Types of Satellite Orbits: LEO, MEO, Geostationary, Polar and Sun-Synchronous

Orbit Basics: Gravity, Kepler's Laws and Orbital Velocity

Why a Satellite Does Not Fall Down

A satellite does not fall down because it is falling all the time. Isaac Newton's thought experiment shows why. A stone thrown from a high mountain, with no air to slow it, lands farther away the harder it is thrown. Thrown at the right speed, the Earth's surface curves away beneath it as fast as it drops, so it keeps circling and comes back to the mountain.

Take a circular orbit at a distance r from the Earth's centre, where r is the Earth's radius plus the height of the satellite. Gravity supplies exactly the centripetal force the satellite needs to keep turning, and equating the two forces gives the orbital speed, v = √(GM/r), where G is the gravitational constant and M the Earth's mass. Two results follow. The speed falls as the height rises, so a low satellite moves fast and a high one moves slowly. And the speed does not depend on the satellite's own mass.

Weightlessness in a satellite has the same cause. An astronaut floats not because gravity is weak up there, but because the astronaut and the spacecraft are both in free fall towards the Earth.

Two orbital speeds are worth remembering:

• Orbital speed in low Earth orbit: About 7.8 km per second.
• Orbital speed at geostationary height: About 3.07 km per second.

Kepler's Laws Applied to Satellites

Kepler's laws were framed for planets going round the Sun, and they apply equally to satellites going round the Earth. The first law says orbits are ellipses with the central body at one focus; a circle is the special case in which the two foci merge. The second law says the line joining the satellite to the Earth sweeps out equal areas in equal times, so the satellite moves fastest when nearest the Earth and slowest when farthest. The third law says the square of the period is proportional to the cube of the semi-major axis.

For a circular orbit the third law takes a simple form, T² = (4π²/GM) r³, where T is the period and r the distance from the Earth's centre. The same relation holds for an elliptical orbit if r is replaced by the semi-major axis, with the Earth at one focus. Put in words, a higher orbit has a longer period, and doubling the radius of a circular orbit multiplies the period by about 2.8.

The law also fixes the shortest possible period. For a satellite skimming the surface, its height is negligible beside the Earth's radius, and the period works out to about 85 minutes. No Earth satellite can go round faster than this. The Moon, the Earth's only natural satellite, is far out and takes about 27.3 days.

Orbital Speed, Period and Escape Speed

Height sets both the speed and the period of an orbit.

Escape speed, often called escape velocity, is a different quantity from orbital speed. It is the least speed with which an object must be thrown up from the Earth's surface to leave the Earth's gravitational pull for good. For the Earth it is about 11.2 km per second; for the Moon it is only 2.3 km per second, about one fifth as much, which is why the Moon holds no atmosphere, since gas molecules moving faster than that escape.

A satellite stays bound because its total energy is negative; if an orbit's energy rose to zero, the object would escape to infinity. A mission that has to leave the Earth's pull altogether therefore needs enough speed to raise its energy to zero.

Altitude, Eccentricity and Inclination of an Orbit

Three things fix a satellite's path and what it can see.

• Height (altitude): The distance of the satellite above the surface. It sets the orbital speed and the period, and it sorts orbits into low, medium and high classes.
• Shape (eccentricity): The eccentricity is 0 for a circle and close to but always less than 1 for a long ellipse. The satellite moves around one focus of the ellipse, not its centre. In an elliptical orbit round the Earth, the nearest point is the perigee and the farthest is the apogee.
• Tilt (inclination): The angle between the orbit's plane and the equator. At 0 degrees the orbit lies directly above the equator, at 90 degrees it passes over the poles, and at 180 degrees it runs above the equator against the Earth's spin.

Changing height has a result that surprises many students. To make a satellite go faster, the operator fires the thrusters against its direction of motion, not along it. Firing forward would raise the orbit and slow the satellite; firing backward drops it to a lower orbit, where it moves faster.

