Installation 04 is the official name of the fictional ring-shaped space station called Halo (or Alpha Halo in full) in the Halo: Combat Evolved video game.

The station

Halo is one of seven giant space stations built by the Forerunners. It is first and foremost a weapon, intended to destroy the parasitic Flood threat by wiping out all possible hosts, i.e. all intelligent life in its range. While that range is "only" approximately 25,000 light years, the activation is thought to trigger the other six Halo stations, extending the range to the whole Milky Way. Ironically, the Flood is apparently still alive only on those space stations. Aside from the Flood the only thing alive on Installation 04 is its "Monitor", 343 Guilty Spark. Discovering the true purpose of the Halo Fortress Worlds, the Master Chief and Cortana decide to blow up the engines in the Pillar of Autumn, which creates a explosion big enough to destroy Halo. Its shattered fragments are still entering Threshold's atmosphere and littering its moon, Basis.

Astronomy

A diagram showing the five Lagrangian points in a two-body system. Threshold is the yellow circle, Basis is the blue circle, and Alpha Halo is positioned at L1.

The "Ringworld" of Halo is much smaller than Larry Niven's Ringworld, and in fact is much closer in scale and structure to Iain M. Banks' Culture Orbitals. While the diameter of Niven's Ringworld is close to the diameter of Earth's orbit of 300 million km, and the standard Banks Orbital is around 14 million kilometers in diameter, the 10,000 km diameter of Halo is much closer to the diameter of Earth itself, which is 12,756 km. Ringworld has a star similar to our Sun in its center, whereas Halo is orbiting a planet but does not encircle it.

Besides the very beautifully sculpted landscape on Halo's surface, the sky is also worthy of attention. Depending on the viewpoint, you can see the star, a moon (called Basis), a gas giant (called Threshold), and the band of Halo itself, which looks like a street into the sky.

In the course of the game, neither Threshold nor Basis is reachable, all action takes place on or in Halo.

Halo is positioned on a Lagrange point of Threshold and Basis, directly between the two celestial bodies, but closer to Basis.