Timeline of artificial satellites and space probes
A distinction is made between two types of unmanned space missions, the spacecraft and the Earth satellites.
Decisive difference is the speed of 11.2 km / s, which must exceed a spacecraft to leave the Earth's gravitational field entirely and, for example fly to Mars can ( escape velocity ). If the escape velocity is not reached, the spacecraft in the gravitational field of the earth and will remain so either one, the moon comparable, however artificial, satellite or falls back to the earth. In addition, also be spacecraft, the switch from the gravitational field of the earth to the moon, called space probes. For leaving our solar system (eg, Voyager 1 ) an escape velocity of about 42 km / s is needed.
And even spacecraft, without regard to the mission specifications, in a halo orbit at one of the Lagrange points L1 and L2 are counted as space probes, as these vehicles are no longer in orbit around the Earth.
Spacecraft ( incomplete)
(* ) End of primary mission