Inclination decides how hard the launch is. A satellite with a low inclination can use the Earth's rotation to help boost it into orbit, and a launch near the equator helps most. A polar satellite gets no help from the Earth's momentum, so it needs more energy to reach the same height. The International Space Station flies at an inclination of about 51.6 degrees, chosen to make it easier for the Space Shuttle and Russian rockets to reach it.

Orbits Classified by Altitude: LEO, MEO and High Earth Orbit

Low Earth Orbit (LEO): Imaging, Space Stations and Constellations

Low Earth orbit (LEO) is an orbit with a period of 128 minutes or less and an eccentricity below 0.25; in height terms it is the region below about 2,000 km, near the lower edge of the inner Van Allen radiation belt. Most artificial objects in space are here. A LEO satellite travels at about 7.8 km per second, which is about 28,000 km per hour.

• Imaging: Earth observation satellites use LEO because being close lets them see the surface in finer detail.
• People in space: Every space station so far has flown in LEO. The International Space Station goes round about every 90 minutes, about 15.5 times a day. India's Gaganyaan crew capsule is designed to orbit at 400 km for up to seven days with a crew of three.
• Telescopes: The Hubble Space Telescope orbits at about 540 km.
• Constellations: Satellite internet systems such as Starlink use large groups of LEO satellites working together.

LEO has two costs. Below about 300 km, drag from the thin upper atmosphere makes an orbit decay quickly, and the drag grows when the Sun is active. When the Sun is quiet a LEO satellite needs to boost its orbit about four times a year; near the peak of solar activity it may need a manoeuvre every two to three weeks. The second cost is crowding, because collisions at orbital speed create more fragments, a chain reaction known as the Kessler syndrome.

Medium Earth Orbit (MEO): GPS, GLONASS, Galileo and BeiDou

Medium Earth orbit (MEO) is the wide band between LEO and the geostationary height, from 2,000 km up to just below 35,786 km. Its best-known use is navigation. At about 20,200 km a satellite takes 12 hours to go round, so it passes over the same two points of the equator every day. This regular, predictable path is the semi-synchronous orbit used by the American GPS.

The band also holds the Van Allen belts, which endanger satellites whose sensitive components are not protected with adequate shielding.

High Earth Orbit and How the Three Classes Compare

High Earth orbit is any orbit at or beyond the geosynchronous height of 35,786 km. High orbit begins about one tenth of the way to the Moon. The most valuable high orbit is the geosynchronous one, where a satellite keeps pace with the turning Earth.

Many weather and some communication satellites sit in high Earth orbit, farthest from the surface. Most scientific satellites, including NASA's Earth Observing System fleet, fly in low Earth orbit.

Geostationary and Geosynchronous Orbits: Conditions and Uses

Conditions for a Geostationary Orbit

A geostationary orbit is a circular orbit 35,786 km above the equator, 42,164 km from the Earth's centre, running from west to east. The satellite takes one sidereal day, 23 hours 56 minutes and 4 seconds, to go round, exactly the time the Earth takes to turn once, so from the ground it seems to hang over one spot. Three conditions must hold together.

1. Geosynchronous: The period equals one rotation of the Earth.
2. Circular: The eccentricity is zero.
3. Equatorial: The inclination is zero, so the orbit lies in the plane of the equator.

The orbit runs in the direction of the Earth's rotation, at about 3.07 km per second. Only one height works, because the period is fixed by the height, so a satellite a little lower would go round faster than the Earth turns, and one a little higher would lag behind.

The idea was popularised by the writer Arthur C. Clarke in a 1945 paper, and Syncom 2 was placed in a geosynchronous orbit in 1963.

Geostationary vs Geosynchronous Orbit: The Difference

Every geostationary orbit is geosynchronous, but not every geosynchronous orbit is geostationary. Geosynchronous orbit means only that the period equals one sidereal day. If the orbit is tilted, the satellite drifts north and south of the equator during the day; if it is elliptical, it drifts east and west. Together the two drifts trace a figure‑8 in the sky, called an analemma.

