Satellite question

[AngryAmish]

I was looking at the stars last weekend and noticed something. Why do all the satellites seem only to travel in an north-south or south-north direction? Is it just that I can't see the others?

[IADEMO2004]

1. imaging satellites have polar orbits

and in a few days they can cover all of the earths surface.

[soothsayer]

2. well, the geostationary (or geosynchronous) ones don't move, per se

from our vantage anyway. But I'm not sure about the others.

on edit, ok, here:
Most people think of satellites as flying east-west, circling the globe above the equator. While many satellites do fly in such low equatorial orbits, there also are many polar-orbiting satellites. which travel a north-south path across the poles.

Different satellites fly around Earth on different orbital paths. The choice of north-south or east-west route depends upon the job assigned to the satellite. Polar-orbiting satellites provide a more global view of Earth.

more at link http://www.spacetoday.org/Questions/PolarSats.html

[JohnnyRingo]

3. I believe it's for the same reason the sun travels from North to South...

...and back again as the seasons change. (This holds true for the moon, a satelite itself)

The Earth is tilted on it's axis and a satelite moves through space irrespective of the planet.

This only holds true for some satelites though and it's true that you only see some of them. the only visible orbits are those at about 150 (or so) miles altitude.

Since a certain speed is required to maintain orbit, altitude is used to alter the apparent movement through the sky. Some satelites orbit at over 15,000 miles, and remain stationary as viewed from the Earth.

It's easier to understand if we imagine throwing a baseball so hard that as Earths gravity pulls it toward the center of our planet, the baseball continuously "falls" over the horizon.

An object in orbit, therefore is in a continuous freefall toward the earth as the planet revolves beneath it. The object must be moving, however, and above drag inducing atmosphere. (100,000+ feet)

I have to note, however, I'm not an astrophysicist and barring a stay in a "Holiday Inn Express" the above may contain some scientific innacuracies.

[XNASA]

4. There's not much point in a satellite travelling east to west.

The satellite needs to downlink to a point on the Earth, usually, so orbiting doesn't really get you anything. It also receives signal from a point on Earth, as well. Most communications satellites are in geo-synchronous orbits, which is why you can't see them, or rather you could see them at dusk and dawn but since they appear not to move, it's hard to pick them up. Imaging SATs have polar orbits so that they can do there thing as the Earth spins below (as mentioned previously in this thread).

[AngryAmish]

5. This is another question for my rocket-scientist friends

It is my understanding that there are angular-momentum benefits to launching near the equater. Wouldn't it take a lot of fuel to put the satellite into polar orbit?

[XNASA]

6. Yep. You are correct.

The launch vehicle in a 90 degree inclination launch must provide all of the energy to get the satellite into a polar orbit. There is no benefit from launching at or near the equator.