Igneous rock

Rock that has crystallised from a molten state (magma).

Illuminated (back/front)

Describes a type of electromagnetic wave detector. In solid-state detectors such as CCD silicon chips, the electrodes are all laid on one face of the chip. The incoming X-rays, for example, can enter the silicon detector either on the electrode (front-illuminated) side or the rear (back-illuminated) surface of the detector.


(1) Figure formed by rays of light or other electromagnetic radiation at a mirror's focal plane.

(2) A picture obtained by a telescope, camera, or other imaging device.


A device for producing an image.


Making an image.

Impact crater

See Crater

Inclination angle

The angle between the rotation axis of a planet and the perpendicular to the ecliptic plane.

Inflationary Universe

A model of the early evolution of the Universe involving its exponential expansion. This hypothetical phase in the evolution of the early Universe has been introduced to account for the observed isotropy of the Universe on a large scale and the fact that its density is within a factor of ten of the critical density. In the most popular version of the theory, the exponential expansion is associated with a phase change which occurred about 10-34 seconds after the beginning of the Big Bang. According to grand unified theories of elementary particles, the strong force decoupled from the electroweak force (a force which unifies the electromagnetic and weak nuclear forces) at this time. This event released enormous energy, stored until then in the vacuum of space-time. This scenario can account for the present vast extent of the Universe and its uniformity.

Infrared light

Infrared radiation (or infrared light) is invisible to the human eye, but can be sensed as 'heat', or thermal radiation. Even cold objects emit infrared radiation. It has a wavelength between 7000 Angstroms (less than a micron) and several hundred microns. (See also Electromagnetic radiation.) Only a small fraction of the infrared light coming from astronomical objects can go through the Earth's atmosphere: to detect the full range of infrared wavelengths a space telescope is needed. Cold and dusty astronomical objects – such as planets, asteroids or star forming regions – are best observed with infrared telescopes. ESA's Infrared Space Observatory (ISO), operating at wavelengths from 2.5 to 240 microns, could observe objects that remain hidden for optical telescopes. The next ESA infrared space telescope, Herschel, will peer even deeper into these objects.

Injection (into orbit)

The placing of a satellite into its required orbit or trajectory.


Apparatus capable of registering information with a precise objective. A science spacecraft can carry several instruments such as cameras, spectrographs, magnetometers, gas analysers, etc.

Integration (satellite, instrument)

Construction phase for a satellite, instrument or piece of equipment, when its constituent parts are assembled.


Measurement device in which a beam of electromagnetic radiation is split and subsequently recombined after travelling different pathlengths so that the beams interfere and produce an interference pattern. This pattern can be used to measure a wide variety of physical parameters.


Experimental technique making use of an interferometer to measure a physical parameter.

Interplanetary voyage

The journey of a spacecraft through the planets of the Solar System.


Between celestial objects; often refers to matter in space that is not a star, such as clouds of dust and gas.


An electrically charged atom. It has either lost or gained one or more electrons.


A chemical element is characterised by the number of protons in its atomic nucleus. Isotopes are atoms of the same element, i.e. with the same number of protons, but with different numbers of neutrons. Isotopes have the same atomic number but differ in their mass number.


Having the same value of some physical property (e.g., density) when measured in any direction.

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