Satellites

Satellites and Projectile Motion

In previous sections, the movement of an object within a Gravitational field was was shown to follow a parabolic path. If this theory is extended to an object moving horizontally at a great enough speed, then the curve of the Earth needs to be taken into account.

For an object with a large enough horizontal velocity (~7000ms-1), its curved path would match that of the Earth's curvature, and the object would never hit the ground. The object would be in Orbit around the Earth.

As long as no external Forces act on the object, it will follow this orbital path forever.


Types of Orbit

There are two main types of Orbit an artificial satellite can be placed in :-

Low-Earth Orbit - Orbital Period = ~ 90 minutes

Geostationary Orbit - Orbital Period = 1 day

Since the 1950's thousands of satellites have been launched and most remain in orbit to the present day. This means that the space around the Earth is a complex tangle of functional and nonfunctional satellites and debris. The image below shows a representation of the satellites and debris over 10 cm in orbit as of 2013 :-

This debris can pose a major hazard to other spacecraft as due to the high orbital speeds involved, even very small objects can cause major damage on collision. The photo below shows the damage caused to the window of a Space Shuttle when a fleck of paint collided with it :-

The video below shows how the European Space Agency is working to prevent problems of space debris:-

Low-Earth Orbit

Low-Earth Orbit is the region of near space up to an altitude of 2,000 km from the Earth's surface. Low-Earth Orbit is the main area in which almost all manned spaceflight has occurred in (The exception being the Apollo Missions). It is also the location of most satellites in Orbit. Most Earth-observation (weather etc.) satellites and spy satellites are found in this region, as being close to the Earth allows them to observe the Earth to a very high resolution. The International Space Station is also found in Low-Earth Orbit (~400 km altitude).

The link below will open a page showing a live HD video feed from the International Space Station (If appears Black, I.S.S. is on the Night Side of the Earth) :-



Most Low-Earth Orbit satellites are still affected by atmospheric drag to a small degree. In order to prevent their orbits from decaying (and the satellite falling back to Earth) the satellite requires occasional readjustment boosting with on-board engines) to ensure it remains in orbit.


Geostationary Orbit

Although most satellites are found in Low-Earth orbit, there is a large group of satellites are a much further distance from Earth. At a distance of 35,786 km from the Earth's surface, these satellites form a ring around the Earth's equator as can be seen in the satellite tracking image below :-

Geostationary Orbit is useful due to the period of the satellites orbit at this altitude. It takes exactly 1 day (23.934461223 hours) for the satellite to orbit the Earth. This means that for an observer on the Earth, a Geostationary satellite will appear stationary in the sky.

The above animation shows two Geostationary satellites in orbit as the Earth rotates. Note that the line connecting the satellite to the Earth is unmoving on the Earth's surface.

Geostationary orbit is a very useful position to place communication satellites as you do not have to track them across the sky, allowing fixed position antenna to receive a continuous signal (eg, Sky TV dishes).


Satellite communication

In order to send signals over short distances, it is possible to send a direct signal. Over larger distances, however, the curve of the Earth can prevent 'Line of Sight' communication. In order to send a signal to anywhere on Earth, a relay satellite system can be used.

As each individual satellite can observe a cone of 120° ,a system of this type containing three satellites can cover the entire Earth's surface.

The video below shows 'Line of Sight' communication as described by Jeff Goldblum :-