A spacecraft enters orbit when itscentripetalacceleration due togravity is less than or equal to thecentrifugal acceleration due to the horizontal component of its velocity. For alow Earth orbit, this velocity is about 7.8 km/s (28,100 km/h; 17,400 mph);[2] by contrast, the fastest crewed airplane speed ever achieved (excluding speeds achieved by deorbiting spacecraft) was 2.2 km/s (7,900 km/h; 4,900 mph) in 1967 by theNorth American X-15.[3] The energy required to reach Earth orbital velocity at analtitude of 600 km (370 mi) is about 36 MJ/kg, which is six times the energy needed merely to climb to the corresponding altitude.[4]
Spacecraft with aperigee below about 2,000 km (1,200 mi) are subject to drag from the Earth's atmosphere,[5] which decreases the orbital altitude. The rate of orbital decay depends on the satellite's cross-sectional area and mass, as well as variations in the air density of the upper atmosphere. Below about 300 km (190 mi), decay becomes more rapid with lifetimes measured in days. Once a satellite descends to 180 km (110 mi), it has only hours before it vaporizes in the atmosphere.[6] Theescape velocity required to pull free of Earth's gravitational field altogether and move into interplanetary space is about 11.2 km/s (40,300 km/h; 25,100 mph).[7]
The following words may have more than one definition or other non-Earth specific definition(s). In the spirit of brevity some of the definitions have been altered ortruncated to reflect only their usage on this page.
as used here, the minimumvelocity an object withoutpropulsion needs to have to move away indefinitely from the Earth. An object at this velocity will enter aparabolic trajectory; above this velocity it will enter ahyperbolic trajectory.
Low (cyan) and Medium (yellow) Earth orbit regions to scale. The black dashed line is the geosynchronous orbit. The green dashed line is the 20,230 km orbit used forGPS satellites.
Geocentric orbits ranging in altitude from 160 km (100 mi) to 2,000 km (1,200 mi) abovemean sea level. At 160 km, one revolution takes approximately 90 minutes, and the circular orbital speed is 8 km/s (26,000 ft/s).
Geocentric circular orbit with an altitude of 35,786 km (22,236 mi). The period of the orbit equals onesidereal day, coinciding with the rotation period of the Earth. The speed is approximately 3 km/s (9,800 ft/s).
Geocentric orbits with altitudes at apogee higher than that of the geosynchronous orbit. A special case of high Earth orbit is thehighly elliptical orbit, where altitude at perigee is less than 2,000 km (1,200 mi).[9]
A satellite that passes above or nearly above both poles of the planet on each revolution. Therefore it has an inclination of (or very close to) 90degrees.
A nearlypolar orbit that passes theequator at the same local time on everypass. Useful for image-taking satellites because shadows will be the same on every pass.
An "orbit" with eccentricity greater than 1. The object'svelocity reaches some value in excess of theescape velocity, therefore it will escape the gravitational pull of the Earth and continue to travelinfinitely with a velocity (relative to Earth) decelerating to some finite value, known as thehyperbolic excess velocity.
Escape Trajectory
This trajectory must be used to launch an interplanetary probe away from Earth, because the excess over escape velocity is what changes itsheliocentric orbit from that of Earth.
Capture Trajectory
This is the mirror image of the escape trajectory; an object traveling with sufficient speed, not aimed directly at Earth, will move toward it and accelerate. In the absence of a decelerating engine impulse to put it into orbit, it will follow the escape trajectory after periapsis.
An "orbit" with eccentricity exactly equal to 1. The object'svelocity equals theescape velocity, therefore it will escape the gravitational pull of the Earth and continue to travel with a velocity (relative to Earth) decelerating to 0. A spacecraft launched from Earth with this velocity would travel some distance away from it, but follow it around the Sun in the sameheliocentric orbit. It is possible, but not likely that an object approaching Earth could follow a parabolic capture trajectory, but speed and direction would have to be precise.
an orbit in which the projection of the object onto the equatorial plane revolves about the Earth in the direction opposite that of the rotation of the Earth.
Thelibration points for objects orbiting Earth are at 105 degrees west and 75 degrees east. More than 160 satellites are gathered at these two points.[10]
An orbit which combines altitude andinclination in such a way that the satellite passes over any given point of theplanet's surface at the same localsolar time. Such an orbit can place a satellite in constant sunlight and is useful for imaging,spy, andweather satellites.
^Kennewell, John; McDonald, Andrew (2011),Satellite Lifetimes and Solar Activity, Commonwealth of Australia Bureau of Weather, Space Weather Branch,archived from the original on 2011-12-28, retrieved2011-12-31.
^Williams, David R. (November 17, 2010),"Earth Fact Sheet",Lunar & Planetary Science, NASA,archived from the original on October 30, 2010, retrieved2012-05-10.
^McDowell, Jonathan (24 May 1998)."Jonathan's Space Report".Transatmospheric orbit (TAO): orbital flight with perigee less than 80 km but more than zero. Potentially used by aerobraking missions and transatmospheric vehicles, also in some temporary phases of orbital flight (e.g. STS pre OMS-2, some failures when no apogee restart)
