When a satellite or spacecraft is launched, it is usually placed in one of several orbits around Earth - or it may be sent on an interplanetary journey, which means it does not orbit Earth anymore, but instead orbits the Sun until it arrives at its final destination, such as Mars or Jupiter. Many factors influence which orbit is best for a satellite to use, depending on what the satellite is designed to accomplish. In this article, we will discuss in detail regarding Orbit Types which will be helpful for UPSC exam preparation.

What is an Orbit?

• An orbit is the curved path that an object in space (such as a star, planet, moon, asteroid, or spacecraft) takes due to gravity around another object.
• Gravity attracts objects in space that have mass to other nearby objects. If they are drawn together with enough force, they can sometimes begin to orbit each other.
• Objects of similar mass orbit each other, with neither at the centre, whereas small objects orbit larger objects. The Moon orbits the Earth and the Earth orbits the Sun in our Solar System, but that does not mean the larger object remains motionless.
• Because of gravity, the Moon pulls Earth slightly away from its centre (which is why tides form in our oceans), and our Sun is pulled away from its centre by Earth and other planets.
• Our satellites are launched into orbit in space by rockets. Gravity keeps the satellite in its required orbit, just as gravity keeps the Moon in orbit around Earth.
• A satellite is placed hundreds or thousands of kilometres above the Earth's surface (as if in a very tall tower) and then given a 'push' by the rocket's engines to begin its orbit.

Classification of Orbits by Altitude

The orbits of satellites are classified as High Earth orbit, Medium Earth orbit, and Low Earth orbit based on their height above the earth.

High Earth Orbit

• About one-tenth of the way to the moon, high Earth orbit begins. Many weather satellites and some communications satellites have a high Earth orbit, farthest away from the surface.
• When a satellite reaches 42,164 kilometres from the Earth's centre (about 36,000 kilometres from the surface), it enters a "sweet spot" in which its orbit matches the rotation of the Earth.
• Because the satellite orbits at the same speed as the Earth, it appears to stay over a single longitude, though it may drift north to south. This high Earth orbit is known as geosynchronous.
• A satellite in a circular geosynchronous orbit directly over the equator (eccentricity and inclination at zero) will have a geostationary orbit in which it does not move relative to the ground at all. It always passes directly over the same spot on the Earth's surface.

Medium Earth Orbit

• Medium Earth orbit encompasses a broad range of orbits between LEO and GEO.
• It is similar to LEO in that it does not require specific paths around Earth, and it is used by a wide range of satellites with a wide range of applications.
• Navigation satellites, which are designed to monitor a specific region, are among the satellites that orbit in a medium (mid) Earth orbit.
• The semi-synchronous orbit and the Molniya orbit are two notable medium Earth orbits.
• The semi-synchronous orbit is a near-circular (low eccentricity) orbit 26,560 kilometres from the Earth's centre (about 20,200 kilometres above the surface).
  • An orbit of a satellite at this altitude takes 12 hours. The Earth rotates beneath the satellite as it moves.
  • Every day, the satellite crosses the equator at the same two points. This orbit is reliable and predictable.
  • The Global Positioning System (GPS) satellites use this orbit.
• The Molniya orbit is the second most common medium Earth orbit. The Molniya orbit, invented by the Russians, is useful for observing high latitudes.
  • The constant view provided by a geostationary orbit is valuable, but because geostationary satellites are parked over the equator, they don't work well for far northern or southern locations, which are always on the edge of view for a geostationary satellite. The Molniya orbit is a viable alternative.
  • The orbit of the Molniya satellite is highly eccentric: it moves in an extreme ellipse with the Earth close to one edge.
  • The satellite moves very quickly when it is close to the Earth because it is accelerated by our planet's gravity.
  • Its speed slows as it moves away, so it spends more time at the top of its orbit, farthest from the Earth.