India uses both. NavIC keeps three satellites in geostationary orbit and four in inclined geosynchronous orbit. The Tundra orbit is another geosynchronous orbit, eccentric and tilted, that lets a satellite spend most of its day over one high-latitude region.

Uses and Limits of Geostationary Satellites

A geostationary satellite is valuable because it never moves in the sky, and it sees a large part of the Earth at once. For weather it gives a constant view of the same area, sending images of clouds, water vapour and wind every few minutes. For communication it lets a dish on the ground point permanently at one spot in the sky, with no need to turn and track, for telephone, television and radio. India's INSAT system, commissioned in 1983 with the launch of INSAT-1B, is a series of multipurpose geostationary satellites for telecommunication, broadcasting, meteorology and search and rescue.

• Delay: A signal takes about 240 milliseconds to go up and back, a problem for voice calls and other time-sensitive uses.
• Poles: Parked over the equator, these satellites see far-northern and far-southern places only at the edge of the view.
• Crowding: Satellites must be spaced apart to avoid radio interference, so orbital slots are limited. The International Telecommunication Union allocates slots and frequencies under its Radio Regulations.
• Sovereignty claims: In the 1976 Bogota Declaration, eight equatorial countries claimed the geostationary orbit above their territory; the claims were not accepted.

Polar and Sun-Synchronous Orbits for Earth Observation

Polar Orbit: Pole-to-Pole Coverage of the Whole Earth

A polar orbit passes above or nearly above both poles on each revolution, with an inclination of about 80 to 90 degrees. Many Earth-observing satellites fly nearly polar low orbits, going round in about 99 minutes. The Earth turns underneath while the satellite travels north and south, so each pass covers a new strip beside the last. In 24 hours a polar satellite sees most of the Earth twice, once in daylight and once in darkness.

• Polar orbit missions: Mapping, reconnaissance and some weather satellites; the Iridium telephone constellation also uses polar orbits.
• Contrast: A low inclination lets a satellite concentrate on a band of the Earth. NASA's Tropical Rainfall Measuring Mission (TRMM) used an inclination of 35 degrees to watch the tropics.

Sun-Synchronous Orbit: Same Local Time, Same Lighting

A sun-synchronous orbit is a special near-polar orbit whose plane turns once a year, keeping step with the Earth's journey round the Sun. The result is that the satellite crosses the equator at the same local solar time on every pass. NASA's Terra satellite, for example, crosses the equator at about 10:30 in the morning in Brazil and, about 99 minutes later, at about 10:30 in Ecuador or Colombia.

• Height and tilt: Usually about 600 to 800 km, with periods of 96 to 100 minutes and an inclination of about 98 degrees, slightly retrograde.
• Same lighting: The angle of sunlight stays nearly the same, so images from different years can be compared without shadows creating false changes. Without such an orbit, tracking change over time, including climate change, would be very difficult.
• Narrow path: Any change in height or inclination takes the satellite out of the sun-synchronous orbit. Air drag and the pull of the Sun and Moon alter the orbit, so operators adjust the inclination about every year or two.
• Dawn-dusk variant: A satellite can ride the line between day and night, crossing the equator near sunrise and sunset, so its solar panels see the Sun almost all the time. This suits radar satellites and solar observatories.

Geostationary vs Polar Satellites: Choosing Between Constancy and Detail

The two kinds work as a pair. A geostationary weather satellite watches a cyclone every few minutes but from far away; a polar satellite sees the same storm in far finer detail but only when it passes overhead.

Transfer Orbits, Molniya and Tundra Orbits and Lagrange Points

Geostationary Transfer Orbit (GTO) and Orbit Raising

A rocket rarely carries a satellite all the way to 35,786 km. It leaves the satellite in a geostationary transfer orbit (GTO), a long ellipse whose perigee is as low as LEO and whose apogee is as high as the geostationary orbit. At the apogee the satellite fires its own thruster to make the orbit circular, a burn often called the apogee kick. The same burn removes any tilt left by a launch site away from the equator. The method is a Hohmann transfer, which links a lower and a higher circular orbit with one elliptical orbit; Walter Hohmann described it in 1925.

