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What ia an Orbit?
1 An orbit is a regular , repeating path that one object in space takes around another one . An object in an orbit is called a satellite . A satellite can be natural , like Earth or the moon . Many planets have moons that orbit them . A satellite can also be man-made , like the International Space Station .
2 Planets , comets , asteroids and other objects in the solar system orbit the sun . Most of the objects orbiting the sun move along or close to an imaginary flat surface . This imaginary surface is called the ecliptic plane
WHAT SHAPE IS AN ORBIT?
1 Orbits come in different shapes .
All orbits are elliptical ,
which means they are an ellipse ,
similar to an oval .
For the planets ,
the orbits are almost circular .
The orbits of comets have a different shape .
They are highly eccentric or "
squashed ."
They look more like thin ellipses than circles .
2 Satellites that orbit Earth ,
including the moon ,
do not always stay the same distance from Earth .
Sometimes they are closer ,
and at other times they are farther away .
The closest point a satellite comes to Earth is called its perigee .
The farthest point is the apogee .
For planets ,
the point in their orbit closest to the sun is perihelion .
The farthest point is called aphelion .
Earth reaches its aphelion during summer in the Northern Hemisphere .
The time it takes a satellite to make one full orbit is called its period .
For example ,
Earth has an orbital period of one year .
The inclination is the angle the orbital plane makes when compared with Earth's equator .
HOW DO OBJECTS STAY IN ORBIT?
1 An object in motion will stay in motion unless something pushes or pulls on it .
This statement is called Newton's first law of motion .
Without gravity ,
an Earth-orbiting satellite would go off into space along a straight line .
With gravity ,
it is pulled back toward Earth .
A constant tug-of-war takes place between the satellite's tendency to move in a straight line ,
or momentum ,
and the tug of gravity pulling the satellite back .
2 An object's momentum and the force of gravity have to be balanced for an orbit to happen .
If the forward momentum of one object is too great ,
it will speed past and not enter into orbit .
If momentum is too small ,
the object will be pulled down and crash .
When these forces are balanced ,
the object is always falling toward the planet ,
but because it's moving sideways fast enough ,
it never hits the planet .
Orbital velocity is the speed needed to stay in orbit .
At an altitude of 150 miles (
242 kilometers )
above Earth ,
orbital velocity is about 17 ,
000 miles per hour .
Satellites that have higher orbits have slower orbital velocities .
WHERE DO SATELLITES ORBIT EARTH?
1 The International Space Station is in low Earth orbit ,
or LEO .
LEO is the first 100 to 200 miles (
161 to 322 km )
of space .
LEO is the easiest orbit to get to and stay in .
One complete orbit in LEO takes about 90 minutes .
2 Satellites that stay above a location on Earth are in geosynchronous Earth orbit ,
or GEO .
These satellites orbit about 23 ,
000 miles (
37 ,
015 km )
above the equator and complete one revolution around Earth precisely every 24 hours .
Satellites headed for GEO first go to an elliptical orbit with an apogee about 37 ,
015 km .
Firing the rocket engines at apogee then makes the orbit round .
Geosynchronous orbits are also called geostationary .
3 Any satellite with an orbital path going over or near the poles maintains a polar orbit .
Polar orbits are usually low Earth orbits .
Eventually ,
Earth's entire surface passes under a satellite in polar orbit .
When a satellite orbits Earth ,
the path it takes makes an angle with the equator .
This angle is called the inclination .
A satellite that orbits parallel to the equator has a zero-degree orbital inclination .
A satellite in a polar orbit has a 90-degree inclination .