The 150-foot-wide meteor that flew by Earth Friday and the smaller one that exploded over Russia hours before were too small to be easily detected. They are not even targets in NASA's surveillance of potentially hazardous objects.
"The surveys are not designed to differentiate all the small ones like that. That would require a way bigger effort and probably more extensive technology," said Robert McMillan, a pioneer in the search for near-Earth objects.
McMillan, who co-founded the University of Arizona's Spacewatch program in 1980, said the various NASA-funded programs that search for hazardous objects have a goal of finding meteors 140 meters or greater in diameter.
NASA was originally given a goal by Congress in 1998 to identify potentially hazardous objects a full kilometer (3,280 feet) or more in diameter - that's the size capable of having a civilization-altering impact.
The meteor that is believed to have wiped out the dinosaurs 65 million years ago, for example, was estimated at 10 kilometers in diameter.
The 140-meter threshold was ordered by Congress in 2005 to detect objects that could be regionally devastating.
You have to draw the line somewhere, said McMillan.
"Big objects are more dangerous and small objects more numerous," said McMillan, "but we follow up on anything that looks like a potential impacter."
Meteor 2012 DA14, for example, was well under the threshold at 45 meters, but once it was detected by amateur astronomers at an observatory in southern Spain, it was added to the watch list. It flew by Friday, as predicted, a full 17,200 miles from Earth.
The object that exploded over Russia was too small to spot when it was farther out in space, and it made its approach toward Earth from the direction of the sun where it was undetectable, said Bill Cooke, lead for the Meteoroid Environments Office at NASA's Marshall Space Flight Center in Huntsville, Ala.
He said small objects strike the Earth all the time. "The Earth intercepts about 80 tons of meteoritic material per day. Millions of very small millimeter-sized objects are striking every day."
Most go unnoticed. A large proportion of the Earth's surface is either covered by water or uninhabited.
Objects the size of the one that exploded over Russia hit every 50 to 100 years, said Paul Chodas, of the Near-Earth Object Program Office at NASA's Jet Propulsion Laboratory in Pasadena, Calif. Chodas and Cooke spoke at a Web conference hosted by NASA Friday.
"Small objects the size of a basketball hit the Earth every day; car-sized objects every month or two. Little stuff hits the Earth all the time," said Chodas.
"Defending the Earth from tiny asteroids is a challenging issue and something that is not currently our goal," Chodas said.
That goal could change after this week's amazing coincidence.
Russian officials called Friday for a cooperative effort to better detect hazardous asteroids.
Rep. Lamar Smith, R-Texas, issued a news release Friday that said: "Developing technology and research that enable us to track objects like Asteroid 2012 DA14 is critical to our future."
Smith, chairman of the House Science, Space and Technology Committee, said he will hold hearings in the coming weeks "to examine ways to better identify and address asteroids that pose a potential threat to Earth."
The subject is also a topic at this month's meeting of the U.N. Committee on the Peaceful Uses of Space in Vienna.
Contact reporter Tom Beal at email@example.com or 573-4158.