Low Earth Orbit

• A low Earth orbit (LEO) is, as the name implies, an orbit that is close to the Earth's surface.
• It is normally less than 1000 km above Earth, but it can be as low as 160 km above Earth, which is low in comparison to other orbits but still very far above the Earth's surface.
• In comparison, most commercial planes do not fly at altitudes much higher than 14 km, so even the lowest LEO is more than ten times higher.
• Unlike GEO satellites, which must always orbit along the equator, LEO satellites do not always have to follow the same path around the Earth - their plane can be tilted.
• This means that satellites in LEO have more available routes, which is one of the reasons why LEO is such a popular orbit.
• Because of its close proximity to Earth, LEO is useful for a variety of reasons.
• It is the most commonly used orbit for satellite imaging because being close to the surface allows for higher resolution images.
• It is also the orbit used by the International Space Station (ISS), as it allows astronauts to travel a shorter distance to and from it.
• Satellites in this orbit travel at approximately 7.8 km per second; at this speed, a satellite takes approximately 90 minutes to circle Earth, implying that the ISS travels around Earth approximately 16 times per day.
• Individual LEO satellites, on the other hand, are less useful for tasks such as telecommunication because they move so quickly across the sky and thus require a lot of effort to track from ground stations.
• Instead, communications satellites in low-Earth orbit (LEO) frequently operate as part of a large combination, or constellation, of multiple satellites to provide continuous coverage.
• To increase coverage, constellations like this, which consist of several of the same or similar satellites, are sometimes launched together to form a 'net' around Earth. Working together, they can cover large areas of the Earth at the same time.
• The majority of scientific satellites, including NASA's Earth Observing System fleet, operate in low Earth orbit.

Factors Affecting Satellite Movement in Orbit

Height

• The distance between the satellite and the Earth's surface, or the height of the orbit, determines how quickly the satellite moves around the Earth.
• The motion of an Earth-orbiting satellite is primarily controlled by Earth's gravity.
• As satellites get closer to Earth, the gravitational pull becomes stronger, and the satellite moves faster.
• NASA's Aqua satellite, for example, takes about 99 minutes to orbit the Earth at about 705 kilometres above the surface, whereas a weather satellite about 36,000 kilometres above the surface takes 23 hours, 56 minutes, and 4 seconds.
• The Moon completes a single orbit in 28 days at 384,403 kilometres from the centre of the Earth.
• Changing the height of a satellite affects its orbital speed. This creates an odd paradox.
• If a satellite operator wants to increase the orbital speed of the satellite, he cannot simply fire the thrusters. This would boost the orbit (increase the altitude), slowing the orbital speed.
• Instead, he must fire the thrusters in the opposite direction of the satellite's forward motion, which would slow a moving vehicle on the ground. This modification will lower the satellite's orbit, increasing its forward velocity.

Eccentricity

• The shape of the orbit is referred to as eccentricity. A satellite in a low eccentricity orbit circles the Earth in a near circle.
• An eccentric orbit is elliptical, with the distance between the satellite and Earth varying depending on where it is in its orbit.
• The eccentricity (e) of an orbit denotes the orbit's deviation from a perfect circle.
• The eccentricity of a circular orbit is zero, whereas the eccentricity of a highly eccentric orbit is closer to (but always less than) one.
• An eccentric orbit is one in which a satellite moves around one of the ellipse's focal points rather than the centre.

Inclination

• Inclination is the angle of the orbit in relation to the equator of the Earth.
• A satellite with zero inclination orbits directly above the equator.
• The inclination of a satellite orbiting from the north pole (geographic, not magnetic) to the south pole is 90 degrees.
• The angle between the plane of an orbit and the equator is known as orbital inclination.
• A 0° orbital inclination crosses directly above the equator, a 90° orbit crosses directly above the pole, and a 180° orbit crosses directly above the equator in the opposite direction of Earth's spin.

Types of Orbits

Many factors influence which orbit is best for a satellite to use, depending on what the satellite is designed to accomplish.

Geostationary Orbit (GEO)

• Satellites in geostationary orbit (GEO) circle Earth above the equator from west to east, taking 23 hours 56 minutes and 4 seconds and travelling at the same rate as Earth.
• Satellites in GEO appear to be 'stationary' over a fixed position as a result of this.
• To perfectly match the rotation of the Earth, GEO satellites should travel at a speed of about 3 km per second at an altitude of 35 786 km. This is much farther away from the Earth's surface than many satellites.
• Satellites that must remain constantly above one location on Earth, such as telecommunication satellites, use GEO. This allows an antenna on Earth to be fixed so that it is always pointed at the satellite.
• Weather monitoring satellites can also use it because they can continuously observe specific areas to see how weather trends emerge there.
• Satellites in GEO cover a large area of the Earth, so as few as three satellites spaced evenly can provide near global coverage. This is because a satellite at this distance from Earth can cover large areas at once.
• To see the entire Earth from GEO, far fewer satellites are required than at a lower altitude.

