How Satellites Work

GPS Satellite

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In 1957, the Soviet Union launched the world’s first satellite, known as Sputnik. This changed the course of world history and led the United States, their chief rival in the Space Race, to mount a massive effort of its own to put manned craft in orbit and land a man on the moon. Since then, the presence of satellites in our atmosphere has become commonplace, which has muted the sense of awe and wonder involved. However, for many, especially students studying in engineering and aerospace programs, the question of How Satellites Work is still one of vital importance.

Satellite perform a wide array of functions. Some are observational, such as the Hubble Space Telescope – providing scientists with images of distant stars, nebulas, galaxies, and other deep space phenomena. Others are dedicated to scientific research, particularly the behavior of organisms in low-gravity environments. Then there are communications satellites which relay telecommunications signals back and forth across the globe. GPS satellites offer navigational aid and tracking aides to people looking to transport goods or navigate their way across land and oceans. And military satellites are used to observe and monitor enemy installations and formations on the ground while also helping the airforce and navy guide their ordinance to enemy targets.

Satellites are deployed by attaching them to rockets which then ferry them into orbit around the planet. Once deployed, they are typically powered by rechargeable batteries which are recharged through solar panels. Other satellites have internal fuel cells that convert chemical energy to electrical energy, while a few rely on nuclear power. Small thrusters provide attitude, altitude, and propulsion control to modify and stabilize the satellite’s position in space.

When it comes to classifying the orbit of a satellite, scientists use a varying list to describe the particular nature of their orbits. For example, Centric classifications refer to the object which the satellite orbits (i.e. planet Earth, the Moon, etc). Altitude classifications determine how far the satellite is from Earth, whether it is in low, medium or high orbit. Inclination refers to whether the satellite is in orbit around the equatorial plane, the polar regions, or the polar-sun orbit that passes the equator at the same local time on every pass so as to stay in the light. Eccentricity classifications describe whether the orbit is circular or elliptical, while Synchronous classifications describe whether or not the satellite’s rotation matches the rotational period of the object (i.e. a standard day).

Depending on the nature of their purpose, satellites also carry a wide range of components inside their housing. This can include radio equipment, storage containers, camera equipment, and even weaponry. In addition, satellites typically have an on-board computer to send and receive information from their controllers on the ground, as well as compute their positions and calculate course corrections.

We have written many articles about satellites for Universe Today. Here’s an article about the satellites in space, and here’s an article about exploring satellites with Google Earth.

If you’d like more info on satellites, check out these articles:
National Geographics article about Orbital Objects
Satellites and Space Weather

We’ve also recorded an episode of Astronomy Cast about the space shuttle. Listen here, Episode 127: The US Space Shuttle.

Sources:
http://en.wikipedia.org/wiki/Satellite
http://en.wikipedia.org/wiki/List_of_orbits
http://www.gma.org/surfing/sats.html
http://science.howstuffworks.com/satellite5.htm
http://www.howstuffworks.com/satellite.htm

How Satellites Stay in Orbit

GPS Satellite

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An artificial satellite is a marvel of technology and engineering. The only thing comparable to the feat in technological terms is the scientific know-how that goes into placing, and keeping, one in orbit around the Earth. Just consider what scientists need to understand in order to make this happen: first, there’s gravity, then a comprehensive knowledge of physics, and of course the nature of orbits themselves. So really, the question of How Satellites Stay in Orbit, is a multidisciplinary one that involves a great of technical and academic knowledge.

First, to understand how a satellite orbits the Earth, it is important to understand what orbit entails. Johann Kepler was the first to accurately describe the mathematical shape of the orbits of planets. Whereas the orbits of planets about the Sun and the Moon about the Earth were thought to be perfectly circular, Kepler stumbled onto the concept of elliptical orbits. In order for an object to stay in orbit around the Earth, it must have enough speed to retrace its path. This is as true of a natural satellite as it is of an artificial one. From Kepler’s discovery, scientists were also able to infer that the closer a satellite is to an object, the stronger the force of attraction, hence it must travel faster in order to maintain orbit.

Next comes an understanding of gravity itself. All objects possess a gravitational field, but it is only in the case of particularly large objects (i.e. planets) that this force is felt. In Earth’s case, the gravitational pull is calculated to 9.8 m/s2. However, that is a specific case at the surface of the planet. When calculating objects in orbit about the Earth, the formula v=(GM/R)1/2 applies, where v is velocity of the satellite, G is the gravitational constant, M is the mass of the planet, and R is the distance from the center of the Earth. Relying on this formula, we are able to see that the velocity required for orbit is equal to the square root of the distance from the object to the center of the Earth times the acceleration due to gravity at that distance. So if we wanted to put a satellite in a circular orbit at 500 km above the surface (what scientists would call a Low Earth Orbit LEO), it would need a speed of ((6.67 x 10-11 * 6.0 x 1024)/(6900000))1/2 or 7615.77 m/s. The greater the altitude, the less velocity is needed to maintain the orbit.

So really, a satellites ability to maintain its orbit comes down to a balance between two factors: its velocity (or the speed at which it would travel in a straight line), and the gravitational pull between the satellite and the planet it orbits. The higher the orbit, the less velocity is required. The nearer the orbit, the faster it must move to ensure that it does not fall back to Earth.

We have written many articles about satellites for Universe Today. Here’s an article about artificial satellites, and here’s an article about geosynchronous orbit.

If you’d like more info on satellites, check out these articles:
Orbital Objects
List of satellites in geostationary orbit

We’ve also recorded an episode of Astronomy Cast about the space shuttle. Listen here, Episode 127: The US Space Shuttle.

Sources:
http://en.wikipedia.org/wiki/Satellite
http://science.howstuffworks.com/satellite6.htm
http://www.bu.edu/satellite/classroom/lesson05-2.html
http://library.thinkquest.org/C007258/Keep_Orbit.htm#