Earth orbits
- required orbital velocity v = sqroot(GM/r) = 7.8 km/s
- cf. Moon, r = 60 earth radii, v = 1.0 km/s
- Therefore to launch an earth satellite
- go up 200 km (to avoid air friction)
- Hubble Space Telescope needs periodic boosting
- rocket sideways to v = 7.8 km/s
- Since earth is rotating ~ 1000 mph ~ 1500 km/h ~ 0.5 km/s at the equator it is easier to orbit in E direction at equator
- v = 7.3 km/s from rocket
- many launch sites located near equator
- The period P of the orbit
- given by distance travelled divided by velocity P = 2?r/v
- for low earth orbit r = 6400 km, v = 7.8 km/s, P = 90 min.
- for Moon, (r = 360,000 km, v = 1 km/s) P = 27 d (sidereal)
- In between (r ~ 42,000 km) P exactly 1 day -- geosyncronous
- Launch satellite toward E at equator and it will stay fixed above one spot on the Earth