by Nancy Fulda
Few science fiction authors have maintained a career without, eventually, needing to position some sort of manned habitat in space.
This is not a trivial task. Space habitats face challenges such as orbital decay, radiation exposure from the Van Allen Belts, and the potentially astronomical cost of shipping supplies up from Earth. The habitat’s altitude also determines the period of its orbital cycle and the apparent size of Earth through the viewports, significant details that can add authenticity to your work.
This article is your handy reference guide to orbital living at every altitude. Whether your space station is a supply depot for interstellar warships or the humble abode of a single resident, one of the orbital regions described below will surely suit your needs.
Low Earth Orbit (160-2,000 km)
Snuggled along the lower edge the inner Van Allen Belt, low-earth orbit is the safest, cheapest, and most popular orbit in use today. The Space Shuttle travels in low-earth orbit, as do the International Space Station and the Hubble Space Telescope.
Any station in this region will need thrust mechanisms to combat atmospheric drag. It will also need shielding to protect it from both macro- and microscopic orbital debris. The inner Van Allen belt extends to this area, and radiation exposure increases with altitude.
At this proximity, Earth looms in the viewports, taking up nearly half of a spacewalking astronaut’s view. The sensation that one might tumble down onto the planet is powerful. Orbital periods range from 87 minutes (at 160km) to 2.1 hours (at 2,000km).
If your space station hosts a lot of interstellar traffic, low-earth orbit is probably not the right choice for you. Sand-blasting effects from microscopic orbital debris aside, the Earth’s pull due to gravity is nearly as strong here as it is on the planet’s surface. Vehicles departing for destinations elsewhere in the solar system--or beyond--would expend massive amounts of energy escaping Earth’s gravity.
Medium Earth Orbit (2,000-35,786 km)
If your station requires a longer orbital period or communication access to the poles, medium-earth orbit might be a good choice. Atmospheric drag at this altitude is negligible, and orbital debris is scarce. GPS and other navigation satellites inhabit this region, often on a semi-synchronous orbit (20,200 km) that circles the Earth twice per day.
The lower and upper Van Allen belts both affect this area, although there is a low-radiation ‘sweet spot’ between 10,000 and 13,000 km. Space habitats that pass through high-radiation areas will require shielding.
At this altitude, Earth continues to dominate the scenery, blocking between 20% and 40% of visible space. Orbital periods range from 2.1 to 23.9 hours.
Medium-earth orbit is a logical location for early space colonies because orbital paths won’t necessarily intersect the Van Allen belts. Access to Earth is difficult at this range, but a space elevator or whirling space tether could lower the cost of cargo transports.
Geosynchronous Orbit (35,786 km)
A space station in geosynchronous orbit will circle the Earth exactly once per twenty-four hours. To an observer on the planet surface, such satellites appear to oscillate along a constant meridian, returning to a specific point in the sky at the same time each day.
If a geosynchronous orbit is in the same elliptical plane as the Earth’s equator, and if the orbit is circular, then it is also a geostationary orbit, meaning that it maintains a constant position relative to the Earth’s surface.
Geosynchronous orbits fall within the outer Van Allen Belt, although not within the region of highest radiation. They are also susceptible to orbital drift due to the solar wind and the moon’s tidal effects.
From geosynchronous orbit, the Earth is a massive globe, 40 times larger than the moon appears when viewed from Earth. The orbital period is identical to Earth’s sidereal rotation period: approximately 23.9 hours.
High-Earth Orbit (>35,786 km)
High-earth orbits denote any cycle with an apogee greater than 35,786 kilometers. This includes highly elliptical orbits such as the Molniya orbits, which facilitate sustained line-of-sight to the poles.
At this altitude, tidal effects from the moon contribute significantly to orbital instability. For this reason, the Earth/Moon Lagrangian points are intriguing potential locations for a space habitat. Of particular interest are the L4 and L5 Lagrangian orbits, also called Trojan points. These points respectively trail and precede the Moon in its orbit by 60 degrees, resulting in a dynamically stable system.
Viewed from this distance, Earth hangs in the sky like a decorative bauble. Its apparent diameter ranges from the looming globe of geosynchronous orbit to the size of a large moon. Orbital periods range from 23.9 hours at 35,786km to 27.6 days at 380,000km.
Because Earth’s gravitational pull is relatively weak at this altitude, high-earth orbits are a logical location for military outposts and interstellar transfer stations. If the orbit is highly elliptical, cargo and passengers may also be transported efficiently between the station and the planet’s surface.
Polar Orbits
Space stations do not necessarily have to orbit around Earth’s equator. Polar orbits allow close inspection of different areas of the globe on each orbital cycle, and are thus ideal for cartographic or reconnaissance space stations. Circling the poles also reduces radiation exposure because the charged particles in the Van Allen belts are concentrated near the plane of Earth’s equator.
Launching supplies or personnel into polar orbit requires a higher delta-v than equatorial launches, however, and space habitats in these orbits would require extra cooling mechanisms during the times of the year when they receive constant sunlight.
Further Reading
Still haven’t found the perfect trajectory for your space habitat? Never fear. Wikipedia has a comprehensive list of orbits and orbital terminology. The Polaris Project offers a similar list with helpful illustrations.
You might also like this online orbital calculator, which will provide the relative speed and orbital period of satellites at various altitudes. The escape velocity calculator is pretty nifty, too. And, if you‘re a math junkie, this orbital mechanics site has enough equations to set anyone‘s head spinning.