Why does a satellite constantly require orbital corrections?
A satellite is subjected to various forces of nature. The Sun and the Moon constantly exert pulls on it. Above 1600 km, solar radiation can, known as solar wind, affects it. The Equator’s bulge causes gravitational perturbations. The Sun and the Moon exert gravitational pulls that keep on varying. Another factor is the Earth’s magnetic field which will reduce the spin rate of a satellite and affect its stability. Changes in the volume of on-board gas and liquid propellants can also tilt a satellite. Also the air molecules in the atmosphere of the Earth stretch far into space and when a satellite moves at 8 km a second, collisions with air molecules are frequent enough.
What is orbital decay of a satellite?
So long as the Earth’s gravity is unable to pull a satellite closer to the atmosphere, it can remains in orbit for years. However, its useful life depends on its stability in orbit and availability of on-board fuel for correction of any instability. If a satellite’s perigee is about 100 km, its orbit begins to ‘decay’ and it begins its final plunge into the lower atmosphere. The orbit will decay if its period is about 87 minutes or less.
Can artificial space satellites and other objects fall out of orbit?
Yes. It has been estimated that in the first 25 years of artificial satellites, over six thousand artificial objects have fallen out of space orbit. In fact, at any time, there are over 50,000 small and large objects in orbit. About 10 per week are slowed down to the point of re-entry. Most of them get burnt up in the heat of re-entry, while a very few such as Skylab have burnt out incompletely and fragments have hit the Earth.
What is meant by inertial guidance system of a spacecraft?
It is a system comprising gyroscopes and accelerometers that enables automatic plotting of the position of a moving spacecraft. This is because movement of the spacecraft and the distance travelled by it can be calculated if its acceleration and velocity are known. A specified star is usually taken as the reference point for this purpose. The inertial guidance system also takes corrective action when the errors in speed and position accumulate to unacceptable levels.