# Orbit

by on July 19, 2009

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Artist concept of LRO in lunar orbit. Credit: NASA

Everything in the universe circles or “orbits” something else. It’s true for meteors and planets, stars and galaxies. They’re all under the influence of each other, moving in an ageless and eternal dance. So what exactly is is orbit? What causes this phenomenon? And why is it so important to spaceflight and space exploration? In this article I’ll try to briefly answer these questions.

First of all, orbit is the circular – or more accurately – ovoid path of an object around a center of mass. As we all learn in high school physics, Force equals mass times acceleration. When a mass changes speed and direction (which is acceleration), it produces a force. The main expression of this force in space is gravity. Gravity is the pull of every object in existence on each other. The bigger the difference between two objects in proximity to each other, the greater the force of gravity between them. This is why a person is held to the ground by the gravitational force of the much more massive Earth. This is true for any two objects with a similar mass ratio. The only way to escape the gravitational pull of a massive object is to have considerable mass yourself and or move at a high speed. If you were to give a short explanation of why orbits happen it would be that an object isn’t massive enough and or fast enough to most escape the gravitational pull of its center of mass.

Orbit happens as an object is able to normally have enough mass and speed to avoid the pull of gravity from the much larger object near it. If we are to use the Earth as example we find out why. Say you launched a baseball into the air. Under normal circumstances the ball will quickly fall back to the ground. But what happens if you were to throw the ball higher? It would still fall but take longer depending on how high it is. However if you were to throw the ball fast enough and high enough it would go into orbit. That is because as an object goes in a straight line at a fast enough speed past a planet, the gravity of that planet pulls on the object. This create the object’s orbit. Another good example is a ball on a string being swung by a child. As long as the child pulls on the string while swinging the ball it goes in a circle, but when the child lets go the ball goes in a straight line.

Orbit is important to space flight for several reasons. First, the observation of orbit gave aeronautics the principles that helps space craft escape the earth’s atmosphere. Second, several instruments and craft used by spaceflight rely on the use of orbits in one way or another. So as you can see orbit are very important for a lot of reasons.

If you enjoyed this article there are others on Universe Today you can read. Here some recommendations.
Here is an interesting article on the strange orbit of a planet. There is also an interesting article about the rare collision of two satellites.

There also great resources on the web. NASA has an article about the successful launch of it lunar orbiter. There is also another interesting article about the Hubble Space telescope.

If you liked this article you might enjoy Astronomy Cast, the October 30 Question episode.

Source: NASA

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