Even though radio astronomy began at 20 MHz (corresponding to a wavelength of 15 meters) with Karl Jansky, until recently ionospheric effects severely limited the angular resolution and sensitivity of most low-frequency (less than 100 MHz) telescopes. Other barriers included radio frequency interference (RFI), the need for non-coplanar-array imaging, and other problems that until only recently were exacerbated by limitations in computational power. As a result, the low-frequency universe, particularly the region below 100 MHz, has been relatively neglected in the race for higher angular resolution and sensitivity.
The observing limitations due to the ionosphere and the move to ever shorter wavelengths to achieve higher resolution with fixed dish and array sizes has meant that most areas at low frequencies are still relatively unexplored with modern techniques and instruments, leaving a region rich in scientific potential ripe for exploitation.
It is a region that favors studies of non-thermal and coherent emission processes, and provides an intrinsic link to shock physics, high-energy phenomena, and the high-redshift Universe. It can provide unique insights into the interaction of thermal and non-thermal sources through absorption and scattering processes, and the intrinsically large field-of-view and high surface-brightness sensitivity can be of great advantage. Moreover the ability to utilize fully electronic, digital, rapidly reconfigurable multi-beaming arrays opens the possibility to develop entirely new fields of astrophysical study. Exciting possibilities include harnessing studies of clusters of galaxies to constrain the cosmological evolution of Dark Matter and Dark Energy, the detection of the radio signature from the Epoch of Reionization or even earlier from the Dark Ages, and the first detailed study of the poorly explored Radio Transient Universe that might bring the detection of magnetized, extra-solar planets.
The demonstration that self-calibration and increasingly more sophisticated means of ionospheric calibration can remove the ionospheric limit on baseline length even at the lowest frequencies (below 100 MHz) is being elegantly demonstrated at the NRL-NRAO 74 MHz system at the VLA.. The success of that system and the Moore's law driven leaps in both computational and digital signal processing power have launched a renaissance in this long neglected field that is now seeing the emergence of a new generation of large, electronic arrays, including the Long Wavelength Array, the Low Frequency Array, and the Low Frequency Demonstrator at the Murchison Widefield Array. At the same time, the ability to realize Earth-orbiting, deep space, and lunar arrays promises the possibility to pursue even lower frequency astrophysics below the ionospheric cutoff near ~10 MHz.
These exciting development herald a rebirth of work in low frequency astrophysics, and NRL's Radio Astrophysics & Sensing Section is playing a leading role in realizing both ground (through the Long Wavelength Array,), and Space- & Lunar-based arrays and in developing the techniques needed to fully exploit them. An early and compact presentation of the new astrophysics to be done and instruments to carry it out is in the book Low Frequency Astrophysics from Space, which is the proceedings of a meeting held on 8 & 9 January 1990 in Crystal City, Virginia. There more than fifty participants from all parts of the world presented the scientific need for coordinated ground, space, and lunar programs to pursue the many areas of astrophysics which can only be probed at the lowest radio frequencies.
"Unfortunately and quite surprisingly, some astronomers tend to regard low frequencies as an area of astronomy where nothing has ever happened in the past and nothing is likely to happen in the future. Besides being a very narrow mind set which disregards the need of modern astrophysics for observations across the entire electromagnetic spectrum, such a view is quite wrong in all respects. ...the past five decades have seen many of the most exciting discoveries in astronomy made at low frequencies (including two Nobel prizes). Now, with new (ground-based) instruments, and, particularly, space and lunar initiatives, the field promises a bright future."
(From Kassim and Weiler's Preface to Proceedings of the Low Frequency Astrophysics from Space meeting held in January 1990)