Probes in a Lissajous Orbit
There are a few probes which are kind of in an Earth orbit, but not really. This type of orbit is called a Lissajous orbit. It is a path around the L1 or L2 Lagrangian points of a two-body system. Lissajous orbits are utilized by certain spacecraft that are required to be in a stable position relative to the Earth and Sun while making long-term observations. In a Lissajous orbit the spacecraft follows a natural (but complex) motion that requires the minimum amount of energy for station-keeping, unlike a halo orbit, in which the craft follows a simple circular or elliptical path.
(USA) 1 November 1994. Built to study radio and plasma that
occur in the solar wind..
- Advanced Composition Explorer (ACE) (USA)
25 August 1997. ACE studies matter comprised of energetic particls, such as
those found in the solar wind.
- Wilkinson Microwave Anisotropy Probe (WMAP) (USA) 30 June 2001.
- Herschel Space Observatory (ESA) 14 May 2009. Launched along
with the Planck spacecraft. This is an infrared telescope looking for the
coldest and dustiest objects in space.
- Planck (ESA) 14 May 2009. Launched along with the Herschel
Space Observatory. Built to study cosmic wave background.
- LISA Pathfinder (ESA) 3 December 2015. Bound for L1, the Laser Interferometer Space Antenna is an experimental spacecraft designed to test technologies for the detection and measurement of gravitational waves.
Lagrange points are locations in space where gravitational forces and the orbital motion of a body balance each other. They were discovered by French mathematician Louis Lagrange in 1772 in his gravitational studies of the ‘Three body problem’: how a third, small body would orbit around two orbiting large ones.
There are five Lagrangian points in the Sun-Earth system and such points also exist in the Earth-Moon system. L1 and L2 in the Sun-Earth system are stable and most used.
L1 is located about 1.5 million km from Earth, between the sun and Earth, along a line from the sun to Earth. L2 is located about 1.5 million km from Earth along the same line, but on the other side of Earth.