The Nova Cannon is one of the most exceptionally powerful weapons employed by the military forces of the Imperium of Man. Mounted below the heavily armoured prows of Imperial Navy Cruisers and Battleships, Nova Cannons have few equals in terms of their range or destructive power. Very few weapons are capable of creating a blast effect that can encompass multiple Warp-capable starships. These rare and massive weapons are distinguished by that capacity.
The Nova Cannon is a weapon of ancient providence dating back to the Dark Age of Technology, though its exact origins remain unknown. But what is a well established fact is that Nova Cannons are a classification of exceptionally powerful weaponry, easily able to wreak vast destruction upon enemy vessels at great range. However, a Nova Cannon is inaccurate and requires a stable vessel to serve as a platform from which it can fire effectively. While variation exists, a typical Nova Cannon consists of an array of potent gravimetric impellers designed to accelerate a projectile to a fraction of the speed of light. These projectiles vary, ranging from sophisticated plasma warheads which burn with the ferocity of a small star for a fraction of a second, to implosive devices which exert destructive gravitational forces upon all those caught within several thousand kilometres of the detonation. In any case, a well-used Nova Cannon is a terrifying thing to face, as much a psychological tool as a weapon. Despite this, these weapons are often ill-favoured by the Imperial Navy, with most starship captains preferring to utilise the more traditional torpedoes. Those few who favour the weapon understand that it is difficult to use and often rendered useless at close ranges.
The Nova Cannon itself is massive; the barrel is the length of all but the largest Imperial vessels. The projectiles have a diameter of 50 metres and are fired at close to light speed after being accelerated by gravimetric coils. When the projectile has travelled the predetermined distance, the warhead implodes with a force equivalent to several plasma bombs. This creates a blast zone the size of a small planet, powerful enough to destroy a Light Cruiser in a single hit or cripple even a Battleship.
While the exact principle of the weapon remains unknown, there have been several arguments about the kinetic energy of the impact. Unfortunately, while the kinetic energy can be estimated with some accuracy, the weapon, primarily due to the extreme range and unreliable aiming, is unlikely to directly hit anything smaller than a planetoid. The primary effect of the Nova Cannon is derived from the explosive force of its projectile, which is produced by an unknown method, though it is presumably a very large thermonuclear fusion warhead. Due to the lethal nature of their warheads, Nova Cannon shells are not armed for a fraction of a second after firing, allowing them to travel many tens of thousands of kilometres through the void before they become truly deadly. For these reasons, a Nova Cannon is only fired from an Imperial warship according to specific rules which do not apply to other naval weapons.
- Mars Pattern Nova Cannon - Though Nova Cannons are quite rare, even by standards of starship construction, the Mars Pattern is the most common construction template. These massive cannons -- hundreds of metres in length -- fire an enormous shell that echoes a traditional explosive warhead, though on a much larger scale. These shells are accelerated to near relativistic velocities, causing an explosion that detonates with more force than dozens of plasma warheads.
- Ryza Pattern Nova Cannon - This variant of the Nova Cannon replaces the massive explosive shells with a highly unstable plasma-based macro-bomb. These weapons are even more power-hungry than a Mars Pattern Nova Cannon, but are believed to be slightly more effective as well. With this additional power does come some additional questions of stability, as weaponising such a huge quantity of plasma carries its own inherent dangers.
- Jovian Pattern Nova Cannon - The Jovian Pattern Nova Cannon is one of the rarest and the most feared of its kind. Most commonly found mounted on Cruisers, the Jovian Pattern replaces the traditional explosive warhead with vortex warheads; the discharge of which results in a rift being opened to the Warp. These resultant rifts have been known to rend vessels cleanly in half. As powerful as it is, the ammunition utilised by the Jovian pattern is quite unstable, and damage that would normally render another pattern of Nova Cannon unusable would outright destroy the Jovian instead.
- Battlefleet Gothic Rulebook (Blue Book) (Specialty Game), pg. 22
- Rogue Trader - Battlefleet Koronus (RPG), pp. 15-16, 36-37, 42
- Warriors of Ultramar (Novel) by Graham McNeill