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A system of about 100 billion stars. Our Sun is a member of the Milky Way Galaxy. There are billions of galaxies in the observable universe. Exactly when and how galaxies formed in the Universe is a topic of current astronomical research.

Milky Way Galaxy
Galilean Moons

Jupiter's four largest moons: Io, Europa, Ganymede and Callisto; discovered independently by Galileo Galilei in 1610.


A substance, such as oxygen or hydrogen, that has no fixed shape and that can expand without limit.

gas giants

The outer solar system planets: Jupiter, Saturn, Uranus, and Neptune, composed mostly of hydrogen, helium and methane gas, and have a density less than 2 grams per cubic centimeter.

geodetic control network

An ellipsoid or sphere-shaped "net" that is used as a type of base-map or control to correlate co-ordinates on a flat image with their actual location on the spherical planetary body in order to obtain accurate measurements from images.

geologic activity

The expression of the internal and external processes and events that affect a planetary body.

geologic time

The time extending from the end of the formative period of the Earth to the beginning of human history.

geologic unit

A body of rock (or ice, in Europa's case) that has a distinct origin and consists of dominant, unifying features that can be easily recognized and mapped.

geosynchronous orbit

A direct, circular, low inclination orbit in which the satellite's orbital velocity is matched to the rotational velocity of the planet; a spacecraft appears to hang motionless above one position of the planet's surface. In other words, the satellite and planet move at the exactly the same speed.

Diagram of satellite orbiting Earth.

A depressed segment of the crust of the earth or a celestial body (as the moon) bounded on at least two sides by faults.


The acceleration produced by the mutual attraction of two masses, and of magnitude inversely proportional to the square of the distance between the two centers of mass.

gravity gradient

Refers to the difference in the acceleration of gravity from one side of an object to another, because gravity is a force that changes with distance between two points.

great circle

The line of intersection of the surface of a sphere and any plane which passes through the center of the sphere.

greenhouse effect

Increase in temperature caused when incoming solar radiation is passed but outgoing thermal radiation is blocked by the atmosphere (carbon dioxide and water vapor are the major factors). Very important on Venus and Earth but very weak on Mars.

Diagram of greenhouse effect
ground truth

The on-site gathering of data at a particular location, for the purpose of calibrating and interpreting remotely acquired data.

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Last Updated: 25 Oct 2013