Astroengineering



Astroengineering is name given to the construction of megastructures in space by technologically advanced beings. It is a form of megascale engineering, but deals with structures that are "easily observable" from interstellar or even intergalactic distances with 20th century astronomical instruments: e.g. a space elevator is an example of megascale engineering, but it is not an example of astroengineering. Typically proposed feats of astroengineering are on the scale to remake an entire stellar system.

Construction of these megastructures seems to be restricted to either Tier One or Tier Two civilizations. Although listed as a Tier Three civilization, the UNSC has no known astroengineered structures.

Forerunners
The Forerunners are renowned for their mimicry of natural environments, but it is their engineering achievements that stand as monuments to their extraordinary prowess, centuries after they have passed on. Their Halo Installations, Shield Worlds, and the Ark are all artificial habitats on a vastly larger scale than anything that either humanity or the Covenant could have accomplished. Forerunner megastructures were typically designed by the Builders and constructed by Engineers who were also tasked with stellar and planetary engineering.

Halo Array
The Halo installations are planet-sized artificial worlds, shaped like a ring. The interior surface contains a breathable atmosphere and seemingly natural terrain, including oceans, lakes, mountains, forests, and tundras. Underneath this is a support layer, upon which this terrain has been put in place, consisting of a vast grid network of metallic plates that provide the base. Though centripetal forces may provide at least some of a Halo installation's gravity, it also uses artificial gravity generators to generate Earth-like gravity.

The Halos were constructed as weapons of mass destruction on a galactic scale, designed to kill all sentient lifeforms in their radius of effect, approximately 25,000 light-years, by firing a harmonic pulse of cross-phased supermassive neutrinos.

Artificial planets


The Forerunners were also adept at creating artificial planets in a variety of ways. They were capable of speeding up natural planetary formation by collapsing an asteroid field into a molten mass of a twenty-megameter range, and then form it into a protoplanet which could be fully cooled in less than ten thousand years.

In addition, they employed other methods to construct planetary-scale megastructures, such as Onyx. Internally, the structure is maintained by, and even composed of, trillions of Sentinels and at least one immense Sentinel constructor factory, while externally the planet can appear like many other terrestrial planets, with a wide variety of terrain. A variation of the concept also exists, built in a more conventional fashion. It's a structure whose surface is composed of an outer shell, with a hollow interior and a habitable inner surface, and a miniaturized artificial star in the center. The interior resembles a Dyson sphere, though on a far smaller scale.

Onyx
The artificial planet Onyx was used as a cover for the Shield World in it's core. It was later colonized by the UNSC, oblivious as to its true origins until 2552, when its Sentinels activated to defend the Shield World. It was later damaged by FENRIS nuclear warheads, and destroyed as its Sentinels burst free and set up defensive stations around the Shield World portal.

Shield 0459
Unlike Onyx, this Shield World was a hollow spherical shell with a terraformed inner surface and an internal sun. The installation's outer surface was heavily infested by the Flood.

It was destroyed by the UNSC forces in 2531 when the overloaded Slipspace drive of sent the internal star to Supernova state.

Micro-Dyson spheres
The Forerunners also constructed massive Dyson spheres, with a diameter of 2 astronomical units, encapsulated them in a Slipspace field to protect them from the Halo Array as it activated. Essentially a sphere, with its habitable surface on its internal surface rather than external, Shield Worlds are also equipped with their own stars, providing warmth, light and energy to the installation. Originally intended as a way to circumvent the Halo effect, the Shield Worlds were not put into use by the Forerunners due to the rampant actions of one of their own AI constructs.

The interior of Shield 0459 was essentially a variation of this concept, albeit on a far smaller scale and not enveloped in Slipspace.

Installation 00


Installation 00 is a Forerunner megastructure vaguely similar to an Alderson disk in overall shape.

With a diameter of between 90,000 and 100,000 kilometers, and a larger total surface area than a Halo installation, it incorporates many similar elements, including Earth-like gravity and atmosphere and a rich and diverse ecosystem. Located in the center of the installation is a small planetoid, used for raw materials, and a star orbiting nearby despite its location: 218, or 262,144 light years outside of the Milky Way Galaxy, where stars cannot naturally form. It's possible that this is an artificial sun created by the Forerunners.

