Aerosols: Gaseous suspensions of ultramicroscopic particles of liquids or solids.

Asteroid belt: The region between 2.1 and 3.5 astronomical units (AU) from the sun where most of the asteroids are found. Asteroids are small planetary bodies revolving around the sun; most of the orbits are between the planets Mars and Jupiter.

Comets: Nebulous celestial bodies having fuzzy heads surrounding bright nuclei; comets are one of two major types of bodies moving in closed orbits about the sun, the other being the planets; in comparison with the planets, comets are characterized by their more eccentric orbits and greater range of inclinations to the ecliptic plane.

Conjunction: 1. The situation in which two celestial bodies have either the same celestial longitude or the same sidereal hour angle (i.e., the angle along the celestial equator formed between the hour circle of a celestial body and the hour circle of the vernal equinox). 2. The time at which the above situation takes place.

Cryo: Pertaining to cryogenic temperatures, which are temperatures within a few degrees of absolute zero (0 degrees Kelvin, or about -273 F).

Deep Space Network: A communications network managed by the Jet Propulsion Laboratory in Pasadena, California, for command and control of all planetary flights.

Descent Imager/Spectral Radiometer (DISR): A Huygens probe instrument that will make a range of imaging and spectral observations of the Saturn moon Titan using several sensors and fields of view. By measuring the upward and downward flow of radiation, the radiation balance (or imbalance) of the thick Titan atmosphere will be measured. Solar sensors will measure the light intensity around the Sun due to scattering by aerosols in the atmosphere. This will permit the calculation of the size and number density of the suspended particles. Two imagers (one visible, one infrared) will observe the surface during the latter stages of the descent and, as the probe slowly spins, build up a mosaic of pictures around the landing site. There will also be a side-view visible imager to get a horizontal view of the horizon and the underside of the cloud deck. For spectral measurements of the surface, a lamp that will switch on shortly before landing will augment the weak sunlight.

Ejecta: Material which is cast out.

Ethane: (CH3CH3) A colorless, odorless gas belonging to the alkane series of hydrocarbons, with freezing point of -183.3 C and boiling point of -88.6 C; used as a fuel and refrigerant and for organic synthesis.

Giant planets: The planets Jupiter, Saturn, Uranus, and Neptune of our solar system. Sometimes called gas giant planets.

Gravitational wave: Also known as gravitational radiation. A propagating gravitational field predicted by general relativity, which is produced by some change in the distribution of matter; it travels at the speed of light, exerting forces on masses in its path.

Heat flux: The amount of heat transferred across a surface of unit area in a unit time. Also known as thermal flux.

Hubble Space Telescope: An astronomical reflecting telescope with a mirror 94.5 inches (2.4 meters) in diameter; placed in orbit above the Earths atmosphere in April 1990.

Hydrocarbons: A large group of chemical compounds composed only of carbon and hydrogen. I

Imaging Science Subsystem (ISS): A remote sensing instrument that captures images in visible light, and some in infrared and ultraviolet light. The ISS has a camera that can take a broad, wide-angle picture and a camera that can record small areas in fine detail. Scientists anticipate that Cassini scientists will be able to use ISS to return hundreds of thousands of images of Saturn and its rings and moons. The sensitivity of Cassini's higher-resolution camera is such that it is able to see a penny, 1.5 cm (0.5 in) across, from a distance of nearly 4 km (2.5 mi).

Ionosphere: That part of a planets atmosphere that is sufficiently ionized by solar ultraviolet radiation so that the concentration of free (i.e., unattached) electrons affects the propagation of radio waves.

Magnetosphere: The region around a rotating magnetized sphere, such as a planet or star, in which the spherical bodys magnetic field plays a dominant part in controlling the physical processes that take place.

Meteoroid: Any solid object moving in interplanetary space that is smaller than a planet or asteroid but larger than a molecule.

Methane: (CH4) A colorless, odorless, and tasteless gas, lighter than air and reacting violently with chlorine and bromine in sunlight, a chief component of natural gas; used as a source of methanol (wood alcohol), acetylene, and carbon monoxide.

Near-infrared: A region in the infrared radiation spectrum in which the wavelengths are relatively short, between 0.75 and about 3 micrometers (millionths of a meter).

Occultation: The disappearance of the light of a celestial body by intervention of another body of larger apparent size; especially, a lunar eclipse of a star or planet.

Orbit transfer: In interplanetary travel, the process in which an elliptical trajectory tangent to the orbits of both the departure planet and the target planet takes place. An orbit transfer can also occur between two orbits of the same planet.

Plasma: A highly ionized gas which contains equal numbers of ions and electrons in sufficient density so that the Debye shielding length is much smaller than the dimensions of the gas. The Debye shielding length is the characteristic distance in a plasma beyond which the electric field of a charged particle is shielded by particles having charges of the opposite sign.

Plasma wave: A disturbance of a plasma involving oscillation of its constituent particles and of an electromagnetic field, which propagates from one point in the plasma to another without net motion of the plasma.

Radiation belt: An envelope of charged particles trapped in the magnetic field of a spatial body.

Radiometry: The detection and measurement of radiant electromagnetic energy, especially that associated with infrared radiation.

Solar wind: The supersonic flow of gas, composed of ionized hydrogen and helium, which continuously flows from the sun out through the solar system with velocities of 180 to 600 miles (300 to 1000 kilometers) per second; it carries magnetic fields from the sun.

Tectonic: Pertaining to tectonics, the branch of geology that deals with regional structural and deformational features of the earth's (or other planet's) crust, including the mutual relations, origin, and historical evolution of the features.

Visual and Infrared Mapping Spectrometer (VIMS): A remote sensing instrument that is made up of two cameras in one: one is used to measure visible wavelengths, the other infrared. VIMS captures images using visible and infrared light to learn more about the composition of moon surfaces, the rings, and the atmospheres of Saturn and Titan. The VIMS also observes the sunlight and starlight that passes through the rings to learn more about ring structure. The VIMS is designed to measure reflected and emitted radiation from atmospheres, rings and surfaces over wavelengths from 0.35 to 5.1 micrometers. It will also help determine the compositions, temperatures, and structures of these objects.