Halo orbit
An orbit that loops widely around a Lagrange point. It is used to keep a spacecraft stationed near the point.
A spacecraft placed exactly at the Lagrange point between the Sun and Earth would sit in front of the Sun as seen from Earth, and the Sun's radio noise would drown out its signal. Robert Farquhar showed that an orbit circling the Lagrange point could be used instead[1]. Seen from Earth it traces a ring, a halo, which gave the orbit its name.
The orbit is unstable: left alone, the spacecraft drifts away. Correcting the drift, however, takes very little fuel. ISEE-3 was the first to fly one, in 1978; today the solar observatory SOHO and the James Webb Space Telescope circle their respective Lagrange points. Satellites that relay communications with the far side of the Moon use orbits of the same kind.
Related terms
- Lagrange pointFive points where the gravity of two bodies, such as the Sun and Earth, balances the motion of orbiting around them, so a small object can hold its position.
- Three-body problemThe problem of finding how three bodies move under their mutual gravity. Two can be solved with a formula; three in general cannot, and tiny differences can change the future completely.