Astronauts in low orbit float despite feeling almost full Earth gravity
Up where the space station flies, gravity is only slightly weaker than on the ground, because a few hundred kilometres is tiny compared with Earth's radius. Astronauts float because they are falling all the time, moving sideways at about 28,000 kilometres per hour so fast that they keep missing the planet.
Low Earth orbit covers any path that circles the planet in 128 minutes or less, which works out to altitudes below roughly 2,000 kilometres, about a third of Earth's radius. Staying there takes around 7.8 kilometres per second, a little less the higher you go, and a rocket must supply about 9.4 kilometres per second of velocity change to get there. Every human spaceflight has stayed in this zone apart from the Apollo trips to the Moon, and every space station has orbited within it.
Its appeal is practical. It costs the least energy to reach, keeps radio signals short and fast, and leaves spacecraft within reach of crews and servicing. Being close also means seeing the ground sharply, so imaging and spy satellites cluster here, many in Sun-synchronous orbits near 800 kilometres that pass over each spot at the same local time. The catch is a narrow view: covering the whole planet continuously takes a constellation of many satellites, as with the Iridium phone network near 780 kilometres.
The thin upper atmosphere still drags. Below about 300 kilometres orbits decay fast. The International Space Station, at around 400 to 420 kilometres, sinks roughly 2 kilometres a month and needs boosting a few times a year. Japan's Tsubame holds the lowest altitude ever for an Earth-observation craft, which flew at 167.4 kilometres. Satellites that burn up add vaporised metal to the stratosphere, with effects not yet understood.
Crowding is the growing worry. NASA tracks more than 25,000 objects over 10 centimetres, with an estimated 500,000 between 1 and 10 centimetres and over 100 million specks above a millimetre. At orbital speed, even a fleck can cripple a spacecraft, and each collision breeds more fragments, a cascade called Kessler syndrome. Some junk lingers for ages: Vanguard 1, launched in 1958, is expected to stay up for 240 years.
Source: Low Earth orbit
