Satellite Basics

Satellites vary in size from a few pounds to many tons. Some remain in orbit for only a few weeks, while others have an expected lifetime of hundreds of years. They are packed with scientific instru­ments, usually miniaturized to save weight and space. Manufacturing is closely monitored and takes place in germ-free conditions. All systems are thoroughly tested-once launched, satel­lites must continue to work under remote control for long periods of time.

SATELLITE LAUNCHERS

A satellite can be carried into space in the cargo bay of a Space Shuttle, or it may be blasted into space on top of a multistage rocket or launch vehicle. An expendable launch vehicle, or ELV, is used only once. An ELV has two or more booster stages; each stage falls away when its engine burn is completed. The final stage sends the pay­load (the satellite) into orbit.

The launch vehicle Pegasus is itself launched from beneath a converted Lockheed L-1011 aircraft. The launcher can place a satellite weighing up to 970 pounds (440 kilograms) into near-Earth orbit. Pegasus launched the Solar Radiation and Climate Explorer and the Galaxy Evolution Explorer satellites in 2003.

A larger satellite, or a satellite intended for high orbit, requires a more powerful launcher, such as a ground-launched Athena or Delta rocket. Delta rockets have launched many satel­lites since the 1960s, including TIROS, Nimbus, ITOS, and Landsat satellites. The big Delta IV can launch a payload of 50,800 pounds (23,070 kilo­grams) into low-Earth orbit.

O A huge Delta IV rocket stands at Cape Canaveral, ready to carry an observation satellite

V

Most satellites are fitted with panels of solar cells that convert energy from sunlight into electricity. They use solar energy to power instruments and the communication systems that send data, such as video images, back to Earth.

A satellite is directed by remote con­trol from mission centers on the ground,
where scientists monitor its orbit, send instructions, and receive data from the satellite’s instruments. Under command from Earth, a satellite can be shifted in orbit by firing small thruster rockets.

A satellite may be in constant com­munication with mission control. If it is in a low orbit, it may be contacted only

when it passes overhead. Each pass may last just 10 minutes, but the satellite may fly by ten or twelve times a day, depend­ing on its orbit. Some satellites are visi­ble at night as they pass overhead.

A satellite that has malfunctioned, or has come to the end of its operational life, is normally shut down and then allowed to burn up as it reenters the atmosphere. Some defective satellites, however, have been picked up by Space Shuttle astronauts for repair.