x66 is a period-4 spaceship with speed c/2 that was found by Hartmut Holzwart in July 1992. One of the symmetric halves can alternatively be escorted by a heavyweight spaceship. Its name refers to the fact that every cell (alive or dead) has at most 6 live neighbours (in contrast to spaceships based on lightweight spaceships, middleweight spaceships, or heavyweight spaceships). In fact, this spaceship was found via a search with this restriction. From this follows that the spaceship is stable also in Life-like rules allowing birth or survival for a cell with 7 or 8 neighbors.
x66 was first synthesized on January 11, 2015, requiring 162 gliders. After the emergence of this spaceship in a symmetrical soup on March 8, 2015, this was drastically reduced to 13 gliders, and then to 12 gliders in 2018.
Half of x66 can be supported by a heavyweight spaceship. A 9-glider synthesis for this variant was found on March 8, 2015, from the same reaction used to create the full x66 in 13 gliders. Since the heavyweight spaceship is glide symmetric, it can support two halves at once, or one half alongside a sidecar.
Half of x66 supported by a heavyweight spaceship RLE: here
- 34P4H2V0.23 at Heinrich Koenig's Game of Life Object Catalogs