This is the Way the World . . . Begins
The term “megastructure” refers to a huge artificial structure for which one of its three special dimensions is 100 kilometers or greater. Both SF and speculative science have contemplated large-scale constructs for years, such as the Dyson Sphere and Star Trek’s Borg Unimatrix; even the planet Earth, as represented in The Hitchhiker’s Guide to the Galaxy, would qualify. The first literary use of a ring-shaped megastructure occurred in Larry Niven’s 1970 Hugo and Nebula award-winning novel Ringworld. Niven elaborates on the details of such a Ringworld, also known as a Niven Ring, in his 1974 book A Hole in Space:
“I myself have dreamed up an intermediate step between Dyson Spheres and planets. Build a ring 93 million miles in radius—one Earth orbit—which would make it 600 million miles long. If we have the mass of Jupiter to work with, and if we make it 1,000 miles wide, we get a thickness of about a thousand meters. The Ringworld would thus be much sturdier than a Dyson Sphere.”
Although Forerunner Halos are also huge ring-shaped habitats, they are comparatively smaller by several orders of magnitude: the radii of the Halo megastructures are a “mere” 5,000 kilometers—more similar to Earth’s average radius, 6,371 kilometers, than that of a ringworld. In fact, because the Halos we have seen to date orbit gas giant planets instead of encircling stars, they are less ring worlds than they are ring satellites.
A 5,000 kilometer radius would yield a circumference of roughly 31,400 kilometers. If the Halos had a width-to-radius ratio similar to that of Niven’s Ringworld, they would be approximately 5.37 kilometers wide. They are significantly wider, though, at 320 kilometers. The Halos, then, would have a surface area of 10 million square kilometers— slightly larger than the surface area of Canada, and approximately 2 percent of the surface area of Earth. Of course, since we know that there are lakes, seas, and rivers on the Halos, the livable surface area would be fractionally less.
What raw materials would it take to construct a Halo, and in what quantities? In order to determine the amount of raw materials required, and what elements may exist in the necessary abundances, we first must calculate the volume of the structure. While a Halo is proportionally wider than a Niven Ring, it is thicker in absolute measure. Niven proposed that a Ringworld be 1 kilometer thick, whereas the Halos are quite a bit sturdier at 22.3 kilometers thick. The total volume of a Halo would be roughly 224 million cubic kilometers, a bit more than 0.02 percent of the volume of Earth.
Of what would it be composed, then? Almost since the genre began, science fiction authors have resorted to the invention of new and exotic materials to endow their structures/spacecraft/armor with the desired combinations of weight, strength, and other material properties.
The practice is so common that a term has even been coined for fictitious substances that have such improbable combinations of material properties: unobtanium. If we dare to imagine how a Halo might plausibly be built, one constraint must be that we shy away from unobtanium and consider only materials that exist in practical abundances in the real universe. In the book Halo: Fall of Reach, however, spectroscopic analysis of the composition of Installation 04 is “inconclusive,” which seems to imply quite strongly that the Halos are, in fact, composed of unobtanium. Let’s go out on a limb, then, and assume that the Halos have a thin outer protective sheath composed of a super-strong, heretofore unknown, alloy that envelopes an internal structure composed of more universal elements.
Iron, in addition to being the principle component of the cores of terrestrial— or Earth-like—planets (Mercury, Venus, Earth, and Mars), is also common in asteroids. In fact, in the solar system many asteroids are composed almost entirely of iron and nickel. Carbon is a fairly common element as well. Then it would be a reasonable assumption that the primary Halo structure is composed of steel—which is an alloy of iron and carbon—with perhaps other elements in smaller amounts. Although less universally abundant, nickel and magnesium, also common in steel, exist in amounts abundant enough to create a very strong and comparatively light steel alloy.
We now know the approximate volume of a Halo and the density of its principle component (a reasonable average density for steel is 7.7 grams per cubic centimeter). Normally, these values would be enough to calculate its approximate mass. We need still one more quantity, though. Views of the exterior surfaces of Installations 04 and 05 clearly reveal direct-vision ports (read: windows) and what appear to be docking hatches. The obvious implication is that the inner surface of the ring is not the only habitable portion of a Halo—obviously a fraction of the ring structure itself is hollow and used for living space, laboratories, even the hardware, maintenance, and pulse generator spaces for the Halo’s weaponry. If we assume that the primary ring structure is roughly 50 percent empty space, then we end up with a total mass of a Halo of about 1.7x1017 kilograms, or 1,700 million billion kilograms.
In A Hole in Space, Larry Niven calculates that it would take the mass of Jupiter to build his ringworld. A major complication, however, is that jovian, or Jupiter-like, planets represent the bulk of the mass of a planetary system like ours, yet they are composed largely of very light materials such as hydrogen and helium. Each has several Earth masses worth of solid material, rock and metals, at their core, but the sum total of all the rock and metal in the solar system—that of the inner planets, the asteroids, the jovian planets and moons—would equal less than one-sixth of one Jupiter’s mass worth of potential construction materials. The mass we calculated for a Halo is approximately twice the mass of Ceres (the largest asteroid in the solar system's Asteroid Belt), a bit less than Pluto's moon Charon, or the mass of a sphere of solid iron roughly 57 kilometers in radius. The entire asteroid belt between Mars and Jupiter would have just about enough mass to construct one Halo.