In 2006, a 5-meter diameter asteroid (2006RH120) was discovered temporarily orbiting Earth. For about one year the Earth had two known moons. Recent simulations by ATLAS astronomer Robert Jedicke and his colleagues Mikael Granvik (U. Helsinki) and Jeremie Vaubaillon (Paris Observatory) suggest that the Earth always has a retinue of "minimoons" along with the much larger Moon. The largest minimoon is probably only about one meter (3 ft) in diameter compared to the Moon which is 3.5 million meters in diameter (about 2,000 miles).
Why are minimoons interesting? The primary reason is that they could be targets for robotic spacecraft that would bring the entire asteroid back to Earth! Astronomers and geologists would love to have a 1-meter diameter chunk of rock in their laboratories that has never traveled through the atmosphere. Scientific opportunities for detailed analysis would be unprecedented.
Though astronomers have lots of rocks from space, from the moon, and from Mars, all the meteorites in collections around the world are heated when they pass through the atmosphere and then damaged by years of weathering and erosion while sitting on the Earth's surfaces. Also the rocks from the Moon are just like rocks from the Earth's surface because the Moon was formed from the same material. Second, the total mass of rocks brought back from the Moon is far less than the mass of a single 1-meter diameter minimoon.
Minimoons are also interesting because even if we can't send a spacecraft to snag one and bring it back to Earth, we can still use ground-based telescopes to study it in detail. If we know where the minimoons are we can determine how fast they rotate and -- to some extent -- what they're made from by using spectrographs. Planetary astronomers have thousands of spectra of large asteroids but none of the smallest objects. While we certainly expect them to be made of the same types of material, the details of surface types can tell us a lot about how asteroids move from the main belt of asteroids into the region near Earth.
Finding minimoons is challenging because they are faint and we don't know where they are. Enter ATLAS. Since it surveys the entire sky every night, ATLAS will find them. We think ATLAS can find a few minimoons every year and it might be possible to devise a spacecraft mission to bring one back to Earth.