Satellite Orbits: LEO, MEO, GEO and Sun-Synchronous

Orbital Speed, Escape Speed and Weightlessness

A satellite is falling all the time; it just never lands. Fired sideways at about 7.9 km/s, a body drops exactly as fast as the curved ground falls away beneath it. Gravity supplies the centripetal force of circular motion, and equating the two gives everything else.

Speed falls with height because gravity weakens as 1/r² (see our page on gravity and the variation of g), and a weaker pull can only bend a slower path into a circle. The satellite's mass cancels: a 1 kg cubesat and the ISS at one height move equally fast.

Escape speed is the least launch speed with which a body never returns (ignoring air). From ½mv² = GMm/R, v_e = √(2GM/R) = √(2gR) ≈ 11.2 km/s, exactly √2 times the low-orbit speed. It does not depend on the body's mass or launch direction. Putting M = (4/3)πR³ρ gives v_e ∝ R√ρ. The Moon's escape speed, about 2.4 km/s, is too low to keep an atmosphere.

Weightlessness. At 400 km, g is still about 8.7 m/s² (89% of its surface value). Astronauts float because they and the station fall together, so the floor pushes on no one: the normal reaction is zero. A pendulum clock stops in orbit.

Kepler's Three Laws and the Period Sums

Newton showed that Kepler's planetary laws follow from inverse-square gravity, so satellites obey them too.

LEO, MEO, GEO and the Transfer Orbit

Orbits are grouped by height.

Geosynchronous versus geostationary. Any orbit with a period of one sidereal day (23 h 56 min 4 s) is geosynchronous. Not 24 hours: while spinning once, the Earth also moves about 1° along its orbit and needs about 4 minutes more to face the Sun again; one true rotation is 23 h 56 min. A geostationary orbit is the special case that is circular, over the equator and west to east, so the satellite seems fixed. An inclined geosynchronous satellite returns to the same longitude daily but swings north and south in a figure of eight. Arthur C. Clarke proposed geostationary relays in 1945; three satellites 120° apart cover the whole Earth except the polar regions.

The transfer orbit. Indian rockets do not place satellites directly in GEO. The GSLV and LVM3 release them into a GTO. At apogee the satellite fires its own liquid apogee motor, over several days, to round off the orbit and remove its tilt. If that fails, it is stranded: in February 2025 the oxidiser valves of NVS-02's engine did not open, leaving it in its transfer orbit. An eastward launch from Sriharikota (about 13.7° N) also borrows roughly 0.45 km/s from the Earth's spin.

Polar and Sun-Synchronous Orbits, and the Lagrange Points

A polar orbit is tilted about 90° to the equator and crosses both poles, usually in LEO. As the Earth turns beneath it, each pass covers a new strip, so it sees the whole globe within days.

A sun-synchronous orbit (SSO) is a near-polar orbit at about 600–800 km, tilted at about 98°. The Earth's equatorial bulge makes the orbit's plane turn slowly; at this height and tilt it turns about 1° a day eastward, keeping pace with the Earth's yearly circuit of the Sun. So the satellite crosses each latitude at the same local solar time every day. Identical lighting makes images taken weeks apart directly comparable. Cartosat, Resourcesat and Oceansat use it.

At the five Lagrange points, Sun and Earth together let a spacecraft circle the Sun in step with the Earth.

Aditya-L1 was launched by PSLV-C57 on 2 September 2023 and entered a halo orbit around L1 on 6 January 2024. From there the Sun is never eclipsed.

Matching Orbit to Job, and Rocket to Orbit

Each job has a best orbit.

EOS-05, imaging from geosynchronous orbit, trades sharpness for a constant watch (see GSLV-F17 and geo-imaging). Crowded low orbits now force ISRO into routine debris-avoidance manoeuvres.

GPS versus NavIC. GPS spreads at least 24 satellites over six planes tilted at 55°, about 20,200 km up, so at least four, the minimum a receiver needs (three for position, one to correct its clock), are always in view. NavIC serves only India and about 1,500 km around it, so ISRO chose orbits that never leave the region: three geostationary satellites and four geosynchronous ones inclined at 29°, whose north–south swing improves the geometry of a fix. Seven satellites do what a global system needs 24 for. More in our article on NavIC and satellite navigation.

All fly from Sriharikota; the cryogenic stage is what lets GSLV and LVM3 lift heavy loads to GTO.

How UPSC Has Asked It: Past Papers Solved

Actual questions since 2020: NDA leans on sums and concepts, CDS on missions.

Practice MCQs (with answers)

1. Which ISRO launcher can place a 4,000 kg communication satellite in transfer orbit?
   1. LVM3
   2. PSLV
   3. SSLV
   4. GSLV Mk II
2. If a satellite's orbital radius becomes 9 times larger, its period becomes
   1. 3 times
   2. 9 times
   3. 27 times
   4. 81 times
3. A geostationary satellite must orbit
   1. over the poles, every 12 hours
   2. over the equator, west to east, once per sidereal day
   3. at any inclination, every 24 hours
   4. over the equator, east to west, once per sidereal day
4. NavIC differs from GPS in that it
   1. uses only medium Earth orbit
   2. needs just three satellites for a fix
   3. is only for military users
   4. is regional, using geostationary and inclined geosynchronous satellites
5. Imaging satellites use sun-synchronous orbits mainly because these
   1. stay above one point
   2. need no rocket
   3. pass each place at the same local time
   4. are the fastest
6. Which Sun–Earth Lagrange point is stable?
   1. L4
   2. L1
   3. L2
   4. L3
7. A satellite's speed in circular orbit suddenly rises by about 41.4%. It will
   1. fall to the Earth
   2. drop to a lower orbit
   3. stay in the same orbit
   4. escape the Earth's gravity
8. A planet has the Earth's density but twice its radius. Its escape speed is about
   1. 5.6 km/s
   2. 22.4 km/s
   3. 11.2 km/s
   4. 15.8 km/s

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