Type of orbits

 There are several types of Earth orbit. Each offers different advantages, and possibilities. 

Let's talk about what is orbit? An orbit is the curved path that an object in space, such as a star, planet, moon, asteroid or spacecraft, takes around another object due to gravity.  Gravity is the force by which a planet or other body draws objects toward its center. 

In our Solar System, the Moon orbits Earth, and Earth and other planets orbit the Sun. The larger object doesn't remain still, but moves due to the smaller object as well. Because of gravity, Earth is pulled slightly from its center by the Moon and our Sun is pulled slightly from its center by Earth and other planets. 

credit: ESA, putting satellite into orbit

A satellite is simply put into orbit by being placed hundreds or thousands of kilometers above Earth’s surface and then being given a push by the rocket’s engines to make it start on its orbit. As the figure above shows, the difference between simple falling back to the ground and orbiting is the power of the push of the engines. 

If a ball is propelled in a certain speed, it will never fall. Why? The ball falls in the same rate as the Earth's surface curves. This certain velocity is called orbital velocity. 

credit: ScienceABC

The velocity decreases with r, the orbit’s distance from the center of Earth. This means that satellites orbiting closer to Earth’s surface must travel faster than satellites orbiting further away.

We define many different types of orbits around Earth. There are many factors that decide which orbit is good to se, more to which purpose is the particular satellite designed for. 

Geosynchronous Orbit (GSO) & Geostationary Orbit (GEO)

Objects in GSO have an orbital speed that matches the Earth’s rotation, yielding a consistent position over a single longitude. GEO is a kind of GSO. It matches the planet’s rotation, but GEO objects only orbit Earth’s equator, and from the ground perspective, they appear in a fixed position in the sky. 

Source: ESA, geostationary orbit

GEO is used by satellites that need to stay constantly above one particular place over Earth, such as telecommunication satellites. Satellites in GEO cover a large range of Earth so as few as three equally-spaced satellites can provide near global coverage. 

Low Earth Orbit (LEO)

LEO is commonly used for communication and remote sensing satellite systems, as well as the International Space Station (ISS) and Hubble Space Telescope.

A low Earth orbit (LEO) is an orbit that is relatively close to Earth’s surface. It is normally at an altitude of less than 1000 km but could be as low as 160 km above Earth. To remind, Karman line is set about 100 km above Earth. 

Source: ESA, Low Earth Orbit, for example ISS

Satellites in this orbit travel at a speed about 7.8 km/s. At this speed, a satellite takes approximately 90 minutes to circle Earth, which means that the ISS travels around Earth about 16 times a day.

Advantages of LEO include reduction of transmission delay, elimination of need for bulky receiving equipment. However there are several disadvantages of this orbit, including smaller coverage area, shorter life span (between 5 and 8 years) than GEOs (about 10 years).

Medium Earth Orbit (MEO)

MEO is commonly used for navigation systems, including the U.S. Global Positioning System (GPS). Medium Earth orbit takes a wide range of orbits anywhere between LEO and GEO.

Source: ESA, Medium Earth Orbit

Polar Orbit

Satellites in polar orbits usually travel past Earth from north to south rather than from west to east, passing roughly over Earth's poles. Polar orbits are a type of low Earth orbit, as they are at low altitudes between 200 to 1000 km.

Within 30 degrees of the Earth’s poles, the polar orbit is used for satellites providing reconnaissance, weather tracking, measuring atmospheric conditions, and long-term Earth observation.

Sun-Synchronous Orbit (SSO)

A type of polar orbit, SSO objects are synchronous with the sun, such that they pass over an Earth region at the same local time every day.

Source: ESA, Sun Synchronous Orbit

Sun-synchronous orbit (SSO) is a particular kind of polar orbit. Satellites in SSO, travelling over the polar regions, are synchronous with the Sun. This means they are synchronised to always be in the same fixed position relative to the Sun. This means that the satellite always visits the same spot at the same local time: for example, passing the city of Paris every day at noon exactly.

