There are two main types of orbits: open and closed. Closed orbits are the easier to understand, as they can be either circular or elliptical (oval) in shape. A body on a closed orbit constantly travels around another, such as a planet orbiting the Sun or the Moon orbiting the Earth.
All four classes of orbit are known as ‘conic sections’ because slicing a cylindrical cone in a different way can make each of their shapes. Closed orbits are achieved by cutting a
cone from one side to another at different angles. Open orbits are
achieved by slicing the cone from one side down through the base.
Spacecraft can be put into a number of different closed
orbits around a planet. These are defined by a number of orbital
characteristics, such as the height above the planet’s surface, the
inclination to the planet’s equator and the direction in which the spacecraft
orbits the planet.
Defined by orbital altitude, these are:
Low-Earth orbit – As the name implies, this is the lowest altitude a
spacecraft must achieve to in order to orbit the Earth. This is around 520 km
altitude and spacecraft in these orbits circle the Earth once every ninety
minutes or so.
Geostationary (or geosynchronous) orbit – This is a much higher orbit and so
takes a lot more energy to reach. However, once at the altitude of 36,000 km,
it takes the spacecraft a full 24 hours to orbit the Earth. Thus, the
spacecraft moves at the same speed with which the Earth rotates and therefore
appears to ‘hover’ over the same spot on the ground
Understanding orbits
There are a few common ways of understanding orbits.
>As the object moves sideways, it falls toward the central body. However, it
moves so quickly that the central body will curve away beneath it.
>A force, such as gravity, pulls the object into a curved path as it attempts
to fly off in a straight line.
>As the object moves sideways (tangentially), it falls toward the central
body. However, it has enough tangential velocity to miss the orbited object,
and will continue falling indefinitely. This understanding is particularly
useful for mathematical analysis, because the object's motion can be described
as the sum of the three one-dimensional coordinates oscillating around a
gravitational center.
>As an illustration of an orbit around a planet, the Newton's cannonball model
may prove useful (see image below). Imagine a cannon sitting on top of a tall
mountain, which fires a cannonball horizontally. The mountain needs to be very
tall, so that the cannon will be above the Earth's atmosphere and we can
ignore the effects of air friction on the cannonball.
If the cannonball is fired with sufficient velocity, the ground curves away from the ball at least as much as the ball falls so the ball never strikes the ground. It is now in what could be called a non-interrupted, or circumnavigating, orbit. For any specific combination of height above the center of gravity, and mass of the planet, there is one specific firing velocity that produces a circular orbit, as shown in (C).
As the firing velocity is increased beyond this, a range of elliptic orbits
are produced; one is shown in (D). If the initial firing is above the surface of the Earth as
shown, there will also be elliptical orbits at slower velocities; these will
come closest to the Earth at the point half an orbit beyond, and directly
opposite, the firing point.
At a specific velocity called escape velocity, again dependent on the firing
height and mass of the planet, an infinite orbit such as
(E) is produced a
parabolic trajectory. At even faster velocities the object will follow a range
of hyperbolic trajectories. In a practical sense, both of these trajectory
types mean the object is "breaking free" of the planet's gravity, and "going
off into space".
ESA/Wikipedia
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