Watching the latest space shuttle launch and reading the news about the astronauts space walking to repair the International Space Station (ISS), I was reminded of some of the oddities of low-earth orbit.
The ISS and space shuttle orbit earth little more than two hundred miles up. At this altitude, in what is called low-earth orbit, objects coast with little resistance just above the upper reaches of earth’s atmosphere.
Two hundred miles is not very high. Consider that the earth itself is about four thousand miles in radius. Two hundred miles on top of four thousand miles amounts to just five percent. The fuzz on a Michigan peach stands out in about the same relief as does the ISS orbit above earth.
It is a common misconception that there is no gravity in space. While this is true, to some extent, in the deepest reaches of outer space, it is decidedly not true just two hundred miles above earth’s surface. The force of gravity exerted by earth on an astronaut in low-earth orbit is about ninety percent as strong as it is when the astronaut is back home.
What explains all the floating around?
Spacewalking astronauts, and everything else in orbit around the earth, are “weightless” because they are freely falling toward the center of the earth. You can be weightless right now, for a brief period of time, in exactly the same way if you jump. The higher you jump, the longer you can be weightless.
Us earthbound folks, who experiment with weightlessness when we jump, have to face the abrupt end of our orbits when we hit the ground. As the saying goes: “it’s not the fall that kills you, it’s the sudden stop at the end.”
Weightless astronauts don’t eminently face the sudden stop because they are zipping along in orbit around the round earth in just such a way that they can fall forever and never get any closer to earth’s center or surface. In orbit, astronauts move horizontally at a speed the matches the distance they fall in any interval of time to the distance earth’s surface curves away below.
At what horizontal speed does something need to move in order to stay in orbit two hundred miles above earth’s surface? Really fast. Relative to earth’s center, things in low-earth orbit have to move about eighteen thousand miles per hour! At that speed, they scoot around earth in just ninety minutes.
All the fire and fury of a space shuttle liftoff is aimed at one thing: to get the shuttle and everything inside moving at eighteen thousand miles per hour. The energy required to reach an altitude of two hundred miles is tiny compared to the energy required to accelerate to the proper orbital speed.
You may have heard that one of the astronauts let a tool kit slip from her hands on a recent ISS spacewalk. Ultimately, like everything else in low-earth orbit, the tool kit will come back down.
For something to return to earth gracefully, however, it must shed the tremendous amount of energy it gained during launch and slow back down to a stop. That little tool kit, for example, has now as much energy as the average US home uses in two days. Before too long, after maybe a couple of years, the air molecules in the upper atmosphere will begin to tear at the toolkit, sapping its energy ever faster. When the kit falls into the thicker parts of the atmosphere, in a brief flash, it will be vaporized.
Let’s wish the astronauts a safe return.
This column originally appeared in the Grand Haven Tribune on 21 November 2008.
