NOVA bomb

"This is the prototype NOVA Bomb, nine fusion warheads encased in lithium triteride armor. When detonated, it compresses its fusionable material to neutron-star density, boosting the thermonuclear yield a hundredfold. I am Vice Admiral Danforth Whitcomb, temporarily in command of the UNSC military base Reach. To the Covenant uglies that might be listening, you have a few seconds to pray to your damned heathen gods. You all have a nice day in hell..."

- Danforth Whitcomb's recording on the NOVA bomb.

The NOVA Bomb is a thermonuclear weapon of the United Nations Space Command, which has a high explosive yield, having the ability to destroy moons.

Theory
The NOVA Bomb itself is nine fusion warheads encased in lithium triteride. When the warheads are activated, the lithium triteride shell focuses the blast upon itself, multiplying the destructive power a hundredfold, giving the NOVA Bomb an unprecedented payload for its size. Its capability to destroy a planet gave it the nickname of "planet-killer". This may be misleading in the fact that it will do nothing more than superficial damage to a planet, but instead wipe out the entire population on it. The full power of this weapon has never been thourougly mentioned in the Halo Series. Also, its explosive power is quickly gone due to the lack of particles or atmosphere around it when detonated. This results in a much quicker release of energy, possibly even further multiplying its damage to ships, planets, and moons.

History
It was intended for the NOVA Bomb be used in space against the Covenant fleet during the Battle of Reach. In the novel, they were left on Reach and "repurposed" by Admiral Whitcomb. His reasoning was that "Either the Covenant pack those bombs up and take them home for study, a possibility that I hope to God happens. A bomb like that could crack their homeworld in two. Or, the bombs stay here, and they'll stop the Covenant on Reach..."

The NOVA Bomb has only been used once, when it was accidentally detonated by a group of Huragok, who, through thier curiosity and obsession, fixed it, despite a grunt's attempts to stop them, resulting in its detonation. The resulting explosion detonated between a Covenant outpost planet Joyous Exultation and its moon, Malhiem, scorching a quarter of the planet and shattering the moon, and the resulting winds on Joyous Exultation flattened cities and spawned tidal waves. Every ship within the NOVA Bomb's range boiled and vaporized in an instant. This was actually extremely unfortunate as the Elites, lead by Imperial Admiral Xytan 'Jar Wattinree could have shattered the Brute fleet, thus effectively destroying the Covenant. This could have turned the tide of the war for both the UNSC and the Covenant Separatists. It severely damaged three-quarters of the fleets that were there, which was over 300 ships. Only eighteen Destroyers, two Cruisers and one Carrier and the Flagship Incorruptible escaped the NOVA bomb explosion due to Slipspace jumps just prior to the explosion.

Delta Halo
In Ghosts of Onyx, Cortana requests that the UNSC launch a NOVA Bomb around Delta Halo and High Charity to stop the spread of the Flood and destroy the Covenant Holy Capital. However, this request was not granted likely due to the low number of nuclear warheads left and their need to protect the inner colonies. High Charity was not bombed and landed on the Ark. It is also possible that this request was granted, but that High Charity went into slipspace before the warhead could be launched.

Trivia

 * The Nova Bomb was a bomb in the book Starship Troopers, wherein it is described as "being able to split a planet in half." It is likely that this inspired the Nova Bomb.
 * A Nova Bomb is also the common name for the heavy bomb used in Starfox games. Though not nearly as powerful as the Halo and Starship Trooper bombs of the same name, the Nova Bomb is one of the most powerful weapons you can use in the Starfox series.
 * The Nova Bomb also appears in the Andromeda TV series. In that show it is a weapon capable of causing stars to go into the supernova stage, therefore destroying the system around it.