Space-based visible/IR telescopes offer considerable advantages over ground-based and airborne telescopes in that the effects of the terrestrial atmosphere are almost completely eliminated (although even the HST at an altitude of 600 km experiences some sensitivity to terrestrial UV airglow). Hence, the complete, unobscured spectra of the giant planets may be observed and angular resolution is, if the telescope and detection system are correctly constructed, diffraction-limited. Of course the angular diameters of the planets are unchanged and so very fine resolution, highly sensitive instruments are still required to image the farthest giant planet, Neptune. Although a number of space observatories have now been launched, only five have been used for planetary observations: the Hubble Space Telescope (HST), the Infrared Space Observatory (ISO), the Submillimeter Wave Astronomy Satellite (SWAS), the Spitzer Telescope, and the AKARI space telescope, which will now be described.
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