographic stills of the long-lasting dust cloud left by the fireball are gathered. An initial path through the atmosphere is calculated. The lake area and adjacent forest along Taku Arm, Tagish Lake, where the initial meteorites were recovered is searched in an effort to recover more pristine material. However, the heavy snow cover proves insurmountable. The decision is made that, if more meteorites are present, they probably can't be found until the spring melt arrives.

April 6 - 15, 2000 - Second expedition - Additional fireball data are gathered. From the information obtained during the first field investigation a more accurate path has been derived for the fireball trajectory. With velocity data from satellite observations, calculations were performed as to where meteorites of various sizes would have fallen to narrow the potential search area.

April 15 - 19, 2000 - Spring thaw accelerates - Searching of the fall area begins again despite continued snow cover. Snow depths decrease during these five days as temperatures increase. Searching bare spots on land yields no meteorites.

April 20, 2000 - Meteorites found - The first meteorites are found and a race against time begins. The Taku Arm lake ice would soon melt and ever changing conditions complicated field work. In the first few days less than 10 meteorites were recovered per day. These meteorites were absorbing sunlight and rapidly sinking through the meter-thick ice. The recovery team wondered how much longer meteorites could be found and retrieved. Then searching conditions improved and totals found soared, reaching a high of 94 meteorites in one day.

May 8, 2000 - Unsafe conditions and an exhausted team - The ice in the fall region had become unsafe, and recovery efforts stop. Approximately 500 meteorites had been found on Taku Arm in a strewn field 16 kilometres long and three kilometres wide. Thousands more fell on the ice and the surrounding hills and mountains, but none have yet been found on land. Approximately 200 meteorites were recovered totaling five to 10 kilograms in mass, but most of this material remains frozen and a tonne of meteorite-bearing ice is now in storage. A field effort consisting of 234 person field days is now over. This recovery effort is believed unique in the history of meteoritics.

May 28, 2000 - Meteorites "drown" - Jim Brook reports that the ice of Taku Arm is now gone.

Terminology

Carbonaceous Chondrites: A rare class of meteorites that have suffered exposure to water on their original parent body surfaces in space. This meteorite group is among the most primitive material in the solar system, having generally escaped from high-temperature processing.

Carbonaceous chondrites are also unique in that they contain significant carbon, primarily in the form of organic compounds similar to those found in living organisms on Earth. Amino acids, for example, have been identified in carbonaceous chondrites, including a large number which do not occur naturally on Earth. These meteorites are metal-poor and water rich, in contrast to almost all other meteorite classes.

Satellite Systems: The U.S. Department of Defense maintains two satellite systems that can detect fireballs caused by asteroidal and cometary fragments entering Earth's atmosphere. One system consists of visible light sensors which 'stare' continuously at the Earth; they have high temporal resolution of transient flashes and measure the total energy released at visible wavelengths. A second system of detectors is sensitive in the infrared (IR) and scan across the visible face of the Earth at intervals; the IR detectors can provide location and velocity information for fireballs.

On January 18, 2000, residents of Western Canada were surprised when a fireball as bright as the Sun streaked across the morning sky. Exploding with an estimated yield of 5-10 thousand tons of TNT, the brilliant meteor attracted the attention of defense satellites, seismic monitoring stations, and just about anyone who happened to be standing outdoors within 700-800 km of the dazzling meteor's path.

"People described it as coming over the mountains, over their heads, and then disappearing over the horizon," says Dr. Peter Brown of the University of Western Ontario. "It was very long-lasting and unusual. We estimate that this object was about 7 meters across and 200 to 250 metric tons. This wasn't your average meteoroid -- it was basically a C-class asteroid detonating in the atmosphere over the Arctic!"

The first fragments of the object were discovered in January by a local resident near the spot where the meteorite hit.

"He was driving his truck across a frozen lake [Tagish Lake] when he noticed some black rock on the snow-covered ice," recounted Brown during a recent presentation at the NASA/Marshall Space Flight Center. "Fortunately, we had been in contact with him beforehand [because he lived near the expected fall-zone] so he knew how to collect the samples. He placed them in clean plastic bags and kept them continuously frozen -- they've never been touched by human hands. He found several dozen pieces after looking for 90 minutes."

"The fragments have been positively identified as carbonaceous chondrites," says Brown. "This is very important. Carbonaceous chondrites are the most pristine, organically-rich meteorites known. The ones that we find soon after a fall are even better than Antarctic meteorites, which have been sitting out for a long, long time -- in some cases 10,000 years or more. This is the first time a meteorite has fallen in a cold arctic area and been quickly recovered."

In April, 2000, Brown and a team of scientists returned to the icy lake to look for more fragments that might have been uncovered as the snow began to melt with the coming of Spring.

"We were out on the lake on April 20 when we came across a hole in the snow with dark material at the bottom. It looked like wolf droppings but it was actually a carbonaceous chondrite! We spent days harvesting and came out with over 400 fragments. The biggest single piece was 200-300 grams; the total mass collected was 5 - 10 kilograms. The only reason that we were able to recover these things is that they were frozen on ice. Water turns these carbonaceous chondrites into mush -- it looks like a black organic sludge when you add water."

Even though the fireball streaked across the sky in a fiery-looking blaze, the fragments were probably ice-cold when they hit the ground, said Brown.

"The outer layers were hot [due to friction with the atmosphere], but carbonaceous chondrites are very porous and don't conduct heat very well," he explained. The inside of the object was still frozen by the icy cold of space when the pieces reached the ground.

These are the only freshly fallen meteorite fragments ever recovered and transferred to a laboratory without thawing. Keeping the fragments continuously frozen minimized the potential loss of organic materials and other volatile compounds in the fragments.

Carbonaceous chondrites, which comprise only about 2 percent of meteorites known to have fallen to Earth, are typically difficult to recover because they easily break down during entry into Earth's atmosphere and during weathering on the ground.

"They are rare because they are so very fragile," continued Brown. "You need an incoming meteorite that's huge -- something that can afford to lose hundreds of metric tons as it blazes through the atmosphere and still deposit many kilograms on the ground."

The fragments -- lumps of crumbly rock with scorched, pitted surfaces -- resembled partly used charcoal briquettes: black, porous, fairly light -- about the same density as lightweight pumice.

Brown and colleagues are trying to compute an accurate orbit for the meteorite to discover where it came from.

"The data we have now indicates the object followed a low-inclination orbit coming from the asteroid belt," says Brown. "Its incoming velocity was 15-16 km/s -- if we can pinpoint the velocity with a precision a few hundred m/s, we might be able to tell which asteroid family this object came from. So far we know that the object has a typically asteroidal orbit, though it is remotely possible that it might be related to short-period comets."

Although the Yukon meteor was spectacular, Brown notes that it didn't add much to the amount of extraterrestrial material that falls to Earth every day.

"Daily the Earth is bombarded by 80 to 100 metric tons of microscopic space dust (in the form of 10-5 gm particles)," said Brown. "Thus, the Yukon meteorite was only 2 or 3 days worth of space dust."

Editor's note: The Nomenclature Committee of the Meteoritical Society has officially designated the name Tagish Lake Meteorite for the Yukon fall specimens.