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IAPETUS Iapetus is one of the 18 moons of Saturn. It is a rocky moon that is partly bright and partly reddish in color. Craters mark the surface. Iapetus has a diameter of about 880 miles (1,420 km). Iapetus orbits at about 3,561,300 km from Saturn. Its orbital period is 79 days. It was discovered by Cassini in 1671.

ICE AGE An ice age is a time lasting thousands of years during which the Earth is very cold and largely covered by ice and glaciers.

IGNEOUS ROCK When molten rock cools, igneous rock is formed.

IMPACT BASIN A impact basin is an impact crater that has a rim diameter greater than 185 miles (300 km). There are over 40 impact basins on the Moon, including Schrodinger (above). These catastrophic impacts produce faulting and other crust deformations. Material ejected from impact basins is distributed over wide areas.

IMPACT CRATER Impact craters are the remains of collisions between an asteroid or meteorite and a planet or moon.

IMPACT MELT Impact melt is rock that melts during impact crater formation. When an objects strikes a planet or moon, forming a crater, the energy released at impact causes extremely high temperatures which can melt some surrounding rock, forming pools or sheets of lava at the bottom of the crater. Impact melt varies greatly in texture, but is uniform in its composition, made mostly of the planet's or moon's rocks, but also containing a bit of the impactor (asteroid, comet, or meteorite) material.

INCANDESCENT An incandescent material is so hot that it glows, producing light. Incandescent solids, liquids, and compressed gases produce a continuous spectrum; other gases produce a line or emission spectrum (only a few wavelengths are emitted).

INCLINATION Inclination is the angle between the plane of an orbit and the plane of the ecliptic. Orbital inclination is abbreviated as i.

INERTIA Inertia is a property of matter: a mass at rest remains at rest and a mass in motion remains in motion as long as no outside force acts upon it.

INDEX CATALOG The Index Catalogs are two supplements to the New General Catalog, listing nebulae and star clusters with an IC number. These supplements were published in 895 and 1908.

INFERIOR PLANETS Mercury and Venus are called inferior planets because they are closer to the Sun than Earth. [Planets that are farther from the Sun than Earth are called superior planets.]

INFRARED RADIATION Infrared radiation is electromagnetic radiation that we can feel as heat.

INNER PLANETS The inner planets are those planets that orbit close to the sun. They are: Mercury, Venus, Earth, and Mars. They are relatively small, composed mostly of rock, and have few or no moons.

INTERFEROMETRY In interferometry, the data from two separate telescopes are used silmutaneouly to create a very good image (much better than either telescope alone).

INTERGRANULAR LANES Intergranular lanes are dark, cool areas between granules where material is descending below the surface of the Sun.

INTERNATIONAL SPACE STATION The International Space Station is an orbiting structure where people can live in space. Construction (in orbit) on the new International Space station began in 1998. The first crew will be launched on a Russian Soyuz spacecraft for a three-month stay beginning January 2000.

INTERSTELLAR DUST Interstellar dust is composed of mictoscopic bits (on the order of a micron in diameter) of carbon and/or silicates. The origin of interstellar dust in unknown, but it seems to be associated with young stars. Interstellar dust is not at all like the dust we have in our houses (which is mostly bits of organic debris and lint).

INTERSTELLAR MEDIUM The interstellar medium is the dust and gas (mostly hydrogen) that are between stars in a galaxy. The interstellar medium is no very dense at all; at its densest, it is emptier than the best vacuum we can produce on Earth.

IO Io is a large, rocky, volcanically active moon of Jupiter. Its volcanoes spew out molten sulphur, making Io a very colorful moon. It is the closest of Jupiter's four large moons and the third largest. It has a diameter of 1,942 miles (3,636 km). Io's mean distance from Jupiter is 220,000 miles (422,000 km). It has a mass of 8.93x1022 kg. It takes Io 1.77 days to orbit Jupiter. Io was discovered by Galileo and Marius (independently) in 1610.

IO TORUS The Io Torus is a doughnut-shaped plasma cloud around Jupiter near Io's orbit (also known as the "Io plasma torus") This torus is caused by Jupiter's strong magnetic field, which strips ions from Io as it rotates; Io acts like an electrical generator.

ION An ion is an atom that is missing one or more electrons; ions have an electrical charge.

ION ENGINE An ion engine is an engine that ionizes a gas in order to propel a spacecraft. A electrical charge is used to ionize xenon gas (an inert gas like neon or helium, but heavier). In an electrical field the ions are accelerated to a speed of about 30 km/second. The high-speed xenon ions are expelled as exhaust, pushing the spacecraft in the other direction (since momentum is conserved). This efficient type of engine delivers 10 times the thrust of conventional engines for the same mass of fuel. This engine is only used in outer space. The ion drive is used in NASA's Deep Space 1; it carries 181 pounds of xenon.

IONOSPHERE The ionosphere is one of the highest layer of the Earth's atmosphere. The ionosphere starts at about 43-50 miles (70-80 km) high and continues for hundreds of miles (about 400 miles = 640 km). It contains many ions and free electrons (plasma). The ions are created when sunlight hits atoms and tears off some electrons. The ionosphere is located between the mesosphere and the exosphere (and is part of the thermosphere). Auroras occur in the ionosphere.

IRIDIUM ANOMALY Iridium is an element that is rare on the Earth's surface, but abundant on chondritic meteors. The presence of excess Iridium at the K-T layer (the Iridium anomaly) supports the Alvarez asteroid theory.

IRON METEORITE An iron meteorite is a meteor made of the metal iron that has fallen to Earth.

IRREGULAR GALAXY An irregular galaxy is a galaxy with no rotational symmetry (it is neither spital, elliptical nor lenticular). Irregular galaxies usually contain only 100 million to 10 billion stars. Some are cloud-lie (with no apparent structure) and others are more standard shapes that have been distturbed. For example, the Magellanic Cloud is an irregular galaxy.

IRREGULAR GALAXY CLUSTER An irregular galaxy cluster is a group of any type of galaxies (not just irrgular galaxies) that forms an irregular (non-spherical) shape. The local group is an irregular galaxy cluster.

ISOTOPE An isotope of an element is another form of the same element, that has a different number of neutrons in the nucleus (giving it a different atomic weight).

ISOTROPY Isotropy is a state in which a physical characteristic (like temperature) is constant in value along axes in different directions. A physical measurement made in one direction is the same as the measurement made in another direction.