system
Given the following post, respond as if you're commenting on Reddit:
human
In Earth travel, we use North, South, East, and West, plus altitude for three-dimensional travel. Since those are all relative to the Earth, what do they use for space travel?
gpt
Pretty much on every planet or moon it is possible to define North, South, East and West based on the body's rotation. Alternatively, they can be defined based on the Solar System's North and South (as the IAU did with Uranus, whose extreme axial tilt makes the rotational North opposite to the Solar System's North, but for all the other planets it's the same).

However this isn't very precise to plan spacecraft trajectories. There are more precise reference frames based on longitude and latitude, so a prime meridian is defined pretty much arbitrarily as they did on Earth. It's also quite common to use Cartesian reference frames centered on a planet but with the axes based on far away celestial objects because they don't rotate with the planet, so you have an inertial reference frame.

In deep space, far away from any planet, we use a reference frame centered on the Sun, with the X axis pointing along the ecliptic plane parallel to the vernal equinox (the intersection between Earth's equatorial plane and the ecliptic plane), the Z axis pointing North from the Sun, and the Y axis pointing 90° ahead of the X axis following the right hand rule.

It is possible to define others, it depends on each particular trajectory. Basically you need a center (a planet, the Sun, etc), a fundamental plane (the ecliptic plane, the planet's equatorial plane, etc) and a direction for the X axis.