Today, many researchers believe we are on the brink of finding convincing evidence for extraterrestrial life – a discovery that would generate widespread public interest and be of profound philosophical and scientific importance. Several avenues of research could lead to this discovery. The first is the quest for the physical evidence of biology, living or remnant, on the planets and moons of our solar system. Mars, Europa, and Titan are nearby worlds where life may have sprung up, and all are the targets of current or planned space missions. Needless to say, such missions are costly, and substantial resources will be expended during the coming decades in the effort to learn if any of these cosmic neighbors has been, or is, a home to living things (NASA 1997).
A second approach to finding extraterrestrial life is inspired by the recent discovery of planets around nearby stars (Mayor et al. 1997, Marcy and Butler 1998). We now know that at least a few percent of solar-type stars have planets, nd the fraction could be far higher. This discovery has spurred major new experiments that will use large, space-borne telescopes to find the small, Earth-like planets that are still beyond our reach. These telescopes would be able to collect the light of such planets, providing scientists with the opportunity to study their spectra. This leads to an intriguing possibility, the opportunity to detect primitive life at a great distance. Earth’s atmosphere contains a high proportion of oxygen and methane, a direct consequence of the presence of life on this planet. If studies of other planets also reveal the spectral signature of these gasses, we could infer that at least simple life exists on these distant worlds.
The third approach to finding biology in the cosmos is, for both the public and much of the scientific community, the most interesting. By looking for artificially-generated radio or light signals from other star systems, we can hope to prove the existence of not just life, but intelligent, technically sophisticated life. This is the only feasible method we have to find out if we share the universe with other sentient beings. Nearly all searches for extraterrestrial intelligence (SETI) have been made using large radio telescopes operating at microwave frequencies. Table 4.3 summarizes the basic parameters of the most sensitive of recent SETI programs.
Surveys that are not yet complete are shown in italics. The parameters for these searches are estimated. The Phoenix survey targets roughly one thousand nearby sun-like stars, and achieves high sensitivity and wide frequency coverage because it can spend many hours per star. The Berkeley and Harvard programs are scanning surveys, observing large portions of the sky but at the cost of lower sensitivity and narrower frequency coverage.
The last columns of the table show the required equivalent isotropic power (EIRP) for a transmitter to be detected at the distance of the nearest star (4 light-years) and at 1,000 light-years. There are about one million solar-type stars within the latter distance. For comparison, note that a typical TV station has an EIRP of 106 watts, while Earth’s most powerful transmitter, the Arecibo planetary radar, has an EIRP of 1013 watts.
Perusal of the table shows that current SETI efforts operate at sensitivity levels that leave much to be desired. The SKA would substantially improve our ability to detect signals, and consequently might lead to an early SETI success. The table lists the expected performance of a “first cut” SETI survey that could be undertaken with the SKA in a shared mode, using 20% of the beams over five years. This SKA survey is markedly better than current searches in several ways. To begin with, the sensitivity would be improved by two orders of magnitude. For the first time, we would be able to detect civilizations radiating signals similar to our own at interstellar distances, and be capable of finding modest radio beacons from a significant fraction of our galaxy. In addition, the availability of many beams and the large instantaneous bandwidth allow an improvement in search speed of about two orders of magnitude. This would allow an increase in the number of star systems examined from 103 to 105 or even 106, as well as permitting a search across a wider bandwidth. Finally, the expected interference mitigation schemes planned for the SKA, combined with the ability to make observations of many stars simultaneously, would likely permit SETI experiments to overcome the growing problem of terrestrial interference, a problem that might make such observations impossible with conventional radio telescopes.
The SKA would enable an extraordinary advance for one of humankind’s most exciting explorations, the effort to learn our place in the biological universe.