Geocentric orbit

• 1. Earth's Orbital Neighborhood

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  Geocentric orbit is the technical name for any path around Earth, whether traced by the Moon or by a human-made machine. By 1997, NASA estimated 2,465 artificial satellite payloads circled the planet. The Goddard Space Flight Center tracked 6,216 additional pieces of space debris alongside them. Together, those figures capture only a snapshot of a neighborhood that had been filling since the first rockets flew. Why does anything stay in orbit at all? Why are some satellites clustered barely above the atmosphere while others sit tens of thousands of kilometers away? And what eventually happens to a machine when its working life is over? Earth's Moon, the original resident of geocentric space, circles at an average altitude of 384,403 kilometers. That places it at the outermost edge of a system whose inner boundary barely clears the upper atmosphere.

• 2. The Velocity Threshold

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  Reaching a low Earth orbit requires a horizontal speed of about 7.8 kilometers per second. A spacecraft at that velocity finds Earth's surface curving away at the same rate it falls. The result is a continuous arc that never reaches the ground.

  The fastest crewed airplane speed ever recorded, excluding deorbiting spacecraft, was 2.2 kilometers per second. That record was set in 1967 by the North American X-15. The gap between 2.2 and 7.8 kilometers per second underscores how different atmospheric flight is from orbital flight.

  Reaching orbital velocity at 600 kilometers altitude demands about 36 megajoules per kilogram. That figure is six times the energy needed simply to climb to that altitude without any forward speed. Beyond orbital velocity lies one more threshold. At about 11.2 kilometers per second, Earth's gravity can no longer hold an object back. That boundary marks the outer edge of geocentric space.

• 3. The Altitude Ladder

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  At 160 kilometers above Earth's surface, a satellite completes one orbit in approximately 90 minutes. That altitude marks the lower boundary of low Earth orbit, a band stretching upward to 2,000 kilometers. Circular orbital speed at that lower boundary reaches about 8 kilometers per second.

  Medium Earth orbit begins at 2,000 kilometers and climbs to the geosynchronous altitude of 35,786 kilometers. Within that band sits the semi-synchronous orbit at approximately 20,200 kilometers, where one circuit takes roughly 12 hours. Geosynchronous orbit occupies that upper boundary, and its period equals exactly one sidereal day. A sidereal day lasts 23 hours, 56 minutes, and 4.091 seconds, slightly shorter than the 24-hour calendar day. At geosynchronous altitude, orbital speed drops to approximately 3 kilometers per second.

  A geosynchronous orbit with an inclination of exactly zero degrees is called geostationary. From the ground, a satellite in that position appears motionless, fixed at a single point in the sky. This orbit also carries a second name: the Clarke orbit, named after the writer Arthur C. Clarke.

  Highly elliptical orbits extend their apogee above 35,786 kilometers while dropping their perigee as low as about 1,000 kilometers. Satellites using these paths spend the majority of their time near apogee, gaining prolonged coverage of high-latitude regions.

  Satellites that outlive their usefulness are often moved into the graveyard orbit. This disposal zone sits a few hundred kilometers above geosynchronous altitude, keeping retired spacecraft clear of active ones.

• 4. Shape, Angle, and Purpose

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  A perfectly circular orbit has an eccentricity of exactly zero. Even a small deviation produces an ellipse, with a perigee closest to Earth and an apogee farthest from it. At higher eccentricities, orbits stretch into long ovals; above eccentricity 1, the path becomes hyperbolic and the object escapes Earth's gravity entirely.

  Walter Hohmann gave his name to a particular orbital maneuver: a two-impulse path that moves a spacecraft from one circular orbit to another. A variant called the geosynchronous transfer orbit applies the same principle to carry satellites from low Earth orbit up to geosynchronous altitude.

  The Molniya orbit combines a 63.4-degree inclination with a period of roughly half a sidereal day. A satellite in that configuration spends most of its time near apogee, hovering over a designated area of Earth. The Tundra orbit shares the 63.4-degree inclination but extends its period to a full sidereal day. Both designs cause satellites to spend most of their time over a single designated region of Earth.

  A polar orbit passes above or nearly above both poles on each revolution, requiring an inclination of 90 degrees or very close to it. This crossing pattern ensures the satellite eventually covers every part of Earth's surface. The sun-synchronous orbit pairs altitude and inclination so that the satellite crosses any given point at the same local solar time on every pass. That consistency makes it useful for imaging, spy, and weather satellites.

  More than 160 satellites have gathered at the two libration points, located at 105 degrees west and 75 degrees east. Spacecraft whose perigee dips into low Earth orbit, however, face a force no mission planner can work around: atmospheric drag.

• 5. Orbital Decay

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  More than 16,291 objects launched into Earth orbit have already re-entered the atmosphere. Atmospheric drag is the mechanism at work. It steadily reduces the altitude of any spacecraft whose path passes through the upper reaches of the atmosphere. How quickly drag acts depends on the satellite's cross-sectional area and mass, and on how dense the upper atmosphere happens to be.

  Below about 300 kilometers, the pace of orbital decay quickens sharply, and satellite lifetimes shrink to days. At 180 kilometers, the remaining lifespan is measured in hours, after which the spacecraft vaporizes in the atmosphere. An object arriving from outside Earth's gravity well with sufficient speed traces a capture trajectory, accelerating as it swings around the planet. Without an engine to slow it into a stable orbit, it departs along an escape trajectory and does not return.