The satellites thrown up out in the deep space, revolving around the earth forever... This usually conjures a romantic image in our minds. Add to these the excitement of building, launching, and finally getting the satellite to operate... it is easy to forget the silent, invisible forces acing on the heavenly bodies, moving them from one direction to other, making them spin, rotate, tumble. The immediate neighborhood of earth is not free from these forces, either. The people who design the satellites know, however, that these forces are worthy foes which sometimes make our life difficult, sometimes impossible.

Well, why so? The satellites have a number of far and near neighbours which disturb its motion. Its final path around the earth is the result of cumulative effect of these perturbing forces. These forces include sun's and moon's attraction, atmospheric drag, effect of earth's magnetic field, solar radiation. How these forces affect the satellite is what I intend to discuss in this essay.

The plane of earth's orbit around the sun is called the ecliptic plane. The plane in which earth's equator lies is called the equatorial plane. The ecliptic and the equatorial planes are inclined at angle of about 23.5° to each other. Now, a satellite rotating around the earth will be nearer to sun on the one side of the orbit and farther away on the other side. As a result, the forces acting on the satellite on diagonally opposite of the orbit will not balance. This will produce a torque, and will try to rotate the orbit of the satellite about the nodal line. This will change the inclination of the orbit (the angle between satellite's orbital plane and equatorial plane). Moon's attraction has the same effect .

Thus a satellite's inclination will increase due to sun's attraction if no controlling is done. This would be a periodic motion, the inclination first increasing and then decreasing. Now consider the effect of earth's oblateness(this term means that earth is not a perfect sphere and has a bulge at the equator). Due to earth's oblateness, the satellite can no longer follow the ideal Kepler's orbits. Thus the orbit of the satellite begins to rotate around the earth, the inclination remaining the same.

The effect of sun's radiation (that surprises you , doesn't it?) is two fold: It rotates the satellite, as well as it increases the eccentricity of the satellite's orbit. A satellite, howsoever much we may try, cannot be built perfectly symmetrical. Sun's radiation falling on the satellite thus produces a torque which may in long run start to rotate the satellite. To understand the change in eccentricity, imagine a circular ring made of a thin wire. When the ring is circular, the eccentricity is 1. If you press it (such that the new shape is still symmetric ), however, the shape changes to that of a ellipse, and eccentricity increases from 1. The effect of sun's radiation is more or less the same.

There are other perturbing forces also, including atmospheric drag (important for only LEO satellites), interaction with earth's magnetic field, solar wind etc; but their effect is slightly more complicated, so I'll avoid discussing that. When a satellite is designed, all these effects are taken into account.