Expert: Philip Stahl - 2/17/2004

Question
Why do planets and other bodies go into orbit except for crashing straight into eachother? (ex. the moon orbits the earth, why doesnt it just crash into the earth?)

Answer
Hello.

Planets and other celestial objects are really in a state of free fall.

Consider a satellite orbiting Earth. Although at each second the satellite has an acceleration toward the center of the Earth (called 'centripetal') it has NO vertical or *downward* velocity. The reason is that it falls from each position at the same rate the Earth's surface falls away underneath it.

Thus, *relative to Earth's surface*, the velocity in a VERTICAL (downward) direction is ZERO. Since the distance between the satellite and Earth's surface remains constant. Hence, we say the satellite - or any orbiting body - is in a constant state of "free fall'.

This condition of perpetual free fall is what we mean be being "in orbit".

The same applies to other bodies orbiting larger ones. Thus, a planet orbiting the Sun is also in a similar state of free fall, with respect to the surface of the Sun.

While it is being pulled in toward the Sun (by the Sun's gravity)it has a speed in its orbit large enough so there's no vertical velocity component. Thus, planets orbit in a circle rather than falling into the Sun (which they would do if their orbital speeds suddenly halted or slowed drastically).

In the same exact fashion, the Moon orbits the Earth (free fall) rather than 'crashing down' into it, because the speed of the Moon in its orbit is large enough to overcome the inward pull of Earth gravity. The Moon keeps continually falling in its orbit relative to the Earth - marking out an orbit or circular path- rather than faling ONTO th Earth.

Two final points need to be made:

1) When bodies far out in space collide with each other, is is usually because their orbits intersect.

This is the case, for example, with asteroids and planets. For example, through Earth's history numerous asteroids- some quite large- have collided with Earth. One of these, 65 million years ago, wiped out the dinosaurs.

2)When an orbiting body does finally come crashing to its parent body, say a satellite to Earth, this is because its orbit has destabilized - departing too much from a circular path. Thus, when the Skylab crashed to Earth in 1973 it did so because its orbit became too elongated- too close to Earth at its apogee point.

This meant it encountered friction with the outer regions of Earth's atmosphere, which slowed its orbital speed, which in turn kept bringing the low point of the orbit down lower- until it finally burnt up in the atmosphere.