ver 7,000 objects in space for which orbital
information is kept by military and civilian organizations. Most of these objects are small but
quite a few are large enough to be seen with the unaided eye. And some, like the Space Shuttle
and International Space Station are very large and appear very bright.
Exercise I :Why could we see artificial satellites only at brief period after sunset and before
dawn?
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Satellites are designed to serve one of three general purposes: space science, applications, or
communications.
Space science satellites carry instruments to study the Sun, measure magnetic field or to examine
the universe in the different energy wavelength of electromagnetic spectrum. Application
satellites survey the Earth's resources and supply weather photographs and other information
forecasters. Communication satellites relay telephone calls and television signals, transmit
scientific information from other satellites down to Earth, and relay voice communications
between the astronauts orbiting on the Space Shuttle and mission controllers on Earth.
On some Space Shuttle missions, there are opportunities for many observers in the continental
United States to see the obiter fly overhead. Shuttles are lunched from the Kennedy Space Center
in Florida, which is at a latitude of 28.5 degrees north. That means that for most missions the
orbiters path is a circle around the Earth that inclined (tilted) 28.5 degrees relative to Earth s
equator. Observers more than a little bit farther north than 28.5 degrees will not be able to see the
orbiter. It will never rise above their local horizon.
Exercise II : Hubble telescope has orbital inclination of 57
0
and International Space Station
has 51.6
0
. Where on the Earth is it possible to see both of them? the telescope only?
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Satellites will orbit from west to east or from nearly pole to pole. Constellations pattern will help
to direct view. For example, to say it is moving through Cygnus the Swan would be very
instructive.
Each satellite has a set path in space above the Earths atmosphere called an orbit. If a satellite
traveled through the atmosphere, air would push against it and slow the satellite down. To be
completely free of the atmosphere, satellites orbit at least 180 miles (300 km) above sea level.
The velocity and inclination to the equator, or angle, with a satellite launched determine the
satellites orbit.
Orbits are divided into several categories, though there are plenty of satellites that dont fall
conveniently into any of these grouping. Low-Earth orbit(LEO) is restricted to the first 100 to 200 miles of space. LEO is the easiest orbit
to get satellite into, and this is where the Shuttle and ISS conduct their operations. One complete
orbit in LEO takes about 90 minutes.
Geosynchronous Earth orbit(GEO): Satellites orbit about 23,000 miles above the equator and
complete one revolution around the Earth precisely every 24 hours. Communication and
meteorological satellites are often geosynchronous. Satellites headed for GEO first go to a
geostationary transfer orbit, an elliptical orbit with an apogee then makes the orbit circular.
Geosynchronous orbits are also called geostationary.
Polar orbit is an orbital path going over or near the pole.
Exercise III :More than 3,500 satellites orbit the Earth today. How they can fit space above?
How all this crowd is placed around the Earth?
Chart below contains information about different satellites and its orbiting altitudes.
By using foam ball as Earth model and wire to represent altitudes in scale place pictures of
different spacecrafts according information from the chart.
NASA Vehicle Altitude
Altitude(km)
Blended Wing Body
12
BOOMERanG
37
Chandra
9,942
F-15 ACTIVE
18
GOES
36,000
Helios Prototype
31
Hubble Space Telescope
595
International Space Station
309-463
KC-135
8-11
SOFIA
13
Sounding Rocket
209
Space Shuttle
195-556
Terra Satellite
705
TOPEX/Poseidon
1,336
TRMM
350
X-37
400
An orbit is the result of a precise balance between the forward motion of an object in space and
the pull of gravity from the Earth. Without gravity, an Earth-orbiting satellite would go off into
space along a straight line. With gravity, it is pulled back toward the Earth. A satellites
momentum and the force of gravity have to be perfectly balanced. If the forward momentum of
the satellite is too great, it will speed past the Earth and not enter into right orbit. If momentum is
too small, the satellite will be pulled into the Earth. You can think of it as the satellite is
constantly falling into the Earth, but because its moving sideways fast enough, it never hits.
Orbital velocity is the speed needed to maintain balance between gravitys pull on the satellite
and the momentum of the satellites motion. Exercise IV : Let calculate velocity required to keep an object into the circular orbit at the
altitude 242km.
G*M
earth
*m
stllt
m
stllt
*V
2
____
Gravity force = ( R
earth
+ h
orbit
)
2
Momentum force = R
earth
+ h
orbit
Step1. Gravity force = Momentum force _________________________________________
Step2. Solve for V___________________________________________________________
I.

Which type of the orbit requires higher velocity LEO or GEO? Explain.
II.

Does orbital velocity dependence on the mass of satellite? Explain.
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III. *** What should be the height of an orbit to keep satellite above the same spot on the
Earth?
Show your work.
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