Nothing gets your attention quite like a meteor screaming in at 40 miles a second.
- By Tony Reichhardt
- Air & Space magazine, May 2009
Phil Bland/Imperial College London
(Page 2 of 3)
In the 20 hours between the discovery of 2008 TC3 and its detonation over Sudan, 27 observatories tracked its orbit. When TC3 hit the atmosphere, visual and infrared sensors on the European Meteosat 8 satellite recorded the flash. So too, according to NASA's Near Earth Object office at the Jet Propulsion Laboratory in California, did unspecified U.S. government spacecraft—presumably the Defense Support Program satellites that watch for infrared signatures of ground-launched missiles.
Data from military satellites are in fact our best evidence for the actual rate of space rocks hitting the planet each year. In 2002, Peter Brown of the University of Western Ontario and colleagues published in Nature magazine an analysis of flashes that military satellites recorded from February 1994 to September 2002. Some 300 events were identified as probable meteors. The scientists were able to reconstruct the energy of the explosions in the atmosphere, and to extrapolate the rate of larger and smaller objects hitting Earth. By their estimate, an object with 50 kilotons of energy (the bomb that exploded over Hiroshima in 1945 yielded about 15 kilotons) appears on average every 10 years. An impact with the force of 0.33 kiloton occurs monthly. The Sudan fireball, about a kiloton, was on the lower end of the range.
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.