Big Asteroid (With Its Own Moon) To Have Closest Approach With Earth Today

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).'

Re:Any luck...

[cusco]

No, not much chance of that. Gravity, like other forms of energy, falls off according to the inverse square rule. If object B is twice as far away as object A it is only attracted at 1/4 (1/2^2) the force. Object C is five times further away as Object A, only gets attracted 1/25th as much (1/5^2). These are far enough away that their mutual gravity is a much stronger force than that of the Earth/Moon system, so our gravity applies effectively equal force on both objects. Clear as mud?

Re:Intellible units...

[gmuslera]

Scientific units are basically metric, those are the intellible units, not the arbitrary cultural ones that are used just for 3 or 4 countries in all the world. And putting the distance in lunar distances (or, maybe, Earth diameters that are around 12000km) puts in the right perspective how far they will be and how little we should worry about them, at least this pass (should you worry about a collision if a grain of sand passes 1km away from you at the closest point?)