Current and planned efforts focus on larger objects
Scientists have begun piecing together the characteristics of the meteor that exploded over Russia on the morning of February 15, using data from seismic instruments that track earthquakes and microphones designed to detect sonic booms from nuclear explosions. Unlike the asteroid DA14, which narrowly but predictably missed Earth later that day, the meteor was too small to detect before its contrail appeared in the dawn skies over the Russian city of Chelyabinsk.
Yet even an object too small to detect can produce an impressive amount of destruction. The meteor was 15 meters across (compared with 50 meters for 2012 DA14) and weighed more than 7,000 metric tons when it entered Earth’s atmosphere, says Margaret Campbell-Brown, an astronomer at Western University in London, Ontario. She estimates that it was streaking through the sky at supersonic speeds of about 18 kilometers a second before exploding at an altitude of 15 to 20 kilometers, creating a shock wave that shattered glass in a deafening boom once it reached the surface. Various news sources have reported hundreds of buildings damaged and about 1,200 injuries.
Coincidentally, the largest observed meteor to enter the atmosphere since 1908 arrived just hours before a much larger object passed the planet uneventfully at a distance of about 27,000 kilometers.
“The fireball is not related in any way to 2012 DA14,” says Paul Chodas, a planetary scientist with NASA’s Near Earth Object Program at the Jet Propulsion Laboratory in Pasadena, Calif. Among other reasons, the meteor buzzed through the sky from north to south, the opposite trajectory of DA14.
The explosion had the equivalent of up to 500,000 tons of TNT, Campbell-Brown says. That’s about 30 times the energy output of the Hiroshima atomic bomb but only 5 percent of the energy of the famous 1908 Tunguska meteor that downed trees over a 2,000-square-kilometer area in Siberia, some 2,400 kilometers from the February 15 event. The Tunguska meteor is estimated to have been about the size of 2012 DA14.
It is no surprise that scientists did not detect the Russian meteor in advance, says Alan Chamberlin of the Near Earth Object Program. Astronomers around the world use ground-based telescopes that can find only those objects that are large enough to reflect sufficient light for detection. A telescope at La Sagra Observatory in southern Spain spotted DA14 on February 22, 2012 because the asteroid was making a close pass to Earth during its 366-day orbit. Astronomers were not so lucky with the Russian object: It was smaller and approached from a sunward direction, Campbell-Brown says, meaning the sun’s glare eliminated any chance of telescope detection in advance.
In general NASA is hunting much larger objects. In 1998, Congress requested that NASA identify 90 percent of near-Earth asteroids a kilometer or more across, a benchmark NASA has achieved. An impact by such an asteroid would threaten civilization. In 2005, that requirement expanded to asteroids 140 meters or larger. So far NASA has identified more than 9,600 near-Earth objects, most of them larger than 100 meters across and none of them a threat to Earth.
The relative blindness to smaller asteroids and other objects, which would not wipe out humanity but could still cause major damage, has many astronomers calling for a more ambitious search. The B612 Foundation, founded by scientists including former astronauts Ed Lu and Rusty Schweickart, plans to launch a space telescope by 2018 to hunt for near-Earth objects as small as 30 meters in diameter. Astronomers have cataloged only about a half-percent of the million-plus near-Earth asteroids in that size range, Schweickart says.
B612’s telescope likely could spot a Tunguska-like asteroid before it strikes, but identifying a 15-meter rock would be a long shot. For the foreseeable future, these events, while very rare, will continue to come by surprise. “It certainly reminds us what even a smaller asteroid is capable of,” Chodas says. “This is Mother Nature shooting across the bow.”
S. Perkins. Tunguska, a century later. Science News. Vol. 173, June 21, 2008, p. 5.