It can be said that remote sensing began with Galileo's invention of the telescope and covers the entire history of observational astronomy to this day. Recently, the subject has rapidly advanced with the development of observing techniques made possible by the invention of new detectors and telescopes over the entire electromagnetic spectrum from gamma rays to meter-wavelength radio emission. Superbly quantitative measurements of distant planets, satellites, comets and asteroids are now routinely made using the techniques of visible, IR and microwave spectrometry in addition to radar mapping and broadband photometry in all bands. Beautiful results, often reaching the sublime, have been obtained with ingenious instruments aboard robotic spacecraft, in particular the Voyager and Magellan vehicles. Equally ingenious are observations and experiments made from Earthbound astronomical facilities operated by distant relatives of the great master and innovator. Here, we limit this 5-year review to these latter efforts, partly because there have been few spacecraft successes in this period (except for Magellan), but also because our understanding of the solar system has continued to grow in this period with the use of Earthbased techniques. The Earth orbiting Hubble Telescope is now a magnificent success in the visible and near IR bands and is proving to be a powerful tool for solar system imaging, easily exceeding that of the Earthbound telescopes. This subject is too vast to review here. There is as yet no real progress to report on the search for other solar systems. These investigations await the development of arrayed telescopes in all wavelength bands, (such as the two Keck telescopes on Mauna Kea), a program that will probably remain ``Earthbound'' for the next decade or so. Preliminary work has been carried out with arrayed telescopes in the millimeter wavelength band. Many ideas and very preliminary results can be found in Beckwith and Sargent [1992], and Sargent and Beckwith [1993].
Photographic images in visible reflected sunlight of all the planets except Pluto and many satellites have been collected by the Explorer, Mariner and Viking spacecraft and little progress at visual wavelengths is possible from beneath the atmosphere, even on our highest mountains such as Mauna Kea. Rather, progress has primarily been made using spectroscopic techniques in the near- and far- IR and the microwave region. Radar studies of the terrestrial planets, the Galilean satellites and Titan have revealed new mysteries about the surfaces of these objects, surfaces that we can not sense in any other way. In this short review, we limit the discussion to new discoveries on Mercury, Mars, Titan, Pluto and Triton; the latter three objects being fascinating subjects for the next decade of planetary science. We also report on the discovery of a new class of solar system bodies called Kuiper Belt Objects [ Jewitt and Luu, 1993; Kuiper, 1951].