An object that moves like a spaceship, except that it leaves
debris behind. The first known puffers were found by Bill Gosper and
travelled at c/2 orthogonally (see diagram below for the very first
one, found in 1971).
Not long afterwards c/12 diagonal puffers were found (see
switch engine). Discounting wickstretchers, which are not
puffers in the conventional sense, no new velocity was obtained after
this until David Bell found the first c/3 orthogonal puffer in April
1996. Other new puffer speeds followed over the next several years.
Many spaceships that travel orthogonally at a speed less than c/2
have useful side or back sparks. These can be used to perturb
standard spaceships that approach from behind. A common technique
for creating puffers for a new speed uses a convoy of the new
spaceships to create debris from an approaching standard spaceship
such that a new standard spaceship is recreated on the same path as
the original one. This forms a closed loop, resulting in a
high-period puffer for the new speed.
As of June 2018, puffers have been found matching every known
velocity of elementary spaceship, except for c/6 and c/7 diagonal
and (2,1)c/6. It is also generally easy to create puffers based on
macro-spaceships, simply by removing some part of the trailing
cleanup mechanism.