## How do you measure a planet's mass? (Beginner)

We would like to know how scientists calculate a planet's mass. Please explain it to us in a way that a 4th grader can understand it.

The only way we can measure a planet's mass is through its gravity. This has been the way Earth's mass was measured, too. (We can't directly probe what's in Earth's interior, but we can measure the gravity on the surface.) Since no human ever visited other planets and measured their gravity on the spot, we usually have to resort to other methods. The most commonly used technique is to observe a body orbiting or passing close to the planet and see how its path is affected by the planet's gravity.

For example, if we see a moon orbiting a planet at certain distance from it, the orbital period of the moon at that particular distance will mainly depend on the planet's mass. The more massive the planet, the more strongly it attracts the moon and faster the moon moves. It is straightforward for astronomers to calculate the planet's mass after we have observed the motion of one of its moons for a while.

Mercury and Venus have no moons, so their exact masses were not known until a few decades ago. Before space flight was developed, the only way to measure their gravity was to see how they affect other planets' orbits. Astronomers would measure very small changes in, say, Earth's orbit, that were caused by the attraction of Venus. These changes are small and it was hard to get the exact mass of Venus by this technique. But once spacecraft were launched to Venus and they flew close to it, scientists could easily measure its mass by tracking how these probes were deflected while passing by Venus. The same technique was used for Mercury when the Mariner 10 spacecraft flew by it in 1974.

#### Matija Cuk

Matija works on the orbital dynamics of the lesser moons of Jupiter and Saturn. He graduated with his PhD from Cornell in November 2004 and is now working at the University of British Columbia in Canada.

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