The Universe in the Classroom

No. 34 - Spring 1996

© 1996, Astronomical Society of the Pacific, 390 Ashton Avenue, San Francisco, CA 94112.

Up, Up, and Away

by James J. Secosky, Bloomfield Central School
and George Musser, Astronomical Society of the Pacific

Want to take a tour of space? Then just flip around the channels on cable TV. Weather Channel forecasts, CNN newscasts, ESPN sportscasts: They all depend on satellites in Earth orbit. Or call your friends on Mauritius, Madagascar, or Maui: A satellite will relay your voice. Worried about the ozone hole over Antarctica or mass graves in Bosnia? Orbital outposts are keeping watch. The challenge these days is finding something that doesn't involve satellites in one way or other.

And satellites are just one perk of the Space Age. Farther afield, robotic space probes have examined all the planets except Pluto, leading to a revolution in the Earth sciences -- from studies of plate tectonics to models of global warming -- now that scientists can compare our world to its planetary siblings. Over 300 people from 26 countries have gone into space, including the 24 astronauts who went on or near the Moon. Who knows how many will go in the next hundred years?

In short, space travel has become a part of our lives. But what goes on behind the scenes? It turns out that satellites and spaceships depend on some of the most basic concepts of physics. So space travel isn't just fun to think about; it is a firm grounding in many of the principles that govern our world and our universe.

Life in Orbit
Hooking Up
Inclined to Agree
To the Red Planet
Thanks for the Lift
How Fast Do You Have to Go?
Activity: An Orbit in Your Hand
Activity: Orbital Inclination
Orbital Ornithology
Finding Space Birds
Observing Tips


The science of space travel takes off with Newton's third law of motion: To every action there is always opposed an equal reaction. This is why rockets ascend. The action of the hot exhaust gases rushing out the rocket engines creates an equal but opposite reaction, which pushes the vehicle forward. The higher the velocity or mass of the exhaust gases, the greater the thrust. Rockets do not move because they are pushing against ground or air. After all, they function in the vacuum of space where there is nothing to push against.

The same Newton's third law applies when you stand on a skateboard or rollerblades and throw a basketball: The board or blades roll in the direction opposite the way you're throwing. The harder you throw the ball or the heavier the ball is, the faster you'll roll backwards.

Following liftoff, the rocket climbs nearly straight up in order to get out of the thickest part of the atmosphere. It reaches an altitude (200 kilometers, or 120 miles) where the air is so thin that there is little atmospheric drag. Once above the bulk of the atmosphere, the rocket turns onto its side and accelerates to 28,000 kilometers per hour (17,400 miles per hour). The horizontal speed is crucial. It ensures that the rocket will keep moving forward while gravity pulls it down. If the speed were any lower, the rocket would crash into the ground somewhere.

In this sense, launching a rocket is like hitting a baseball. A lightly hit ball might drop near second base; a harder hit ball would go deep into the outfield; a home run would fly over the fence. If you hit the baseball hard enough, it would fly over the fence, over the parking lot, over the highway, over the harbor, over the Atlantic Ocean, Africa, the Indian Ocean, the Pacific Ocean -- right back over the ball park again, over the parking lot, over the highway, and so on, and so on. You would have hit that ball into orbit (see diagram). The ball would always be falling toward the ground, but would have enough forward momentum to keep it from ever reaching the ground. With no forces except gravity, the ball would stay in orbit forever (see activity). (In practice, a small amount of atmosphere is still present even at 200 kilometers, so every satellite will eventually slow down and fall to Earth.)
Going, going, gone
Going, going, gone. During a 1960 baseball game between the New York Yankees and Detroit Tigers, Mickey Mantle knocked a home run out of Briggs Stadium. According to Guinness, it was the longest home run ever: 643 feet (196 meters). In principle, if The Mick had swung his bat a bit faster -- 150 times faster -- he could have hit the ball into orbit. (In practice, air resistance would have melted the ball, shrapnel from the shattered bat would have killed the infielders, and Mantle would have sat out the rest of the season with severe shoulder injuries.) Orbits are just like the flight path of a ball. The Earth's gravity pulls the ball down while the ball's initial velocity carries it forward. The higher the initial velocity, the farther the ball can go before hitting the ground. At a high enough initial velocity, the ball can go all the way around the world -- and that's what we call an orbit.

Like the orbiting baseball, a satellite or space shuttle is always falling toward the ground -- but always missing it. Since it is falling, everything in it is falling; the sensation of weightlessness, or zero-g, results. People seem to love the feeling. They'll even pay to experience it: skydiving, bungee jumping, trampoline jumping, roller-coaster rides, Free Fall or Demon Drop rides.

Life in Orbit

Just because astronauts are weightless does not mean there is no gravity in space. Indeed, the force of gravity keeps the Moon in orbit around the Earth and the planets in orbit around the Sun. Weightlessness simply means that astronauts are in the free fall of orbital motion. Skydivers and bungee jumpers also feel weightless, but who would deny that gravity is pulling on them?

To get astronauts used to weightlessness, NASA takes them for a ride on a KC-134 airplane. Like a flying roller coaster, the KC-134 goes into a steep dive that produces 30 to 40 seconds of weightlessness. All the weightless scenes in the movie "Apollo 13'' were filmed aboard such an aircraft. Astronauts also simulate working in space while underwater in large tanks.

Despite these preparations, it takes astronauts a while to get used to the strange condition of weightlessness. When entering orbit, they do not immediately float up to the ceiling. As Newton's first law of motion says, a force is required to get people or objects moving. In zero-g, objects stay where they are put, until a force is applied to them. You could leave a pencil hanging in mid-air and pick it up later. If astronauts find themselves sitting in the middle of the cabin, too far to push off a wall, they're stuck until another astronaut comes to push them into motion.

Sometimes objects in orbit are disturbed by forces that we don't normally think about on Earth. Weightless water, for example, curls up into a sphere because of its internal cohesion forces --the same forces that cause beads to form on a newly waxed car. Astronauts have to drink using special straws. The straws close when not in use; otherwise the liquid would continue to squirt out the straw even when the astronaut was not sucking on it.

As much as we curse weight on Earth, weightlessness can cause physiological problems for astronauts in orbit. Half of all astronauts suffer motion sickness in the first few days. Some vomit. Space flights lasting months weaken people's muscles and bones. The body reckons it no longer needs the extra strength, breaks down the muscles and bones, and excretes the chemicals via the kidneys. The same thing happens on Earth when people are confined to bed; those of us who have had to wear a cast know how weak the limb becomes. Exercise counteracts this breakdown of the body.

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