Thomas Pynchon said it best, years before: “A screaming comes across the sky.” Midmorning today, near the city of Chelyabinsk in western Siberia, a meteor came in from the northeast. (Phil Plait has compiled many videos and pictures of the stunning event.) The meteor burned brightly and created a powerful shock wave. Nearly 1,000 people were injured, according to wire reports, and a 20-foot crater has been found in the town of Cherbakul.
Could anyone have seen the meteoroid coming?
To answer that question, we must first ask how big the meteoroid was, before most of it burned in the atmosphere. It will be a while before more reliable estimates can be made, but the Associated Press cites Richard Binzel, an MIT professor, who estimated 2 meters; the Russian Academy of Sciences estimates the size at 10 tons. (These two estimates are off by about a factor of about 3, which, for preliminary estimates, is close enough. A 2-meter-diameter object made entirely of iron would weigh about 30 tons.)
If those estimates are in the right ballpark, it was probably too small to have been seen with present methods. However, these techniques have been improving rapidly. A study released last year by NASA found that there are roughly 5,000 “potentially hazardous asteroids” 100 meters (330 feet) or larger whose orbits come near Earth’s. NASA released this rather scary-looking diagram of Earth’s orbit passing through the PHAs. (Because the PHAs are themselves moving, the situation is not quite as dire as the diagram makes it look.) A similar diagram showing objects the size of the Chelyabinsk meteor would be even scarier though. There are many, many more small near-Earth objects than large ones. NEO is another acronym devised to get around the fact that none of the plain English words—asteroid, comet, meteor, meteorite, and meteroid—exactly describe space objects that pose a danger. The same methods that astronomers use to search for larger objects, as they have been refined, make the threshold of detection smaller and smaller.
The good news is that, in the last 20 years, astronomers have gotten far, far better at discovering PHAs. This NASA chart from January of this year illustrates the dramatic increase in knowledge. We now track nearly 100 times more PHAs than we did in 1993. Up until the late 1990s, the only people looking systematically for such hazardous objects were a group at the University of Arizona called Spacewatch, which has been around since 1980. Spacewatch was a pioneering group: They were the first to really take the threat seriously and were also methodological pioneers.
Astronomers' most powerful telescopes, because they tend to have narrower fields of view, are poorly suited to the task of scanning the sky for danger. Normal, small telescopes—Spacewatch uses 0.9 meter and 1.8 meter telescopes—can do the job reasonably well, so long as they are coupled with capable detectors and software for automated scanning of the sky. From 1985 to 2009, Spacewatch discovered just under 2,500 PHAs. The number they found increased dramatically as the technologies they relied upon—electronic detectors and computer software improved in the late 1990s.
But the real revolution in discovering PHAs came when NASA and the Air Force began to take the threat seriously. In early 1996, LINEAR, a joint NASA-Air Force program run by MIT’s Lincoln Labs, started field tests on a pair of telescopes at the White Sands Missile Range in Socorro, N.M. LINEAR’s telescopes are 1 meter in size—similar to Spacewatch’s—but LINEAR uses what the Air Force describes as “highly sensitive digital camera technology, known as Deep STARE.” LINEAR has sent more than 231,000 discoveries of new objects to the Minor Planet Center (a body in Cambridge, Mass., with responsibility for keeping track of things in the solar system.) The vast majority of LINEAR’s discoveries were of distant asteroids and other objects which pose no danger to Earth. About one-third of the PHAs known today were found by LINEAR.