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The Gravity of the Situation

Date:  16th February, 2004

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Gravity is one of those things that we never really question. It's just there, binding all but a privileged few to our planet's surface for our entire lives. I've always imagined gravity to be a hugely powerful force, but a new book made me think again...

I was reading Isaac Asimov's Extraterrestrial Civilizations, (a truly fascinating and inspirational tome). The book examines the possibilities of advanced civilizations having evolved on other planets. Asimov, as always, does a exceptional job writing about astronomy and physics in a language that the average person can understand. Like all good scientists, Asimov justifies his conclusions with logical arguments, the laws of physics and indisputable math. Where humankind's knowledge is too limited to support an argument, he uses the principle of mediocrity to extrapolate from the little that we do know. The text is progressive, leading the reader towards a fantastic conclusion without provoking cynicism.

During a discourse on the topic of gravity, Asimov caught my attention when he wrote that "gravity is an incredibly weak force."

This seems to be an incredible statement when we remember that the sun's gravity is powerful enough to keep Pluto in orbit - with almost six billion kilometers between them.

Consider what to it takes to defeat the "incredibly weak" gravity of Earth. The Space Shuttle orbiter weighs around 80,000kg, depending on crew and payload. In order to get this weight into orbit, the shuttle must reach "escape velocity" - a speed of 11.3 km/second (25,277mph). Its engines will have to deliver 3,269,378Kg of thrust to achieve this speed and a phenomenal amount of energy is required. Hence, the shuttle will consume 1,723 tons of fuel (103 tons liquid hydrogen, 616 tons liquid oxygen and 1,004 tons of "solid fuel") from launch-pad to orbit (ie: twenty times its own weight in fuel).

Once in orbit, the shuttle has to travel at approximately 28,292 km/h (17,580mph) to stop gravity pulling it back down to the surface.

These mind-numbing figures seem to indicate that gravity is anything but weak. In order to validate Asimov's statement, we need to understand what gravity is:

grav-i-ty (n.): The natural force of attraction between any two massive bodies, which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

Therein lies the kicker, mass. The mass of our planet is (approximately) 5.9742 × 10 24 (6,000,000,000,000,000,000,000,000) kilograms. The mass of the sun, 1.98892 × 10  30 (2,000,000,000,000,000,000,000,000,000,000) kilograms.

It is the sheer enormity of scale that I find so mind-blowing. Asimov is quite right, gravity is weak - we can break entirely free of the gravity of the entire mass of Earth with a "simple" rocket. The further away we get, the weaker it becomes. Hell, we can temporarily defeat this huge mass by simply jumping into the air!

The force of gravity diminishes so rapidly with distance, yet the gravity of our planet has an effect, however imperceptible, on every other body in the incomprehensible vastness of the universe.

Feel free to become a gibbering wreck now, I'm already inside the asylum waiting for you.