A halo orbit is a large looping path around a Lagrange point, letting a spacecraft stay near L1 or L2 without sitting exactly on the point.
Spacecraft at L1 or L2 do not park on the point itself. They loop around it on a big, slightly tilted path. Seen from Earth, the loop looks like a ring floating around the point — a halo. ESA notes that spacecraft at L1 and L2 fly halo orbits or a looser kind of loop called a Lissajous orbit.
NASA points out that this is not an orbit in the usual sense. There is nothing at the Lagrange point to pull on the spacecraft. In truth the spacecraft is orbiting the Sun, taking one year, just like Earth; the halo is how its path looks relative to L1.
Aditya-L1 entered its halo orbit around the Sun–Earth L1 point on 6 January 2024, about 1.5 million km from Earth. One lap takes about 178 days. ISRO gives the orbit's size as about 209,200 km along one axis, 663,200 km along another and 120,000 km along the third.
Aditya-L1 completed its first lap on 2 July 2024. ISRO chose the orbit to support a five-year mission with as few correction burns as possible, saving fuel.
That size is enormous. The longest semi-axis, 663,200 km, is about 1.7 times the Moon's average distance from Earth.
ISRO lists the advantages. From its halo orbit Aditya-L1 can view the Sun continuously and keep steady contact with ground stations. It sits outside Earth's magnetosphere, so it can sample the solar wind directly. And its speed relative to the Sun changes smoothly, which helps studies of the Sun's vibrations, called helioseismology.
Halo orbits around L1 are unstable: left alone, a spacecraft drifts off. So it fires small thrusters every few months. Aditya-L1 made station-keeping burns on 22 February, 7 June and 2 July 2024; the last one put it back on the planned path just as it completed its first halo.
Look for the 6 January 2024 halo-orbit insertion on Aditya-L1's mission timeline; the site's globe does not show it, because SGP4 only models Earth orbits. Follow Aditya-L1
In the rotating frame that keeps the Sun and Earth fixed, L1 is an equilibrium point, but an unstable one. Linearising the motion around it gives an in-plane oscillation, an out-of-plane oscillation and an exponential drift. A halo orbit is a special periodic solution in which the in-plane and out-of-plane frequencies are locked together, so the path closes on itself. Without that lock you get a Lissajous figure that never quite repeats.
ISRO's numbers for Aditya-L1 describe the halo by its semi-axes: A_x = 209,200 km (along the Sun–Earth line), A_y = 663,200 km (sideways, in the plane of Earth's orbit) and A_z = 120,000 km (out of that plane).
Worked comparisons:
The instability sets the maintenance schedule. NASA notes that L1 and L2 are unstable on a timescale of about 23 days, so small errors grow, and station-keeping burns are needed regularly. NASA also notes that most station-keeping manoeuvres happen near the far ends of the halo. Aditya-L1 needed three burns in its first lap, about six months.
There is no mass at the Lagrange point to pull the spacecraft round. The spacecraft actually orbits the Sun; the halo is only its path as seen relative to L1.
About 178 days. It completed its first one on 2 July 2024.
Orbits around L1 are unstable, so small errors grow and the spacecraft would drift away without regular corrections.