Launch vehicles are therefore rated by how much they can put into GTO. The Geosynchronous Satellite Launch Vehicle (GSLV) of the Indian Space Research Organisation (ISRO) is built around this task. Its cryogenic upper stage gives the high speed at perigee that a geostationary transfer orbit needs, and the satellite's own engine then circularises the orbit at apogee. Where a launcher leaves a satellite short of the full GTO, ISRO calls the result a sub-GTO, and the satellite raises itself in stages with its own engine. Each burn at perigee raises the apogee, and Aditya-L1 was lifted in this way, from an orbit of 245 km by 22,459 km to one of 256 km by 1,21,973 km, before it left for its Lagrange point.

Highly Elliptical Orbits: Molniya and Tundra

A geostationary satellite serves the far north poorly, so the Soviet Union devised the Molniya orbit. It combines a high inclination of 63.4 degrees with a high eccentricity of about 0.72 and a period of 12 hours, half a sidereal day. Near the Earth the satellite races; far away it slows, so it spends about two-thirds of each orbit over one hemisphere. Russian communication satellites and the Sirius radio satellites have used it.

The related Tundra orbit is a highly elliptical geosynchronous orbit, with the same tilt of about 63.4 degrees, a period of one sidereal day and a typical eccentricity of 0.2 to 0.3. A satellite in it spends most of its time over a chosen area, a feature called apogee dwell. Both orbits serve communication with, and surveillance of, high-latitude regions that a geostationary satellite sees badly.

Lagrange Points and Halo Orbits: Aditya-L1 and JWST

Every pair of large bodies, such as the Sun and the Earth, has five Lagrange points, L1 to L5, where their pulls and the motion of the pair combine so that a spacecraft keeps the same position relative to both. L1 lies between the Sun and the Earth; L2 lies behind the Earth, away from the Sun; L3 is on the far side of the Sun; L4 and L5 sit 60 degrees ahead of and behind the Earth on its orbit. L1 is about 1.5 million km from the Earth towards the Sun, 0.01 astronomical unit; L2 is about the same distance on the far side.

• Stability: Only L4 and L5 are stable. A craft at L1, L2 or L3 needs regular small corrections, like a ball balanced on a hilltop.
• L1: A constant view of the Sun. Aditya-L1, India's first space-based mission to study the Sun, was launched on 2 September 2023 by PSLV-C57, a flight of the Polar Satellite Launch Vehicle (PSLV), and was inserted into a halo orbit around L1 on 6 January 2024.
• L2: The Sun and Earth stay in one line behind the craft, so one heat shield blocks both. The James Webb Space Telescope (JWST) works here, about 1.5 million km from the Earth.
• Halo orbit: A craft near L1 or L2 does not sit on the point; it circles it in a halo orbit, a periodic looping path that is not flat.

Graveyard Orbit, Space Debris and Orbital Safety

Graveyard Orbit and End-of-Life Disposal

Satellites that finish their work must be moved out of the way. A retired geostationary satellite is pushed into a graveyard orbit, a supersynchronous orbit above the geostationary belt, so it cannot collide with working satellites. The recommended rise follows a formula of the Inter-Agency Space Debris Coordination Committee: 235 km plus 1,000 km multiplied by the satellite's reflectivity coefficient and its area-to-mass ratio. In low Earth orbit the end comes differently, because drag pulls a satellite lower until it burns up in the atmosphere.

How long debris stays up depends on height. Debris below about 600 km normally falls back within several years; at 800 km decay often takes centuries; above 1,000 km it normally keeps orbiting for 1,000 years or more. This is why a low orbit clears itself in time while a geostationary satellite has to be lifted out of the way.

