This YouTube video shows the asteroid's path.
By Stewart Bishop, Globe Correspondent
Although Harvard University astronomer Timothy Spahr spends his life tracking asteroids and minor planets, it's rare when he gets the chance to see one up close.
On Friday night, Spahr received word that an asteroid was headed our way. Though it received little publicity, the asteroid passed by Earth early Monday. At its closest, the asteroid, named 2009 DD45, came within 45,000 miles of Earth, which is around twice as high as some satellite orbits and about one-fifth of the distance between the moon and Earth.
The cosmic object, which was estimated to be 20 yards to 30 yards across, came closest to Earth near the equator somewhere over the Pacific Ocean.
It was about the same size as the one that burned up over Siberia in 1908, leveling nearly 800 square miles of forest in the infamous "Tunguska Event" event.
"It's pretty unusual to see one this close," said Spahr, director of the minor-planet center at the Harvard-Smithsonian Center for Astrophysics in Cambridge. "If an object of this size were to impact the Earth, it would be equivalent to a small nuclear explosion."
Brian Marden, a senior astronomer at the center, said that many such objects pass this close but go unobserved.
"No one is watching the whole sky all the time," Marden said.
He said that light and celestial objects like the moon can affect astronomers' visibility. "If the moon is full, no one is watching," Marden said.
In October 2008, astronomers for the first time tracked an asteroid from space to its impact, when it burned up over Sudan, a mere six hours after it had been discovered.
Spahr said that while asteroids the size of DD45 would pose a catastrophic risk for the planet, they are not the ones that concern him the most.
"Now if an asteroid that was, say, a kilometer across were to strike the earth, that could result in mass extinction," Spahr said. Fortunately for humanity, such asteroids are usually visible several years in advance.
Of the known asteroids, the next time one of such size will come in such close proximity to Earth is in 2029, when the asteroid known as 99942 Apophis, an 885-square-foot space rock, is expected to come within 20,000 miles of the planet.
Material from the Associated Press was included in this report.