- describe the concept of a satellite
- identify the types of satellites and their orbits
- describe two main issues in keeping a satellite in orbit for long periods of time
Most people can identify a satellite
as an object orbiting the Earth, but they may not realize the broader
implications of the word. A satellite is correctly defined as any
object that is orbiting another object. This includes both natural
objects and artificial objects, and it extends beyond the Earth.
RADARSAT-2, launching summer 2007
Earth is in fact a satellite around the Sun, as are the other planets.
The Moon is a satellite around the Earth. Comets are satellites around
the Sun, although they have more irregular orbital paths.
These are all natural satellites
, whereas the metallic objects
we have placed in orbit around the Earth are called artificial satellites
includes all the satellites that are currently circling the Earth, even the
ones that no longer work, but are still in orbit. It also includes the
International Space Station, which qualifies currently as the largest
A view of a communication satellite, ANIK C2, as seen from
Did you know?
a shuttle window.
The ANIK series of satellites are Canadian, with ANIK I
being the first ever communication satellite launched.
are now more than 2500 artificial satellites in orbit around the Earth.
Although this number is quite large, there are still only eight
countries (as of 2007) that have the capability to launch satellites:
Russia, United States, France, Japan, China, United Kingdom, India and
There are several ways of classifying artificial satellites.
The most obvious way is by function. Many of the first satellites were used to
conduct scientific exploration of outer space and the atmosphere. Many
satellites still function this way, such as the Hubble Space Telescope, but
there is a huge variety of function now. Here is a brief list of some:
are used to create the Global Positioning System (GPS)
are used for transmitting signals over large areas for TV, radio, internet, and more
are used to observe storms and other weather formations
are used for reconnaissance purposes or for anti-satellite purposes as well
use various kinds of telescopes
contain types of organic matter for scientific study in orbit
Astronomical satellites were discussed under the topic of
Space Telescopes in great detail in Unit 3, Lesson 5. As well, few more
details about satellites are worthy of mention here.
Did you know?
an object is in orbit around an object it is actually in continuous
freefall. Recall that for an orbit to occur the mass of the large body,
known as the ‘primary’, is exerting a force of gravity on the
satellite. The satellite needs to be moving at a high enough velocity
so that it is not pulled down towards the primary. It also cannot be
going too fast or it would just fly off into space. So it must be going
at just exactly the right velocity so that as it is pulled down towards
the primary it is also moving forward around in the path of the orbit,
thereby continuously ‘falling’ towards the primary, but never hitting
is a large satellite equipped with a variety of Earth observing tools.
Launched in 1995, it already provided valuable information for much of
the world on such topics as cartography, hydrology, coastal monitoring,
climate change and many other topics. It can observe day or night, and
in any weather conditon, making very efficient and reliable. Thanks to
RADARSAT-1, Canada has become a leader in satellite remote sensing
data. RADARSAT-2, a more powerful and updated version, has just been
launched in 2007. This will ensure that Canada continues to lead in
this field and provide high quality data on resource management and
Artist's rendering of RADARSAT-1
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launched in Nov. 1995, is an Earth observation satellite developed by
Canada. With a primary objective of monitoring environmental changes,
RADARSAT-1 is able to obtain images, day or night and even through
thick cloud cover.
Launch the RADARSAT-1 website!
Satellites can also be classified according to the
types of orbits they follow. Not all satellites simply run circles
around the Earth. Some satellites have elliptical orbits, while others
have a geo-stationary orbit where they stay above the same point over
the Earth. The altitude of the orbit is also variable, from Low Earth
Orbit (LEO) which ranges up to 2000km, Medium Earth Orbit (MEO), and
High Earth Orbit (HEO) which starts at 35,786km above the planet’s
As a general rule the
lower the orbit, the more quickly the satellite needs to be moving to
maintain its path around the Earth and not tumble down to the surface.
This gives us the opportunity to see satellites in the night sky fairly
easily. A close satellite will mean that it is in a position to reflect
enough light from the Sun to be easily seen, but is also moving
quickly, making it stand out better against the stationary stars. On a
good dark night you can often see a satellite cross the night sky
almost every two minutes!
This image shows a GPS satellite during a test. It is easy to see the solar panels used
to power the satellite. These satellites maintain a geo-stationary orbit to remain in
a specifically designed orbit in relation to the other GPS satellites.
Satellites are now designed to stay in orbit for long periods of time.
This presents two main difficulties: power and control. For a satellite
to keep functioning for a long period of time, it needs a continual
source of power. Solar panels and batteries are required to harvest
energy from the Sun when its rotation takes it to that side of the
A more recent development is the control of satellites. For
many years, satellites were launched by rocket, or by the Space Shuttle. After
that, they were left to continue their orbits as best they could. Some
satellites were equipped with fuel to make a limited amount of corrections to
problems with their orbit. Recently a further refinement called ion drive
been developed. This is a sort of miniature rocket engine that is about 10
times more efficient than traditional rocket fuel; however, it is limited to
smaller accelerations, so it is restricted to smaller vehicles thus far.
This photo shows a test fire of a Xenon Ion Thrust done by NASA at the Jet Propulsion
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Laboratory. It is the first non-chemical propulsion engine used to propel spacecraft.
only applies a force equivalent to the weight of a piece of paper, it burns fuel
exceptionally efficiently resulting in an efficiency about 10 times greater than
traditional rocket fuel over long periods.
Space 1 launched in Oct. 1998. Over the course of its mission, it
tested 12 new technologies. After returning the best images and data
ever recorded of a comet, it was retired in Dec. 2001
Launch the Deep Space 1 website!