NOVA bomb

"This is the prototype NOVA Bomb, nine fission warheads encased in lithium triteride armor. When detonated, it compresses its fissionable 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..."

- Vice Admiral Danforth Whitcomb's recording on the NOVA bomb, right before it is activated in the space adjacent to the planet Joyous Exultation.

The NOVA Bomb is a Human very high-yield combination of numerous fusion warheads, used by the United Nations Space Command.

Background
The NOVA Bomb is a cluster of nine fission warheads encased in lithium triteride. When the warheads are activated the lithium triteride shell focuses the blast in upon itself, multiplying the destructive power a hundredfold. This gives the NOVA Bomb an unprecedented payload for its size. Its capability to destroy a planet, thus causing the nickname "Planet-Killer" to be derived. It should be noted, that on the one recorded event when a NOVA Bomb was detonated in the midst of a fleet close to a planet, it scorched one-quarter of the planet due to a concentration of thermal energy inside the planet's atmosphere. Any further detailed analysis on the full power of this bomb has yet to be mentioned in the Halo series, but it is known that it has the most destructive capabilities in the UNSC arsenal.

Theory
The NOVA Bomb's explosive power is quickly depleted due to the lack of particles or atmosphere around it when it's detonated in space. This results in a much quicker release of energy, with the possibility of even further multiplying its damage to ships, planets, moons, and other celestial objects due to the lack of energy absorbing particles in space. However if detonated inside a planet's atmosphere, a portion of the total energy generated would be dispersed depending on the atmosphere. Even if the power of the Nova bomb was not mentioned in the book, it can be loosely calculated: nine warheads multiplied by thirty megatons (though it is not known if this is the actual yield of one NOVA fission warhead) each and this multiplied again by the 100 fold yield of the lithium triteride shells. So we can state the Nova bomb could be around that of a twenty-seven gigaton yield (if the multiplier was exactly one hundred-fold the equation is: 9 warheads x 30 megaton x 100 = 27 gigaton yield)

History
It was intended for the NOVA Bomb to be used in space against the Covenant Fleet during the Battle of Reach. In the novel Halo: First Strike it was 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 only documented triggering of a NOVA Bomb occurred when it was accidentally detonated by a group of Huragok. The Huragok, through their curiosity and frenzied obsession, fixed the circuitry despite Kwassass, a Grunt, who attempted to stop them. This resulted in its detonation. The resulting explosion occurred between a Covenant outpost planet Joyous Exultation and its moon, Malhiem, scorching a quarter of the planet and shattering its nearby moon. 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 for the humans, as the Elites, led by Imperial Admiral Xytan 'Jar Wattinree could have shattered the Brute fleet, thus effectively destroying the Covenant. This could have immediately turned the tide of the war for both the UNSC and the Covenant Separatists. However, the Elite Imperial Commander there, along with all of the station elites still believed in the Original Covenant lies, and would have attacked the humans at earth, and possibly declare Thel 'Vadam (the arbiter) and Rtas heretics. The NOVA Bomb explosion severely damaged three-quarters of the fleet that was there, totaling over 300 ships. Only eighteen Destroyers, two Cruisers, one Carrier and the Flagship Incorruptible escaped the NOVA Bomb explosion due to a Slipspace jump en masse just seconds prior to the explosion. The novel described the explosion as though "a small star erupted between Joyous Exultation and its moon."

Delta Halo
During the Battle of Installation 05, and the subsequent Quarantine of the Installation and High Charity, Cortana requested that UNSC HIGHCOM deploy a NOVA Bomb around Installation 05 and High Charity to stop the outbreak of the Flood in order to 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 later landed on Installation 00. It is also possible that this request was granted, but that High Charity went into Slipspace before the warhead could even be launched. However this last possibility is highly unlikely because a nuclear warhead is very easily spotted when transitioning from a slipspace jump.

Trivia

 * The NOVA Bomb was a bomb mentioned in the novel Starship Troopers, wherein it is described as "being able to split a planet in half." It is likely that this inspired the Halo Universe's own 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 Troopers bombs of the same name, the NOVA Bomb is one of the most powerful weapons you can use in the Starfox series. It 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 entire system around it.
 * Only a handful of these enormously powerful weapons have ever been made in the history of the UNSC. However, it is unknown if the explosion of the NOVA bomb, actually killed Xytan 'Jar Wattinree because he was in a congress with 30 Ship Masters (including Voro 'Nar Mantakree, but he survived by transitioning into Slipspace aboard his ship). It is unknown if he was in the part that was obliterated or not.
 * The NOVA Bomb was frequently referenced in Halo: Ghosts of Onyx.
 * While it was first used in Halo: Ghosts of Onyx, it was first mentioned in an earlier novel Halo: First Strike.