The unexpected appearance and explosion of a small asteroid over Russia's Ural Mountains on Friday highlights the challenges astronomers face as they try to get a better handle on the risk Earth faces from objects whose orbits bring them uncomfortably close to Earth, or even cross Earth's orbit.
In their hunt to identify such near-Earth objects wider than half a mile across – potential civilization busters if one were to strike Earth – astronomers have cataloged about 95 percent of the objects in this size class during the past 15 years.
But they have found less than 1 percent of the objects 100 feet across or larger, a class that includes the asteroid 2012 DA14. This object flitted past Earth Friday afternoon Eastern Standard Time a scant 17,200 miles from Earth – a record for a known object of its size.
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At 150 feet across, 2012 DA14 is comparable in size to the object that exploded over the Tunguska River in Siberia in 1908. The shock wave flattened 820 square miles of forest – an area about the size of Greater Tampa-St. Petersburg.
The asteroid over Russia's Chelyabinsk region Friday was smaller still, estimated to span about 50 feet and weigh about 7,000 metric tons before it exploded into fragments high above the ground. The shock wave shattered windows in three major cities in the region, damaged a zinc factory, and inflicted mostly minor injuries on more than 950 people.
Nobody saw the object coming.
It's enough to make some lawmakers wince. On Friday, Lamar Alexander (R) of Tennessee and chairman of the House Science, Space, and Technology Committee, issued a statement regarding the two events that noted that the committee will hold hearings in the near future to explore ways to improve efforts to detect asteroids as well as to deal with any deemed a potential threat to the planet.
Given the size of the Chelyabinsk asteroid, astronomers estimate that an event like this occurs on average every 100 years. Yet smaller objects also can arrive with little or no warning, and explode in a loud, spectacular fashion, even with no damage on the ground. And they hit more frequently.
These surprise visitors are among the near-Earth objects that keep Kalait Ramesh awake at night.
"Historically, we've had relatively low population density. These things tended to happen in areas where nobody sees them or nobody remembers them," says Dr. Ramesh, a professor of mechanical engineering at the Johns Hopkins University whose studies of stress on various materials has led him to include asteroids in the mix of materials.
"But as our population's gone up, it's gotten to the point where these things can have a big impact. My biggest worry is that this will happen in a place where there is significant political instability or two countries on the verge of a war," he says.
An event like Friday's Chelyabinsk asteroid explosion could be mistaken for an attack, he says.
The potential isn't lost on the international community at large. A subcommittee of the United Nations Committee on the Peaceful Uses of Outer Space is meeting in Vienna through Feb. 22 to tackle several space-related issues, including hazards from comets or asteroids.
The Scientific and Technical Subcommittee is building a plan to better coordinate searches for near-Earth objects as well as a plan to mount coordinated warnings and responses to objects with Earth in their cross hairs.
The need for such coordination became apparent in 2008, with the sudden appearance of 2088 TC3, an 80-ton object 5-to-7-feet wide that blew apart with an energy equivalent of 2 kilotons of high explosives in a brilliant fireball. The explosion occurred above a spot roughly 60 miles south of the border between Sudan and Egypt.
The Catalina Sky Survey, a near-Earth-object surveillance effort run by the University of Arizona, spotted the object 20 hours before it entered the atmosphere. Scientists were able to get a good bead on its orbit and track.
NASA alerted the White House, the National Security Council, the Pentagon, the State Department, and the Department of Homeland Security, as well as issuing a press release. But no one from the United States alerted Sudan because the two countries did not have diplomatic relations, according to an account by Donald Yeomans, who heads NASA's Near Earth Objects Program office at the Jet Propulsion Laboratory in Pasadena.
Over the past five years, the US has chalked up some 90 to 95 percent of all near-Earth-object discoveries, notes Tim Spahr, director of the International Astronomical Union's Minor Planet Center in Cambridge, Mass.
Currently, three major observing efforts are under way, with a fourth slated to start using a new space-surveillance telescope the US Air Force has built to track satellites and orbital debris. But virtually all of the major survey projects use telescopes in the Northern Hemisphere.
Last November, Australia and the US inked an agreement to install the telescope, as well as a space-surveillance radar, in Australia. In addition, the US is building the Large Synoptic Survey Telescope, a telescope that can gather panoramic images of the entire southern sky from its perch atop Cerro Pachon in Chile's Atacama Desert.
Yet despite the respectable amount of telescope glass aimed at the night sky, ground-based observations have their limits, notes Russell Schweickart, a former NASA astronaut who has spent much of his post-Apollo-9 career focusing on asteroid hazards and planetary defense.
Earthbound telescopes tend to take their observations at visible wavelengths. But the amount of light reaching a telescope – assuming good weather – depends on the size and distance of an asteroid, as well as its composition.
By some estimates most near-Earth objects reflect about as much light as dark, wet soil, while the darkest reflect about as much light as a charcoal briquette.
The toughest asteroids to spot until they are very close are the ones that are small and dark.
Moreover, if an asteroid's orbital period is close to that of Earth's, "you get less information than you want for tracking," Mr. Schweickart says.
The reason: The closer the orbital periods, the longer orbital mechanics keeps the asteroid out of view for Earthbound observers.
If an asteroid completes one circuit around the sun in 360 days, for instance, "for five or 10 years, it's relatively close to you every year. But for the next 30 or 40 you don't get any data, so you don't know what's going on," Schweickart says.
For Schweickart and other planetary-defense specialists, it's time to take detection into space using telescopes that scan the skies at infrared wavelengths – making them sensitive to the heat asteroids give off from sunlight the objects have intercepted. Such telescopes not only can pick out the harder-to-detect asteroids, they also can lead to better estimations of an asteroid’s diameter compared with estimates derived from visible-light observations.
NASA's WISE satellite, an infrared telescope launched in December 2009 and designed for multiple research programs, has given researchers a taste of what's possible. It bagged 130 near-Earth asteroids, 21 potentially hazardous objects, and 17 comets before its detectors ran out of coolant in late 2011.
The B612 Foundation, a nonprofit, independent group of near-Earth asteroid specialists, is wrapping up design work on an infrared telescope to be placed on orbit around the sun the same distance away as Venus. The design is based on NASA's highly successful Kepler planet-hunting spacecraft. From that vantage point, it can look back at Earth and see clearly objects that cross or come near Earth's orbit – the heat signatures of the objects shining against the cold, black expanse of space in the background.
The mission, whose goal is to detect 90 percent of near-Earth objects about 460 feet across or larger, represents the first privately funded craft for asteroid detection. If all goes well, the group anticipates launching the craft in 2018, says Schweickart, a founder and former chairman of the group's board of directors. (Schweickart still sits on the board.)
In addition, the newly formed company Deep Space Industries, which aims to mine asteroids, has proposed a squadron of small "FirefFly" satellites in Earth orbit to hunt for prospecting asteroids to mine as well as search for potentially hazardous objects.
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