BOX 1-1 Examples of Heavily Used Orbital Regions

Low Earth Orbit (LEO): A majority of the world's spacecraft operate in LEO because these orbits have characteristics that are advantageous for a wide array of missions. First, less energy (and thus a smaller launch vehicle) is required to launch a spacecraft into LEO than to put it into any higher orbit. Second, proximity to Earth allows remote sensing missions to receive higher resolution images. Finally, the Earth's magnetic field protects spacecraft in some LEOs from cosmic radiation and solar flares; this is of particular importance for human operations in space.

Sun-Synchronous Orbit: These LEOs precess in such a way that they do not experience changes in Sun angle due to the movement of the Earth around the Sun. This means that the lighting conditions for points on the Earth as the spacecraft passes overhead do not change over the course of a year—a useful feature for some remote sensing missions. Sun-synchronous orbits have inclinations greater than 90 degrees (the exact inclination varies with altitude). Although spacecraft can occupy Sun-synchronous orbits at most altitudes, for a number of reasons the altitudes near 900 and 1,500 km are the most widely used.

Geosynchronous Earth Orbit (GEO): GEOs are circular with orbital periods of approximately 1,436 minutes (about 24 hours), so spacecraft in them remain above roughly the same longitude on the Earth throughout their orbit. A special type of GEO is the geostationary Earth orbit, which has an inclination close to zero degrees. From the surface of the Earth, spacecraft in geostationary Earth orbits appear to be fixed in the sky. Communications with the spacecraft are thus simplified—both because the spacecraft is in view at all times and because ground antennas do not have to follow the spacecraft's movement. Inclined GEOs are also useful for some missions, although they require ground stations that are able to track a spacecraft's north-south as well as its apparent east-west movement.

bital corrections. These spacecraft are placed into orbits from which they can accomplish their particular mission effectively, resulting in a highly nonuniform distribution of spacecraft about the Earth. Box 1-1 lists three heavily used orbital regions and some of the reasons why they are used. (Additional information about these and other orbital regions is contained in the Glossary.) A few spacecraft each year are launched out of Earth orbit and into interplanetary space; the hazard to future space operations from these probes is utterly negligible.

The distribution of spacecraft around the Earth at the start of 1994 is displayed in Figure 1-1. This distribution is not static; as missions, technologies, and available launch vehicles change, the distribution of functional spacecraft also changes. For example, over the past three decades, the annual percentage of new space missions to orbits above LEO has been increasing; in 1993, High Earth Orbits (HEOs) were the final desti-



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