NOVA bomb

"This is the prototype NOVA Bomb, nine fusion 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 nine nuclear fusion warheads, used by the United Nations Space Command.

Background
The NOVA Bomb is a cluster of nine nuclear 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 has earned it the nickname "Planet-Killer". 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's surface due to a concentration of thermal energy inside the planet's atmosphere; it also shattered a moon. 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 is the most destructive weapon in the UNSC's 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 throughout the atmosphere. The density as well as composition of the atmosphere are the largest contributing factors when determining how much energy is dispersed. Even though the power of the NOVA Bomb is not mentioned in the book, a low end can be calculated:

Assumptions: the moon is 2km, and the NOVA bomb is only 5,000 km away.

Calc: Apply the Inverse Squared Law: Source Energy / (4 * Pi * R2) the radius is the distance from the source to the range.

x/ (4 * 3.14195... * 5,0002) = 4 Megatons per square kilometer to fragment a 2km moon which requires 8 Megatons to be fragmented.

x = 1.2 Petatons.

Some may find this yield hard to believe for a fusion device, but the stated effects to the planet and nearby moon require explosive power of this magnitude.

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 stationed on the Covenant Sublime Transcendence. Immediately after Kwassass, a fellow crew member, deciphered Admiral Whitcomb's message, the 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, 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. It is unknown what the Covenant did with the planet, but it is unable to support life any more.

The NOVA Bomb explosion severely damaged three-quarters 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 the 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."

Exactly what effect this had on later events is unknown. If the NOVA hadn't detonated and the fleet had survived, the Elites would certainly have had a larger attack force, and been better able to crush the Brute defenses during the Battle of Installation 00. Imperial Admiral 'Wattinree seems to have shown no reconsideration of the humans status as heretics, even after rejecting the Prophets, sending a fleet to Onyx to claim it for the Elites - if he had survived, especially given the amount of reverence given to someone of his stature, it is unknown whether he would have allied with the Arbiter and humanity.

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 (hence the name); a nearly identical weapon is also found in X-COM: Interceptor, though that version is launched from a fighter and takes around a minute to accelerate to hyperspeed before impact in order to burrow deep enough into the star's core.
 * Only a handful of these enormously powerful weapons have ever been made in the history of the UNSC.
 * 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.
 * The bomb may be named after novae -- cataclysmic nuclear explosions of stars.