Lunar cycler approaching Earth. Computer artwork of a Lunar cycler (centre) rounding the south pole of the Earth near perigee, its closest approach to the Earth, while a trailing future generation space shuttle (right) prepares for a rendezvous. A lunar cycler (or Earth-Moon cycler) is a hypothetical spacecraft on a special trajectory that encounters the Earth and the Moon on a regular basis, typically every 9 or 14 days. Cyclers are potentially useful for transporting people or materials using little propellant. Instead, they rely on gravity assist manoeuvres to keep them going.