Orbital bombardment refers to the act of bombarding targets on a planetary surface from orbit.
- Main article: Glassing
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's 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 Magnetic Accelerator Cannon round impacting a planetary surface can deal equally devastating damage within an extremely localized radius. However, instances of the UNSC using MACs to bombard a planet are rare; only the UNSC Spirit of Fire and UNSC Infinity are known to have engaged in such practices. Judging by its effects, the Spirit of Fire's MAC is seemingly less powerful than the MAC guns on most UNSC ships, as the damage done by a normal MAC round would be immensely greater.
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.
In addition to using weapons specifically designed for the task, orbital bombardment can be performed by de-orbiting asteroids or other objects onto 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.