Thrusters are small rocket engines that raise a satellite's orbit, keep it in its slot, turn it and finally move it out of the way at end of life.
A satellite carries its own small rocket engines. They push by throwing gas out one way so that the satellite moves the other way.
Most satellites have two kinds. A single big engine for large changes of orbit, and a set of small thrusters for fine adjustments and for turning.
Rockets often leave a satellite in a temporary, stretched orbit called a transfer orbit. The satellite must finish the climb itself with a series of engine burns. This is called orbit-raising.
ISRO's IRNSS-1A shows the usual layout: one 440 N liquid apogee motor for orbit-raising and twelve 22 N thrusters. GSAT-19 reached its geostationary orbit by firing its liquid apogee motor in stages.
Worked example: a 440 N motor pushing a 1,425 kg IRNSS satellite gives an acceleration of 440 ÷ 1,425 ≈ 0.31 m/s². That is gentle — about a thirtieth of the pull of gravity on the ground — but an hour of firing adds more than 1 km/s of speed.
Once in place, thrusters make small corrections to hold the satellite in its slot. This is station-keeping. They also help turn the satellite and unload its reaction wheels.
They can save a mission, too. In January 2017 the eight 22 N thrusters on ISRO's Mars Orbiter Mission fired for 431 seconds to shift its orbit away from an 8-hour eclipse.
ISRO's NVS-02 navigation satellite was placed in its transfer orbit by GSLV-F15 on 29 January 2025. Its solar panels opened and it talked to the ground. But ISRO reported that the valves admitting oxidiser to the orbit-raising engine did not open.
ISRO's Apex Committee later found that the electrical signal never reached the oxidiser-line pyro valve, most likely because a connector contact came loose in both the main and backup paths. NVS-02 was left in a 170 × 37,785 km orbit instead of its planned circular one. The fix was flown on CMS-03 in November 2025, whose pyro systems worked.
Find NVS-02 in the satellite list: its altitude swings between a few hundred and tens of thousands of kilometres, because its engine never fired to circularise the orbit. Open the NavIC tracker
A thruster's push is
F = ṁ × v_e
where ṁ is the mass of propellant expelled per second and v_e is the exhaust speed. The change in velocity a burn gives is set by the rocket equation:
Δv = v_e × ln(m_start ÷ m_end)
For small burns this is close to Δv ≈ F × t ÷ m.
Check with ISRO's MOM numbers. Eight thrusters of 22 N rated thrust, 431 s: total impulse ≤ 8 × 22 × 431 ≈ 75,900 N·s. ISRO reports Δv = 97.5 m/s. Dividing, the implied spacecraft mass would be at most 75,900 ÷ 97.5 ≈ 780 kg if every thruster gave its full rated thrust for the whole burn. ISRO does not give the mass, so treat this only as a rough upper bound.
Why two engine sizes? Big burns need high thrust so they finish near the right point in the orbit. A 440 N apogee motor on a 1,425 kg satellite such as IRNSS-1A gives an acceleration of 440 ÷ 1,425 ≈ 0.31 m/s², or about 1.1 km/s of Δv per hour of firing. Fine control needs small, repeatable pulses, which is what 22 N thrusters provide.
NVS-02 shows the failure mode of a bipropellant engine. Fuel and oxidiser must both flow. ISRO's original update said the oxidiser valves did not open; the Apex Committee traced it to the pyro valve's firing signal. Without oxidiser there is no combustion, and so no Δv.
Small thruster firings that keep a satellite in its assigned orbit or slot, correcting the slow drift caused by natural forces.
The valves that should have let oxidiser into its orbit-raising engine did not open, so the engine could not fire to circularise the orbit.
The 440 N liquid apogee motor gives large velocity changes for orbit-raising; the small 22 N thrusters give fine corrections and attitude control.