Inclination of the Orbit Plane

Inclination of the Orbit Plane

The satellite’s path is always in a plane; that means you could draw it on a piece of paper. And whatever kind of orbit we’re talking about, its plane always goes through the center of the Earth.

An orbit plane can be inclined at any angle between equatorial and polar, and we choose the inclination to suit whatever purposes we have for the satellite. Sometimes we choose the inclination that is the easier – that is so we can put the most payload into orbit with a given rocket. Many US satellites and manned spacecraft are put in orbit inclined at or near 33 degrees to the equator, because that is the easiest inclination for launches from Cape Canaveral (which is at 33 degrees latitude). Many Russian satellites are in orbits inclined in the neighbourhood of 57 degrees to the equator because that is the easiest for them; their principal launch site is at 57 degrees latitude. In both cases that is the easiest inclination because it takes advantage of the rotation of the Earth; you launch the satellite in the same direction that the Earth is already rotating (east), and that gives you some extra push.

Loops of Emission Nebulosity in NGC 3576. Photo in public domain

Putting the same satellite into a polar orbit takes a more powerful rocket, because now you get no benefit at all from the spin of the Earth. And unless you can launch from the Equator, putting it into an equatorial orbit takes some extra thrust because you have to change the plane of the orbit by firing a rocket engine in the right direction just as the satellite is crossing the equator.

Polar orbits are useful for weather satellites, because while the satellite is going around and around over the north and south poles, the Earth is turning below, inside the satellite’s orbit, and each north-south (or south-north) pass of the satellite scans a new part of the planet’s surface until it has all been scanned. If you make the orbit almost polar but not quite, you can make the orbit plane itself rotate at the same rate the Earth is going around the Sun (about 1 degree per day). This near-polar plane is also useful for weather satellites because if you start at the right time you can keep the Sun always behind the satellite. This will allow the satellite’s cameras to take pictures in daylight during the whole year.

For communication satellites like TRW’s Intelsat III, it is best to use a so-called geostationary or synchronous orbit. We already saw that there is a particular altitude for each speed, and therefore for each period (time it takes for one revolution around the Earth), from the hour-and-a-half period of Mercury spacecraft to the 28-day period of the Moon. Somewhere in between there must be an altitude that will give you a period of exactly 24 hours, and this turns out to be a little over 22,000 miles. If you put a satellite in circular orbit in the plane of the equator at that altitude, it will be going around the Earth at the same rate the Earth is going around its own axis. That means that the satellite will remain over the same place on the Earth at all times, and to someone on Earth it would appear to be standing still.