From New Scientist:

Computer models of asteroid orbits are showing that small space rocks a few metres across can lodge in Earth’s gravitational field if they stray too close. Only a tiny fraction of them break up and hit our planet. Most orbit unseen for months or years, somewhere beyond the moon, before slipping safely and silently back into deep space. But while they remain close, they are mini-moons of Earth.

Apparently these mini-moons are rich in rare minerals, and small enough that we could go up there with a rocket and bring back a whole one to study.

Two thoughts:

  • What if we brought one back and it was full of alien spores?
  • (For children) What if we brought one back and it turned out to be made of cheese?