A horizontal velocity
of 8 km/s will maintain an object placed just above the Earth's
surface in a circular orbit.
The orbit will not decay as long as the satellite is high enough to avoid atmospheric
- The required altitude is about 200 miles -- a tiny fraction
of the Earth's diameter.
A satellite in "low Earth orbit," and thus traveling at 8 kilometers/second takes 90 minutes to complete
- Satellites are regularly visible as they pass over in the evening or pre-dawn sky.
- Although it is dark at the surface, the Sun is still shining
on the space high above illuminating the satellite.
Satellite orbits follow Kepler's Laws. The higher the orbit
the longer the orbital period.
The Moon orbits 400,000 km "above the ground" and requires
27.3 days to complete an orbit.
An artificial satellite in a circular orbit 42,000 km above the
Earth's surface takes about 24 hours to orbit the Earth.
In an equatorial orbit, such a geostationary satellite will
keep station over a single point on the ground.
This is the principle behind communication and weather satellites.
Interplanetary spacecraft are artifical satellites of the Sun.
After their launch they often follow a simple elliptical orbit to
Sometimes the spacecraft's trajectory takes advantage of a gravitational
NotesRevised September 29, 2006