“How little mass could an object in space have and still hold an astronaut in orbit around itself?”
We put your question to scientists who plan space missions to the asteroids, our solar system’s least massive worlds. They said that any two bodies in isolation can orbit each other because bodies with mass attract each other — no matter how small. That means, in theory, that a 100-kilogram astronaut could slowly orbit around a 100-ton space station.
But few objects in space exist in isolation. So an astronaut can’t orbit around a space station in Earth orbit — because Earth’s gravity interferes. An astronaut who tried would escape the space station’s feeble gravity — and enter an independent orbit around the Earth. Scientists call the region around a body where an object can safely orbit its Hill Sphere . . .
A typical asteroid a few kilometers or miles across — orbiting the sun at twice Earth’s distance — has a Hill Sphere a few hundred kilometers or miles in radius. In other words, an astronaut could safely orbit that far from the asteroid’s center. If she moved beyond that distance, she’d slip away into orbit around the sun.