Why the space station flies at a tilt of 51.6 degrees.

What orbital inclination means, why a launch site's latitude limits which orbits are easy to reach, and how the International Space Station ended up at 51.6 degrees, with what that means for who can see it.

Select the International Space Station in Orbit Atlas and watch its path for a while. Its orbit forms a great tilted ring around Earth, and the ground track below swings like a wave between northern and southern latitudes. The station never goes further north than about southern England or further south than the Falkland Islands. That limit comes from one number: the orbit's inclination of 51.6 degrees. This guide explains what inclination means and how that particular value was chosen.

What inclination means

Inclination is the angle between a satellite's orbital plane and Earth's equator.

• 0 degrees: the satellite orbits directly above the equator, traveling in the same direction Earth rotates. Geostationary satellites are close to this.
• 90 degrees: the orbit passes over both poles.
• More than 90 degrees: the satellite travels slightly against Earth's rotation. Many Earth-observation satellites use this, such as Landsat 9 at 98.2 degrees, as explained in our guide to sun-synchronous orbits.

The inclination sets the highest latitude the satellite passes over. NASA's Spot the Station explains that because the space station orbits at 51.6 degrees, the furthest north and south of the equator it goes is 51.6 degrees latitude. Places beyond that never have the station directly overhead, though they can still see it lower in the sky.

Why launch sites matter

A rocket launched straight east from any site gets a free boost from Earth's rotation, which carries the launch pad eastward at hundreds of meters per second near the equator. Launching due east also puts the satellite into an orbit whose inclination equals the latitude of the launch site. That is the lowest inclination the site can reach directly.

To reach a lower inclination, a rocket must change the plane of its orbit, which costs a great deal of fuel. To reach a higher inclination, it launches in a more northerly or southerly direction. That is relatively easy, though it gets a little less help from Earth's rotation.

Europe's spaceport at Kourou in French Guiana, at about 5 degrees north, shows the other side of this. Its closeness to the equator makes it well suited to launches toward geostationary orbit, which lies above the equator.

Two more practical constraints shape the choice of launch direction. Rockets drop spent stages along their path, so routes are chosen to avoid populated areas and neighboring countries. And range safety rules limit which directions are allowed from each site.

How the station ended up at 51.6 degrees

The space station had to be reachable by the spacecraft of all its partners. Russia's Soyuz crew ships and Progress cargo ships launch from the Baikonur Cosmodrome in Kazakhstan, at about 46 degrees north.

According to an answer from NASA's Mission Control team, preserved in NASA's archived question pages, launching from Baikonur directly east into an orbit near 46 degrees would send spent rocket stages down in China. Launching on a more northeasterly path, into a 51.6 degree orbit, keeps the stages over Russian and Kazakh territory. That made 51.6 degrees the practical choice for Russian access to the station.

There is a trade-off. Rockets from the Kennedy Space Center in Florida, at about 28.5 degrees north, need more performance to reach a 51.6 degree orbit than a lower-inclination one, which reduced how much the Space Shuttle could carry to the station. But the higher inclination brought other benefits: Russian vehicles could serve as crew transport and lifeboats, and the station flies over more of the inhabited world.

What 51.6 degrees means for observers

• Most people can see it. Most of the world's population lives between 51.6 degrees north and south, so most people can have the station pass directly overhead at times.
• Higher latitudes see low passes. In places like Scotland, Scandinavia or Alaska, the station stays lower in the sky, but it can still be bright.
• The ground track drifts. Earth turns beneath the orbit, so each pass crosses the equator further west than the last. Over the course of a day the station passes over most places within its latitude band.

Our guide to seeing the space station explains how to catch a pass.

Inclination as a fingerprint

Inclination is one of the six numbers that define an orbit, and it is included in every two-line element set. It is a useful clue when you meet an unfamiliar satellite in Orbit Atlas:

• Inclinations near 0 degrees suggest geostationary or near-equatorial orbits.
• Around 51.6 degrees often means the space station or spacecraft that visit it.
• Around 55 degrees is typical for navigation satellites such as GPS.
• Inclinations near 97 to 99 degrees usually indicate sun-synchronous Earth-observation satellites.

Inclination alone is not proof of a satellite's purpose, but together with altitude, which our guide to orbit heights describes, it tells you a lot about what a satellite is doing up there.