Ask the Astronaut: What makes moons and planets round?

Pretty close to spherical: Ceres in false color, as seen by NASA’s Dawn spacecraft. (NASA/JPL-Caltech/UCLA/MPS/DLR/IDA)
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Q: What makes planets and moons appear perfectly round and not irregularly shaped like asteroids? (Phil Gartner, Benton, Louisiana)

Gravity is at work here. A body as massive as a planet or large moon has sufficient gravity to pull its solid rock, liquid oceans, and gaseous atmosphere into the shape of a sphere. Any hills or mountains sticking up above that “equipotential surface” eventually slump down under gravity until they are at the same gravitational potential as their neighboring particles of matter. In other words, gravity is stronger than the strength of the rock, over time.

Smaller bodies like asteroids lack the mass—and thus the gravity—to pull their rocky surfaces into a spherical shape. The rocks resist the weak gravitational tug and retain the lumpy-looking, potato or dumbbell shapes we see in asteroid photos from spacecraft or Earth-based radar observations. As an example, dwarf planet Ceres is just about spherical with a diameter of 950 km (590 miles). Asteroid Vesta is 525 km (326 miles) in diameter; its gravity is too weak to drag its egg-like profile into a spherical shape.

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About Tom Jones

Former astronaut Tom Jones is a scientist, author, and pilot. In more than eleven years with NASA, Tom flew on four space shuttle missions to Earth orbit. On his last flight, he led three spacewalks to install the centerpiece of the International Space Station, the American Destiny laboratory. He has spent 53 days working and living in space. See his full bio here.

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