ABSTRACT
Astronomy, to a much greater degree than most other fields of science, is dependent on remote sensing measurements, because most of the objects under study are inaccessible to direct, in situ measurements or sample returns. Until the advent of space-based observations, astronomical observations (including planetary astronomy and remotesensing measurements of Earth’s upper atmosphere) were limited to the wavelength ranges of the electromagnetic spectrum that can penetrate Earth’s lower atmosphere, which is opaque to radiations having wavelengths below about 300 nm=3000 Å (see Fig. 1). This precluded observations of nearly all of the ultraviolet, x-ray, and gamma ray regions of the spectrum.
