system
Given the following post, respond as if you're commenting on Reddit:
human
When sending things to other planets, why do we choose not to use the moon as a mini gravity-assist? Surely it would just reduce fuel (even slightly) and thus reduce the mass of spacecraft?
gpt
We can and do! The STEREO probes used the Moon for instance.

One thing that makes it tricky is that the Moon moves so fast in *angular speed*. The Moon is orbiting the Earth at 1 km/s, compared with other planets that are moving at 10s of km/s, but the Moon is much closer, so its angular speed is much higher. The Moon moves around the Earth at about 12 degrees a day. Venus at its most rapid moves across the sky at about 1 degree a day.

This means that if you have to delay your launch by a couple of days due to weather, the Moon is in a completely different location in its orbit, and a gravity assist might not work so well. The Moon is also so close that you can't fix this with small adjustments along the way. You can wait until the next month, but by that point the other planets will have moved, and you're probably in an even worse situation.

With Venus or Jupiter or whatever, delaying by a few days only makes a small difference in the orbital configuration of the solar system. And because your probe is travelling for months, you can make small adjustments to somewhat make up the difference.

So yeah, we can use the Moon, and we do use it fairly often, but it's a little tricky, so we don't use it every time.