Aditya-L1: India’s Solar Observatory at Sun–Earth L1
Why L1, the Earth-bound phase, trans-L1 injection and cruise, halo-orbit insertion, and the observatory at work
6 January 2024, about 16:00 IST
For 109 days Aditya-L1 had been coasting away from the Earth, its path trimmed twice by small correction burns. On the afternoon of 6 January, as it crossed the – plane of the rotating Sun–Earth frame 1.5 million kilometres from home, ISRO's controllers commanded a short burn of its control engines.
The burn had a precise job. It cancelled the spacecraft's velocity along the Sun–Earth line and perpendicular to the orbital plane, and set the one component a halo orbit has at that crossing. At about 16:00 IST ISRO announced that Aditya-L1 was in its halo orbit around Sun–Earth L1 — a periodic orbit with a period of about 177.86 days. Without the burn, ISRO noted, the spacecraft would have kept drifting away.
This lesson follows the mission through every step of this unit, using ISRO's own numbers.
💡Why L1
Aditya-L1 — Aditya is a Sanskrit name for the Sun — carries seven instruments. Four look at the Sun: the VELC coronagraph, the SUIT ultraviolet imaging telescope, and the SoLEXS and HEL1OS X-ray spectrometers. Three sample the space around the spacecraft: ASPEX and PAPA measure the solar-wind particles, and a magnetometer on a 6-metre boom measures the interplanetary magnetic field.
ISRO's reasons for L1 are the reasons of this unit:
- an unbroken view of the Sun, with no eclipses and no occultations;
- a place outside the Earth's magnetosphere, in the undisturbed solar wind;
- continuous communication with ground stations.
📐Worked Example: the view from the halo
The third lab flies one revolution of the computed halo:
- the distance from the Earth varies from 1,256,006 to 1,674,562 km;
- the spacecraft is always sunward of the Earth, so the Earth's shadow never reaches it: no eclipses;
- seen from a ground station it stays between 4.5° and 25.8° from the Sun — never in front of the solar disk (0.27° in radius), where the Sun's radio noise would drown the signal;
- solar wind at 400 km/s passes Aditya-L1 63 minutes before it reaches the Earth; a fast coronal mass ejection at 2,000 km/s, 13 minutes before.
| Event | Date | Perigee × apogee altitude | Period | Burn (approx.) |
|---|---|---|---|---|
| PSLV-C57 injection | 2 Sep 2023 | 235.6 × 19,502.7 km | 5.7 h | — |
| Earth-bound burn 1 | 3 Sep 2023 | 245 × 22,459 km | 6.5 h | 104 m/s |
| Earth-bound burn 2 | 5 Sep 2023 | 282 × 40,225 km | 12.0 h | 374 m/s |
| Earth-bound burn 3 | 10 Sep 2023 | 296 × 71,767 km | 24.1 h | 269 m/s |
| Earth-bound burn 4 | 15 Sep 2023 | 256 × 121,973 km | 48.5 h | 165 m/s |
The PSLV-C57, in its XL configuration, put the 1,480.7 kg spacecraft into an orbit inclined 19.2° to the equator. From there the spacecraft's own liquid apogee motor took over. Each burn was made near perigee, where the spacecraft is fastest and a burn raises the apogee most for the least propellant — the Oberth effect of the lesson on bi-elliptic transfers. The four Earth-bound burns add up to about 912 m/s. ISRO's strategy also kept the number of perigee passes small, limiting the time spent crossing the Van Allen radiation belts. The last of these orbits reached a third of the way to the Moon and took two days to go round.
💡Trans-L1 injection and the cruise
The fifth burn, the trans-Lagrangian point 1 insertion on 19 September, sent the spacecraft out. A two-body estimate (second lab) puts it at about 249 m/s at perigee, raising the apogee from 121,973 km to the distance of L1.
The two-body model is good for the first days and then fails, exactly as this unit predicts:
- it says the spacecraft passes the edge of the Earth's sphere of influence (929,000 km) 10.1 days after perigee; ISRO announced the exit on 30 September, 11 days after the burn;
- it says the apogee near L1 is reached after 37.6 days; Aditya-L1 arrived after about 109 days. As the spacecraft approaches L1 the Sun's pull almost cancels the Earth's, and it creeps the last stretch.
⚠A modelling trap the lab exposes
The Sun–Earth restricted problem usually puts the Moon's mass into the Earth's, because for locating L1 and computing the halo it is the Earth–Moon pair that orbits the Sun. For the departure from a 256 km perigee that is badly wrong: the same burn, with the Moon's mass added to the Earth's, gives an apogee of 395,258 km instead of 1,497,621 km. Near the Earth the Moon is a separate body. Every model has a domain; know which one you are in.
💡Halo-orbit insertion
ISRO describes the insertion as a burn made as the spacecraft crossed the – plane, cancelling the and velocity components and setting the component the halo needs. The second lab shows why that point is chosen. On the computed Aditya halo the two crossings are:
- on the sunward side, 171,701 km from L1 towards the Sun and 132,369 km out of the plane, with velocity (0, +266.9, 0) m/s;
- on the Earthward side, 246,126 km from L1 towards the Earth and 106,348 km below the plane, with velocity (0, −292.4, 0) m/s.
There are some who question the relevance of space activities in a developing nation. To us, there is no ambiguity of purpose.
🛰The first revolution
On 2 July 2024 ISRO announced that Aditya-L1 had completed its first revolution of the halo, after station-keeping burns on 22 February and 7 June 2024; a third burn that day sent it on into its second revolution. The orbit was designed for a mission life of five years with as few station-keeping burns, and as little propellant, as possible. Aditya-L1 joined the observatories that have watched the Sun from Sun–Earth L1 since ISEE-3 in 1978 — SOHO, ACE and DSCOVR among them.
↓But there is a problem
Everything in this unit belonged to the Sun and the Earth. The same mathematics governs every place where two bodies share control of a third — and nowhere more than at the giant planets, where a spacecraft orbiting a moon is always pulled by the planet too.
But first a spacecraft has to get there. Jupiter is five times as far from the Sun as the Earth; Neptune, thirty. In 1965 a graduate student at JPL noticed that, for a few years in the late 1970s, the outer planets would line up in a way that happens only once every 175 years.