An asteroid breezed by Earth closer than moon Wednesday morning, and a second space rock will do the same at 5:12 p.m. EDT.
There is no danger of impact, says NASA, even though at closest approach asteroid 2010 RF12, the afternoon visitor, will be less than 48,000 miles away. The moon, in comparison, averages about 239,000 miles away.
The asteroid is estimated to be between 20 feet and 46 feet in diameter.
Asteroid 2010 RX30, which is between 32 and 65 feet in diameter, flew by at 5:51 a.m. EDT, passing about 154,000 miles during its closest approach to Earth. Both space rocks were discovered on Sunday by the University of Arizona's Catalina Sky Survey.
"These are so small we don't consider them a hazard," Lindley Johnson, head of NASA's Near-Earth Object Observations Program, told Discovery News. "Objects this small pass by Earth pretty frequently."
Even if the asteroids were to hit, they likely would break apart in Earth's atmosphere, Johnson added. "They wouldn't do any damage."
Amateur and professional astronomers have been combing the skies for a look at the asteroids, which will be useful in calculating their future orbit.
"It will be at least a year before the next closest approach," Johnson said.
NASA says both objects should be observable with moderate-sized amateur telescopes as they make their closest approaches.
About 20 percent of the known near-Earth asteroids are considered potentially hazardous because of their size and because their orbits come within 4.6 million miles of the planet. NASA is in the process of building a catalog of near-Earth objects that are at least a kilometer, or .62 miles, in diameter.
"The number of NEOs is something over 7,000," Lindley Johnson, head of NASA's Near-Earth Object Observations Program, said in an interview with Discovery News. "We don't have the current assets to find the entire population of those objects. We know of less than 5 percent of the total number."
Under a White House proposal pending before Congress, NASA would shift its Constellation moon-bound program into a more flexible approach to space exploration, with the goal of sending astronauts to visit a near-Earth object by 2025.
In addition to honing expertise for an eventual mission to Mars, studying asteroids may provide useful information about how to deflect an object threatening Earth.
A group of scientists led by Apollo 17 astronaut Harrison Schmitt urged Congress in 2003 to begin preparations on how to handle a potential asteroid encounter.
"We cannot rely on statistics alone to protect us from catastrophe; such a strategy is like refusing to buy fire insurance because blazes are infrequent. Our country simply cannot afford to wait for the first modern occurrence of a devastating NEO impact before taking steps to adequately address this threat," Schmitt wrote.
A 7.5-mile wide rock that slammed into Yucatan, Mexico, 65.5 million years ago is believed to have triggered a global change that killed off the dinosaurs.
The impact of that asteroid is estimated to have been a billion times more powerful than the Hiroshima atomic bomb, according to a March 2010 report in Science. A follow-up study published in the journal Geology last month speculates that it was at least two asteroids crashing into Earth that were responsible for the dinosaurs' extinction.
Studies of plant growth at a second impact site in the Ukraine suggest that the two events were several thousands of years apart.