sentences:  If we ignore these factors, and approximate Mars's orbital period as 1.875 Earth years, then 15 Earth years is 8 Martian years.In the diagram opposite, a spacecraft in an Aldrin cycler orbit that starts from Earth at point E1 will encounter Mars at M1.When it gets back to E1 just over two Earth years later, Earth will no longer be there, but it will encounter Earth again at E2, which is 51.4 degrees, of an Earth orbit, further round.