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From Earth, the Sun disappears every night and is often hidden by clouds or blurred by the atmosphere. To watch it all the time, India sent Aditya-L1 to a special point in space about 1.5 million km away. This post explains what the mission studies, why that point exists, and how to calculate where it is using the gravitation chapter you already study.
What is Aditya-L1?
ISRO's Aditya-L1 page calls it the first space-based Indian mission to study the Sun. It was launched by PSLV-C57 on 2 September 2023 and inserted into a halo orbit around the Sun-Earth Lagrange point 1 (L1) on 6 January 2024. ISRO notes that a satellite in a halo orbit around L1 can view the Sun continuously without any occultation or eclipses, which lets scientists watch solar activity and its effect on space weather in real time.
It carries seven payloads. Four look at the Sun directly: the Visible Emission Line Coronagraph (VELC), the Solar Ultraviolet Imaging Telescope (SUIT), and two X-ray spectrometers, SoLEXS and HEL1OS. Three measure the particles and magnetic field around the spacecraft: ASPEX, PAPA and a magnetometer. Their targets include coronal heating, coronal mass ejections and flares.
What is a Lagrange point?
NASA's explainer gives the precise definition: Lagrange points are positions where the gravitational pull of two large masses precisely equals the centripetal force required for a small object to move with them. There are five for the Sun-Earth system. L1 lies on the line between the Sun and Earth.
Here is the puzzle L1 solves. A body closer to the Sun than Earth should orbit faster, completing a lap in less than a year. At L1, Earth's pull partly cancels the Sun's, so the net pull is just right for a one-year orbit. The spacecraft then keeps pace with Earth, always between Earth and the Sun.
Worked example: how far away is L1?
Let the Sun's mass be M, Earth's mass m, the Earth-Sun distance R, and the distance from Earth to L1 r. A body at L1 orbits the Sun at radius (R − r) with Earth's angular speed ω, where ω² = GM/R³.
Step 1: the balance. Net inward pull = required centripetal force (per kg): GM/(R − r)² − Gm/r² = ω²(R − r) = GM(R − r)/R³.
Step 2: approximate for small r. Using (1 − r/R)⁻² ≈ 1 + 2r/R, the left term is about (GM/R²)(1 + 2r/R). The right side is (GM/R²)(1 − r/R). Subtracting gives (GM/R²)(3r/R) = Gm/r², so r³ = (m/3M)R³, and
r = R × (m/3M)^(1/3).
Step 3: numbers. NASA's Sun fact sheet gives the Sun-to-Earth mass ratio as 332,900, so m/3M = 1 ÷ 998,700 = 1.001 × 10⁻⁶. Its cube root is 0.0100. With R = 149.598 million km from NASA's Earth fact sheet, r = 0.0100 × 149.598 million km = 1.50 million km.
ISRO's mission details say exactly this: about 1.5 million km from Earth, about 1% of the Earth-Sun distance.
Step 4: how much warning does it give? Light from L1 reaches Earth in 1.5 × 10⁹ m ÷ 3.0 × 10⁸ m/s = 5 s. A cloud of solar particles is far slower. If one moved at, say, 500 km/s, it would take 1.5 × 10⁶ km ÷ 500 km/s = 3,000 s, or 50 minutes, to travel from L1 to Earth. That is why ISRO stresses that L1 lets the satellite sample solar radiation and magnetic storms before they are influenced by Earth's magnetic field and atmosphere.
How did Aditya-L1 get there?
ISRO's mission updates record four Earth-bound manoeuvres in September 2023, each raising the orbit, ending in a 256 km by 121,973 km orbit on 15 September. The Trans-Lagrangean Point 1 Insertion burn on 19 September sent it towards L1, and on 30 September ISRO said it had escaped Earth's sphere of influence. The mission details page describes the cruise as about 110 days. At L1 it flies an irregular halo orbit in a plane roughly perpendicular to the Earth-Sun line. NASA's explainer notes that L1, L2 and L3 are unstable, which is one reason spacecraft orbit around L1 rather than sitting on it.
What mistakes do students make with L1?
Equating the two pulls. Setting GM/(R − r)² = Gm/r² gives the equal-pull point, r = R√(m/M) ÷ (1 + √(m/M)) ≈ 259,000 km. That ignores the orbit, as the callout says.
Thinking Aditya-L1 flies to the Sun. ISRO is explicit that it will neither land on the Sun nor approach it any closer.
Mixing km and m. Keep R and r in the same unit; the mass ratio has none.
For orbit and energy practice, see our guide to gravitation and satellite problems. For another ISRO mission worked through with physics, read the science of Chandrayaan-3.
What can a student take from the mission?
Aditya-L1 is a reminder that position is a choice. By choosing where to put the observatory, ISRO removed nights, eclipses and the atmosphere from the problem at once. In your own problem solving, a smart choice of frame or origin often does the same.
If you want to see how Earth's pull and the Sun's pull combine along the Sun-Earth line, the Fermi Simulation Lab has interactive simulations for ideas easier to see than read.
Use the same formula with the Earth-Moon mass ratio of about 81 (from the 0.0123 Moon/Earth ratio on NASA's Moon fact sheet) to find the Earth-Moon L1 point, and check whether your answer is closer to the Moon or to Earth.
