Since the launch of the first Earth satellite, Sputnik 1, in 1957, Earth-orbiting spacecraft have found many uses. They give us a view of the Earth that can't be had from mountaintops or airplanes; they provide a base above the Earth's atmosphere for telescopes to study the planets, stars, and the universe; and they serve as outposts for antennas high above the Earth that make modern communications possible.

The uses of Earth satellites are readily appreciated and understood. Less understood are the orbits of these satellites.

On this page we describe how satellites get into orbit. On the following pages we describe the laws governing those orbits.

Getting Into Orbit

The key to getting a spacecraft into orbit is to give it enough horizontal speed so that the Earth's gravity can't pull it down to the ground the way it pulls down everything else.

Imagine a very tall mountain. Put a cannon on the mountaintop and shoot a cannonball in the horizontal direction. The Earth's gravity will pull it down and it will strike the ground some distance away. Next, use more gunpowder so the cannonball's initial speed is greater. The distance the cannonball travels before hitting the ground will be greater.

Now imagine that you have a super cannon that allows you to shoot the cannonball with greater and greater initial horizontal speeds. The ball's trajectory will carry it farther and farther around the Earth before it hits the ground. There is a speed at which the ball will go all the way around the Earth without ever striking the Earth. Gravity still pulls it down, but as it does, the ball's momentum carries it forward. The result is that the ball keeps falling around the Earth. The ball is now in orbit.

The speed required for the ball to orbit the Earth just above its surface, without going either higher or lower, is 4.9 miles/second (17,700 miles/hour). You'd better duck one hour and twenty-four minutes after firing, or the cannonball may hit you in the back of your head. That's how long it takes the cannonball to complete one orbit. The cannonball has become a satellite of the Earth.

In this example, we assumed that there is no atmosphere. In reality, the cannonball -- or satellite -- has to be above the bulk of the terrestrial atmosphere to avoid friction, which would slow it down and make it fall back to Earth.

In reality, we don't use cannons to get satellites into orbit. We use rockets; however, the principle is the same. Rockets carry satellites above the terrestrial atmosphere and accelerate them to speeds at which they "keep falling" around the Earth.

Kepler's Laws of Orbital Motion

So far, we have described a satellite's motion in Earth orbit. Orbital motion is, however, a common phenomenon in our solar system and elsewhere in the universe -- the Moon orbiting the Earth, the Earth and other planetary bodies orbiting the Sun, stars orbiting each other, and so forth. Few of such orbits are circular, as we assumed above. In general, they are elliptical and follow three very specific rules, or laws. These laws -- Kepler's three laws of planetary motion (animated) -- are described in the following pages.

Comments to: Observatorium Curator (curator@rspac.ivv.nasa.gov)

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