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Low Earth orbit
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<p>A low Earth orbit (LEO) is an <a href="/facts/Geocentric_orbit/geC4BF1x">orbit around Earth</a> with a <a href="/facts/Orbital_period/94f56uh2">period</a> of 128 minutes or less (making at least 11.25 orbits per day) and an <a href="/facts/Orbital_eccentricity/WGlB4NBn">eccentricity</a> less than 0.25. Most of the artificial objects in <a href="/facts/Outer_space/R3LRKyFs">outer space</a> are in LEO, peaking in number at an altitude around 800 km (500 mi), while the farthest in LEO, before <a href="/facts/Medium_Earth_orbit/7Qmz5XxG">medium Earth orbit</a> (MEO), have an altitude of 2,000 kilometers, about one-third of the <a href="/facts/Earth_radius/PVsf0g2I">radius of Earth</a> and near the beginning of the <a href="/facts/Van_Allen_radiation_belt/XKnhnWK0">inner Van Allen radiation belt</a>. </p><p>The term <i>LEO region</i> is used for the area of space below an <a href="/facts/Altitude/B5eNVurK">altitude</a> of 2,000 km (1,200 mi) (about one-third of Earth's radius). Objects in orbits that pass through this zone, even if they have an <a href="/facts/Apogee/MSHAk1RU">apogee</a> further out or are <a href="/facts/Sub-orbital_spaceflight/NdLRlGIq">sub-orbital</a>, are carefully tracked since they present a collision risk to the many LEO satellites. </p><p>No <a href="/facts/Human_spaceflight/VgVxsGmq">human spaceflights</a> other than the lunar missions of the <a href="/facts/Apollo_program/IC1Ek1Ao">Apollo program</a> (1968-1972) have taken place beyond LEO. The <a href="/facts/Artemis_program/QHBrKFI8">Artemis program</a> of lunar missions is scheduled to take place beyond LEO. All <a href="/facts/Space_station/2HtaLbDH">space stations</a> to date have operated <a href="/facts/Geocentric_orbit/geC4BF1x">geocentric</a> within LEO. </p>