Astronomy & spaceflight
Satellites
Thousands of machines constantly fall around the Earth yet never fall down. They navigate, relay and photograph for us.
When you navigate with your phone, check the weather or watch a football match from the other side of the world, satellites are involved. Thousands of these machines circle above our heads. The crazy part: they have no running engine holding them up. They are falling. All the time.
Falling yet never landing
Throw a ball horizontally: it falls to the ground in an arc. Throw it faster and the arc gets longer. Now imagine throwing it so fast that the Earth curves away beneath it exactly as quickly as it falls. Then it never hits the ground, it falls around the Earth forever. That is exactly what an is. Near the Earth this takes about 28000 km/h, roughly 7.8 kilometres per second. does not switch off up there, in fact it is the only thing keeping the satellite on its circular path.
The satellite keeps falling toward the planet, but moves sideways so fast that it keeps missing it. Rule of thumb: an orbit is falling around the planet.
Low orbit or geostationary
How high a satellite flies determines its job. In low orbits a few hundred kilometres up, you race around the Earth in about 90 minutes, perfect for sharp Earth observation and fast internet links. The geostationary orbit at about 36000 km is completely different: there one lap takes exactly as long as one rotation of the Earth, that is one day. The satellite therefore moves in step with the Earth and appears to hover over a fixed point on the equator. That is why a fixed satellite dish can simply point at it.
GPS: positioning takes a team
Navigation satellites like those of GPS constantly broadcast their position and a highly accurate time. Your phone measures how long the signals travelled and works out the distances from that. A single satellite is not enough: it only tells you that you are somewhere on a huge sphere around it. Only with signals from at least four satellites can your phone pin down your position and time exactly. That is why whole fleets of navigation satellites circle the Earth.
Space debris
Not everything up there is useful. Retired satellites, discarded rocket parts and fragments from old collisions circle the Earth as space debris. At around 28000 km/h even a piece the size of a bolt can badly damage a satellite, and in a head on collision the two speeds even add up. That is why the ISS and many satellites regularly dodge debris, and new satellites are expected to dispose of themselves at the end of their lives: by burning up in the atmosphere on purpose or moving to a graveyard orbit.
Exercises
0 of 6 solvedTime to try it yourself. You can't break anything, every attempt counts.
Why does a satellite in orbit not crash into the Earth?
What is special about a geostationary satellite?
A satellite in low orbit takes about 90 minutes per lap around the Earth. How many laps does it manage in 24 hours, that is 1440 minutes?
For your phone to pin down position and time exactly, it needs signals from at least … satellites.
The geostationary orbit sits at around 36000 km, a low orbit at around 400 km. How many times as high is the geostationary orbit?
Match each term to its fitting description.