Artificial Satellite

C

Weather Satellites

Weather satellites carry cameras and other instruments pointed toward Earth’s atmosphere. They can provide advance warning of severe weather and are a great aid to weather forecasting. NASA launched the first weather satellite, Television Infrared Observation Satellite (TIROS) 1, in 1960. TIROS 1 transmitted almost 23,000 photographs of Earth and its atmosphere. NASA operates the Geostationary Operational Environmental Satellite (GOES) series, which are in geostationary orbit. GOES provides information for weather forecasting, including the tracking of storms. GOES is augmented by Meteosat 3, a European weather satellite also in geostationary orbit. The National Oceanic and Atmospheric Administration (NOAA) operates three satellites that collect data for long-term weather forecasting. These three satellites are not in geostationary orbit; rather, their orbits carry them across the poles at a relatively low altitude.

D

Military Satellites

Many military satellites are similar to commercial ones, but they send encrypted data that only a special receiver can decipher. Military surveillance satellites take pictures just as other earth-imaging satellites do, but cameras on military satellites usually have a higher resolution.

The U.S. military operates a variety of satellite systems. The Defense Satellite Communications System (DSCS) consists of five spacecraft in geostationary orbit that transmit voice, data, and television signals between military sites. The Defense Support Program (DSP) uses satellites that are intended to give early warning of missile launches. DSP was used during the Persian Gulf War (1991) to warn of Iraqi Scud missile launches.

Some military satellites provide data that is available to the public. For instance, the satellites of the Defense Meteorological Satellite Program (DMSP) collect and disseminate global weather information. The military also maintains the Global Positioning System (GPS), described earlier, which provides navigation information that anyone with a GPS receiver can use.

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E

Scientific Satellites

Earth-orbiting satellites can provide data to map Earth, determine the size and shape of Earth, and study the dynamics of the oceans and the atmosphere. Scientists also use satellites to observe the Sun, the Moon, other planets and their moons, comets, stars, and galaxies. The Hubble Space Telescope is a general-purpose observatory launched in 1990. Some scientific satellites orbit bodies other than Earth. The Mars Global Surveyor, for example, orbits the planet Mars.

III

Satellite Launches

Placing a satellite into orbit requires a tremendous amount of energy, which must come from the launch vehicle, or device that launches the satellite. The satellite needs to reach an altitude of at least 200 km (120 mi) and a speed of over 29,000 km/h (18,000 mph) to lift into orbit successfully. Satellites receive this combination of potential energy (altitude) and kinetic energy (speed) from multistage rockets burning chemical fuels.

The first stage of a multistage rocket consists of rocket engines that provide a huge amount of force, or thrust. The first stage lifts the entire launch vehicle—with its load of fuel, the rocket body, and the satellite—off the launch pad and into the first part of the flight. After its engines use all their fuel, the first stage portion of the rocket separates from the rest of the launch vehicle and falls to Earth. The second stage then ignites, providing the energy necessary to lift the satellite into orbit. It, too, then separates from the satellite and any remaining rocket stages.

The rest of the launch depends on the satellite’s mission. For example, if the mission requires a geostationary orbit, which can be achieved only at a distance of about 35,000 km (22,000 mi) above Earth, a third rocket stage provides the thrust to lift the satellite to its final orbital altitude. After the satellite has reached the final altitude, another rocket engine fires and gives the satellite a circular orbit. All rocket-engine burns occur at a precise moment and last for a precise amount of time so that the satellite achieves its proper position in space.

In 1990 the United States began launching some satellites from aircraft flying at high altitudes. This method still requires a rocket-powered launch vehicle, but because the vehicle does not have to overcome friction with the thick atmosphere found at low altitudes, much less fuel is needed. However, the size of the rocket is limited by the size and strength of the aircraft, so only smaller satellites can be launched this way.

Another method of launching satellites is to have astronauts launch them from the U.S. space shuttle. The space shuttle can carry large satellites and, because the shuttle is already in orbit when the satellite is launched, the astronauts can verify that the satellite has survived the rigors of launch. The space shuttle can also bring satellites back to Earth for repair, or astronauts can repair satellites in space.

The Single Stage to Orbit (SSTO) is a launch vehicle that may lower the cost of launching satellites by decreasing the number of launch stages needed and increasing the reusability of launch vehicles. The SSTO would be a piloted vehicle like the space shuttle, but it would be designed to launch satellites more inexpensively and efficiently than the space shuttle can.

IV

Operations in Space

Because satellites must survive the launch and must operate in the harsh environment of space, they require unique and durable technologies. Satellites have to carry their own power source because they cannot receive power from Earth. They must remain pointed in a specific direction, or orientation, to accomplish their mission. Satellites need to maintain proper temperature in the face of direct rays from the Sun and in the cold blackness of space. They must also survive high levels of radiation and collisions with micrometeoroids (see Meteoroid). Most satellites have onboard computers that help with satellite operations and with the satellite’s mission.