Common Earth Orbits

Typical orbits around the Earth and why missions choose them.

Reference Numbers

• From the circular speed and orbital period with :

Orbit		Speed	Period
Surface of the Earth			1 hour 24 minutes
Altitude of (“LEO”)			1 hour 34 minutes
(“GEO” if )			23 hours 56 minutes and a few seconds

• The GEO period matches one sidereal day (see Solar and Sidereal Time)

Criteria for Choosing Orbits

• Coverage of the ground and/or visibility in space
• Lifetime of the mission: low orbits are brought in by atmospheric drag
• Cost of launch and/or maintenance: lower orbits are generally easier to reach
• Location of launch sites and the orbits that are accessible from them
• Presence of forces that make it easier to maintain desired orbits, such as exposure to the Sun for charging solar panels
• Useful orbits divide into two categories:
  1. Orbit features driven by two-body dynamics
  2. Orbit features driven by perturbations

Orbit Types

Orbit type	Key orbital parameters	Primary use(s)
LEO (Low Earth Orbit)	Altitude < 1,000 km	Earth observation, space stations, crewed missions, high-resolution imaging
GEO (Geostationary / Geosynchronous Orbit)	Altitude ~35,786 km; orbital period matches Earth’s rotation	Telecommunications, TV broadcasting, weather monitoring, continuous coverage over a fixed region
MEO (Medium Earth Orbit)	Altitude 1,000 km to 35,286 km (sic)	Navigation constellations (e.g., GPS, Galileo, GLONASS)
HEO (Highly Elliptical Orbit)	Eccentricity	High-latitude communication, prolonged coverage at apogee, specialized scientific missions

• GEO = “Geosynchronous Equatorial Orbit”
• The MEO upper bound is printed as 35,286 km in the slide; presumably the GEO altitude of ~35,786 km

Groundtracks

• Groundtrack: the path traced on the Earth’s surface by the point directly below the satellite
• 
  • Purple: a low-inclination orbit, oscillating between about latitude
  • Orange: a high-inclination orbit, reaching about latitude
  • The highest latitude reached equals the inclination (for )
  • Each revolution’s track is shifted west of the previous one, because the Earth rotates underneath
• A zoomed map (shared over Zoom) shows a figure-eight groundtrack centered on the equator: the track of a geosynchronous orbit that is not exactly equatorial