September 09, 2006
From “MECO” (main engine cutoff) until entry, the crew of STS-115 will enjoy being weightless. The reason they are weightless is not because there isn’t any gravity in space. Weightlessness is a result of falling, in their case, in a big circle around the Earth. I bring this up because I have discovered that the cause of weightlessness is the number 1 misconception about space, and I’m hoping my fellow NSS members will help clear up this issue while discussing the mission with their friends and families.
To help your friends visualize what is going on, toss a ball into the air. Gravity pulls the ball back down. Note that the more energy you put into the toss, the higher the ball goes, but gravity still pulls it back down. So why doesn’t the shuttle fall back down? Actually it does! To understand what is happening, toss the ball in a high arc to your friend. You have given the ball horizontal speed as well as vertical so that its fall will intersect their position. As they move farther away from you, you have to give it more horizontal energy so the ball won’t be pulled down to the ground before reaching them. If you shoot the ball out of a gun, it will go a long way before impacting the ground. In order for the ball to fall without impacting the ground, it has to be high enough not to run into any buildings or mountains, and it has to be going 5 miles/second. To stay up there, it needs to be above the atmosphere (defined as about 50 miles) so the friction won’t slow it down. (The atmosphere extends beyond 50 miles and is part of the reason that the space station has to be boosted regularly.) Gravity never stops pulling the ball or the shuttle down, but its hold is reduced by the square of the distance from the center of the Earth. At the altitude of the space station, gravity is about 95 percent of what it is on Earth. It is NOT zero!
So if gravity is 95 percent of surface gravity, why are the astronauts weightless? Because they are falling. (It’s FREE fall because they don’t pay the price of hitting the ground.) All objects, regardless of mass, are pulled towards Earth with the same force, and fall at the same rate. If you and the ball were both thrown into the air, you would both fall together. When you looked over at the ball, you would see it floating beside you. If you had tossed a bathroom scale up with you, it too would be falling. It would not measure any weight for you or the ball. (To demonstrate this, put a heavy can of something on a bathroom scale and drop it over a bed: the can will be weightless while falling.)
Those of us who have had the thrill of flying on a parabolic flight have had the fun of floating around and gobbling up juice bubbles falling with us. But anyone who has ever jumped off a bed has been weightless for at least a second. That’s not long enough to really feel the effects of the blood rushing to your head. The excess fluid in your head triggers your body to get rid of fluids, so body temperature rises and causes you to sweat and have to go to the bathroom (and sometimes throw up). These effects are the origin of space adaptation syndrome (SAS) or space sickness. Most people get rid of their water and adjust within a day.
A freefall environment is fantastic. Besides the ease of movement, women especially love being in space. No bras required! And forget about wrinkles: the fluid shift provides a natural facelift! The fluid shift also reduces the size of thighs and waistlines, though gentlemen should note that the neck expands, so collars will become tight. I’m sure none of the men will miss wearing ties anyway! The women could wear high heels without hurting their feet, but no shoes are required, so why wear them?
Though freefall is the “correct” term for weightlessness, “zero-g” has become part of the lexicon. Unfortunately as a result, millions of children now have it in their heads that there is no gravity in space (even on the Moon). Everyone from astronauts to non-technical people (including journalists) use this term and reinforce the misconception.
Freefall is fantastic, and even more so when you realize what we humans have to do to achieve it: build rockets that can reach the altitude and horizontal speed necessary to go into orbit, and ships that can protect us from the airless vacuum. We have to go even faster to escape from Earth’s gravity and then let ourselves freefall down the Moon’s gravity well. It is very hard to do, and equipment and Mother Nature are constantly challenging us to find better ways to do it. Today, the space shuttle Atlantis succeeded in a spectacular launch. For 11 days, the crew will take advantage of that special environment to do some amazing feats of engineering and science. I hope NSS members will be out spreading the word about their accomplishments in zero-g, I mean freefall!
Posted by m_dyson at September 9, 2006 01:51 PM