Satellites: from Sputnik to GPS

Sputnik 1, the first satellite, was 58 cm across, weighed 83.6 kg and went round Earth in about 98 minutes.

The beep heard round the world

On 4 October 1957 the Soviet Union put the first artificial satellite into orbit. NASA's history office describes Sputnik 1 as about the size of a beach ball, 58 cm across, weighing 83.6 kg and taking about 98 minutes to go round Earth on its elliptical path. It did little more than broadcast a radio beep, but anyone with a receiver could hear it pass overhead. A month later Sputnik 2, at 508.3 kg, carried the dog Laika. The United States answered with Explorer 1 on 31 January 1958, and in July 1958 Congress passed the Space Act that created NASA. The space race had begun.

Less than four years after Sputnik, on 12 April 1961, Yuri Gagarin became the first human in space. NASA notes that his Vostok 1 circled Earth at 27,400 km/h, the flight lasted 108 minutes, and Gagarin did not land inside the capsule: he ejected and came down by parachute.

What satellites do

Thousands of satellites now circle Earth, and their jobs follow from their orbits. Low orbits, a few hundred kilometres up, are close enough for sharp pictures, so Earth-observation satellites such as Korea's KOMPSAT (Arirang) series fly there; they also pass over most of the planet as Earth turns beneath them. Medium orbits suit navigation: GPS.gov describes a constellation of at least 24 satellites at about 20,200 km, arranged in six orbital planes so that at least four are in view from almost anywhere. The U.S. Space Force has been flying 31 operational GPS satellites. High geostationary satellites at 35,786 km, like Korea's Chollian weather satellites, watch one whole hemisphere of Earth continuously.

Where would you see a geostationary satellite from Korea? It sits above the equator, so from the northern hemisphere it is always somewhere in the southern sky. Simple geometry gives its height: for a satellite at your own longitude, the altitude is arctan[(cos φ − 6,378 ÷ 42,164) ÷ sin φ]. At Seoul, φ = 37.6°, that is about 46° above the due-south horizon; from Jeju, farther south, about 51°. That is why satellite dishes in Korea all face roughly south and tilt up at about that angle.

A GPS receiver works out where it is from timing. Each satellite broadcasts its position and the exact time from on-board atomic clocks. The receiver measures how long each signal took to arrive, multiplies by the speed of light to get distances, and with four satellites solves for its own position in three dimensions plus the time error of its own clock. Because light covers 300 km in a thousandth of a second, the clocks must be extraordinarily precise: an error of a millionth of a second would already mean 300 m.

Korea in space

Korea's own launcher arrived in 2022. On 21 June, the Nuri rocket (KSLV-II), developed by KARI and launched from the Naro Space Center in Goheung, placed a performance verification satellite into its target orbit at 700 km. The government announced that this made Korea the seventh country to launch a practical satellite of more than one tonne with its own technology. In May 2024 the country opened the Korea AeroSpace Administration (KASA) in Sacheon to lead future rocket, satellite and lunar programmes.

More satellites also mean more junk: spent rocket stages, dead satellites and fragments from collisions. A paint flake at orbital speed carries a great deal of energy, so agencies track larger debris and spacecraft sometimes fire their engines to dodge it. New missions are now expected to plan how their satellites will be removed from orbit when they finish.