Space & Rocket Glossary

Short, plain-English meanings of the technical words used in our ISRO mission and rocket stories — from burns and stages to transfer orbits and Lagrange points. Each term links to a deeper guide where we have one.

Apogee and perigee: Perigee is the point of an orbit closest to Earth; apogee is the farthest. Burns at perigee are used to raise apogee, and the other way round. What is an orbit?
Atomic clock: An extremely precise clock. Navigation satellites depend on them: a timing error of a millionth of a second puts your position off by about 300 metres.
Burn: Firing a rocket or spacecraft engine for a set time to change its speed — and therefore its orbit. Speeding up raises the orbit; slowing down lowers it. What is an orbit?
Crew escape system: Small, powerful rockets on top of a crew capsule that can pull it away from a failing launcher within seconds, then let it parachute down.
Cryogenic engine: An engine that burns liquid hydrogen (at about −253 °C) and liquid oxygen (about −183 °C). It gets more push per kilogram than other fuels, which upper stages need for heavy satellites — but it is one of the hardest rocket technologies to build.
Docking: Two spacecraft meeting in orbit and locking together. It is needed to build space stations, refuel spacecraft and bring samples back from the Moon.
Geostationary and geosynchronous orbit: An orbit about 36,000 km up that takes exactly one day, matching Earth's spin. A geostationary satellite sits over the equator and seems fixed in the sky; a geosynchronous one may drift north and south in a figure-of-eight. What is an orbit?
Geosynchronous transfer orbit (GTO): A long oval orbit, low at one end and about 36,000 km at the other. Rockets drop communication satellites here; the satellite then fires its own engine at the high point to circle at that height. What is an orbit?
Halo orbit: A large looping path around a Lagrange point. The spacecraft circles the empty balance point rather than sitting on it, which keeps it in sunlight and in contact with Earth.
HLVM3 (human-rated LVM3): The version of ISRO's LVM3 rocket made safe enough to carry people, with extra safety margins, health monitoring and a crew escape system. It will launch Gaganyaan. Gaganyaan launch tracker
Lagrange point (L1): A spot where the gravity of two large bodies balances for a small spacecraft. Sun–Earth L1 is 1.5 million km from Earth towards the Sun, with a non-stop view of the Sun.
Low Earth orbit (LEO): Orbits from about 160 to 2,000 km above Earth. Satellites here circle the planet in roughly 90 minutes — the ISS, Starlink and most imaging satellites live here. What is an orbit?
NavIC (IRNSS) and the NVS satellites: India's own satellite navigation system, developed and operated by ISRO — an alternative to GPS over India and 1,500 km around it. The first-generation satellites are named IRNSS-1A to 1I; the second generation are NVS-01, NVS-02 and NVS-03. NavIC satellites and status
Orbit insertion: The burn on arrival that slows a spacecraft enough for its destination's gravity to capture it into orbit. Miss it, and the spacecraft flies past.
Orbit raising: A series of burns, usually at perigee, that stretch an orbit further out step by step. ISRO uses it to send spacecraft to the Moon, Mars and L1 on rockets too small to send them there directly.
Payload: Whatever a rocket carries — satellites, a spacecraft, experiments. On a spacecraft, "payloads" are its science instruments.
Payload fairing: The nose cone that protects the satellite from air friction during the climb through the atmosphere, then splits open and falls away once the rocket is high enough.
Propellant (solid and liquid): What a rocket burns: a fuel plus an oxidiser that supplies the oxygen. Solid propellant is a rubbery block cast inside the motor and can't be switched off; liquid propellant is pumped from tanks and can be throttled or restarted.
Propulsion module: The part of a spacecraft that carries its main engine and fuel. On Chandrayaan-3 it carried the lander to the Moon, then was sent back towards Earth.
Range safety: The team and on-board system that destroy a rocket deliberately if it veers off course, so it can't fall on populated areas.
Re-entry: Coming back into Earth's atmosphere from space. Friction heats the object to thousands of degrees, so anything meant to survive needs a heat shield.
Rocket stage: A section of a rocket with its own engine and fuel. When its fuel runs out it drops away, so the rest of the rocket is lighter and can go faster. PSLV has four stages.
Sounding rocket: A small rocket that flies straight up into the upper atmosphere or space and falls back, without reaching orbit. Used for quick science experiments.
Sphere of influence: The region around Earth, out to about 925,000 km, where Earth's gravity matters more than the Sun's. Leaving it means a spacecraft is truly travelling around the Sun.
Station-keeping: Small, regular burns that keep a spacecraft in its planned orbit against the tugs of gravity and sunlight.
Strap-on booster: Extra rockets attached to the sides of a launcher to add thrust at lift-off. They fall away once empty. PSLV-XL has six; LVM3 has two very large ones.
Sun-synchronous orbit: A near-polar orbit timed so the satellite passes over any place at the same local time every day. Lighting stays consistent, which makes images easy to compare — so most imaging satellites use it. What is an orbit?
Trajectory correction: A small burn during a long cruise that fine-tunes a spacecraft's path so it arrives at exactly the right place and time.
Trans-lunar / trans-Mars injection: The burn that gives a spacecraft enough speed to leave Earth orbit and head for its destination — the Moon, Mars or a Lagrange point.
Turbopump: A turbine-driven pump that forces propellant into the engine at very high pressure. If it stops, the engine stops.
Vikas engine: ISRO's workhorse liquid-fuel engine, named after Vikram Sarabhai. It powers PSLV's second stage, GSLV's strap-ons and second stage, and LVM3's core.
Vyommitra: A half-humanoid robot built by ISRO to fly in the crew seat on Gaganyaan's uncrewed test flights. It operates panels and reports cabin conditions so the life-support systems are proven before astronauts fly. Gaganyaan launch tracker