Geocentric orbit
In 1967, the North American X-15 reached a speed of about 2,000 miles per hour. This was the fastest speed ever achieved by a crewed airplane before that moment. A spacecraft needs to move much faster than this to stay in orbit around Earth. The required velocity for low Earth orbit is approximately 17,500 miles per hour. Gravity pulls the object down while its forward motion tries to carry it away into space. These two forces balance each other when centripetal acceleration equals centrifugal acceleration due to horizontal velocity. Reaching this orbital speed requires an energy expenditure of about 36 megajoules per kilogram at an altitude of 200 kilometers. That amount of energy is six times greater than what is needed just to climb to that same height.
NASA tracked 2,465 artificial satellite payloads and 6,216 pieces of space debris as of 1997. Low Earth orbits range from 160 kilometers to 2,000 kilometers above mean sea level. At 160 kilometers, one complete revolution takes roughly 90 minutes. Medium Earth orbits sit between 2,000 kilometers and the geosynchronous belt at 35,786 kilometers. Geosynchronous satellites circle Earth at exactly 35,786 kilometers with a period matching one sidereal day. High Earth orbits extend beyond that 35,786 kilometer boundary. The average distance to the Moon is approximately 384,400 kilometers. Objects below 200 kilometers face significant atmospheric drag that lowers their altitude over time.
A polar orbit passes directly over both poles of the planet on every single revolution. This path has an inclination angle of 90 degrees relative to the equatorial plane. Sun synchronous orbits pass the equator at the exact same local time during each cycle. These paths allow imaging satellites to capture images under consistent shadow conditions. Prograde orbits move in the same direction as Earth rotates around its axis. Retrograde orbits travel opposite to the rotation of the planet. Inclination defines the angle between any reference plane and another plane or axis. An inclined orbit exists whenever this angle differs from zero degrees.
Eccentricity measures how much an orbit deviates from a perfect circle shape. A circular orbit has an eccentricity value of exactly zero. Elliptic orbits possess an eccentricity greater than zero but less than one. Hyperbolic trajectories occur when eccentricity exceeds one and velocity surpasses escape speed. Parabolic trajectories have an eccentricity equal to one precisely. The perigee marks the nearest point of approach to Earth where orbital velocity reaches maximum levels. Apogee represents the farthest distance from Earth where velocity drops to minimum values. Walter Hohmann developed a maneuver using two engine impulses to shift spacecraft between circular orbits.
Spacecraft with a perigee below 200 kilometers experience drag from Earth's upper atmosphere. This atmospheric resistance decreases the orbital altitude continuously over time. Below 160 kilometers, decay becomes rapid enough that lifetimes measure only in days. Once a satellite descends to roughly 80 kilometers, it vaporizes within hours. The rate of decay depends on the cross-sectional area and mass of the object. Variations in air density of the upper atmosphere also influence how quickly objects fall. More than 16,291 previously launched objects have undergone this process and entered the atmosphere since tracking began.
A Molniya orbit features an inclination of 63.4 degrees and lasts about 12 hours. Such satellites spend most of their time over designated areas of the Earth. Tundra orbits share the same 63.4 degree inclination but last one full sidereal day. Graveyard orbits sit a few hundred kilometers above geosynchronous positions for end-of-life disposal. Supersynchronous storage orbits allow satellites to drift westward after mission completion. Subsynchronous drift orbits cause satellites to move eastward when placed just below GSO levels. Geosynchronous transfer orbits connect low Earth altitudes with geosynchronous heights through elliptical paths. Escape trajectories launch interplanetary probes away from Earth by exceeding escape velocity thresholds.
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Common questions
What speed is required for a spacecraft to stay in low Earth orbit around Earth?
The required velocity for low Earth orbit is approximately 17,500 miles per hour. This speed allows forward motion to balance the pull of gravity and maintain an orbital path.
When did NASA track 2465 artificial satellite payloads and 6216 pieces of space debris as of 1997?
NASA tracked 2,465 artificial satellite payloads and 6,216 pieces of space debris as of 1997. These figures represent the count of objects monitored at that specific time.
How does atmospheric drag affect satellites with a perigee below 200 kilometers above mean sea level?
Spacecraft with a perigee below 200 kilometers experience drag from Earth's upper atmosphere which decreases the orbital altitude continuously over time. Below 160 kilometers decay becomes rapid enough that lifetimes measure only in days.
Why do Molniya orbits feature an inclination of 63.4 degrees relative to the equatorial plane?
A Molniya orbit features an inclination of 63.4 degrees and lasts about 12 hours so such satellites spend most of their time over designated areas of the Earth. This specific angle allows for extended coverage over high latitude regions.
What is the average distance to the Moon compared to geosynchronous satellite positions around Earth?
The average distance to the Moon is approximately 384,400 kilometers while geosynchronous satellites circle Earth at exactly 35,786 kilometers. This difference places the Moon far beyond the boundary of medium Earth orbits.