A spacecraft enters orbit when its centripetalacceleration
due to gravity
is less than or equal to the centrifugal
acceleration due to the horizontal component of its velocity. For a low Earth orbit
, this velocity is about 7,800 m/s (28,100 km/h; 17,400 mph);
by contrast, the fastest manned airplane speed ever achieved (excluding speeds achieved by deorbiting spacecraft) was 2,200 m/s (7,900 km/h; 4,900 mph) in 1967 by the North American X-15
The energy required to reach Earth orbital velocity at an altitude of 600 km (370 mi) is about 36 MJ
/kg, which is six times the energy needed merely to climb to the corresponding altitude.
Spacecraft with a perigee
below about 2,000 km (1,200 mi) are subject to drag from the Earth's atmosphere,
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.
The escape velocity
required to pull free of Earth's gravitational field altogether and move into interplanetary space is about 11,200 m/s (40,300 km/h; 25,100 mph).
List of terms and concepts
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 or truncated
to reflect only their usage on this page.
as used here, the height of an object above the average surface of the Earth's oceans.Analemma
a term in astronomy
used to describe the plot of the positions of the Sun on the celestial sphere
throughout one year. Closely resembles a figure-eight.Apogee
is the farthest point that a satellite or celestial body
can go from Earth, at which the orbital velocity will be at its minimum.Eccentricity
a measure of how much an orbit deviates from a perfect circle. Eccentricity is strictly defined for all circular
and elliptical orbits
, and parabolic
and hyperbolic trajectories
as used here, an imaginary plane
extending from the equator on the Earth to the celestial sphere
as used here, the minimum velocity
an object without propulsion
needs to have to move away indefinitely from the Earth. An object at this velocity will enter a parabolic trajectory
; above this velocity it will enter a hyperbolic trajectory
of a force
over the time during which it acts. Measured in (N
between a reference plane
and another plane
. In the sense discussed here the reference plane
is the Earth's equatorial plane
the six parameters of the Keplerian elements
needed to specify that orbit uniquely.Orbital period
as defined here, time it takes a satellite to make one full orbit around the Earth.Perigee
is the nearest approach point of a satellite or celestial body from Earth, at which the orbital velocity will be at its maximum.Sidereal day
the time it takes for a celestial object
to rotate 360°. For the Earth this is: 23 hours, 56 minutes, 4.091 seconds.Solar time
as used here, the local time as measured by a sundial
an object's speed in a particular direction. Since velocity is defined as a vector
, both speed and direction are required to define it.
Geocentric orbit types
The following is a list of different geocentric orbit classifications.
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 for GPS
Low Earth orbit (LEO)
Geocentric orbits ranging in altitude from 160 kilometers (100 statute miles) to 2,000 kilometres (1,200 mi) above mean sea level
. At 160 km, one revolution takes approximately 90 minutes, and the circular orbital speed is 8,000 metres per second (26,000 ft/s).Medium Earth orbit (MEO)
Geocentric orbits with altitudes at apogee ranging between 2,000 kilometres (1,200 mi) and that of the geosynchronous orbit
at 35,786 kilometres (22,236 mi).Geosynchronous orbit (GEO)
Geocentric circular orbit with an altitude of 35,786 kilometres (22,236 mi). The period of the orbit equals one sidereal day
, coinciding with the rotation period of the Earth. The speed is approximately 3,000 metres per second (9,800 ft/s).High Earth orbit (HEO)
Geocentric orbits with altitudes at apogee higher than that of the geosynchronous orbit. A special case of high Earth orbit is the highly elliptical orbit
, where altitude at perigee is less than 2,000 kilometres (1,200 mi).
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) 90 degrees
.Polar sun synchronous orbit
A nearly polar orbit
that passes the equator
at the same local time on every pass
. Useful for image-taking satellites because shadows will be the same on every pass.
This trajectory must be used to launch an interplanetary probe away from Earth, because the excess over escape velocity is what changes its heliocentric 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's velocity
equals the escape 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 same heliocentric 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 same direction as the rotation of the Earth.Retrograde orbit
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.
Earth orbital libration points
The libration points
for objects orbiting Earth are at 105 degrees west and 75 degrees east. More than 160 satellites are gathered at these two points.
Tangential velocities at altitude
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Last edited on 6 April 2021, at 20:02
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