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Space and Sky · entry 35 of 52
Objects in low Earth orbit travel fast enough to circle the planet in about ninety minutes.
The longer version
Orbital speed depends on altitude: closer to Earth, gravity is stronger and a satellite must travel faster to keep missing the ground. At a few hundred kilometers up, that speed carries a craft around the planet in roughly an hour and a half. An object there completes many circuits in a single day.
Such orbits decay slowly because traces of atmosphere remain and produce drag. A satellite must periodically boost itself or it will reenter and burn up. Higher orbits move more slowly and take longer, which is why distant communications satellites appear to hang in place relative to the ground.
Where this entry sits
The night sky looks calm, yet almost everything up there is moving, burning, or slowly changing. These facts cover planets, stars, the Moon, gravity, black holes, comets, and the story of how people began exploring beyond Earth. The full list sits on the Space and Sky page, where every entry is listed in order.
More From Space and Sky
Eight more entriesSpacecraft launched decades ago still return faint signals from far beyond the planets.
Read 37Rovers on Mars drive across the surface under instructions delayed by many minutes.
Read 38Telescopes in orbit avoid the blurring caused by Earth's atmosphere when observing galaxies.
Read 39The first artificial satellite reached orbit in 1957 and transmitted a simple radio signal.
Read 40Humans first walked on the Moon in 1969 during a brief burst of exploration.
Read 41The Sun rotates faster at its equator than at its poles because it is gaseous.
Read 42Neutron stars pack more mass than the Sun into a sphere the size of a city.
Read 43White dwarfs are slowly cooling remnants of stars that once resembled our Sun.
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