Despite the predictions that no one would ever put build data centers in space, Google is starting on Thursday. Google’s prototype satellite will be one of 130 payloads on SpaceX’s Transporter 18 mission on October 1.
The server will follow a dawn-dusk orbit, a special case of a sun-synchronous orbit (SSO), following the terminator line between daylight on dark on the earth below. A dawn-dusk orbit allows the satellite’s solar panels to stay in nearly continuous daylight, while also being in a relatively inexpensive low earth orbit (LEO). Geostationary orbit (GEO) would allow solar panels to always receive sunlight, but launching a satellite into GEO requires more fuel and so is more expensive.
A dawn-dusk orbit would not be possible if the earth were perfectly spherical. The earth’s equatorial bulge makes it possible to design an orbit that precesses once per year. David Hammen explains this in an answer to a question on the Space Exploration Stack Exchange site.
If the Earth had a spherically distributed gravitational field, a satellite’s right ascension of ascending node would be constant. … Fortunately, the Earth’s gravitational field is not spherical. The Earth’s rotation results in an equatorial bulge. This equatorial bulge causes RAAN to precess (or recess). …
Sun synchronous orbits are chosen so that RAAN precesses by 360 degrees per year, or a bit less than one degree per day. …
A dawn-dusk satellite is a special case of a sun synchronous orbit. … A dawn-dusk orbit typically does not quite follow the terminator. Following the terminator would require a rather high orbit.
While there’s a lot of room in space (I read that somewhere) in LEO orbit, it seems like it’ll be a lot tighter if all the data centers want to be in a single orbit, only choosing different slots from the daylight hours. How much space is there there?
Google plans to have their server satellites in a tight formation. I don’t imagine they’d be in danger of collision with other satellites, but they have sorta the opposite problem: how to keep their cluster of satellites together. That’s got to take periodic adjustment, which means propellant.