Scientists call them "near-Earth objects"—the giant space rocks that whiz by our planet every 5 years or so. The one that passed us early this morning came within an unsettling 334,000 miles of Earth. Not to worry, experts say, the asteroid, which may be up to 2,000 feet in diameter, isn't close enough to do any harm, and besides, NEOs that size are likely to hit us only once every 37,000 years.
But the flyby gave professional and amateur sky watchers alike a rare chance to bring out the big glass to catch a glimpse of the object. In a dark and cloudless sky, the asteroid, named 2007 TU24, was be observable with 3-inch aperture telescopes. At magnitude 10.3 it will glimmer about 50 times fainter than an object just visible to the naked eye.
In the night sky over rural Puerto Rico, the world's largest radio telescope at Arecibo Observatory prepared to take first-time images of TU24. "We don't know anything about this asteroid," said Mike Nolan, head of radar astronomy at the observatory." Such objects pass near Earth with relative frequency, but it's rare that astronomers have enough advance notice to plan for rigorous observing," he said.
Astronomers with University of Arizona's Catalina Sky Survey first discovered TU24 last fall. It is one of an estimated 7,000 near-Earth objects of its size or larger, and most have never been closely studied.
"We have good images of a couple dozen objects like this, and for about one in 10, we see something we've never seen before," said Nolan. "We really haven't sampled the population enough to know what's out there."
At its nearest, the object is only 1.4 times the distance between the Earth and the moon. By bouncing radar off the rock, a team of scientists led by Steve Ostro, a NASA Jet Propulsion Laboratory astronomer and principal investigator for the project, are using the Arecibo radar telescope to measure TU24's size, speed and spin and then photograph the asteroid to make a detailed map of its surface.
Radar astronomy studies celestial bodies—planets, moons, asteroids and comets—in our solar system. Directed by the scope's 1,000-foot reflector, a powerful beam of radio energy is transmitted in the direction of the asteroid. The target reflects a very small portion of the energy back toward Earth, where it is collected, focused and analyzed to yield information about the roughness, composition, size, shape, rotation and path of the object.
TU24 is classified as an Apollo asteroid, a group of near-Earth objects whose orbits cross that of Earth, making a collision at least possible as both bodies orbit the Sun. Most asteroids are amassed in the so-called asteroid belt that circles the Sun between the orbits of Mars and Jupiter.
Most asteroids and smaller meteoroids heading toward Earth burn up in the atmosphere before they hit ground. Some were large enough to survive the fall through the atmosphere and smashed into the Earth, releasing atom-bomb amounts of energy. The most famous, which hit the Yucatan Peninsula about 65 million years ago, is blamed for wiping out the dinosaurs. That asteroid left a crater some 180 miles in diameter and spewed enough material into the sky to block most of the sunlight for about a year, drastically cooling the Earth and decreasing or preventing photosynthesis. The blast set off tsunamis, earthquakes, volcanoes, forest fires, changed climate and left the Earth unlivable for most plants and animals.
Today, there are about 170 known or suspected impact craters on Earth. Most are from 1 to 10 miles in diameter and fewer than 600 million years old.
Arecibo Observatory is part of the National Astronomy and Ionosphere Center, a national research center operated by Cornell for the National Science Foundation. The Jet Propulsion Laboratory is a division of the California Institute of Technology.
—Leslie Fink, NSF
This report is provided by the National Science Foundation, an independent federal agency that supports fundamental research and education across all fields of science and engineering, in partnership with U.S. News and World Report.