An asteroid about a third of a mile in diameter will whizz past Earth at roughly 12:48 p.m. on Sunday, scientists said.
The object, the largest ever observed passing so close, will miss Earth by about 279,000 miles - a close call in astronomical terms.There is no chance of an actual collision, scientists said. Since the asteroid was detected, astronomers at several observatories have carefully monitored its position and precisely calculated its four-year orbit around the sun.
It will come closer to Earth than all but five other objects ever detected, but will not come as close as the moon, which is 240,000 miles from Earth.
Carl Hergenrother, a University of Arizona senior majoring in atmospheric sciences, and Timothy Spahr, an astronomy graduate who now attends the University of Florida, made the latest discovery Wednesday.
They were using a 16-inch diameter telescope on Mount Lemmon, just outside Tucson, and detected it on photographs each of the two previous nights.
The object, called 1996 JA-1, is traveling through space at about 10 miles per second, or about 36,000 mph, and will cross a distance about 18 times the width of the full moon in one hour.
Spahr, Hergenrother and other astronomers who have plotted its course since its discovery say that makes its approach the sixth-closest known.
This asteroid is roughly one-tenth the size of one believed to have killed all dinosaurs and most other living things when it struck Earth 65 million years ago.
"This asteroid wouldn't destroy civilization if it hit Earth, but it sure would mess things up," Spahr said.
If it struck land, it might leave a crater several miles wide, devastating an area perhaps hundreds of miles across. If it struck the ocean, it would produce tsunamis, or tidal waves, that could travel hundreds of miles and devastate coastlines with waves towering nearly 200 feet high.