Residents with telescopes in the United States and Canada had the best view of 2004 XP14, which appeared early Monday as a streaking dot in the northern sky.
Astronomers tracking the space rock's path since its discovery in 2004 had determined that it would pose no risk to Earth during the encounter nor in the next 100 years. Judging by its brightness, 2004 XP14 was estimated to be a quarter-mile to a half-mile wide.
An asteroid that size, if it smashed into Earth, would probably cause regional destruction. Scientists have said it would take a mile-wide or larger asteroid to cause widespread devastation that could threaten civilization.
Asteroid encounters are not uncommon. More than three dozen, mostly smaller, asteroids have flown closer to Earth in the last few years. But the latest was unusual because it was thought to be among the largest to have flown by.
Scientists believe an asteroid or comet impact probably wiped out the dinosaurs 65 million years ago and gave rise to the age of mammals. But the probability of an asteroid hitting the Earth and causing a global disaster in the near future is extremely low, they say.
Asteroids the size of 2004 XP14 collide with Earth about every 84,000 years. Scientists said it is hard to predict what would happen if such an event occurred because it depends on the object's makeup, its angle and speed, and whether it was headed for the ocean or land.
An asteroid similar in size to 2004 XP14 would probably punch through the atmosphere and cause destruction on a regional scale, one expert said. If it smashed into the United States, it would probably destroy several states, but not the entire continent, said Don Yeomans, who heads the Near Earth Object Program at NASA's Jet Propulsion Laboratory.
"It would be quite serious, but not a global catastrophe," he said.
If it hit an ocean, it would likely create killer waves like the deadly 2004 Indian Ocean tsunami, said Brian Marsden, director of the Minor Planet Center in Massachusetts.
"Something as large as this clearly does make a bit of a mess," Marsden said.
Besides devastation in the immediate impact zone, another concern is the extent to which debris from the collision would spread in the atmosphere, like ash from a volcanic eruption, possibly dimming or blotting out the sun.
Astronomers will analyze radar data on 2004 XP14 over the next several days to get a better idea of its shape and future trajectory.