A satellite of Saturn, discovered by Giovanni Cassini in 1671. The Voyager probes confirmed a hypothesis, proposed by Cassini after he noticed variations in Iapetus’s brightness, that one hemisphere is very much darker than the other. The satellite always keeps the same face towards Saturn and, as it orbits the planet, its dark and light hemispheres alternately face the Earth. The bright surface is cratered and probably ice-covered; the dark part is blanketed with a material ten times darker whose nature and origin are unknown. The density of 1.2 times that of water suggests that Iapetus contains a high proportion of ice, possibly including frozen methane and ammonia.

Abbreviation for International Astronomical Union.

Abbreviation for Index Catalogue.

Asteroid 1566, diameter 1.4 km, discovered in 1949 by W. Baade. A member of the Apollo group, its highly elliptical orbit takes it closer to the Sun than Mercury.

Abbreviation for International Cometary Explorer.

A planetary body characterized by relatively small size and a composition consisting of a mixture of ices and rock. Examples include Pluto, planetary moons such as Triton, and objects populating the Kuiper Belt.

Abbreviation for intracluster medium.

Asteroid 243, a member of the Koronis family, measuring 58 km by 23 km. Close-up images of Ida were obtained on 28 August 1993 by the Galileo spacecraft en route to Jupiter. Galileo discovered that Ida has a small satellite, subsequently named Dactyl, which measures about 1.6 km by 1.2 km. Observations of Dactyl’s orbital motion made it possible to determine that Ida’s density lies in the range 2.2-2.9 g/cm3. The two bodies do not have an identical composition, suggesting that the system may result from the collision and break-up of larger bodies that created the Koronis family. The surfaces of both bodies are heavily cratered.

Abbreviation for interplanetary dust particle.

A rock formed directly by cooling and solidification from a molten state.

Abbreviation for International Geophysical Year.

Comet Ikeya-Seki.

An electronic device for amplifying the brightness of an image. Amplification is achieved by the multistage production of photoelectrons. A photocathode is coated on the inside of the entry window. The electrons produced when a light signal strikes the window are accelerated through a potential of about 40 kV. Focusing is by the magnetic field of a surrounding solenoid combined with the electrostatic fields of ring electrodes; alternatively, the focusing may be entirely electrostatic. Further amplification is introduced by the use of more than one photocathode stage. The image finally formed on a phosphor screen is many times brighter than the original. The final detector may be a television camera or a charge-coupled device. A drawback of the system is the unavoidable distortion in the image; nevertheless, such devices have found wide application in astronomy.

An electronic imaging light detector sensitive enough to record and count the arrival of single photons.

An alternative name for an image intensifier.