Satellite Orbital Speed Calculator

Below the circular orbital velocity, the satellite cannot maintain its altitude and begins falling. It enters an elliptical orbit with apogee at its current position and perigee lower. If the deceleration is sufficient, it will reenter the atmosphere. This is exactly how deorbit maneuvers work: a retrograde burn reduces velocity, lowering the perigee into the atmosphere.

For a theoretical surface orbit (ignoring mountains): v = sqrt(GM_Moon/R_Moon) = sqrt(4903/1737) = 1.68 km/s (about 6050 km/h). A ball thrown horizontally at 1.68 km/s on the Moon's surface would orbit (if the surface were smooth and there was no atmosphere). This is why low-thrust engines can achieve lunar orbit easily.

GPS satellites at MEO move at 3.87 km/s. Due to special relativity (time dilation from high speed), their clocks run slow by about 7 microseconds per day. Due to general relativity (less gravitational time dilation at high altitude), their clocks run fast by about 45 microseconds per day. The net correction of +38 microseconds per day is programmed into GPS satellites to maintain timing accuracy.

The orbital speed at the periapsis or apoapsis of a frozen orbit (one designed to be stable against perturbations) is computed using the vis-viva equation with the actual periapsis or apoapsis radius. For Earth observation satellites in SSO (typically 500-800 km), speeds range from 7.6 to 7.4 km/s.

Any orbit with perigee below about 120 km (for Earth) will decay rapidly due to atmospheric drag. Practical minimum stable orbits are above 200 km. At 150 km altitude, orbital lifetime is only a few hours to days. Above 600 km, atmospheric drag becomes negligible and orbits are stable for decades to centuries (becoming a debris problem).

Orbital decay is the gradual decrease in semi-major axis (and thus altitude) caused by atmospheric drag or other perturbations. At each perigee pass, drag decelerates the satellite slightly, lowering the apogee. Over time, the orbit circularizes at a lower altitude and eventually the satellite reenters. The ISS loses about 2 km per month to atmospheric drag and must be regularly reboosted.

Technically yes for circular orbits. Practically, very low orbits decay rapidly, the Van Allen radiation belts make some medium altitudes hostile to electronics, and GEO is a crowded, valuable orbit. Satellites are placed in LEO (below 2000 km), MEO (2000-35786 km), or GEO (35786 km) based on mission requirements. Very high orbits (above GEO) are sometimes used for high-apogee science missions.

First cosmic velocity is the minimum orbital speed for a circular orbit at Earth's surface (ignoring atmosphere): v1 = sqrt(GM/R_Earth) = sqrt(398600/6371) = 7.91 km/s. This is the speed needed to orbit Earth at the surface. Second cosmic velocity (escape velocity from the surface) = v1*sqrt(2) = 11.18 km/s. Third cosmic velocity (escape the Solar System from Earth) = 16.7 km/s.

LEO satellite phone constellations (Starlink, Iridium) use many satellites because each one moves quickly across the sky (15 orbits/day) and can only see a limited footprint below. GEO satellites are stationary as seen from Earth (1 orbit/day) and can cover a large area with just one satellite, but have higher latency (240 ms signal round-trip) due to the 35,786 km altitude.