Two asteroids swooping past Earth Wednesday may have caught the attention of the public, but events like these are not actually rare, NASA scientists say.
"This is the first time we've seen [two] combined within a 24-hour period, but that's probably because we don't know everything that is out there," said Lindley Johnson, program executive of the Near-Earth Object program at NASA headquarters in Washington.
Single asteroids have been known to make such close passes, but they usually slip by unnoticed, Johnson told SPACE.com.
In fact, with a rough estimate of 50 million unknown asteroids, a 33-foot-wide (10-meter) near-Earth object could pass harmlessly between Earth and the orbit of the moon every day, Johnson added. Such an asteroid might hit Earth's atmosphere once every 10 years, but because of its small size, it would pose no substantial threat to the people or property below.
"They would certainly break up in Earth's atmosphere, or we might get some meteorites on the ground," Johnson said.
In Wednesday's double flyby, the larger of the two space rocks, asteroid 2010 RX30, approached Earth at 5:51 a.m. EDT (0951 GMT), passing within 154,000 miles (248,000 km). It was estimated to be between 33 and 65 feet (10 and 20 meters) wide.
The second asteroid was due to come closer. The 20- to 46-foot-wide (6 to 14 meters) asteroid 2010 RF12 was due to pass within 49,000 miles (79,000 km) of Earth at 5:12 pm EDT (2112 GMT), NASA asteroid trackers said.
Both asteroids flew well inside the orbit of the moon.
Preliminary observations suggested both rocks and their orbits are fairly standard.
"They're pretty common from what we see," Johnson said. "RF12 is in a very Earth-like orbit. If it was larger and worth going to, it might be a candidate for a spaceflight mission in the future, but at 10 meters in size, it's not very interesting."
Astronomers are still unsure where the asteroids originated, but they are hoping that observations taken during the flybys will shed some light.
"We can project their orbits back to see what part of the solar system they came from, and hopefully we'll get some information and be able to relate them to asteroid families," Johnson said.
Because the two asteroids — discovered only three days before their flyby — were moving quickly, finding and tracking them across the sky posed a challenge for seasoned skywatchers. Viewers using a 20-inch (50-cm) telescope should have been able to catch a glimpse, Johnson said.
Asteroid 2010 RF12 was particularly well-positioned for observers in Europe and Africa, he said — "a fairly rapidly moving object across the stellar background, (looking) very much like a satellite that you see crossing through the star field, though not quite that fast."
Using a network of telescopes on the ground and in space, NASA experts and other astronomers routinely track asteroids and comets that may fly uncomfortably near the Earth.
The space agency's Near-Earth Object Observations program is responsible for finding potentially dangerous asteroids and studying their orbits to determine if they pose a risk of hitting the Earth.