Rather than being a superweapon, like the Halos, the Ark functions as the the control center of the Halo Array. It also served as the lead researching station for the Librarian and as a construction facility for Halos, using the planetoid to provide metals, gases, elements and other resources needed for reconstruction. After the construction is complete, the Halo is transported to its destination via a Slipspace portal.

Stars


The Forerunners had the ability to harness the energy output of an entire star by using containment and collection fields; dozens of stars had been harnessed for energy production this way. Forerunners stellar Engineers nicknamed "plasma jockeys" also experimented with controlling young stars, attempting to "tame" them for future use.

The Forerunners could also either create artificial stars, or move existing stars across distances of thousands of light years, either of which would be a tremendous technological feat. Additionally, they seem to be able to extrude whole stars (and probably the surrounding system as well) into Slipspace. One star was used in the construction of the Shield World, Onyx, while an artificial star, created just for it, orbited Installation 00. Shield 0459 also had a miniaturized artificial star in its center.

Construction
"A Halo’s construction is executed by armies of Sentinels: some are the size of cities. Viewing the picture of the reconstituted asteroid material, you can see the process of skinning the Halo. There are four huge terraforming factories moving across the face of the Halo. They hold in the atmosphere which would leak out across the face, eventually pouring through the super structure, tugged into place by centrifugal force."

- The Art of Halo 3

The exact methods used by the Forerunners to construct their artificial worlds is unknown. Presumably, large numbers of Constructors and Sentinels play a role in the construction process, laying down the initial structure. Over this is laid a vast network of scaffolding, over which the artificial environment is placed, including earth, rocks, and significant quantities of water.

The Ark plays a significant and noteworthy part in construction, and is the location where replacement Halo Installations are built and later transported, should one be destroyed by the Flood or opposing forces. Although the Ark constructs smaller installations, it is unknown how the Ark was initially built or if it is capable of repairing itself.

Covenant
Though on a far smaller scale, the Covenant have been known to build super structures.

High Charity
High Charity was the Covenant capital city prior to 2552. Presumed to be built around a chunk of rock taken from the San 'Shyuum's home planet, it was not only an artificial planet, but capable of traveling through slipspace, powered by a surviving Forerunner Dreadnought. It was the largest known Covenant artificial planetoid, at 348 kilometers in diameter, as well as the home of the majority of the San 'Shyuum population, and was infested by the Flood, and eventually destroyed by John-117.

Unyielding Hierophant
The Unyielding Hierophant was a mobile space station used as a fleet command center and refit platform. Approximately 10 kilometers long, with two bulbous sections joined by a docking ring at the middle, it was a significant Covenant military base in the region, capable of resupplying up to 500 warships and carried thousands of Covenant personnel internally, including Banshee and Wraith pilots. It was destroyed by members of SPARTAN Blue Team during Operation: FIRST STRIKE.

Human
While 26th-century humanity does not possess the technology required to construct megastructures on the scale of the Covenant or the Forerunners, they have built several habitats within mined-out asteroids.

Rubble
The Rubble was a complex structure comprised of hundreds of asteroids, located in a Lagrange point between the gas giant Hesiod and its moon, Metisette. The habitats were made up of many asteroids that were linked together with docking tubes. Each of these asteroids had its own means of propulsion, and they were kept together by the machinations and calculations of the AI Juliana. She had been operating far past her normal 7 year life span and was rampant, but she had to remain in operation because the tasks that she performed were too difficult for humans. The asteroids that made up the Rubble were mined and constructed by a mix of Kig-yar and humans. The humans that lived on The Rubble were either insurrectionists, sympathizers or surviving colonists from the colony world Madrigal.

Eridanus Secundus
Built inside a hollowed out asteroid in the Eridanus system, the Eridanus cell of the United Rebel Front hid here until their leader, Colonel Watts was captured by members of the SPARTAN-II Program. It had several factories, schools, hotels and even a hospital built inside it as well as a large docking bay. It was later destroyed by a Covenant fleet in 2552.