How gravity keeps satellites in orbit

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

24 June 2007

Last week article elicited several questions from readers. The first is Tony Kariuki who wants to know whether the weight of an object in orbit matters. “I would have thought the bigger the space station gets as they continue to build it, the heavier it becomes and probably they would have to increase its orbital speed?”

Well, Tony, not quite so. The counteracting force due to the speed of the station (or any other object, for that matter) is directly proportional to the mass. But at the same time, the gravitational pull is also directly proportional to this mass.

Therefore, as the mass increases, the two forces (gravity and the opposing force) go up by similar amounts thereby remaining balanced. For that reason, there is no need to increase the orbital speed. That also explains why the astronauts assembling the Space Station can “float” without needing to stand on anything – they are flying at 28,000km/h as well!

John Njogu adds his voice and says: “You didn't explain why an orbiting satellite makes a circular path rather than a straight one.” The quick answer to that is “gravity”. But that leads to yet another one: how does gravity do it?

First it is obvious that a moving object can only change direction if a force is applied towards the “inside” of the turn. For example, if you want to turn a car to the left, you must twist the steering wheel to that side…and if you let go, the wheel will spin back to the straight direction.

Secondly, an object going round in a circle is constantly changing its direction of motion. It may, for example, start off moving due east, then in a few moments it will be heading southward, then westward, then northward, finally eastwards and starting all over again…

If this were a car, then driver would need to apply a constant force (using the steering wheel) to the right all the time. That would make the car follow a clockwise circular path. The direction of the turning force also changes all the time – initially, it is in a southern direction (when the car is heading east), then in a western direction (car going south), and so on.

Clearly, the turning force is always applied in a direction pointing towards the centre of the circular path. Therefore, if a moving object is acted upon by a force that is directed towards a central point, it will move in a circle around that point. This is what gravity does.

The force of gravity is in a direction towards the centre of the earth. It therefore keeps the International Space Station (or any other satellite) is a circular orbit around the planet.

Finally, some schoolbooks teach that the force of gravity “keeps the Earth moving around the sun”. This is wrong! Gravity only keeps the planet in an orbit. The Earth doesn’t need anything to keep it moving. The real mystery is what or who set it in motion in the first place.