An orbit in which a spacecraft will remain in the vicinity of a Lagrangian point. A halo orbit actually takes the form of a circular or elliptical loop around the Lagrangian point. The first spacecraft to use a halo orbit was the Solar and Heliospheric Observatory (SOHO), which was placed in an elliptical orbit with a semimajor axis of about 650 000 km around the L₁ Lagrangian point between the Earth and Sun.