Polar orbit and Sun-synchronous Orbit (SSO)

• Polar orbit satellites typically travel from north to south rather than west to east, passing roughly over the Earth's poles.
• Polar orbiting satellites do not have to pass exactly through the North and South Poles; a deviation of 20 to 30 degrees is still considered a polar orbit.
• Polar orbits are a type of low Earth orbit because they are at low altitudes ranging from 200 to 1000 kilometres.
• Sun-synchronous orbit (SSO) is a type of polar orbit.
• SSO satellites travelling over the polar regions are synchronised with the Sun.
  • This means that they are always in the same 'fixed' position relative to the Sun. This means that the satellite will always visit the same location at the same time.
• This means that the satellite will always observe a point on Earth as if it were constantly at the same time of day, which has a variety of applications; for example, scientists and those who use satellite images can compare how something changes over time.

Transfer orbits and Geostationary Transfer Orbits (GTO)

• Transfer orbits are a type of orbit that is used to transition from one orbit to another.
• Satellites are not always placed directly on their final orbit when launched from Earth and carried to space by launch vehicles.
• Often, satellites are placed in a transfer orbit, which is an orbit in which the satellite or spacecraft can move from one orbit to another using relatively little energy from built-in motors.
• This allows a satellite to reach a high-altitude orbit, such as GEO, without requiring the launch vehicle to travel all the way to this altitude, which would require more effort - it's like taking a shortcut.
• This method of reaching GEO is an example of one of the most common transfer orbits, known as the geostationary transfer orbit (GTO).
• The eccentricity of an orbit is a measure of how circular (round) or elliptical (squashed) it is.
• The satellite is always the same distance from the Earth's surface in a perfectly round orbit, but in a highly eccentric orbit, the path looks like an ellipse.
• Depending on where the satellite is in its orbit, a highly eccentric orbit like this one can quickly take it from very far away to very close to the Earth's surface.
• In transfer orbits, the payload uses engines to move from one eccentric orbit to another, putting it on track to higher or lower orbits.
• Following liftoff, a launch vehicle travels to space along the path depicted in the figure by the yellow line.
• When the rocket arrives at its destination, it releases the payload, which sends it into an elliptical orbit along the blue line, further away from Earth.
• The apogee is the point on the blue elliptical orbit that is farthest away from Earth, and the perigee is the point that is closest.
• When the payload reaches the apogee at 35,786 km GEO altitude, it fires its engines in such a way that it enters and stays in the circular GEO orbit, as shown by the red line in the diagram.
• The GTO is thus the blue path from the yellow orbit to the red orbit.

Lagrange Points (L-points)

• Being too close to Earth can be detrimental to many spacecraft's missions, even at more distant orbits such as GEO.
• For example, for space-based observatories and telescopes whose mission it is to photograph deep, dark space, being close to Earth is extremely detrimental because Earth emits visible and infrared radiation that prevents the telescope from detecting faint lights such as distant galaxies.
• Attempting to photograph dark space with a telescope next to our glowing Earth would be as futile as attempting to photograph stars from Earth in broad daylight.
• Lagrange points, also known as L-points, allow for orbits that are much, much farther away (over a million kilometres) and do not directly orbit Earth.
• These are specific points in space where the gravitational fields of Earth and the Sun interact in such a way that spacecraft orbiting them remain stable and can thus be 'anchored' relative to Earth.
• If a spacecraft were launched to distant points in space, it would naturally fall into an orbit around the Sun, and those spacecrafts would soon end up far from Earth, making communication difficult.
• Instead, spacecraft launched to these special L-points remain fixed and close to Earth with minimal effort without changing orbit.
• L1 and L2 are the most commonly used L-points. Both are four times farther away from Earth than the Moon - 1.5 million km versus GEO's 36,000 km - but that is still only about 1% of Earth's distance from the Sun.

Conclusion

As there is no air in space and thus no air friction, gravity allows the satellite to orbit Earth with little assistance. By putting satellites into orbit, we can use technologies for communication, navigation, weather forecasting, and astronomy observations.

