An asteroid passed Earth last week, and with the Arecibo radio telescope astronomers got an unprecedented look—along with a couple surprises.

Asteroid 1998 QE2 came within six million kilometers of Earth, about 15 times the distance to the Moon. Researchers used Arecibo’s 300-meter-wide dish to bounce radio signals off the rock. By measuring how quickly the transmitted signals returned, the researchers could map the asteroid’s surface.

The first images, however, came back with a twist: the asteroid has a moon. At one quarter the size of the three-kilometer-diameter asteroid, the moonlet has the same proportional size to 1998 QE2 as our moon does to Earth. The discovery is a pleasant bonus: astronomers can calculate the asteroid’s mass by measuring how quickly the satellite orbits it.

Spectra obtained at NASA’s Infrared Telescope Facility in Hawaii also revealed that the asteroid’s surface has not been significantly altered from its original composition. That makes 1998 QE2 unique among all known asteroids and collected meteorites. So-called “primitive” asteroids can reveal details about the origin and evolution of the solar system.

The image is not a traditional picture, but what astronomers call a “delay-Doppler map.” The vertical axis measures distance. The horizontal axis records a body’s speed away from or toward the telescope. It works the same as speed radar used by law enforcement: radio waves reflected off a moving object come back at slightly different frequencies. By scanning over a spinning body, astronomers can map the variation in speed across the surface. Rapidly rotating asteroids end up looking very spread out; slow rotators get mushed. That’s why 1998 QE2’s satellite looks like a bright, narrow smear: it is spinning slowly and all the reflected light returns at nearly the same frequency.

Discovered in 1998 by LINEAR (Lincoln Near Earth Asteroid Research), a Massachusetts Institute of Technology program that uses automated telescopes to detect and catalogue near-Earth objects, 1998 QE2 orbits the sun once every 3.8 years. This is the first time it has passed close enough to zap it with radar.

The Arecibo Telescope, nestled in a valley in Puerto Rico, is the largest single-dish telescope in the world and one of the very few that can map asteroids. When not tuning in on the stars it is sometimes a star itself, having appeared in several movies like Golden Eye and Contact.

Christopher Crockett