Re: Ok smarty pants...
Size.
The meteor would have to be large and inertially integrated (solid) enough so that the surface could get really hot before the whole body dissociates/vaporizes/flys apart.
Look at videos of the ET or Mir or GRO or sadly Columbia. All white-green with differing compositions.
It's a stiff drink, but I know you can take it ya lush.
Or they could be copper-barium ingots to fuel Superman's spaceship. ;)