Orbital defense platform

"That MAC gun can put a round clean through a Covenant capital ship."

- Avery Johnson

An Orbital Defense Platform is a class of large UNSC space stations used to defend locations of great importance, typically planets of high strategic value. They have the Hull Classification Symbol of ODA.

ODPs are frequently placed in geosynchronous orbit and deployed in groups, or clusters, of two to five platforms. The well-equipped and large bridge of an ODP is suitable for directing large-scale engagements. Such was the case when Fleet Admiral Sir Terrence Hood directed UNSC forces from an ODP during the Battle of Earth. The most effective method to destroy these platforms is via Covenant Boarding Craft. These craft would avoid the heavy slugs, and then plant a bomb. In order to avoid this, the UNSC would attempt to repel the boarders.

Facilities
Internally, the stations are remarkably spacious, complete with high ceilings and a monorail system to move personnel and equipment. In addition to numerous small craft bays, there are two docks for larger ships, such as freighters, or even frigates. These docking bays could possibly be used to hold Longsword fighters used to protect the station from hostile spacecraft. The design incorporates many features against potential hostile boarding action, including security stations and small arms racks situated at strategic locations.

Armament
The primary and crucial armament is a Mk. V "Super" Magnetic Accelerator Cannon capable of causing near-fatal damage to even a shielded Covenant capital warship with a single slug. The slug can weigh up to 3,000 tons and is launched at "point four-tenths" the speed of light, ensuring the complete annihilation of the ODP's target. As with starship-mounted MACs, a platform's cannon is commonly controlled by an AI.

Curiously, no sources indicate the presence of additional weapons, such as point defense cannons commonly mounted on UNSC warships (the absence of such weapons may explain why so many Covenant Boarding Craft were able to latch onto the Platforms orbiting Reach and Earth). However, the possibility that ODPs are not armed with such weapons cannot be ruled out yet. The apparent lack of point-defense cannons may be explained by the fact that transient magnetic bursts tend to magnetize the weapons.

Power plant
ODPs do not possess the power generation equipment necessary to fire their main armament. Instead, they receive power from ground-based generators. A possible strategy to neutralize the formidable ODPs would be to disable the generators via ground assault. This would disable the platform's weaponry without destroying it, allowing this valuable prize to be captured. The Covenant used this strategy during their invasion of Reach, dropping troops to the planet's surface and destroying these generators. This paved the way for Covenant forces to completely overwhelm the UNSC forces in the space battle over Reach.

Reach
By August 2552, Reach was protected by 20 ODPs. At least some of these platforms were in place as early as 2527. During the Fall of Reach, Marines and the SPARTANs were deployed to protect the orbital platforms' planetside power generators but were ultimately unable to do so against determined enemy assaults. The disabling of the defense platforms was a major factor on the battle's outcome.

Earth
The outbreak of the Human-Covenant War prompted a massive expansion of Earth's defenses. Even as late as the Battle of Reach preparations had not yet been completed; they were set to come online on September 14. By October 2552, some 300 ODPs were operational. Many of Earth's ODPs were named after points of interest that they roughly kept station over.

A portion and small section of the 300 platforms first saw action at the early stages of the Battle of Earth. Athens Station and Malta Station were each destroyed by conveniently placed Covenant Bombs installed by Covenant boarding parties, while Cairo Station only narrowly avoided the same fate as the other platforms thanks to Master Chief's removal of the Covenant bomb on board.

The ODPs were also used later in the Battle of Earth after Truth's Fleet arrived, but the Covenant might have already destroyed numerous stations when Truth himself arrived at Earth on the Forerunner Dreadnought in November.

Known ODPs

 * Athens Station UNSCDF-ODA144 (Destroyed)
 * Cairo Station UNSCDF-ODA142 (Damaged)
 * Malta Station UNSCDF-ODA143 (Destroyed)
 * Nassau Station (Unknown)

Trivia

 * All known Orbital Defense Platforms over Earth are named after cities on Earth.
 * The fact that there are 300 ODPs could be a reference to the Battle of Thermopylae, in which 300 Spartans used the phalanx formation to hold their own against an army of more than 200,000 Persians.
 * Two weeks after the Battle of Reach, when the UNSC Security Commitee met at HIGHCOM in Sydney, it is mentioned by Colonel Ackerson that most of Earth's ODP'S would take ten days before being fully operational.
 * The kinetic energy of a single shell from an orbital defense platform would be equivalent to almost 51.6 gigatons of TNT, 1031.08 times the yield of the current world's largest nuclear weapon ever tested.
 * They were originally proposed sometime before 2490 as Avery Johnson mentions that they were being proposed around the same time he shipped out for boot camp.
 * ODPs can also fire downwards into the atmosphere, as shown in Halo: Reach when an ODP's MAC round destroys a Covenant Corvette in atmosphere.