Rendezvous
In spaceflight, rendezvous refers to the event in which two spacecraft meet. This does not neccessarily occur in space: a rendezvous can also happen on the surface of a celestial body.
In orbit
A rendezvous usually takes place in orbit, e.g. when spacecrafts are travelling to a space station. If two spacecraft are close enough to each other (< 300m) and travel in similar orbits they are said to rendezvous. In such a situation, both spacecraft can stay close to the space station with minimal corrections.
There are two ways to bring about rendezvous:
- direct insertion into the space station's orbit, or
- insertion into a catch-up orbit.
A direct insertion needs more propellant and allows only few windows for insertion. If launching from the surface, the spacecraft not only has to align its orbital plane with that of the other spacecraft, but also to do its final insertion burn close to the space station. Because of the very rare launch windows and high propellant demands the alternative way — inserting the spacecraft into a catch-up orbit — may be preferred.
Rendezvous
When the target is reached, the orbital parameters of both craft should be equal. At close distances, its enough to just neutralize any velocity difference.
On the surface
Rendezvous on the surface of a planet is not nearly as difficult as in orbit since the speeds involved are much smaller. The presence of an atmosphere also simplifies maneuvers for aerodynamic vessels. The simplest way to perform a surface rendezvous is to fly directly towards the other spacecraft.
When the other spacecraft is far away (such as on the other side of the planet), a more fuel-efficient way to get there may be via a ballistic trajectory. This can be thought of as "throwing" the spacecraft towards the target, and letting it fall close to it.
