On the Edge of Earth: The Future of American Space Power

Appendix: Elementary Descriptions of Orbits

• APPENDIX ELEMENTARY DESCRIPTION O F ORBITS Terms Achieving Orbit Satellites achieve Earth orbit by being propelled beyond the atmosphere at about 17,000 miles per hour, a velocity that counteracts the force of gravity. At that speed, the earth recedes from the satellite at the same speed that the earth's gravity pulls the satellite toward it. In orbit, the satellite follows a circular or elliptical path around the earth. The properties of orbits can vary greatly. Altitude, inclination, eccentricity, and orbital period are the major aspects or parameters of satellite orbits. Altitude. The minimum altitude for an orbiting satellite is about 60 miles; at lower altitudes, the atmosphere induces drag that causes immediate reentry. Due to the presence of rarefied atmosphere, spacecraft whose entire orbits lie in the 60- to 100-mile altitude band require repeated activation of their engines in order to avoid reentry in less than 24 hours. Satellites operating at 200 miles can remain in orbit for almost a year without use of their engines. Although it may take decades, even at 1,200 miles, atmospheric drag eventually will cause a satellite orbit to decay. Beyond about 1,200 miles is the hard vacuum of space; objects orbiting at that altitude will stay in orbit permanently. At about 22,300 miles altitude , satellites orbiting the earth can remain over the same point on the earth's surface (see Geostationary Earth Orbit). 298 Appendix Inclination. Orbital inclination is measured in terms of degrees above or below the equator. The degree of inclination of a satellite's orbital plane determines the area of the earth a satellite will overfly as it orbits. Satellites in 90-degree (polar) orbits can pass over every point on earth. A satellite in an orbit of 45 degrees inclination will only be able to overfly the tropical and temperate territory ranging from 45 degrees north to 45 degrees south of the equator; it will not be able to view the polar regions directly. An orbital plane, inclined approximately 45 degrees from the equator. Figures courtesy of Ashton Carter and adapted from his article "Satellites and AntiSatellites ," International Security\ Spring 1986,52. An equatorial orbital plane, inclined zero degrees Eccentricity. Satellite orbits follow either circular or elliptical paths. Eccentricity describes the shape of an orbital ellipse, in relative terms of "fat" and "wide." A circle is simply an ellipse that is as "fat" as it is "wide." Orbital Period. The time it takes for a satellite to circumnavigate the body around which it orbits. The higher the altitude of the orbit, the longer the orbital period. In other words, the higher the orbit, the longer it takes to circle the earth. For satellites in low Earth orbit, the orbital period is approximately 90 minutes. For geosynchronous satellites, the orbital period is the same length as the earth's day: just under 24 hours. Appendix 299 Orbital Plane An imaginary two-dimensional surface (of infinite height and length, but no breadth) that contains all the flight points on an orbit in space. Once launched into space, a satellite will normally maintain the same orbital plane as long as it is in orbit, whatever its altitude and degree of eccentricity. The only way a satellite can change orbital plane is by changing its direction, which requires an (often considerable) expenditure of propellant. Multiple satellites can occupy the same orbital plane. Apogee The maximum altitude attained by a spacecraft in elliptical orbit around Earth. Perigee The minimum altitude attained by a spacecraft in elliptical orbit around Earth. Types of orbits include: Low Earth Orbit (LEO) Low Earth orbits range from 60 to 300 miles above the surface of the earth. This area of circumterrestrial space is bounded at the lower limit (60 miles) by the presence of atmosphere sufficiently thick to induce reentry and at the higher limit (250-300 miles) by the Van Allen radiation belts. Between these altitudes, the orbital period of satellites ranges from about 90 minutes to two hours. Because of their closeness to Earth, LEO orbits are optimal for reconnaissance satellites that collect imagery of the earth's surface. Low Earth orbits also are used as an intermediate step in the placement of satellites into higher orbits, especially geosynchronous orbit. Constellation offiveU.S. Transit navigation satellites in polar LEO, infiveorbital planes 300 Appendix Medium Earth Orbit (MEO) Medium Earth orbits range in altitude from just above LEO (300 miles) to...