|Solar day||27 5⁄8 hours|
|Length of year||1762.5 local solar days|
|5.56 Earth years|
|Axial Tilt||60 degrees|
|Surface Gravity||0.00979 g (0.096 m/s2)|
|Population||between 2000 and 5000 (30th century)|
Hygiea is the fourth largest asteroid in the main asteroid belt between Mars and Jupiter. It is the largest asteroid in the asteroid belt with a rotation rate similar to that of the Earth; the three larger asteroids — Ceres, Pallas and Vesta — all have rotation periods of less than ten hours. This Earthlike rotation period was the main reason Hygiea was chosen as the base for mining in the asteroid belt.
The Hygiean colony was founded in 2215 by Asteroid Resources Limited. Other space mining companies soon joined Asteroid Resources Limited with their own infrastructure investments, resulting in the colony being jointly owned by most of the space mining companies, and later some shipbuilding companies. Initially it was mined by the space mining companies. Eventually most of the easily-accessible minerals were depleted, and it then served as a refuelling and maintenance base for habitat ships that were mining elsewhere in the asteroid belt. It grew in importance as it became first an ore and metals depot, and then an industrial centre.
Cosmic radiation makes it necessary to construct most inhabited facilities underground. Cut and cover is the usual method of construction. A pit is excavated first, then the structure is constructed, and then the regolith is piled back on top. Almost all construction uses teleoperated robots, with the operators safely enclosed in a radiation-resistant environment, most usually a crawler. The facilities require very deep foundations to provide a secure footing in the regolith.
The habitats use sunlight by piping the light underground through various forms of light pipes. The Earthlike rotation period greatly reduces the cost of this form of illumination.
Being an industrial world, Hygiea is not a pleasant place to visit. All facilities are underground, and most of them have the drab and utilitarian appearance of industrial sites all over Local Space. Most of the population is transient, either waiting for a habitat ship or passage back to Earth. A small population of permanent residents provide the goods and services that people always require — food, clothing and the like.
Hygiea is a low-gravity world, with a surface gravity less than one per cent of the Earth. All people on Hygiea must spend at least two hours a day taking exercise in a gravity centrifuge. The centrifuges are usually designed to produce one Earth gravity, but some can produce lower gravities if desired. Because everyone must spend so much time in the centrifuges, they are designed to be as pleasant as possible. The largest centrifuges provide a place where competitive indoor sports like basketball can be played. Smaller ones provide various forms of entertainment for the visitors while they are taking their exercise.
The large axial tilt of the asteroid posed some challenges for the residents. About half the surface experiences long periods of darkness lasting for two Earth years or more, alternating with long periods in sunlight. For those parts of the surface that receive daylight every day, the position of the sun varies greatly over the length of the Hygiean year. These challenges were overcome through careful planning. Facilities that don't require sunlight or heat were placed near the poles, whereas facilities where light is necessary were placed near the equator.
Hygiea is an important industrial centre in the Solar System. A lot of shipbuilding takes place there. The low gravity of the tiny world facilitates the construction of ships that are not designed to land on the surface of planets with appreciable gravity.
Hygiea serves its original purpose as a mining support world, where it refuels, maintains and recrews habitat ships, and serves as a depot for ores and metals. Some mining takes place on Hygiea itself, but most accessible reserves of useful minerals are already depleted.
The coming and going of habitat ships is dictated by the location of the target asteroid in its orbit relative to Hygiea, and in some cases habitat ships must remain on Hygiea for extended periods of time while they wait for their launch windows. To berth the habitat ships, the world has very large docks for the ships.
Some hydroponic agriculture takes place on Hygiea, providing fresh vegetables for the population. All agriculture is underground because most plants are as susceptible to the effects of cosmic radiation as humans are. Sunlight is carried inside from collectors on the surface using optical fibre cables and other similar technology. The intensity of sunlight at Hygiea is only ten per cent of that on Earth. To boost the light levels to a more Earthlike level, the hydroponic gardens require a large light collecting area. As a result, less than ten per cent of the space in each farm actually grows plants. Some of the space is used to provide the nutrients to the plants, but the rest of the space in the farms are used for a variety of purposes that don’t require light. This is why the farms are usually placed with the ore silos. The ore storage does not require light but provides useful surface area for the capture of sunlight. The farm in turn uses the sunlight that would otherwise have been wasted. Many farms and ore silos are located near the poles, where they can benefit from the constant light during the two-year-long local summer. The farms are abandoned during the local winter when the facilities spend two years or more in darkness. Unlike the polar farms, the equatorial farms are not abandoned seasonally.
Solar energy is the main source of energy for Hygiea. The axial tilt makes solar energy difficult to generate, but the early engineers found novel solutions to this problem. They mounted solar generators on converted crawlers and these move back and forth on the surface of Hygiea over the course of the Hygiean year so as to remain in sunlight as much as possible. Most of these solar generators generate electricity that is carried to where it is needed. Some of the solar generators capture the sunlight instead, and pipe the sunlight directly all over the asteroid to where artificial lighting is required.