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In gravitationally bound systems, the orbital speed of an astronomical body or object (e.g. planet, moon, artificial satellite, spacecraft, or star) is the speed at which it orbits around either the barycenter (the combined center of mass) or, if one body is much more massive than the other bodies of the system combined, its speed relative to the center of mass of the most massive body.
Escape speed at a distance d from the center of a spherically symmetric primary body (such as a star or a planet) with mass M is given by the formula [2] [3] = = where: G is the universal gravitational constant (G ≈ 6.67 × 10 −11 m 3 ⋅kg −1 ⋅s −2 [4])
[nb 1] Earth's orbital speed averages 29.78 km/s (19 mi/s; 107,208 km/h; 66,616 mph), which is fast enough to cover the planet's diameter in 7 minutes and the distance to the Moon in 4 hours. [3] The point towards which the Earth in its solar orbit is directed at any given instant is known as the "apex of the Earth's way". [4] [5]
A wide variety of sources [5] [6] [7] define LEO in terms of altitude.The altitude of an object in an elliptic orbit can vary significantly along the orbit. Even for circular orbits, the altitude above ground can vary by as much as 30 km (19 mi) (especially for polar orbits) due to the oblateness of Earth's spheroid figure and local topography.
Sketch of a circumlunar free return trajectory (not to scale), plotted on the rotating reference frame rotating with the moon. (Moon's motion only shown for clarity) In orbital mechanics, a free-return trajectory is a trajectory of a spacecraft traveling away from a primary body (for example, the Earth) where gravity due to a secondary body (for example, the Moon) causes the spacecraft to ...
The five species model is only usable for entry from low Earth orbit where entry velocity is approximately 7.8 km/s (28,000 km/h; 17,000 mph). For lunar return entry of 11 km/s, [23] the shock layer contains a significant amount of ionized nitrogen and oxygen. The five-species model is no longer accurate and a twelve-species model must be used ...
For a low 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 the North American X-15. [3]
Orbit Center-to-center distance Altitude above the Earth's surface Speed Orbital period Specific orbital energy; Earth's own rotation at surface (for comparison— not an orbit) 6,378 km: 0 km: 465.1 m/s (1,674 km/h or 1,040 mph) 23 h 56 min 4.09 sec: −62.6 MJ/kg: Orbiting at Earth's surface (equator) theoretical 6,378 km: 0 km