- "You are...all of you...vermin. Cowering in the dirt thinking... what, I wonder? That you might escape the coming fire? No. Your world will burn until its surface is but glass!"
- —The Prophet of Truth, to humans regarding Earth
The process of glassing a planet begins when a fleet has achieved air and space superiority on an enemy-controlled world. The specifics of the glassing process vary depending on the strategic significance of the planet or the orders of the Fleet Master. A Minor Prophet is typically assigned to most Covenant fleets to oversee the destruction of a human world. In some or possibly all instances, before the fleet is permitted to destroy a planet, the Prophet performs a religious ritual - the Prophet declares by which religious ideal of the Covenant, marked by a glyph, will the world be destroyed. There is currently only one known instance of a human world, Kholo, having a glyph burned into its surface.
Before bombardment commences, all Covenant ground operations cease and local forces are evacuated. The Covenant will then begin glassing the planet from orbit by moving their large warships close together, proceeding to blanket the world's population centers and the surrounding areas with a series of crisscrossing orbits to ensure that the planet is rendered uninhabitable.[note 1] The glassing of a planet by the Covenant is a very taxing process, and requires a massive amount of energy even for a Tier 2 civilization; Nevertheless, the Covenant are able to accomplish this feat in an effective matter.
Glassing a planet can take a great deal of time, dependent upon both the tonnage and class of the vessels involved and how many ships are within the fleet. It could take as little as two days or as much as two weeks or longer to complete the process. While partial glassing can be accomplished in a matter of days, covering the entire surface area to the last centimeter would take a prohibitive amount of time. The Assembly believed that, were the Covenant to commit thousands of ships to the task, it would have taken them around thirty years to completely glass Earth alone; Earth's relatively small size, along with a myriad of other factors, would likely cause the thorough glassing of larger planets to take even longer.[note 2]
The Covenant have been known to utilize two methods of glassing.
 Orbital bombardment
Typically, when fleet actions require no deployment of ground forces, the Covenant will glass the planet from orbit; this is accomplished by their larger warships gathering close together, aiming their plasma batteries, located along the lateral lines of their warships towards the surface and firing them. Once discharged, the plasma rounds are guided towards the surface, contained in a magnetic bubble and controlled by the ships automation through miles of atmospheric interference until it strikes its intended target. Resembling gigantic fireballs, these plasma lances are a hundred meters across, and strike the surface with tremendous speed and force.
The impact of the plasma lance is similar to that of a nuclear detonation, though on a much different scale. When the plasma bolt impacts the surface, the magnetic field sustaining and guiding the plasma collapses, and, depending on how powerful the release of energy is, the initial zone of impact is obliterated instantly. The areas outside of the initial impact zone are affected by the heat wave generated by the blast; depending on the range, those closest outside of the blast zone are instantly killed by the intense heat. As thermal expansion takes over, the resulting flames fan out and create a pyroclastic surge which continues to burn the areas it comes into contact with until it has cooled enough that it cannot harm the surface.
The topsoil in the impact zone and other surface geology become molten and are converted into a mineral called lechatelierite, which is similar to glass. The surrounding areas are blasted with intense heat and are burned.
Subsequently, the atmosphere is saturated with millions of tons of ash, debris and soot that are thrown up from the initial impact, blocking sunlight and dramatically lowering the amount of sunlight reaching the planets surface, subjecting it to an impact winter. As the ground zero area cools, the surface is covered by extensive areas of molten soil, and is comparable to active volcanic sites in some parts of the world, on a larger scale.
 Low-altitude bombardment
When ground engagements are in effect, the Covenant will resort to low range bombardment, bringing their ships closer to the surface to destroy the area in question. This is accomplished by either building up or focusing plasma though a magnetic envelope from the energy projector located on the underside of the ship, and then discharging the plasma as a narrow beam. A single CCS-class battlecruiser can glass approximately one acre of a planet's surface in fifteen seconds of sustained fire, with desert taking less time and deep ocean taking much longer to glass. The Covenant use low-range bombardment in many battles where major ground engagements are occurring; smaller warships use this method specifically to deal with human armies and low-level structures during campaigns, while larger Covenant warships use this to level cities while engagements are occurring.
Low-range bombardment is not performed solely by energy projectors; smaller Covenant ships like corvettes are equipped with bombardment guns that fire smaller, less-powerful lances than those utilized by capital ships. Capital ships can also use their energy projector to excavate areas of interest for the Covenant, such as unearthing the Portal to Installation 00 during the Battle of Earth.