Space Debris and the Kessler Syndrome

Space debris is any human-made object orbiting the Earth that no longer serves a useful purpose, such as dead satellites, spent rocket stages, fragments from explosions and collisions, and flecks of paint. An estimated 50,000 large objects that can be tracked orbit the Earth, along with about 100 million millimetre-sized pieces; more than 34,000 objects had been formally catalogued by the US Space Surveillance Network by August 2026. Most debris lies within 2,000 km of the surface, with the greatest concentration near 1,000 km.

Debris in LEO travels at 7 to 8 km per second, and objects strike one another at an average relative speed of about 10 km per second, so even a small piece carries great energy. The International Space Station has made 41 collision-avoidance manoeuvres since 1999, moving away when the chance of a collision exceeds 1 in 10,000.

The Kessler syndrome is the scenario proposed in 1978 by the NASA scientists Donald Kessler and Burton Cour-Palais, in which the density of objects in LEO becomes so high that collisions cascade, each collision creating fragments that cause more collisions. The destruction of China's Fengyun-1C weather satellite in 2007 and the collision of Iridium 33 and Cosmos 2251 in 2009 added greatly to the large debris in orbit. The most important action now is to prevent the unnecessary creation of new debris, since cleaning up the environment remains a technical and economic challenge.

Mission Shakti, FOBS and the Outer Space Treaty

On 27 March 2019 India shot down one of its own satellites in LEO with a ground-launched interceptor. The missile struck its target at an altitude of 283 km, and the interception was kept below 300 km so that the debris would decay quickly. The debris was expected to decay within weeks, but the last catalogued piece stayed in orbit until 14 June 2022. The test made India the fourth country, after the United States, Russia and China, to have tested an anti-satellite weapon.

A Fractional Orbital Bombardment System (FOBS) puts a warhead into a low orbit and brings it down on its target by a retrograde engine burn before it completes a full orbit. The Soviet Union first developed such a system in the 1960s, and China tested one combined with a hypersonic glide vehicle in August 2021.

Article IV of the Outer Space Treaty of 1967 bars placing nuclear weapons or other weapons of mass destruction in orbit around the Earth. It does not bar conventional weapons in orbit, which is why a fractional orbital system carrying a conventional warhead raises a legal question while one carrying a nuclear warhead would breach the treaty.

Indian Launch Vehicles and the Orbits They Serve

PSLV, GSLV, LVM3 and SSLV: Which Rocket Reaches Which Orbit

India has three active operational launch vehicles, the PSLV, the GSLV and the LVM3 (Launch Vehicle Mark‑3, earlier called the GSLV Mk III), with the Small Satellite Launch Vehicle (SSLV) developed for small payloads. Each is matched to a class of orbit.

The PSLV is called the workhorse of ISRO for consistently delivering satellites to low Earth orbits. It has also carried Chandrayaan‑1, the Mars Orbiter Mission, AstroSat and the NavIC satellites. Its PSLV-C37 flight carried the Cartosat‑2 series satellite and 103 co-passenger satellites.

Cryogenic Stages and the Energy Needed for Geostationary Launches

Reaching GTO takes far more energy than reaching LEO, because the satellite must be lifted to an apogee of about 35,786 km. The upper stage must therefore be efficient. A cryogenic stage burns liquid hydrogen with liquid oxygen. The GSLV's third stage is cryogenic, and its Mk II version flew with the indigenous CE‑7.5 engine, India's first cryogenic engine, on 5 January 2014; the earlier GSLV Mk I flights used a KVD‑1 engine. The LVM3's C25 stage carries 28 tonnes of propellant, liquid oxygen and liquid hydrogen, and is powered by the CE‑20 engine, India's largest cryogenic engine.

The payloads show the gap. The same GSLV that lifts 2,250 kg to GTO lifts 6,000 kg to LEO, and the LVM3 lifts 4,000 kg to GTO and 8,000 kg to LEO.

Indian Satellites by Orbit and the NavIC System

Which Orbit for Which Mission

The mission decides the orbit. A satellite that must stare at one region goes high over the equator; one that must photograph the whole Earth in the same light goes low and near-polar; one that must watch the Sun goes far out to L1. India's satellites show every choice.