FAQs

MCQs

1. Which of the following orbits is commonly used for weather monitoring satellites?

A) Low Earth Orbit (LEO)

B) Medium Earth Orbit (MEO)

C) Geostationary Orbit (GEO)

D) Polar Orbit

2. Which orbit is primarily used for Earth observation satellites?

A) Low Earth Orbit (LEO)

B) Medium Earth Orbit (MEO)

C) Geostationary Orbit (GEO)

D) Highly Elliptical Orbit

3. Which of the following orbits is most suitable for navigation satellites like GPS?

A) Low Earth Orbit (LEO)

B) Medium Earth Orbit (MEO)

C) Geostationary Orbit (GEO)

D) Polar Orbit

4. What is the main characteristic of a satellite in geostationary orbit?

A) It moves at the same speed as the Earth’s rotation

B) It orbits at a very high altitude

C) It is used for deep space missions

D) It passes over the poles

5. Which orbit type would be most suitable for a satellite that needs to continuously observe the entire Earth’s surface?

A) Low Earth Orbit (LEO)

B) Medium Earth Orbit (MEO)

C) Polar Orbit

D) Geostationary Orbit (GEO)

GS Mains Questions and Model Answers

Q1: Explain the significance of various satellite orbits in global communication and Earth observation systems.

Answer: Satellite orbits play a crucial role in global communication and Earth observation systems. - **Geostationary Orbit (GEO)** allows satellites to remain stationary relative to a fixed point on Earth, providing continuous coverage of a specific area, making it ideal for communication, weather forecasting, and broadcasting services. - **Low Earth Orbit (LEO)** provides higher-resolution images for Earth observation and environmental monitoring, as the satellite is closer to the Earth’s surface. However, it requires a larger number of satellites for global coverage due to its smaller footprint. - **Medium Earth Orbit (MEO)** is mainly used for navigation satellites like GPS, providing wide coverage and ensuring continuous positioning accuracy. Each orbit serves a specific need, enhancing communication networks, weather systems, and navigation technologies.

Q2: Discuss the challenges and benefits of using Low Earth Orbit (LEO) for satellite communication systems.

Answer: The use of Low Earth Orbit (LEO) for satellite communication systems comes with both challenges and benefits. - **Benefits**: - LEO satellites have lower latency compared to GEO satellites, offering faster communication services. - They provide higher-resolution data for Earth observation as they are closer to the Earth’s surface. - LEO networks can be more flexible, supporting various applications such as global internet services and scientific missions. - **Challenges**: - LEO satellites have a limited coverage area, requiring a large constellation of satellites to provide continuous global coverage. - Their shorter lifespan (due to atmospheric drag) necessitates frequent replacements. - They are also more vulnerable to space debris, which can potentially damage the satellites.

Q3: How can the use of satellite orbits contribute to sustainable environmental monitoring and disaster management?

Answer: Satellite orbits play an essential role in environmental monitoring and disaster management. - **Polar Orbits** allow satellites to pass over all areas of the Earth, providing frequent and comprehensive data on weather patterns, deforestation, and climate change. - **Geostationary Orbits** offer continuous monitoring of specific regions, such as monitoring the development of cyclones, wildfires, or other natural disasters. - These satellite systems support early warning systems, enabling governments and agencies to respond more effectively to disasters, mitigate environmental damage, and plan for sustainable resource management.

Previous Year Questions on Orbit Types

1. UPSC CSE 2022:

Question: Discuss the differences between Low Earth Orbit (LEO), Medium Earth Orbit (MEO), and Geostationary Orbit (GEO) with respect to their applications and benefits.

Answer: LEO is used for Earth observation and communication with low latency. MEO is mainly used for navigation satellites like GPS, offering wide coverage. GEO is ideal for weather and communication satellites, offering fixed positioning over the Earth’s surface. Each orbit serves specific functions based on distance and application requirements.

2. UPSC CSE 2020:

Question: What is the role of Polar Orbits in satellite technology and Earth observation?

Answer: Polar Orbits enable satellites to pass over the Earth's poles, providing global coverage of the Earth's surface. This orbit is used for environmental monitoring, disaster management, and mapping. The continuous orbital motion ensures that the entire surface of the Earth is observed over time, making it crucial for tracking changes in land cover, climate, and ecosystems.