# How do you measure a planet's mass?

We would like to know how scientist 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 nobody ever visited other planets and was able to measure 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 depend on the planet's mass only. Heavier the planet, stronger it attracts the moon and faster the moon moves. It is straightforward for astronomers to calculate the planet's mass after they have observed the motion of one of its moons for a while.

Mercury and Venus have no moons, so their exact mass was not known until a few decades ago. Before spaceflight 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 the spacecraft were launched to Venus and they flew close to it, scientists could easily measure its mass by tracking how were these probes attracted while passing by Venus. The same technique was used for Mercury when the Mariner X spacecraft flew by it.

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