Turns out, nobody really knows.
The surprising truth is that aside from the massive lump of rock we know about, and the various man-made space stations, rocket remnants and other junk floating above the atmosphere, there could be dozens of small 'moons' orbiting our planet - that we've never even seen.
The Earth regularly 'captures' small asteroids and pulls them into an orbit around our planet, sometimes for short periods and - possibly - sometimes for years at a time.
These objects are known as Temporary Captured Objects (TCOs) and are not technically moons, making about three orbits each. Only one - RH120 - has been confirmed, but observations show it was not a one-off.
These 'mini Moons' can be as big as trucks, but are usually smaller - somewhere around 2 feet in diameter.
According to Discovery, simulations show that two asteroids the size of dishwashers and 12 smaller asteroids orbit the Earth at one time. The larger truck-sized moons arrive about twice a century.
Now astronomers have an idea - we've landed on one Moon, so why can't we go to the others?
"We'd eventually like to see a mission to a mini-moon," said astronomer Robert Jedicke, from the University of Hawaii at a meeting last week.
Nasa currently has two spare telescopes donated to it by America's National Reconnaissance Office, and discussions are ongoing about how to use them. The space agency is expected to be handed a list of proposals next week, but in the meantime Jedicke and other researchers are pushing for missions to the other moons.
He told Discovery that bringing such a 'mini moon' back to Earth would give us a "Rosetta stone of the solar system".
"You bring back a chunk of material that's never been processed through the atmosphere, that's not been sitting on the ground. It's going to be a tremendous wealth of information about how the solar system formed," he said.
Meanwhile as private companies prepare to visit asteroids and potentially mine them for resources, some are touting missions to our mini moons as a chance to test the idea.Suggest a correction