Aerospace Orbital Test Review

Eccentricity

Describes the roundness of the orbit and the shape of the ellipse in terms of how wide it is.

Closed vs Open Orbits

Eccentricity between and 0 and 1 is a closed orbit. A value of 0 means it’s a circle and a value of 1 and greater means it’s an open orbit.

Semi-Major Axis

Describes the size of the ellipse and it is half the major axis of the orbit. It starts from the center of the ellipse.

Apogee

Point in orbit farthest to the Earth.

Perigee

Point in orbit closes to the Earth.

Apogee Altitude

Distance between surface of the Earth and apogee.

Perigee Altitude

Distance between surface of the Earth and perigee.

What determines orbital period?

Only the semi-major axis.

What determines the orientation in space?

Inclination, right ascension of the ascending node, and argument of perigee.

Inclination

The angle between the Earth’s equator and the plane of the orbit. It describes the tilt of the orbit. Higher angle means more perpendicular to the equator.

How to determine inclination through ground trace?

Look at the topmost horizontal number and the bottommost horizontal number the ground trace stretches through and that’s the inclination.

Equatorial Orbit

Orbit with an inclination of 0 degrees.

Polar Orbit

Orbit with an inclination of 90 degrees.

How to determine orbital period through a ground trace?

Follow the curve until it makes a full rotation around the Earth in the same way, and then multiply the number between the starting vertical number and the ending one by 4.

Right Ascension of the Ascending Node (RAAN)

The angle measured by the equator and a fixed point in space (known as Aries or the vernal equinox). Ascending node is the point on the orbit where the orbital motion is south to north along the equator.

Argument of Perigee

Describes the location of the perigee in relation to the ascending node.

True Anomaly

Angle between the perigee point and the satellite’s location as it moves through the orbit. 0 degrees at the perigee and 180 degrees at the apogee as the satellite constantly moves around the orbit.

Keplerian Elements

1. Eccentricity

2. Semi-Major Axis

3. Inclination

4. RAAN

5. Argument of Perigee

6. True Anomaly

Epoch

A time stamp that has to be included when providing a Keplerian element set to know when this set of values was accurate for the satellite.

Kepler’s Laws

1. Satellite will travel around the Earth in elliptical paths with the Earth at one foci

2. A line drawn between the Earth and the orbit will sweep out equal areas during equal time periods around the orbit

Low Earth Orbit (LEO)

Only several hundred km from the Earth with short orbital periods that are mainly used for weather and imagery. Most manned space missions fall under this.

Geostationary Orbit (GEO)

Remains over one location on Earth in a special orbit where a period equals exactly a day so the satellite is exactly over one specific area at all times. Roughly 36000 km from the surface at 0 degree inclination. They only exist at the equator and can only see roughly 70 degrees north and south.

Geostationary Belt

Ring of GEOs around the Earth that is a limited resource. When a geobird dies (satellite in the ring) it needs to be removed from its slot and replaced by another satellite.

Station-Keeps

Small orbit adjustment burns to keep geobirds in their orbit as no orbit can be perfectly geostationary because they can drift north to south slightly and east to west as well due to inclination.

Geosynchronous

An interchangeable term for geostationary.

Molniya

A highly inclined and elliptical orbit to cover northern Russia with a very large apogee altitude and is very slow at the apogee to stay above Russia for as long as possible.

A satellite in which orbit passes through the entire world?

A polar orbit.

Sun Synchronous Orbit

A special orbit that passes over the same part of the Earth at roughly the same time every day.

Constellations

A group of satellites working together to accomplish the mission. A GPS is a good example of this.

Energy

Potential energy and kinetic energy are the two main energies that are added to find the total energy of the orbit. With both orbit energies, subtract to determine the energy transfer.

Aristotle

Proposed the idea of a geocentric universe, where everything revolves around the Earth in a perfect circle.

Copernicus

Proposed the idea of the heliocentric model, where everything revolves around the Sun in perfect circles.

Brahe

Recorded planetary positions, especially Mars, which led the foundation of elliptical orbits.

Kepler

Three laws of planetary motion, showing that orbits were ellipses and not perfect circles.

Hooke

Planetary motion is governed by an inverse square force directed to the Sun, which influenced Newton’s creation of universal gravitation.

Newton

Created the law of universal gravitation and explained how bodies orbit due to gravity through math.

Einstein

Redefined gravity as a curvature of spacetime through his general theory of relativity.