AstroSat was launched on a PSLV-C30 on 28 September 2015 and flies at about 650 km with an inclination of about 6 degrees, a low tilt that lets the launch use the Earth's rotation.

Earth Observation Satellites: IRS, Cartosat, RISAT, Oceansat and EOS

The IRS programme began with IRS-1A in 1988; ISRO has since launched many operational remote sensing satellites, and India has one of the largest constellations of such satellites in operation. Their data serve agriculture, water resources, urban planning, rural development, mineral prospecting, environment, forestry, ocean resources and disaster management.

• Cartosat series: Mapping satellites in sun-synchronous polar orbit; Cartosat‑3 was launched by PSLV-C47 on 27 November 2019.
• RISAT series: The Radar Imaging Satellites (RISAT) give all-weather observation, where earlier optical satellites were hampered by cloud cover; RISAT-2B, launched on 22 May 2019, and RISAT-2BR1, launched on 11 December 2019, both fly in low Earth orbit.
• Oceansat‑2 and Resourcesat-2A: Ocean and land-resource satellites in sun-synchronous polar orbit.
• INSAT-3D and INSAT-3DR: Weather and disaster-management satellites in geostationary orbit; INSAT-3DR was launched on a GSLV on 8 September 2016.
• NISAR: The NASA‑ISRO radar satellite, launched on 30 July 2025, flies at 747 km at an inclination of 98.4 degrees, crossing the equator at 6 am going north and 6 pm going south, and repeats its coverage every 12 days.

EOS‑05: India's First Imaging Satellite from Geosynchronous Orbit

Imaging from geosynchronous height narrows the usual trade between constancy and detail. The satellite looks at the same part of the Earth all the time, which lets it image a selected area every 5 minutes and the entire Indian landmass every 30 minutes, at a resolution of 42 metres. Its uses include monitoring of natural disasters such as cyclones, floods, cloudbursts and forest fires, agricultural assessment and security surveillance, all of which gain from frequent looks at a wide area.

NavIC: India's Regional Navigation Constellation

The Indian Regional Navigation Satellite System (IRNSS), called NavIC, gives position and time over India and a region extending up to 1,500 km beyond its borders, the primary service area. The extended service area lies between the primary area and a rectangle from 30 degrees South to 50 degrees North and from 30 degrees East to 130 degrees East. It was built partly because foreign navigation systems are not guaranteed in a crisis, as in 1999 when the United States denied India's request for GPS data for the Kargil region. The government approved the project in May 2006, and IRNSS-1A was launched on 1 July 2013.

• Seven satellites by design: Three in geostationary orbit at 32.5 degrees East, 83 degrees East and 131.5 degrees East, and four in inclined geosynchronous orbit, two crossing the equator at 55 degrees East and two at 111.75 degrees East.
• Why two kinds of orbit: The three geostationary satellites stay fixed over the equator, while the four inclined ones swing north and south of it in a figure‑8 each day, so a receiver in India sees satellites in different parts of the sky. All seven stay in view of the region, whereas a global system needs many more satellites because they circle the whole Earth, as GPS does at 20,200 km.
• How a position is fixed: A receiver needs signals from at least four satellites to fix an accurate position, so enough healthy satellites must always be in view over India.
• Two services: A Standard Positioning Service open to all users, and an encrypted Restricted Service for authorised users such as the military. GPS likewise has a Standard Positioning Service and an encrypted precise service for authorised users.
• Accuracy: Better than 20 m in the primary service area.
• Other regional systems: Japan's Quasi-Zenith Satellite System (QZSS) is the other regional navigation system, with satellites in geosynchronous and quasi-zenith orbits. GPS, GLONASS, Galileo and BeiDou are the global systems.
• NavIC applications: Terrestrial, aerial and marine navigation, disaster management, vehicle tracking and fleet management, integration with mobile phones, precise timing, and mapping and geodetic data capture.

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