An unprecedented look at an asteroid that sailed past Earth last week came with a surprise: the space rock had its own moon.
Researchers in Puerto Rico pointed the Arecibo radio telescope at the asteroid 1998 QE2, which passed within 6 million kilometers of Earth, or about 15 times as far as the moon -- and were surprised to see the small satellite orbiting the asteroid.
By bouncing radio waves off the asteroid, researchers were not only able to map the rock's surface, but determine how fast the "moonlet" was orbiting. Because the moon is approximately proportional in size to QE2 as our moon is to Earth, the researchers are able to calculate the mass of the asteroid.
Spectrum data out of NASA's Infrared Telescope Facility in Hawaii revealed QE2 is a "primitive" asteroid, meaning its surface has not been significantly altered from its original composition, and could provide new insight into the origin of the solar system.
"Asteroid QE2 is dark, red, and primitive -- that is, it hasn’t been heated or melted as much as other asteroids," Arecibo's Ellen Howell said in a statement. "QE2 is nothing like any asteroid we've visited with a spacecraft, or plan to, or that we have meteorites from."
'It's an entirely new beast in the menagerie of asteroids near Earth," Howell said.
The Arecibo Telescope, the largest single-dish telescope in the world at 300 meters wide, works just like radar guns used for law enforcement or pitch speed in baseball. It bounces radio waves off a moving object and then determines the speed and shape of the object based on the reflection of the waves it gets back.
The QE2 asteroid was discovered by an MIT program in 1998, and though it orbits the sun every 3.8 years, last week was the first time it was close enough for radar to get a look.