From Halopedia, the Halo wiki
Orbital bombardment is the act of bombarding targets on a planetary surface from orbit.
While the Covenant sometimes perform glassing from close range near the surface, the most common practice is to fire lances of magnetically contained plasma to the planet surface from orbit. As the plasma bolt impacts the ground, the magnetic field collapses and everything in the impact zone is vaporized instantly. Most commonly, this is performed by Covenant warships covering the entire planet in crisscrossing orbits until the entire surface has been destroyed.
While humanity lacks the ship-borne plasma weapons the Covenant possess, the kinetic energy released by a UNSC vessel's Magnetic Accelerator Cannon round impacting a planetary surface can deal equally devastating damage within a localized radius. Notable instances of MAC strikes include those employed by the UNSC Spirit of Fire, UNSC Majestic and the UNSC Infinity. Judging by its effects, the Spirit of Fire's MAC is seemingly less powerful or fired on a lower power setting than the MAC guns on most UNSC ships, as the damage done by a MAC round fired at full velocity would be immensely greater, although the damage done by Infinity and Majestic would be consistent with such energy being released.
The UNSC may also use nuclear weapons as a form of bombardment. While the UNSC often uses nuclear weapons during ground and orbital engagements, the use of such armaments to bombard a planet's surface from orbit is rare. The only widely known instance of this occurring is during the rebellion on the Far Isle colony in 2492. It should be noted that both conventional and nuclear bombardments are far less effective at neutralizing a planet than either glassing or the use of a NOVA Bomb.
On October 14, 2559, during the Battle of Epsilon Clarion, Veta Lopis has the Spirit of Fire perform an orbital bombardment using EMP MAC rounds on the Ark's Epsilon Clarion installation. The bombardment reduces the facility to molten slag while the EMP fries all electronics in the area. The facility's destruction prevents the attempt by the Keepers of the One Freedom and the Covenant loyalists to fire the Halo Array while at the same time destroying the archeon-class ancilla Intrepid Eye.
In addition to using weapons specifically designed for the task, orbital bombardment can be performed by de-orbiting or dropping asteroids or other objects to a planetary target. Such was the case in the concluding moments of the Battle of Metisette, when the AI Juliana de-orbited the Rubble so that Hesiod's moon Metisette would collide with the Rubble's former asteroid habitats, making them rain down on the Redoubt.
In Halo Wars 2, one of the leader powers granted to Jerome-092 is to direct rubble from the destroyed CAS-class assault carrier, Enduring Conviction, down onto an enemy force as an improvised orbital strike.
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