Fireball!- page 2 | Space | Air & Space Magazine
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(Phil Bland/Imperial College London)


Nothing gets your attention quite like a meteor screaming in at 40 miles a second.

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Ground-based instruments are also useful in catching meteors in the act. Large bolides (another term for impactors) can cause atmospheric pressure waves strong enough to register on seismic detectors. The first, and still largest, of these extraterrestrial seismic impact signals to be captured was caused by a June 1908 blast near the Siberian river Tunguska. Recent estimates put the blast at about 3,000 to 5,000 kilotons of energy, from an object roughly 120 feet across. A reasonable guess of the frequency of such impacts is once every few hundred to a thousand years.

As arms control agencies have set up a global network to monitor compliance with the Comprehensive Nuclear Test Ban Treaty, another tool has appeared to help meteor trackers. A meteor's dying scream falls in the "infrasound" range, below the range of human hearing. Infrasound can be "heard" with microphones tuned very low or barometers tuned very high, and instruments designed to listen for nuclear explosions pick up the low rumble of incoming space rocks as they hurtle through Earth's atmosphere.

Add to these the dedicated networks of all-sky cameras (nearly the entire sky appears in a single frame) set up in Canada, Europe, and the United States over the past 50 years to watch for bright meteors. The most recent is under construction in Australia's Nullarbor desert, a project headed by Phil Bland, a planetary scientist at Imperial College London, with colleagues from the United Kingdom and the Czech Republic. These networks provide photographs taken at different locations, which help researchers triangulate the positions of meteors, plot their paths through the atmosphere, and reconstruct their original solar orbits—like running a movie backward to see how it started.

Only nine times have scientists managed to assemble enough information from cameras, infrasound, seismic detectors, eyewitness reports, and other sources to reconstruct an impactor's original orbit. As a result, only a handful of the estimated 30,000 meteorites in collections come from known orbits.

Bland hopes that his Australian network, which consists of four cameras but is expected to grow to at least 10, will better bridge the gap between the astronomical study of asteroids and the geological study of meteorites. Bland foresees his cameras tracking a fireball so accurately that a search team will be able to find any resulting meteorites quickly, enabling a reconstruction of the original object's solar orbit. For asteroid researchers, that's like mounting a cheap sample-return mission. "Fundamentally, you need to know where that rock came from to understand it," says Bland.

An estimated 40,000 rocks heavier than a half-dollar fall to Earth every year (impactors larger than a couple of feet typically explode from the pressure of ramming through the atmosphere at high speed, dropping meteorites to the ground). Only a small percentage are ever found.

Scientists aren't the only ones interested in more efficient searches: The total haul of meteorites from a "witnessed" fall can be worth tens of thousands of dollars on the collectors' market. Meteorite hunters now know it's possible to predict fireballs. And maybe next time they'll get more than 20 hours' notice.

In that case, says Wayne Hally, a New Jersey-based coordinator for the North American Meteor Network, "many dozens of people would get in their cars and start driving." Among them would likely be McCartney Taylor, a collector in Austin, Texas, who says that such predictions, if they become routine, will "change the meteorite business. We're going to have to pre-deploy if we're going to beat other guys to the fall."

Robert Jedicke of the University of Hawaii's Institute for Astronomy is in charge of asteroid observations for Pan-STARRS, a new telescope network headquartered in Hawaii that will provide fast, frequent sky surveys. Pan-STARRS will outperform today's asteroid searches, but, says Jedicke, finding objects as small as 2008 TC3 on a collision course with Earth is "not going to be something that happens all the time. It's a very rare occurrence. We're going to need bigger telescopes covering much more of the sky on a regular basis."

Bigger survey telescopes are planned. Clark Chapman, who studies asteroids at the Southwest Research Institute in Boulder, Colorado, predicts that by the 2020s, the next generation of asteroid surveys will have tracked a quarter-million orbiting objects 16 feet in diameter—not much bigger than 2008 TC3.

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