Close but no cigar

Astronomers spotted an asteroid this week after it had flown past Earth on a course that took it so close to the planet it was below the orbits of some satellites.

The space rock was relatively small, however, and would not have posed any danger had it plunged into the atmosphere.

The object, named 2004 YD5, was about 16 feet (5 meters) wide, though that's a rough estimate based on its distance and assumed reflectivity. Had it entered the atmosphere, it would have exploded high up, experts figure.

Satellite territory

The asteroid passed just under the orbits of geostationary satellites, which at 22,300 miles (36,000 kilometers) altitude are the highest manmade objects circling Earth. Most other satellites, along with the International Space Station, circle the planet at just a few hundred miles up.

2004 YD5 is the second closest pass of an asteroid ever observed by telescope, according to the Asteroid/Comet Connection, a web site that monitors space rock discoveries. The closest involved a rock that flew by last March and was not announced until August.

2004 YD5 was discovered Tuesday, Dec. 21 by Stan Pope, who volunteers his time to examine images provided by the FMO (Fast Moving Object) project, an online program run by the University of Arizona's Spacewatch Project. After the initial detection, other observers noted the object's position during the day and its path was then calculated back. Closest approach occurred on Dec. 19.

The rock approached Earth from near the Sun and so would have been nearly impossible to detect prior to close passage. It soared over Antarctica -- underneath the planet, Washington State University researcher Pasquale Tricarico told the Asteroid/Comet Connection.