radiation in the frequency range from 50 to 750 kHz, or lightning, which produces strong interference in the range from 1 to 30 MHz and above—preclude observations below 30 MHz (the very low frequency [VLF] range) except under exceptional circumstances, or at special locations and for limited amounts of time.
A variety of astronomical phenomena are expected to emit radiation at the wavelengths affected by terrestrial radio noise. These include non-thermal emission from the Milky Way galaxy, pulsars, interstellar scintillation, active galactic nuclei, and clusters of galaxies, as well as the Sun and Jupiter. Much higher up in frequency, neutral atmospheric gases—particularly atmospheric water vapor—attenuate cosmic radiation increasingly strongly above 10 GHz, with attenuation peaking around 22 GHz. Strong oxygen lines attenuate heavily near 60 and 120 GHz, and water lines around 183 GHz.
For observations at λ ≈ 0.2 to 3 μm, sunlight scattered by zodiacal dust grains is the dominant source of diffuse background emissions and can, hence, be the dominant noise source for observations of faint sources. Observations from Pioneer 101 and Helios 1 and 22 spacecraft suggest that zodiacal brightness declines with heliocentric distance as Iz α r–2.3 or Iz α r–2.5. An observatory at 5 AU could have ≈ 50× lower zodiacal background than current or planned ultraviolet/optical/infrared observatories in Earth-trailing or L2 orbits. Reducing the zodiacal background further is of limited use, as diffuse galactic emission and the mean extragalactic flux are ≈ 10−2 of the 1-AU zodiacal background near 800 nm.
For background-limited observations of unresolved sources of specific flux fυ, the signal-to-noise (S/N) ratio acquired in time T from a diffraction-limited telescope of diameter D scales as:
The last factor is the bandwidth of the observation. The lowered zodiacal background at 5 AU could increase observing efficiency by a factor of 50. This gain is realized only when the diffuse background is the dominant noise source. For brighter sources, shot noise in the source photons is dominant. For 2-meter-class visible telescopes at 1 AU, such as the Hubble Space Telescope (HST) or the proposed Supernova/Acceleration Probe, any source brighter than V ≈ 29 mag is brighter than the diffuse background—nearly every star within 10 kpc, for example. The zodiacal brightness in the near-infrared is similar to that in the visible and drops precipitously into the ultraviolet, so it is unlikely that observing beyond 1 AU would be of use in observations of stars in the Milky Way.
Study of stars beyond the Milky Way, for example in elliptical galaxies, requires reaching V > 29 mag. But such observations also require very high angular resolution, much better than that afforded by the HST, to eliminate crowding of stars and resolve the population. Hence an increase in D to improve resolution (and S/N) would be much more useful than a reduction in Iz. There is hence little S/N incentive to move beyond L2 for the observation of point sources at ultraviolet, optical, and infrared wavelengths.
A major thrust of the astronomy and astrophysics (AAp) decadal survey3 and NASA’s exploration initiative is the detection and study of extrasolar planets. For such observations, there is little S/N incentive to reducing the solar zodiacal background by going to >1-AU orbits, because most of the targets will be embedded in a dust disk about their host stars that is a significantly larger and unavoidable source of background photons. Thus, the first reconnaissance and characterization of extrasolar planets will be done from a near-Earth vantage point.