A crawler or climber is a reaction in which two objects (or groups of objects) collide, leaving one of the objects moved backwards or forwards to a degree, and the other forward. Crawler reactions are commonly used to create engineered spaceships, also known as caterpillars.
Perhaps the first and most well known climber is the infamous 17c/45 reaction, consisting of a pi heptomino running over a blinker, moving it back 6 cells and rephasing it, whilst the pi heptomino moves forward 17 cells. This takes place in 45 generations.
The reaction was known for a long time, however the idea of a spaceship powered by this reaction was mentioned much later in 2002 by David Bell who had discovered reactions between multiple pies, which emitted gliders. After much work, Gabriel Nivasch finished construction of caterpillar, the first 17c/45 orthogonal spaceship, on 31 December 2004.
Research has been conducted into building much smaller 17c/45 spaceships with pi crawlers, analogously to the silverfish. Discussion has been ongoing since as early as 2013.[1]
There is a well-known reaction in which a glider crashes into the side of a half-bakery, shifting the loaves and releasing a glider. This reaction has been used to create Half-baked knightship and parallel HBK, and was later used to create the HBK caterpillar.
Another orthogonal crawler, the 31c/240 reaction, consisting of a Herschel crawling though blocks.
Several spaceships are known that use this 31c/240 crawler, in order of discovery being the shield bug, centipede, silverfish, Leaf bug (related to Leaf bug 2), and Water strider, the latter two formerly being the smallest self-supporting spaceships in life.
The (23,5)c/79 Herschel climber involves a Herschel reacting with a stream of gliders, emitting more gliders behind it in the same way, displacing the Herschel by 23 cells vertically and 5 horizontally in the space of 79 generations. It can also crawl on certain still lifes as well as gliders.
This climber has been engineered and other components assembled by hand to create waterbear, which was the fastest known oblique spaceship until the discovery of Sir Robin in 2018.
Consists of an I-heptomino climbing a stream of gliders, emitting two gliders and a block for every one glider it receives. Discussion started in late 2025; results include a complete (15/11)c/107 ship by Nora Brown in early 2026.[2]
Consists of a pair of Herschels climbing a track consisting of pairs of gliders. While it emits three gliders for every two it receives, it also emits an extra block. Research started in late 2023,[3] and a completed spaceship was provided by user Brithos in summer 2026. [4]
List of crawlers with potential for completed spaceships
Many crawlers exist in life, but relatively few out of those actually have the potential to be made into ships. Below are some crawlers which have not yet been made into spaceships, but modified or not output sufficient outputs of gliders and/or minimal numbers of still-lifes to be considered contenders to be engineered into spaceships.
An interesting climber, the (13,1)c/31 Pseudo-B climber involves a climbing reaction that never isolates itself into a Herschel or B-heptomino. A spaceship based on it was first mentioned by Gabriel Nivasch as an afterthought on his caterpillar article.
Consists of a B-heptomino climbing on gliders. Was initially proposed in 2014,[5] but no development happened until a brief burst of work in early 2021.[6]
Consuming blinkers, a new blinker is produced with a significant shift. However, a large cluster of debris is left in the other direction, making stabilisation appear challenging.[9]