An anonymous reader writes "Asteroid 1998 QE2 has an estimated diameter of 2.7 km. This asteroid will have a close approach with Earth at about 15.2 LD (Lunar Distances = ~384,000 kilometers) or 0.0392 AU (1 AU = ~150 million kilometers) at 2059 UT on 2013 May 31 and it will reach the peak magnitude ~10.8 on May 31 around 2300 UT." Radar images of the asteroid taken Wednesday show that 1998 QE2 has its own tiny moon, about 600 meters wide. Phil Plait explained how the images were taken, and what further information we gleaned from them. 'The very presence of the moon is a good thing. By measuring how long it takes to go around the primary, the mass of the primary can be found using math known for centuries (the more massive the big asteroid, the faster the moon will go around it at a given distance). We also know the size of the primary, so that means we can find its density, and therefore what it’s made of (probably mostly rock).'