Orbital Properties of Planets and Satellites – Learn

Types of Orbits

Spacecraft and satellites placed into orbit will generally be placed into one of two altitudes (orbits):

  • Near Earth orbit (low Earth orbit)
  • Geostationary and Geosynchronous orbit

Near Earth orbit:

Satellites in a near Earth orbit generally orbit the Earth at altitudes of between 250km and 1200km. Any lower than 250kms and atmospheric drag can impact the orbit of the satellite and any higher than 1200kms exposes the satellite to the Van Allen radiation belt. The Van Allen radiation belt is a region of high radiation trapped by the Earth’s magnetic field and poses significant risk to space travellers and electronic equipment.

Most spacecraft or satellites in near Earth orbit exist between 250-400kms depending on their purpose. Spacecraft and satellites orbiting at 250km travel at approximately 27900 km/hr and take about 90 minutes to complete one orbit of the Earth. They can scan the total surface of the Earth every 24 hours.

Near Earth orbit satellites are used for a range of different purposes: These include mapping, monitoring weather and military surveillance. The International Space Station (ISS) is the largest near Earth satellite, orbiting at 400km in an almost circular orbit. The Hubble space telescope orbits at an altitude of 540km which is just above the Earth’s atmosphere, allowing it to collect data that is not possible with Earth-based telescopes.

Geostationary and Geosynchronous Satellites:

A geostationary satellite stays over the same point on the Earth’s surface above the equator. This requires the satellite to have an orbital period that is equal to that of the Earth – 1 day or 24 hours. Geostationary satellites orbit at altitudes of approximately 36,000km. Geostationary satellites are used for communications and global positioning systems (GPS). 

A geosynchronous satellite will travel above any great circle. A great circle is any circle on the Earth’s surface that has a radius which extends from the Earth’s centre so that it has the same circumference as the equator. They have the same orbital properties of geostationary satellites. 

Analysing satellite orbits:

When quantitatively analysing the orbital properties of satellites (period, orbital velocity, orbital radius and altitude), equations previously studied are used with the orbital properties discussed above:

{ v }_{ 0 }=\sqrt { \cfrac { G{ M } }{ { r } } }

T=\cfrac { 2\pi r }{ { v }_{ 0 } }

A useful equation is derived from the equations above to determine the period independent of the orbital velocity:

\cfrac { 2\pi r }{ T } =\sqrt { \cfrac { GM }{ r } }

\cfrac { 4{ \pi }^{ 2 }{ r }^{ 2 } }{ { T }^{ 2 } } =\cfrac { GM }{ r }

{ T }^{ 2 }=\cfrac { 4{ \pi }^{ 2 }{ r }^{ 3 } }{ GM }