The destructive process of bombarding the surface of a planet with plasma leaves the surface unable to recover to its former state without re-terraforming. During the glassing of a planet the Covenant focus on destroying the ecosystem of a planet, and will spend a considerable amount of time vaporizing any bodies of water the planet may possess, or at least reduces the remaining water to small, ash-choked pools. The deeper the body of water is, the more time it will take to vaporize it. Once the Covenant have finished the surface is dead, devoid of any life on the surface. From orbit the world is simply a series of grays, blacks and reds, everything on the surface is charred and the air is littered with ash and dust.
It is possible for the atmosphere of a planet to boil away, or more correctly, suffer atmospheric escape from the process. Large planets, particularly those of which the damage is not enough for it to lose atmospheric gases to space, do not suffer this.
Because of the extensive destruction brought by the Covenant, the only way the world could recover is by re-terraforming efforts. The undertaking of such an effort would be massive and time consuming, ranging from a conservative 110 years to over 300 years given the manpower and technology at the time of such estimates. However, these estimates were made in 2530, and human technology took drastic leaps forward in the wake of the Human-Covenant War over 20 years later, particularly due to advances brought forth by the discovery of Forerunner technology. It is apparent that such figures do not hold true in the post-war era, as Reach was at least partially re-terraformed within thirty-seven years.
Throughout the Human-Covenant War, areas of interest to the Covenant were left intact for reasons of study or retrieval; however, this was not common, as only a handful of planets were partially glassed and only when a Forerunner artifact was discovered. After the Covenant glassed a planet, a shard of glass was removed and was placed in the Sanctum of the Hierarchs within High Charity.
Since the formation of the Covenant in 852 BCE, the Covenant have used their ship-borne plasma weapons to threaten lesser species into a truce and their eventual induction into the Covenant. The Lekgolo were the first to be threatened with such destruction because of their transgressions against Forerunner technology before becoming the first outside species to join the Covenant in 784 BCE. The Unggoy almost faced orbital bombardment for their rebellion in 2462 CE, or the 39th Age of Conflict by Covenant records. Although the Kig-Yar were not threatened with orbital bombardment, they were quick to realize the Covenant had such a capacity to destroy them and thus chose to join the Covenant for greater wealth that the Covenant would provide.
 Human-Covenant War
- Main article: Human-Covenant War
After the Hierarchs' declaration to exterminate humanity, the Covenant put all of their destructive methods to use. The first planet to suffer this fate was Harvest, glassed by the Jiralhanae-operated battlecruiser Rapid Conversion. Prior to the war, the territory of the Unified Earth Government comprised over eight-hundred planets; most of these worlds were glassed during the war.
By the time Vice Admiral Preston Cole arrived with his battlegroup in March 2526, the Covenant had nearly destroyed the entire planet. After Cole's bloody victory, the UNSC waged a five-year campaign to reclaim the planet as it underwent a nuclear winter.
The term "glassing" was coined by the artificial intelligence group known as the Assembly in 2526. The group at the time estimated that it would take 30.3801 years for a Covenant fleet of comparable size to that of the UNSC's to glass Earth. The Assembly chose the term to magnify the horrible act in the hope that it would galvanize humanity into action.
Throughout the rest of 2525 and 2526, the Covenant continued their campaigns in the Outer Colonies, glassing a number of worlds and human establishments including Biko, Green Hills, and Bliss. Admiral Cole and his fleet continued fighting for these colonies but they were lost causes, the Covenant with their determination, superior firepower and technology continued their ruthless campaign until the last outer colony was glassed in 2535. Each battle human forces face with the Covenant were brutal ground forces were often deployed first, but as UNSC forces were routed in space, the bombardment would commence resulting in either loss of life or retreat.
The Covenant's bloody campaign continued as they moved into the Inner Colonies and began glassing major worlds. Throughout the 2540s, the UNSC continued their attempts at slowing the Covenant. Although some of their tactics succeeded, the Covenant continued to swarm human-controlled space, glassing every world they found. After the apparent death of Admiral Cole in 2543, the Covenant briefly halted their advance, then renewed their bloody tenacity and attacked human-controlled worlds with even more ferocity. After the Fall of Reach in the summer of 2552, the Covenant leadership believed that humanity would lose the will to fight. However, this was a miscalculation, as humanity's disastrous defeat at Reach caused them to fight to the last breath, scoring a crushing victory at Installation 04, which would indirectly lead to the Great Schism, and with the crucial help of their new-found Sangheili allies, Humanity would win the Human-Covenant war.
