Astronomy

The Meaning and cause of Orbital Decay



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Interest in the meaning of orbital decay and what causes it is now on the front burner of many people’s minds because of all the satellites and space debris orbiting our planet. The old fable of Chicken Little’s “the sky is falling!” isn’t far from the truth any more. We have had close misses from comets, rogue meteors, natural and artificial satellites, plus space debris—much which we have created. Depending on the size of these pieces of matter, the earth could be devastated and life as we know it completely destroyed if one slips through our atmosphere and hits the earth with severe impact.

Orbital decay refers to the continuous reduction of the altitude of a satellite. The closer it is to Earth, the denser our atmosphere becomes, thus the faster the decay. This means the greater the atmospheric drag, the greater the increased heat (caused by friction) causing burning on re-entry. The more the object comes closer to the earth, the more the gravity of the earth pulls the object into more decay and the higher the speed of the decay. There are two possible outcomes: the object burns up and is completely destroyed by the atmospheric drag. The second alternative is that the object is only partially burned and its remainder hits the earth. It’s the smoking object that is dangerous to the earth because it infiltrates our atmosphere and affects our weather patterns, usually on the cold side because the smoke blocks the sun.

All this can be explained by physics. Newton’s first law of physics is that an object stays in motion until it is acted upon by a force. Therefore, the object that is rotating or orbiting the earth will stay in that circular motion until another force like the atmospheric drag (friction between the object and our atmosphere), slows it down, eventually causing it to stop orbiting. Newton’s Law of Gravity explains how all objects are attracted to each other and pull on each other if they have mass or weight. That is why the smaller object would be pulled to earth since the mass and gravitational pull of the earth is greater.

External factors such as solar activity, tides and gravitation waves affect the rate of the orbital decay. If the object is big enough to cause a tidal bulge on the earth, especially if it is in a retrograde orbit (opposite to how the earth rotates), or in earth’s lower atmosphere, it can cause the earth’s tides to sap the object’s momentum thus slowing it down and increasing the decay. Also depending on the size of the object, gravity’s pulling on both the satellite and earth could cause them to collide and merge into one satellite.

In conclusion, not only do we need to be conscious of keeping the earth clean, we need to seriously keep the earth’s lower atmosphere clean and all satellites out of harms way.

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