image The optical representation of an object produced when light from the object is reflected or refracted by a mirror or lens.

inertia The tendency of an object to continue in motion at the same speed and in the same direction, unless acted upon by a force.

inflation Short period of unchecked cosmic expansion early in the history of the universe. During inflation, the universe swelled in size by a factor of about 1050.

infrared Region of the electromagnetic spectrum just outside the visible range, corresponding to light of a slightly longer wavelength than red light.

infrared telescope Telescope designed to detect infrared radiation. Many such telescopes are designed to be lightweight so that they can be carried above most of Earth's atmosphere by balloons, airplanes, or satellites.

inner core The central part of Earth's core, believed to be solid, and composed mainly of nickel and iron.

intensity A basic property of electromagnetic radiation that specifies the amount or strength of the radiation.

intercrater plains Regions on the surface of Mercury that do not show extensive cratering, but are relatively smooth.

interference The ability of two or more waves to interact in such a way that they either reinforce or cancel each other.

interferometer Collection of two or more telescopes working together as a team, observing the same object at the same time and at the same wavelength. The effective diameter of an interferometer is equal to the distance between its outermost telescopes.

interferometry Technique in widespread use to dramatically improve the resolution of radio and infrared maps. Several telescopes observe an object simultaneously, and a computer analyzes how the signals interfere with one another to reconstruct a detailed image of the field of view.

interplanetary space The space between the objects in the solar system.

interstellar dust Microscopic dust grains that populate the space between stars, having their origins in the ejected matter of long-dead stars.

interstellar medium The matter between stars, composed of two components, gas and dust, intermixed throughout all of space.

inverse-square law The law that a field follows if its strength decreases with the square of the distance. Fields that follow the inverse square law rapidly decrease in strength as the distance increases, but never quite reach zero.

Io plasma torus Doughnut-shaped region of energetic ionized particles, emitted by the volcanoes on Jupiter's moon Io, and swept up by Jupiter's magnetic field.

ion An atom that has lost one or more electrons.

ion tail Thin stream of ionized gas that is pushed away from the head of a comet by the solar wind. It extends directly away from the Sun. Often referred to as a plasma tail.

ionized State of an atom that has had at least one of its electrons removed.

ionosphere Layer in Earth's atmosphere above about 100 km where the atmosphere is significantly ionized, and conducts electricity.

irregular galaxy A galaxy which does not fit into any of the other major categories in the Hubble classification scheme.

isotopes Nuclei containing the same number of protons but different numbers of neutrons. Most elements can exist in several isotopic forms. A common example of an isotope is deuterium, which differs from normal hydrogen by the presence of an extra neutron in the nucleus.

isotropic Assumed property of the universe such that the universe looks the same in every direction.

isotropy Assumed property of the universe such that the universe looks the same in every direction.