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A NOVA Bomb is a very high-yield combination of nine nuclear fusion warheads used by the United Nations Space Command.
The NOVA Bomb is a cluster of nine nuclear fusion warheads encased in a lithium triteride case. 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 has earned it the nickname "planet-killer."
The NOVA Bomb was intended to be used in space against the Covenant Fleet during the Battle of Reach. 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."
A NOVA Bomb was accidentally detonated by a group of Huragok stationed on the Covenant Sublime Transcendence. Immediately after Kwassass deciphered Admiral Whitcomb's message, a Huragok, through their curiosity and frenzied obsession, fixed the circuitry of the bomb, detonating it despite Kwassass's attempts to stop them. The Huragok completed the detonation timer's circuit, which promptly resulted in its activation. The resulting explosion occurred between a Covenant outpost planet Joyous Exultation and its moon, Malhiem, scorching half of the planet and shattering the moon. The resulting winds on Joyous Exultation flattened cities and spawned tidal waves. Radiation flooded the planet and killed everything living on it, penetrating the surface of the planet to its core. Every ship within the NOVA Bomb's range, save for those on the opposite side of the planet, boiled and vaporized in an instant. The planet is no longer capable of supporting life.
The NOVA Bomb explosion completely destroyed three-fifths of the fleet that was there, totaling over 300 ships. 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. Other ships arrived to reinforce Incorruptible arrived from Joyous Exultation, suggesting that they escaped the bomb's detonation as a result of being positioned on the opposite side of the planet. The novel describes the explosion as though "a small star erupted between Joyous Exultation and its moon."
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 and 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 because of the massive amount of Cherenkov radiation that is emitted during the final transition. Cherenkov radiation is observed during a transition of fissile materials such as plutonium and uranium from Slipspace. Fusion requires both deuterium and tritium (or in the case of "dry" bombs lithium which produces tritium when bombarded by neutrons). These elements would not emit Cherenkov radiation and thus a pure fusion bomb would not be detected when exiting Slipspace, but NOVA Bombs are stated to be thermonuclear weapons and thus use fission at various steps to create the temperatures and pressures necessary to initiate fusion. It is also possible that there were few NOVA bombs because it was only a prototype, but a dangerous one at that.