heavyweight spaceship (commonly abbreviated to HWSS) or (rarely) big fish is the fourth most common spaceship after the glider, lightweight spaceship and middleweight spaceship. It was found by John Conway in 1970 and travels at a speed of c/2 orthogonally. Its domino spark can be used to stabilize several tagalongs, including sidecar and half of x66. It is one of only three known spaceships that is a polyomino in any of its phases.
1973, Douglas G. Petrie made the first 8-glider synthesis for a heavyweight spaceship, involving a 3-glider ship, a 2-glider eater 1, and three other gliders from different directions in the final step. Later, a head-on  3-glider collision producing a clean HWSS was found by Dave Buckingham.
A number of different known ways to construct a heavyweight spaceship can be found in
Mark Niemiec's glider synthesis database.
There are four-object
one-glider seeds for the HWSS, consisting of a loaf, a beehive and two blocks. 
There are also three 1G seeds for the heavyweight spaceship in the  octo3obj database.
See also: List of common spaceships
The HWSS is the thirty-fifth most common object on
Adam P. Goucher's Catagolue.
It is the fourth most common natural spaceship, being less common than the
middleweight spaceship but about 355,000 times more common than MWSS on MWSS 1.
There are 26 results in the
octohash database, 322 results in the octo3obj database, and 30 [note 1] three-glider collisions in the octo3g database with at least one escaping HWSS.
Interactions with other objects
The side and tail sparks allow an HWSS to make changes to other objects without being affected itself -- a "
Heisenburp" effect. For example, gliders approaching from behind can be deleted in five different ways, or can be converted into a block, B-heptomino, or beehive, that can be reanimated to make catch and throw technologies, as shown below.
First row: the five ways to delete a forward glider with an HWSS. Second row: three HWSS Heisenburp reactions with a forward glider (click above to open LifeViewer) RLE: here Plaintext: here
Two HWSSes can escort XWSS-like objects that would otherwise be unstable, forming a c/2 spaceship. Two similar examples are shown below.
↑ There are two collisions (headerless RLEs are
10b3o$6b3obo$11bo$2o$2o7bo$9bo$9bo! and 10b3o$6b3obo$11bo$2o$2o$8b3o!), both converging to the same sequence, with two escaping heavyweight spaceships. The final population is 425.
↑ according to Mark Niemiec: