AGI’s Introduction to Orbits

What Is an Orbit?

Johannes Kepler discovered (in 1600s) that planets orbit in ellipses, not circles.Satellites (natural or human-made) also orbit Earth in elliptical pattern.­ Elliptical orbits

remain fixed in space and Earth spins under a fixed satellite orbit.

A closed path around which a planet or satellite travels.

All Sorts of Orbits!

• Low Earth Orbit (LEO)

• Polar Orbit• Medium Earth

Orbit (MEO)• Highly Elliptical

Orbit (HEO)• Geostationary

(GEO)• Molniya Orbit

(Moly)

Low Earth Orbit (LEO)• LEOs orbit relatively close to the Earth (e.g., several hundred kilometers, km) with no minimum altitude.

• LEO orbits are characterized by short orbital periods.• Roughly 90 minutes• Many revolutions per day and limited swath areas (area that a satellite can see)

• All staffed space missions except lunar missions have been LEO.

• Many Earth-observing satellites are LEO orbits.

Geostationary (GEO)

Achieved by placing satellite at altitude where orbital period exactly equals one day.

Orbit is about 22,300 miles above Earth and inclination is exactly zero degrees.

What’s in a name?A geostationary satellite stays in one spot with respect to the Earth.

GEOThere is only one altitude above Earth with an orbital period of 24 hours

All geostationary orbits are in a ring around Earth

Ring is called the geostationary belt

Geostationary belt is a limited resource

GEO

A short lesson on Urban navigation

Q. How can you tell what direction is south if you’re lost in the middle of urban United States with no compass or GPS receiver?A. Look for a building or house with a TV satellite dish. Geostationary satellites can only hover above the equator, therefore all northern hemisphere dishes are communicating with geostationary satellites toward the south.

Molniya (“Moly”)Geostationary satellites for Russian communications pose severe challenges since a majority of its land mass is too far north for geostationary belt satellites to see.

The solution was to create a type of orbit called a Molniya orbit.It allows long-term communications over northern Russian land mass.

MolniyaMolniya ground trace differs from most conventional ground traces.The image below clearly illustrates the satellite hang time over Russia.

Click to begin animation

PolarPolar orbit has a 90 degree inclination

Satellite will eventually pass over all of Earth

Polar orbit satellites can gather information about the entire Earth

E.g., weather satellites

ConstellationsSingle satellites are often insufficient to perform a mission

Groups of satellites in various orbits work together to accomplish the mission

This grouping of satellites is called a constellation

E.g., GPS system

Orbital Pattern Animation

Now That You Know the Basics

• If Norway wanted to obtain satellite imagery for all of its major urban areas, what type of orbit would be appropriate?

• Could researchers at McMurdo Station in Antarctica use geostationary satellites for communications?

Use your new understanding of orbits to answer these questions.

Answer These Questions1. Planets orbit in circles. T / F 2. Name three types of orbits.3. A geostationary satellite can orbit

around the moon. T / F 4. How long is the orbit period of a

geostationary satellite?5. Molniya orbits are commonly used to

view Russia. T / F 6. A satellite in an equatorial orbit

(inclination = 0) will eventually pass over the entire Earth. T / F

Answer Key1. Planets orbit in circles. T / F 2. Name three types of orbits.

3. A geostationary satellite can orbit around the moon. T / F

4. How long is the orbit period of a geostationary satellite

5. Molniya orbits are commonly used to view Russia. T / F

6. A satellite in an equatorial orbit (inclination = 0) will eventually pass over the entire Earth. T / F

Low Earth Orbit (LEO); Polar Orbit; Medium Earth Orbit (MEO);Highly Elliptical Orbit (HEO); Geostationary (GEO); and Molniya Orbit (Moly)

24 hours