A new window for astrophysics
The opening of a new spectral window for astronomical investigations has always resulted in major discoveries, significant insights into astrophysical processes, and an enrichment of our understanding of the universe. Interferometric space or lunar-based arrays for imaging the entire sky below 30 MHz will cover a frequency range which is totally inaccessible or extremely difficult to observe from the ground due to ionospheric absorption and scattering. This is a region of the electromagnetic spectrum which is essentially unexplored by astronomy but which, at ~106 Hz, is likely to display phenomena as different from those at centimeter radio wavelengths (~109 Hz) as centimeter radio phenomena are from infrared (~1012 Hz), infrared are from the ultraviolet (~1015 Hz), or ultraviolet are from the X-ray (~1018 Hz).
Also, this low frequency window from ~30 kHz (just above the local plasma frequency of the interplanetary medium) to ~30 MHz (where high resolution observations from the ground become possible most of the time) spans three orders of magnitude in frequency, wider than the infrared window opened by IRAS and ISO or the ultraviolet window opened by IUE and EUVE. It is the last region of the electromagnetic spectrum which is still largely unexplored.
Because of this large gap in our knowledge, the likelihood of discovering new processes and objects is great, even though many worthwhile projects can already be defined for a new instrument. Jansky sensitivity and subarcminute resolution would be as much of an advance for the field as was the Einstein satellite for X-ray astronomy or the IRAS for infrared astronomy.
The need to go to space
High resolution (less than 1'), very low frequency (less than 30 MHz) observations cannot be done from the ground. The ionosphere is too severe a restriction. (Kassim et al. 1993) have shown that by using dual frequency observations at 327 MHz and 74 MHz with the VLA, ionospheric phase corrections can be introduced. However, they estimate that these techniques will begin to fail below ~30 MHz due to ionospheric scintillation. At these frequencies ionospheric phase and amplitude fluctuations become so large that even the strongest calibration sources become decorrelated and selfcal techniques fail.
The plasma frequency of the ionosphere is ~15 MHz on the day side of the earth near sunspot maximum and ~10 MHz on the night side near sunspot minimum, making the layer opaque to all lower frequencies. Even at preferred sites near the magnetic poles, such as Canada and Tasmania, and near sunspot minimum when ground based observations can be taken as low as ~2 MHz, the available resolution is extremely poor (several degrees). Therefore, to study this last unexplored window on the electromagnetic universe, one must go to space.
The astrophysical need and technical means to open this new window on the universe have been detailed in the proceedings from Workshops held in Green Bank, West Virginia in September 1986 (Radio Astronomy From Space; Weiler 1987) and in Crystal City, Virginia in January 1990 (Low Frequency Astrophysics from Space; Kassim & Weiler 1990).
The goal of the NRL program is to determine the requirements and technical parameters for LFSAs in Earth or lunar orbit, in deep space, or on the surface of the moon to perform high resolution, high sensitivity sky surveying at frequencies below the ionospheric cutoff and open this last unexplored frontier of astronomy.