Highly Elliptical Orbit (HEO)

An HEO is oblong, with one end nearer the Earth and other more distant. Satellites in HEO are suited for communications, satellite radio, remote sensing and other applications.

Source: Astronomía para tontos, comparison of different orbits around Earth

Transfer orbits and geostationary transfer orbit (GTO)

Transfer orbits are used to get from one orbit to another. The reason of use such an orbit is to be more efficient. Instead of taking rocket directly to the final destination, it takes it to lower altitude and uses shortcuts to get to the particular orbit by build-in engines of the satellite. 

source: ESA, Geostationary transfer orbit

After liftoff, a launch vehicle makes its way, following a path shown by the yellow line, in the figure. At the target destination, the rocket releases the payload which sets it off on an elliptical orbit, following the blue line which sends the payload farther away from Earth. The point farthest away from the Earth on the blue elliptical orbit is called the apogee and the point closest is called the perigee. 

When the payload reaches the apogee at the GEO altitude of 35 786 km, it fires its engines to enter the circular GEO orbit and stays there, shown by the red line in the diagram. The GTO is the blue path from the yellow orbit to the red orbit.

Lagrange points

For many spacecraft being put in orbit, being too close to Earth can be disruptive to their mission – even at more distant orbits such as GEO. So there is another specific place, where we can place the spacecraft.

Lagrange points, also known as L-points, allow for orbits that are much, much farther away and do not orbit Earth directly. These are specific points far out in space where the gravitational fields of Earth and the Sun combine in such a way that spacecraft that orbit them remain stable and can thus be fixed relative to Earth.

credit: NASA, five Lagrange points

 If a spacecraft was launched to other points in space very distant from Earth, they would naturally fall into an orbit around the Sun. Instead, spacecraft launched to these special L-points stay fixed, and remain close to Earth with minimal effort without going into a different orbit.

The most used L-points are L1 and L2. These are both four times farther away from Earth than the Moon – 1.5 million km, compared to GEO’s 36 000 km – but that is still only approximately 1% of the distance of Earth from the Sun.

Of the five Lagrange points, three are unstable and two are stable. The unstable Lagrange points - labeled L1, L2 and L3 - lie along the line connecting the two large masses. The stable Lagrange points - labeled L4 and L5 - form the apex of two equilateral triangles that have the large masses at their vertices. L4 leads the orbit of earth and L5 follows.

The L1 point of the Earth-Sun system affords an uninterrupted view of the sun and is currently home to the Solar and Heliospheric Observatory Satellite SOHO.

The L2 point of the Earth-Sun system was the home to the WMAP spacecraft, Planck (mission ended 23 October 2013), and future home of the James Webb Space Telescope. The Launch date is October 2021 on Ariane 5 (ESA) to reach orbit: Lagrange point 2, 1.5 million km from Earth. 

credit: NASA, comparison of Hubble and Webb

NASA is unlikely to find any use for the L3 point since it remains hidden behind the Sun at all times.

The L4 and L5 points are stable orbits as long as the mass ratio between the two large masses exceeds 24.96. This condition is satisfied for both the Earth-Sun and Earth-Moon systems, and for many other pairs of bodies in the solar system.

Objects found orbiting at the L4 and L5 points are often called Trojans after the three large asteroids Agamemnon, Achilles and Hector that orbit in the L4 and L5 points of the Jupiter-Sun system.

In 1956, astronomer Kordylewski discovered large concentrations of dust at the Trojan points of the Earth-Moon system. The DIRBE instrument on the COBE satellite confirmed earlier observations of a dust ring following the Earth's orbit around the Sun. The existence of this ring is closely related to the Trojan points.

In 2010 NASA's WISE telescope finally confirmed the first Trojan asteroid (2010 TK7) around Earth's leading Lagrange point.

credit NASA: This artist's concept illustrates the first known Earth Trojan asteroid, discovered by NASA's WISE mission. The asteroid is shown in gray and its extreme orbit is shown in green. Earth's orbit around the sun is indicated by blue dots. The objects are not drawn to scale.