May 31, 2013

A large asteroid that is swinging past Earth today is bringing along a surprise companion–a moon. Asteroid 1998 QE2 will make its closest approach to Earth at a comfortable distance of 3.6 million miles (5.8 million kilometers) at 4:59 p.m. Eastern Daylight Savings Time. That distance is about 15 times as far as the orbit of Earth’s moon, so there’s no chance that the asteroid could crash into us.

Scientists have been tracking the asteroid since its discovery on August 19, 1998, by the Massachusetts Institute of Technology Lincoln Near Earth Asteroid Research program in Socorro, New Mexico. Radar images of the asteroid suggest that it is about 1.7 miles (2.7 kilometers) in diameter. The moon is an estimated 2,000 feet (600 meters) across, more than 15 times the length of a football field. Only about 16 percent of asteroids larger than 655 feet (200 meters) in diameter belong to systems with two or more objects.

A series of radar images show asteroid 1998 QE2 as it appeared when it was about 3.75 million miles (6 million kilometers) from Earth. The small white dot at lower right in the images is the asteroid's moon. (NASA/JPL-Caltech/GSSR)

Scientists at NASA’s Jet Propulsion Laboratory in Pasadena, California, began observing the asteroid and its moon on May 29 using two radar telescopes—the Deep Space Network antenna at Goldstone, California, and the Arecibo Observatory in Puerto Rico. The radar observations, which will continue until June 9, will allow the scientists to determine the size and shape of the asteroid and its moon and to study the objects’ surface features. The first radar images of the asteroid revealed dark areas that may be impact craters or another kind of depression in the surface.

NASA’s Near-Earth Object Program identifies and tracks comets and asteroids that move around the sun near Earth’s orbit. These comets and asteroids are called near-Earth objects or NEO’s. The major role of the program is to determine if any NEO’s could strike Earth. By the end of 2008, the program had discovered and tracked more than 90 percent of the NEO’s larger than 0.62 mile (1 kilometer). The program is now focusing on finding 90 percent of NEO’s larger than 450 feet (140 meters). As of mid-2013, the program had found 9,858 NEO’s. At least 860 of them have a diameter of approximately 0.62 mile.

Additional World Book articles:

• Dawn
• Eros
• Vesta
• When Worlds and Comets Collide (a special report)
• What Has Caused Mass Extinctions (a special report)

Nov. 7, 2011

In the closest approach of any asteroid since 1976, a space rock known as 2005 YU55 will whiz past Earth inside the orbit of the moon on Nov. 8, 2011, at a speed of 30,000 miles per hour (13 kilometers per second). The asteroid, a dark, slowly spinning object about 1,300 feet (400 meters) wide, will pass no closer than 201,700 miles (324,600 kilometers) to our planet. The moon follows an orbit that is an average of 238,897 miles (384,467 kilometers) from the center of Earth. Asteroid 2005 YU55 is a regular visitor to the inner solar system, but it has not come this close to Earth in 200 years. Another asteroid this large will not approach Earth until 2028, according to scientists at NASA.

The asteroid Kleopatra ranks among the most oddly shaped asteroids. It has a "dog bone" shape, with two rounded knobs connected by a slender center. NASA/JPL/NSF/Arecibo Radio Telescope

Scientists planned to track 2005 YU55 using NASA’s Deep Space Network, a group of satellite tracking stations in California, and the radio telescope at the Arecibo Observatory in Puerto Rico. The Arecibo telescope captured a fuzzy image of the asteroid in 2010, when the space rock was 1.5 million miles (2.3 million kilometers) away. Scientists also hope to learn if the asteroid contains ice or minerals holding water.

The asteroid was not expected to have any detectable gravitational effects on Earth’s tides or tectonic plates, which are affected by the moon. Nor will the asteroid hit the moon, which will be more than 150,000 miles (240,000 kilometers) away as the asteroid makes its closest approach to Earth. An official with NASA’s Near-Earth Object Program Office noted that an asteroid about the size of 2005 YU55 crashes into Earth only about once every 100,000 years. If an asteroid this size were to hit Earth, it would dig a crater 4 miles (6.4 kilometers) wide and 1,700 feet (518 meters) deep.

Additional World Book articles:

• Dawn
• Dinosaur
• Eros
• Vesta