ISROpedia

Imagine throwing a ball so fast that by the time it falls toward the ground, the Earth's surface curves away. The ball keeps falling — but it never hits. That's an orbit. A satellite is always falling toward Earth, just fast enough that it keeps missing.

Most satellites move across the sky. But if you put a satellite at exactly 36,000 km altitude, it orbits once in 24 hours — the same time Earth rotates. To someone on the ground, it appears to hover in one spot. That's how a satellite TV dish points at a fixed point in the sky.

Between any two massive objects — like the Sun and Earth — there are five special spots where the gravitational pulls balance perfectly. A spacecraft parked at one of these spots barely needs any fuel to stay there. Aditya-L1 is at the Sun-Earth L1 point — 1.5 million km from Earth, always facing the Sun.

Rocket engines burn fuel with oxygen. But carrying oxygen as a gas wastes space — it needs to be compressed 800x. The solution: cool it until it liquefies. Liquid oxygen is −183°C. Liquid hydrogen is −253°C. "Cryogenic" means extremely cold. India's CE-20 engine on LVM3 uses both — and India mastered this technology itself after it was denied to us.

Getting from one orbit to another in space is not like driving — you can't just turn. The most fuel-efficient way to move between orbits is an elliptical "bridge" orbit. You fire your engine once to enter the ellipse, coast halfway around the solar system, then fire again to enter the destination orbit. Mangalyaan used this to reach Mars — but with an additional Indian trick: instead of a single direct Trans-Mars Injection from low orbit, ISRO first spent 25 days doing gradual Earth orbit-raising burns, climbing to a very high Earth orbit before the final departure burn. This "slingshot" approach — using multiple smaller burns instead of one massive one — works around the size limitation of India's rockets and is a key reason ISRO missions cost a fraction of equivalent NASA missions.

A spacecraft approaching a planet is like a tennis ball bouncing off a racket. If the planet is moving, the spacecraft gains or loses speed by interacting with the planet's gravity. Mars Orbiter Mission (Mangalyaan) used Earth's gravity to gain extra velocity for free — without using rocket fuel. The spacecraft swung around Earth just above the atmosphere, got "flung" by Earth's gravity, and exited with higher energy. This is why Mangalyaan used gradual Earth orbit-raising burns: each raise brought it closer to the Sun, and the final swing-by released maximum velocity toward Mars.

When a spacecraft arrives at another planet, it's travelling very fast. Slowing down with engines uses enormous amounts of fuel. Instead, you can graze the planet's upper atmosphere — the thin air creates drag and slows you down over many orbits, like a feather gradually settling. Shukrayaan will use this at Venus. It's free deceleration — but one miscalculation means burning up.

Two spacecraft meeting in orbit are both travelling at 7.8 km/s. For one to attach to the other, it must match speed, direction, and position exactly — then approach at a few centimetres per second. The final approach is fully autonomous — signals from Earth are too slow. SpaDeX proved India can do this. It is the technology that makes space stations possible.

Most Earth observation satellites use a special orbit where they always cross the equator at the same local time of day — so the lighting on the ground is consistent in every image. This is a Sun-Synchronous Orbit (SSO). Resourcesat, Cartosat, and Oceansat all use SSO — it means their images of India are always taken at the same sun angle, making comparisons across months and years meaningful.

Geostationary satellites sit 36,000 km above the equator. If you put two satellites at the same longitude, they interfere with each other — their signals jam. So the International Telecommunications Union allocates specific "slots" — positions along the geostationary arc — to each country. India owns eight GEO slots. Each slot must be carefully managed: satellites drift over time, so periodic "station-keeping" maneuvers nudge them back into their assigned slot before they drift 0.1° out of position. If India lets a satellite drift into another country's slot, international incidents can happen.