The Battle of Earth culminated in the glassing of the city of Voi in Africa by Covenant Separatists in an effort to contain a Flood infestation which had come to Earth only hours earlier. Fleet Admiral Terrence Hood accused Shipmaster Rtas 'Vadum and the Fleet of Retribution of glassing half of the continent, but later views of Africa's surface showed only a relatively small area was scorched. Given Hood's distaste for his former enemies, he probably exaggerated about the extent of the glassing.
 List of known glassed worlds
 Halo Wars gameplay
- "Burn, heretics!"
- —Regret, when using his Cleansing ability.
In Halo Wars, the Prophet of Regret is able to call down a "Cleansing Beam" from an orbiting Covenant ship. Rather than a leader power, this is treated as the unit's secondary ability, using the left control stick to guide the beam at the cost of resources every second. It lacks the power of typical glassing maneuvers to balance gameplay, much like Captain Cutter's ability to fire a MAC blast at enemy targets. The upgrades Regret’s Sentence, Regret’s Doom, and Regret's Condemnation increase its blast radius, though require a higher tech level to purchase.
A CCS-class battlecruiser glassing New Alexandria.
The Prophet of Truth’s fleet uncovers the Portal.
 List of appearances
- ^ According to earlier media, such as the original printing of Halo: First Strike, the Covenant glassed every square centimeter of a planet. However, later media show a more practical approach to glassing in which only part, perhaps most, of a planet is glassed in order to render it uninhabitable; the 2011 re-release of Halo: First Strike was modified to accommodate this.
- ^ While the Assembly asserts in Data Pad 10 that glassing a whole planet is a feat too impractical for the Covenant to feasibly achieve, the rest of canon contradicts this. Numerous instances are listed in the novels where glassing and glassed planets are observed, as well as in Halo: Reach where the data pads are from, since Reach's burnt surface is the opening shot of the game. Possible reasons for this discrepancy may be that the Assembly had only observed low-altitude bombardment, which is much slower and precise, or that they had misjudged the size of the full Covenant fleet, since they had based their figures on a fleet the size of the UNSC's. While the Assembly never explicitly retracts its statement, it does however state in Data Pad 14 that terraforming on the first eight colonies lost could take up to 3 centuries, an unusually long figure if those planets' full surfaces were never scorched. Based on the sum of the canon, it is thus assumed here that glassing is indeed a true method of attack by the Covenant.
- ^ Halo: First Strike, page 105; page 136 (2010 edition)'
- ^ Halo: Evolutions - Essential Tales of the Halo Universe, "The Return", pages 494-496
- ^ Halo: First Strike, page 130 (2010 edition)
- ^ Halo: Evolutions - Essential Tales of the Halo Universe, "The Return", page 495
- ^ a b c d e Halo: Reach, Data pad 10
- ^ Halo Legends, Halo: The Story So Far
- ^ Halo: The Fall of Reach (2010), Adjunct - page 389
- ^ Halo: The Fall of Reach, page 8 page 22 2010 edition
- ^ a b Halo: First Strike page 32 page 46
- ^ a b Halo: Evolutions - Essential Tales of the Halo Universe, "The Return", page 505
- ^ Halo: Reach Legendary Edition Guide page 440
- ^ Halo: Reach, campaign level, New Alexandria
- ^ Halo: Reach, campaign level, The Pillar of Autumn
- ^ Halo: First Strike Bonus Content page 417
- ^ Halo: The Fall of Reach, page 8; page 23 (2010 edition)
- ^ Halo: Reach, Data pad 14
- ^ Halo: Reach, campaign level Lone Wolf
- ^ Halo: First Strike page 122; page 150 (2010 edition)
- ^ Halo: First Strike page 338; page 405 (2010 edition)
- ^ Halo: Contact Harvest page 270
- ^ [Halo Encyclopedia]] page 114
- ^ Halo: Contact Harvest, page 151
- ^ Halo Encyclopedia page 142
- ^ Halo: Contact Harvest page 285
- ^ Halo Encyclopedia, "chapter 1", page 33
- ^ Xbox.com: The Halo Timeline
- ^ Halo: The Fall of Reach page 173; page 199 (2010 edition)
- ^ Halo: Evolutions - Essential Tales of the Halo Universe, "The Return", page 499
- ^ Halo 3, campaign level, Floodgate
- ^ Halo 3, campaign level, Halo
- ^ File:PortalOrbit.png