New Results and Techniques in Space Radio Astronomy

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Abstract
The first long wavelength radio astronomical observations from above the ionosphere were performed in the early 1960's using very short antennas on both sounding rockets and satellites. Although these exploratory measurements were sometimes troubled by perturbations arising in the local plasma enivronment and by spacecraft interference, they did develop the fundamental information and techniques on which to build more sophisticated low frequency radio astronomical systems. The first spacecraft designed specifically and exclusively for radio astronomical studies, Radio Astronomy Explorer 1, was launched on July 4, 1968. It carries two gravity-gradient stabilized, 229-m, travelling-wave V-antennas, a 37-m dipole antenna, and a number of radiometer systems to provide measurements over the 0.2 to 9.2 MHz frequency range (λ = 33 to 1500 m) with a time resolution of 0.5 sec and an absolute accuracy of about 25%. Although the observations may seem crude in some cases, when compared with those of ground-based telescopes, the RAE has afforded substantial improvement over the capabilities of the early flight experiments. Through the study of solar radio bursts down to 0.2 MHz, we are getting new information on the density, plasma velocity, and dynamics of coronal streamers out to distances of greater than 50 R⊙. From galactic continuum background maps at frequencies around 4 MHz, we are getting new information on the distribution of the ionized component of the interstellar medium. Cosmic noise background spectra measured down to ~ 0.5 MHz are providing new estimates of the interstellar flux of cosmic rays, of magnetic fields in the galactic halo, and of the nature of radiation from distant extragalactic radio sources.A second RAE spacecraft is now being constructed for launch into a lunar orbit. This satellite will permit radio astronomical measurements at frequencies down to 30 kHz (λ = 10 km), will facilitate the use of the moon as an occulting disk for source position measurements, and will provide new information on the cislunar noise environment required to assess the feasibility of future lunar radio observatories. Studies are in progress on the feasibility and application of orbiting radio interferometers (baselines up to 5 km) and large orbiting filled-aperture telescopes (diameter ~ 1 km).