November 22, 2008
Working in Freefall
The National Space Society's vision is of "People living and working in thriving communities beyond the Earth." The astronauts of STS-126 and Expedition 18 are making major progress this week toward making that vision come true.
Ironically, more progress is often made when things go wrong than when they go perfectly. During a spacewalk last week, an expensive tool kit got away from Heide Stefanyshyn-Piper. This padded white box contained tools, including a grease gun, important to the repair of the solar alpha rotary joint that turns the solar arrays like a paddlewheel to track the sun. Fortunately, they had more than one set of these tools, though they now have to conserve the use of grease.
To rescue the box, Piper could have used her SAFER backpack to fly after it. But those backpacks are not reusable, and therefore are reserved only for life-threatening situations. The loss of a tool box is unfortunate, but not life-threatening.
The lesson here for long-term working in space is that care must be taken to provide backups for everything. Accidents happen. If that tool box had been the only one, the repair would have to wait until a logistics flight could bring up a new one. If the station were on its way to Mars, that might not be possible. If the repair were critical to survival, Piper would have had to use the backpack to go after those tools. This repair was not that critical, and there was also another set of tools. So Piper made the right decision in not going after it.
But how did this loss occur? It was NOT because there is no gravity in space. If you stood on a bathroom scale tower as tall as the station is high, you would weight 90 percent of what you weigh on the surface of Earth. You weigh less because you are farther from the center of mass, but certainly not zero.
The reason that things are weightless and appear to float in space is because they are all falling at the same rate. The effects of falling can be felt on Earth, but only until the falling object or person hits the ground.
NASA's "vomit comet" aircraft falls about 10,000 feet in the sky so that astronauts and scientists can run tests during about 25 seconds of being weightless. (Put a heavy object on a bathroom scale and drop it over a bed to see for yourself that the scale goes to zero while falling.)
So the astronauts and their tools and the space station are floating because they are falling. The correct term for this environment is FREEFALL.
NSS members: Please do not contribute to the extreme science illiteracy in this country by repeating misleading phrases such as "Because there is no gravity in space..." and "In zero-g..." this or that happens. I have discovered through my speaking engagements that at least 95 percent of school children (and their teachers!) believe that there is no gravity in space. NASA's not helping. On a diagram of the new water recycling system that NASA provided to the press this week (and that was published in the Houston Chronicle) it said, "because of the lack of gravity in space..." PLEASE use the term FREEFALL instead!
But if everything is falling together, how did the box get away? Basics physics: objects move in the direction of the force applied to them. Imagine jumping off a diving board while holding a box. You and the box will hit the water at the same time. But while you are falling, if you push that box sideways, it will move away from you until you both hit the water. If you push it down, it will hit before you. If you push it up, it will hit after you. The same thing happens in space, only the astronaut and box never hit the water--they continue to fall in a big circle around the Earth without hitting the ground, and the box continues to move away (without even air to slow it down).
Why don't spacecraft hit the ground? Not because there isn't any gravity! They stay in orbit because they are going fast enough to balance out the pull of gravity. If they went even faster, they could escape (to fall in a big circle around the sun--so they'd still be weightless). It may be helpful to think of twirling a stick (the spaceship) on a string (the pull of gravity). If the spin is too slow, the stick falls. As long as it moves fast, it keeps going around.
Learning to work in the FREEFALL environment is tricky. Small forces that are usually damped out or inconsequential when not falling become very important. The experience the astronauts and ground teams have developed and continue to develop, lost equipment and all, is providing just the lessons we need to realize our vision of people living and working in space.
And the troubleshooting on the water recycling system is another great example of working in space. The motor in the centrifuge that is used to separate contaminants from water in urine is not working correctly. Addressing this problem involves understanding what sensors are needed to diagnose the problem, what tools are required to fix it, and what spares our future lunar and Martian travelers will need to have with them.
The public may not understand what FREEFALL is all about, but hopefully, they will have the opportunity to experience it for themselves in the future thanks to the work of shuttle and station astronauts.
To the stars,
Member of the NSS Board of Advisors
Posted by m_dyson at November 22, 2008 02:25 PM