 Asteroids are relatively small, rocky worlds that revolve in elliptical orbits around the sun. There are billions of them in the solar system, ranging in size from a few meters across to the size of dwarf planets. Asteroids have remained mostly unchanged for billions of years, so research into them could reveal a great deal about the early solar system. To that end, a probe called Osiris Rex, launched in September 2016, will travel to a near-Earth asteroid called Bennu and bring a small sample back to Earth for study. If all goes as planned, the spacecraft will reach Bennu in 2018 and return a sample to Earth in 2023. Most asteroids lie in a vast ring between the orbits of Mars and Jupiter called the asteroid belt. It ranges from 2.2 astronomical units to 3.2 astronomical units from the sun, and is around a million kilometers thick. It's estimated to contain around 30 billion asteroids larger than 100 meters across, but the entire mass of the asteroid belt comes to little more than 4% of the mass of our moon, with Osiris accounting for almost half of that. But with as many as 30 billion asteroids in the belt, it is interesting to calculate the average volume of space each one has around it. Although the density varies, we can get an average by dividing the asteroid belt volume by the number of asteroids. We get 12.7 trillion cubic kilometers of space for each asteroid. That's 3 trillion cubic miles. If we went there, it would look quite empty. So navigating through the belt would be easy, and collisions would be rare. But we do know that lots of asteroids exist outside the asteroid belt. For example, the outer moons of the giant gas planets are all thought to be captured asteroids, and asteroids also orbit gravitational volumes known as Lagrange points.