The goal of an accelerated Delta-class Explorer program is to fly a total of six astrophysics Explorers during the 1990s, two or three more than are currently planned. This recommended flight rate is considered essential for a revitalized program of moderate missions, as discussed in . Although specific missions should be selected by the normal peer review process, the committee believes that three areas of space astronomy are particularly primed for Delta-class experiments: gamma-ray spectroscopy of galactic and extragalactic sources, a complete submillimeter line survey of important astronomical objects, and an x-ray telescope capable of making images with 60-arcsecond resolution in the energy range 10 to 250 keV. These and other possible Explorers are discussed in the Working Papers (NRC, 1991). A related recommendation is the acceleration of the Small Explorer (SMEX) program from its currently planned two or three astronomy missions in the 1990s to a total of five missions in the decade.
Cosmic-ray protons with energies greater than 1019 eV are not confined by the galactic magnetic field, so that their observation can reveal their point of origin, either galactic or extragalactic. The existing Fly's Eye telescope in Utah (see Plate 2.9) has detected some 200 fluorescent trails of highly energetic cosmic rays (1019 to 1020 eV) moving through the atmosphere. The direction, energy, and longitudinal development of the airshower can be measured. The present data suggest an isotropic distribution of particles with a flattening of the spectrum at energies above 1019 and a possible cutoff at energies above 1020. The longitudinal development of the airshower suggests that these particles are protons. These particles may come from outside of the galaxy, but few mechanisms are known that can accelerate particles to these energies, and no mechanisms are known that can fill the universe with such energetic particles. However, the statistics on which these conclusions are based are sparse. A new Fly's Eye telescope would be 10 times more sensitive and would detect many more events than the existing instrument. The statistics of more than 2,000 events in a few years would lead to a better determination of the energy spectrum and the isotropy of these energetic cosmic rays. The improved spectrum would help determine whether the cutoff at energies of 10 20 eV, expected from pion-producing interactions of protons with the 2.7 K cosmic background radiation (the Greisen effect), is real. The improved spatial resolution would be used to make more detailed studies of the longitudinal development of the airshowers and thereby infer the composition of the particles. This modestly priced facility will explore a new domain in cosmic-ray physics and could yield fundamental new insights.