# Low Earth and Geostationary Orbits

This topic is part of the HSC Physics syllabus under the section Motion in Gravitational Fields.

### HSC Physics Syllabus

• Predict quantitatively the orbital properties of planets and satellites in a variety of situations, including near the Earth and geostationary orbits, and relate these to their uses

### Orbital motion depends on the nature of orbit

An object’s orbital motion depends on its velocity, mass of object it is orbiting around and the radius of orbit.

#### Orbital velocity

Greater orbital velocity is required for smaller orbits and when orbiting planets of heavier mass. This implicates on greater energy need and consumption as greater velocity can only be achieved and maintained by greater fuel consumption. Ultimately, this means satellites in bigger orbit paths need to carry more fuel otherwise they are destined to have short orbit time.

Satellites often encounter orbital decay whereby they are unable to maintain the stable orbit and as such fall back down onto Earth’s surface. There are couple of reasons for this:

As indicated by Newton’s law of gravitation, smaller orbital radius also causes the distance between two masses (e.g. Earth and satellite) to become smaller. This causes the gravitational force and acceleration experienced by the satellite to increase. This is a problem for satellites with low altitude orbits as it contributes to orbital decay

#### Orbital Decay Due to Air Resistance

Orbital decay refers to the process by which a satellite's altitude gradually decreases over time, eventually resulting in the satellite re-entering the Earth's atmosphere. This phenomenon occurs due to various factors, primarily atmospheric drag, which is much more significant for satellites in Low Earth Orbit (LEO) compared to those in Geostationary Orbit (GEO).

Other causes of orbital decay include:

• Electromagnetic Forces
• Gravity Perturbations
• Collision with Space Debris

### Low Earth Orbits (LEO)

#### Characteristics

• Altitude: Typically, a LEO is at an altitude of about 160 to 2,000 kilometres above the Earth's surface.
• Velocity: Satellites in LEO travel at very high speeds of approximately 7.8 km/s to maintain their orbit and must complete an orbit around the Earth in about 90 minutes.
• Orbital Period: The lower the altitude, the shorter the orbital period. Satellites in LEO have shorter periods, which means they circle the Earth multiple times a day.
• Atmospheric Drag:  Even at altitudes as high as 2,000 kilometres, the Earth's atmosphere still has trace amounts of gas. Satellites in LEO move at very high velocities, and when they encounter these atmospheric particles, it creates drag. This drag reduces the satellite's kinetic energy, causing it to slow down and lose altitude.

#### Uses

• Observation and Imaging: Due to their proximity to Earth, LEO satellites are ideal for Earth observation, remote sensing, and imaging. They provide high-resolution data for weather forecasting, environmental monitoring, and disaster management.
• Communication: Some communication satellites use LEO to provide services like mobile voice and data services, though they require a network of satellites due to their fast-moving nature.
• Science and Research: LEO is used for scientific experiments in conditions of microgravity, as well as for astronomical observations free from the Earth's atmospheric interference.

### Geostationary Orbits (GEO)

#### Characteristics

• Altitude: A GEO is much higher, situated at approximately 35,786 kilometers above the Earth's equator.
• Velocity: Satellites in GEO orbit above Earth's equator in an easterly direction at a speed that matches the Earth's rotation, which means they have an orbital period of exactly one day (about 24 hours).
• Stationary Relative to Earth: These satellites appear stationary relative to an observer on Earth because they orbit over the same point on the equator at all times.
• Negligible Atmospheric Drag: Satellites in GEO are much farther from the Earth's surface, at an altitude of approximately 35,786 kilometers, where the atmosphere is so thin that atmospheric drag is virtually nonexistent.

#### Uses

• Communication: GEO satellites are widely used for telecommunications and broadcast services, as they can provide consistent coverage to large areas of the Earth.
• Weather Observation: Meteorological satellites in GEO provide continuous weather and climate data as they can observe large portions of the Earth's surface at all times.
• Navigation: Some navigation systems also use GEO satellites to supplement their networks, providing enhanced stability and coverage.

Geosynchronous orbit satellites are similar to geostationary with one key difference: Geostationary satellites orbit the equator whereas geosynchronous orbit at a different latitude. Geosynchronous orbit satellites are not examinable in the HSC Physics syllabus.

### Table Comparison Between LEO and GEO

 Low-Earth (LEO) Geostationary (GEO) Orbital Radius Satellites with low attitude (<2000 km) and short orbital period. Satellites with much higher altitude of ~35000 – 36000 km. ‘Orbits’ at the same velocity as Earth’s rotation.   Thus, it remains at the exactly same spot above Earth’s surface throughout its orbit. Total energy Lower (gravitational potential energy is more negative) Higher Orbital period Shorter Longer. Orbital period is approximately 24 hours. Orbital velocity Faster Slower Advantage ·     Closer to surface of Earth which enables higher resolution photographs and videos ·     Cheaper to establish due to low attitude and lower fuel requirement to reach there ·     Accurate signal in relation to communication ·    Orbital period is in sync with that of Earth which enables several essential applications ·    Easier than low-Earth to maintain the orbit due to greater orbital radius (thus low orbital velocity) ·    Wide coverage Disadvantage ·     Harder to maintain a stable orbit due to low attitude (greater orbital velocity) ·     Limited coverage in relation to communication ·     More expensive to establish due to high attitude, more fuel required to reach the orbit ·     Requires strong signal for communication ·     Vulnerable to sun outages (elevation in radiation, occurs twice a year) Application & uses ·    Cellular communication that only require small coverage e.g. Iridium phone systems ·    Spy satellites ·    International Space Station ·    Television ·    Radio ·    Weather forecast ·    Cell phones

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