A 'spaceship is a finite pattern that reappears (without additions or losses) after a number of generations (known as its period) and is displaced by a non-zero amount. By far the most natural spaceships are the glider, lightweight spaceship, middleweight spaceship and heavyweight spaceship.
It is known that there exist spaceships travelling in all rational directions and at arbitrarily slow speeds (see universal constructor). Before 1989, however, the only known examples travelled at c/4 diagonally (gliders) or c/2 orthogonally (everything else). In 1989, Dean Hickerson started to use automated searches to look for new spaceships, and had considerable success. Other people have continued these searches using tools such as lifesrc and gfind, and as a result there are now a great variety of known spaceships travelling at ten different velocities.