The only way we can measure a planet's mass is through
its gravity. 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 easy for astronomers to calculate the planet's mass
after we have observed the motion of one of its moons for a while.
After knowing the distance between the planet and the moon and the
time it takes to complete one revolution is calculated, the weight of the
planet can be calculated easily.
Mercury and Venus have no moons, the only way to measure their gravity was to
see how they affect other planets' orbits. Astronomers would measure very small
changes in, Earth's orbit, that were caused by the attraction of Venus. So,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.
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