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. For instance, an Orbital elevator is an example of megascale engineering, but not of astroengineering. Typically proposed feats of astroengineering are on the scale to remake an entire stellar system.
Construction of these megastructures seem to be restricted to civilizations that are either tier one or two on the Technological Achievement Tiers. 
==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 seven Halo Installations, multiple Shield Worlds, and Ark are all artificial habitats on a vastly larger scale than anything that either humanity or the Covenant could have accomplished. All known Forerunner megastructures are a part of the Halo Array.
===Orbital===
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.Halo: The Flood, page 14
The exact size of a Halo seems to vary. Alpha Halo has a diameter approximately equal to that of Earth,Halo: The Flood, page 13 but Delta Halo was described as being only as large as a moon.Halo: Ghosts of Onyx, page 188 Because these are the only two known Halo Installations, it remains to be seen whether these are standard sizes or whether each Installation is different in size and structure.
The Halo's 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. The exact means by which they do this is unknown.
===Artificial Planets===
The Forerunners were also adept at creating artificial planets, such as Onyx. Internally, the structure is maintained by, and even composed of, trillions of Sentinels and at least one immense Sentinel manufacturing facility, 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 its 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.Halo: Ghosts of Onyx, pages 377-378
====Shield 0459====
Unlike Onyx, Shield 0459 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. With an approximate diameter of 2900 kilometers, it's one of the smallest Forerunner megastructures. 
It was destroyed by the UNSC forces in 2531 when the overloaded Slipspace Drive of sent the internal star to supernova state.
====Requiem====
Requiem was a Shield World similar to both Onyx and Shield 0459, however it was the first built and was used by the Ur-Didact as his base. It seems to have multiple internal layers, there being at least two. The first interior level has an artificial sun.YouTube - Halo 4 Cutscenes - Shutdown Unlike Shield 0459 and Onyx, its exterior surface seems to be just a metal outer hull.
====X50====
X50 was the moon of the now uninhabitable Draetheus V. It was activated during the Battle of Draetheus V, revealing it to be a Forerunner planet creator/destroyer. It is unknown if the whole moon is this device, or if the device is simply located on it. Either way, it, and other installations like it, is how such artificial planets were likely to have been created by the Forerunners.
===Micro-Dyson Spheres===
The Forerunners also constructed massive Dyson Spheres, with a diameter of 2 astronomical units,Halo: Ghosts of Onyx, page 380 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,Halo: Ghosts of Onyx, page 379 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.Halo 3, Terminals
The interior of Shield 0459 was essentially a variation of this concept, albeit on a far smaller scale and not enveloped in Slipspace.
===Alderson Disks===
The concept of an Alderson Disk includes a massive disk-shaped megastructure with a star residing in a hole in the center.
The only known structure resembling an Alderson disk constructed by the Forerunners is Installation 00. With a diameter of between 90,000 and 100,000 kilometers,Bungie Forum - Size of the Ark 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, presumably used for raw materials, and a star orbiting nearby despite its location: 262,144 light years outside of the Milky Way galaxy, where stars cannot naturally form. Installation 00 is lit by an artificial star created by the Forerunners.Halo 3, campaign level The Ark
Rather than being a superweapon like the Halos, the Ark functions as both a control center for the entire array 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 the site of its predecessor via a Slipspace portal.
Although Installation 00 resembles an Alderson Disk, it has several differences to the original concept: It doesn't have a star in the center, it is far smaller than an actual Alderson Disk, and it has curved arms instead of being a flat disk. However, because of the close outward resemblance, it could be considered an Alderson Disk.
===Stars===
The Forerunners had the ability to either create artificial stars, or move existing stars across distances of thousands of lightyears, either of which would be a tremendous technological feat. Additionally, they seem to be able to extrude whole stars (and presumably 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 at its center. The shield world Requiem also possessed an artificial star, though it was not located at the physical center of the shield world, but within the "mantle" of the artificial planet where much of its artificial biosphere was located.
==Covenant==
Though on a far smaller scale, the Covenant have been known to build structures that almost rival those built by the Forerunners that they so revere.
===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 Spartan-117.
===Unyielding Hierophant===
The Unyielding Hierophant was a space station in Tau Ceti, a former UNSC colonized star system. 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 too was destroyed by Spartan-117.
==Human==
===Rubble===
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 UNSC world Madrigal. 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 A.I. Juliana. She had been operating far past her normal 7 year life span and was approaching rampancy, but she had to remain in operation because the tasks that she performed were too difficult for humans. The Rubble was destroyed in the Battle of the Rubble and the Battle of Metisette where it was used to destroy a Jackal base with hundreds of thousands of Grunt soldiers making up a massive army intended to invade the Rubble and Earth. Habitat Exodus was used to reach UNSC space, but that was destroyed too afterwards.
===Eridanus Secundus===
Built inside a hollowed out asteroid the Eridanus rebels hid here until their leader, Colonel Watts was captured by John-117. It had several factories, schools, hotels and even a hospital built inside it as well as a large docking bay. It was later glassed by a Covenant fleet in 2552.
==Construction==
The exact methods used by the Forerunners to construct these 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.
==Gallery==
File:Installation04 scaffolding.jpg|The foundational scaffolding beneath a Halo installation's terrestrial surface.
File:1209484737 Halo002lowrb9.jpg|Scaffolding islands of a Halo installation.
File:Artificial Planets.jpg|Construction of artificial planets.
File:1209584069 MountainsoftheArk.jpg|A mountain range on the Ark in panorama.
==Sources==
Category:Astronomy
Category:Forerunner
Category:Technology