Terraforming

Terraforming is the process of artificially altering an extraterrestrial planet or moon to be more Earth-like and suitable for terrestrial life. Depending on the starting conditions, this may encompass a wide array of techniques, including atmospheric and climate modification as well as topographic and biological engineering. The term habiforming is also used to refer to alien efforts to alter environments to their preferences. Terraforming is often combined with bioforming, or the introduction of genetically engineered or modified organisms to the terraformed world. On a basic level, this can extend even to the colonists themselves, who may require tailored injections to withstand alien microbiota or non-standard gravity, for example.

Human
Common terraforming measures are more of a nudge toward habitability than the complete remaking of a world. As a general rule, worlds requiring minimal modification are preferred. Typical prerequisites for such worlds include an Earthlike rotation period and thus day length, sufficient gravity, the presence of surface water, and a preexisting nitrogen-dominated atmosphere. Terraforming is never a one-size-fits-all process that can be repeated in the same form everywhere, though some techniques can be useful in more places than one. It is also a question of to what degree it is viable or cost-effective to modify a given planet; sometimes the introduction of magnetic shielding, domes, and localized paragravity generators is considered good enough, whereas some worlds may undergo global atmospheric modification to give them a terrestrial atmosphere. Such processes are usually extensive and are rarely done in under a century. In many cases, terraforming is also not a process that can be considered "done" at any point, as maintaining the planet's habitability requires active systems such as magnetic shields, solar mirrors and shades, artificial magnetospheres, atmospheric processors, and various active climate control measures.

Many terraforming techniques, such as atmospheric modification, climate control, and solar shades, were pioneered in the Sol system prior to the advent of slipspace travel, with several being used to stabilize and repair Earth's climate. Mars is the most notable human colony to be terraformed, and the planet to receive the most extensive terraforming processes; Venus was at one time another prime candidate and was subjected to terraforming experiments for over a century, but it would fall by the wayside as far more attractive worlds were made available via faster-than-light travel. Since the breaking of the light barrier, planets as inhospitable as Mars or Venus are rarely even contemplated for terraforming; for example, using mass ice asteroid bombardment to create oceans as was done on Mars would not be regarded as worth the effort on extrasolar worlds.

Terraforming is always locally tailored and though approaches improve with each effort, various historical failures have shown that the process is never fully predictable. Few worlds are habitable from the outset, but some require only minor tweaks. The relative abundance of worlds with an Earth-compatible biosphere from the outset has puzzled astrobiologists for centuries. It was previously thought highly unlikely that most alien biospheres could support Earth-based life, yet many such worlds have been discovered. Such worlds are generally attributed to ancient biodisplacement. Even in the case of out-of-place biota, it is common for local bacteria to be initially hostile to Earth fauna, though any compatibility issues can generally be solved through medical science.

However, places that are otherwise hostile to life are still colonized for economic or strategic reasons, such as natural riches or unique features not available elsewhere (such as the computing and industrial benefits of cryoworlds). In such cases, alternatives to terraforming include surface arcologies and space habitats, which can be built to be hospitable at a much lower cost than that of terraforming. One popular means of measuring the viability of planetary bodies for terraforming is the CAA's habitability rating system, though experts use far more precise scales when weighing the pros and cons of terraforming.

Worlds with existing biospheres present an ethical case against terraforming, as any attempt to tamper with the local ecology will irrevocably disturb it, potentially with unforeseen and disastrous consequences. Although this is a concern to planetary conservationist groups, it is rare for preexisting planetary ecologies to be completely protected from settlement, though nature preserves are often created.

The human religio-philosophical and political movement known as the Delugers once believed it was humanity's moral imperative to fill an otherwise barren universe with terrestrial life, terraforming even barren worlds through great effort for its own sake. Though active between the 22nd and 24th centuries, the discovery of large numbers of worlds with their own biospheres rendered the Delugers' mission obsolete and the movement died out.

Covenant
The Covenant have access to various terraforming methods, some of which are fairly exotic. This is thanks in part to their scavenging of Forerunner relics, though some proven techniques date back to the Sangheili's pre-Covenant days.

One of the Covenant's quirks is walled terraforming, or the creation of walled oases of atmosphere and biology on otherwise inhospitable worlds. Energy fields projected from the walls are used to hold the atmosphere in place, while gravitic systems may be used to raise or lower the local gravity. Such "walled gardens" are often created on resource-rich worlds or outposts not otherwise warranting full terraforming, or as temporary measures while a planetary body is undergoing large-scale terraforming efforts.

For full planetary terraforming, the Covenant use "life engines", mobile facilities encompassing an array of planetary engineering systems mostly reverse-engineered from Forerunner equivalents. These include incredibly powerful atmosphere processors, genetic databanks and life-seeding units, rather like the terrain-generation systems used on the Halo rings and other Forerunner megastructures.

Lesser terraforming machinery is used to control climate and atmosphere on worlds that are otherwise desirable for settlement, for either resources or strategic location. Sometimes subtle terraforming is also done for aesthetic preferences, though often this takes the form of introducing plants and fauna familiar from one's homeworld, or ones otherwise regarded as pleasing. Notably, San'Shyuum usually seek to replicate the environment of the upper-class districts of High Charity one way or the other. However, this typically occurs on warships and mobile habitats as the San'Shyuum rarely settle on planets. There are also some cultural traditions among Sangheili that seek to appreciate the natural environments of worlds as they are, with any modifications being subtle at best.