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Because the atmosphere severely degrades photometric precision, this detection scheme requires an instrument in Earth orbit. The double differential photometer required for the job is at least one generation of technology away, but this particular method may be worth pursuing because it is best suited to the detection of the small-orbit, short-period planets within any system.

The real utility of this approach may eventually be to search for the inner planets of planetary systems already detected by other methods. In general, all the other methods work best for low-mass stars and high-mass planets. It is Jupiters that will be found if anything in the near future, not other Earths.

To image a planet directly it is necessary to resolve its combined internal luminosity and reflected starlight from the overwhelming luminosity of the nearby star. No system in existence on Earth or planned for orbit comes close to achieving these accuracies.

In the near future the only planets to be directly imaged will be massive, in large orbits about low-mass stars, or much closer than 10 parsecs. Since the SSB reviewed the potential for direct imaging and recommended consideration of low-light-scattering requirements for future telescopes A Strategy for the Detection and Study of Extrasolar Planetary Materials: — , SSB, in press , there have been new studies yielding promising results.

The combination of a coronagraph to reduce the diffraction of starlight by the telescope and a supersmooth mirror to reduce scattered light holds great promise for the future. This telescope should greatly reduce diffracted light in the wings of an image of a point source Airy pattern over the entire field of view of the instrument.

Such an instrument would have to orbit the Earth to get above the detrimental effects of atmospheric viewing. A 2- m -diameter instrument in Earth orbit would allow the direct detection of a Jupiter-sized planet around a solar-type star out to a distance of approximately 10 parsecs. A larger instrument 10 m in diameter would be required to detect the Earth out to these distances, and the technology for such a large mirror has not been demonstrated.

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In the CIT , diffraction is controlled by a Lyot coronagraph. Combining a Lyot coronagraph with apodizing occulting masks in the first focal plane can probably reduce the contribution from diffracted starlight by a factor of However, this factor of improvement in diffracted light can be utilized only if the scattering due to figure error in the primary mirror and to surface dust or scratches is at least times less than the diffraction of a conventional mirror. Supersmooth mirrors have been manufactured for use in the fabrication of microelectronics.

These mirrors 0.

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During the fabrication of supersmooth mirrors, figure errors decrease monotonically until the desired specification is reached and then polishing stops. The metrology used to measure the figure of the mirrors can go beyond the current specifications, and there appears to be no inherent reason why much smoother mirrors could not be fabricated. This promising technology should be pursued in coming years. The study of protoplanetary nebulae and young stellar disk systems also requires direct imaging techniques.

Estimates of the required measurement accuracies for studying solar-system-scale objects at 10 parsecs or at parsecs Taurus molecular cloud star-forming region follow. To resolve a linear dimension of 1 AU at 10 parsecs requires a resolution of 0.

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  7. In the Taurus cloud, the corresponding resolutions become 0. To study dusty disks around young stars or protoplanetary nebulae in the process of collapse requires more than just spatial resolution; spectral resolution and good sensitivity are necessary. To locate the inner cutoff if any of a dust disk around a young star and to characterize the particle size distribution and density as a function of radius require the ability to suppress the luminosity of the central star, as in the case of individual direct planet detection.

    Once again, coronagraphs can be used, but their physical size will ultimately determine how close to the star the measurements can be made. Ground-based interferometers at millimeter, submillimeter, and infrared wavelengths may prove to be the instruments of choice to study the radial dependence of the continuum emission from the dust.

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    Long baselines, large collecting area elements, or many elements are required for sensitivity. To achieve the most ambitious objectives of remotely detecting, characterizing, and searching for evidence of life on an extrasolar planet, direct light from the planet must be examined without contamination from the light of the parent star. Only direct methods of planet detection are suitable for obtaining these observations.

    These large-aperture instruments may be monolithic but will most likely be composed of multiple phased elements. Therefore, additional technologies involved in stabilization of the point spread function of a multiple-element telescope will be needed. An alternative to a very large-aperture telescope has recently been suggested for imaging a distant Earthlike planet and spectroscopically analyzing its atmosphere in search of oxygen and ozone: an optical aperture-synthesis interferometer.

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    In particular, a element array of 2. The stellar light can be satisfactorily suppressed by the combined effects of apodization and precise positioning of the star in a null of the interferometer. If the distance is increased to 10 parsecs, the required integration time increases to hours.

    Is there a more rational way to scan the heavens for alien life?

    Whether such a scheme is feasible will depend ultimately on the accuracy with which the elements of the array can be maintained in relation to one another so that the star can be kept precisely at the interferometer null and the signal phase can be maintained. The positional requirements are extraordinary, far in excess of today's capabilities. Unless they are fortuitously close, the direct imaging and spectroscopic analysis of Earthlike planets are not likely to occur within the time scale envisaged in this report.

    Continued development and research are required to ascertain whether very large smooth mirrors or extreme positional accuracy in space can actually be demonstrated by the next generation of technology. The search for primitive life may not yet be timely, but the development of technology to meet required measurement accuracies is. The problem of finding life beyond the solar system may become more tractable if there exist extraterrestrial technologies engaged in activities that can be detected remotely.

    If intelligent aliens exist, why haven't we seen them?

    One very practical test of the idea that intelligent life exists beyond our solar system is based on the postulate that other technologies have transmitted either deliberately or unintentionally electromagnetic signals that can be received and recognized with extant technology here on Earth. In , it was proposed that transmissions in the neighborhood of the spectral line of neutral hydrogen MHz might be a means by which extraterrestrial technologies communicate with each other over interstellar distances.

    More than two decades of scientific debate and review have expanded this idea into a plan to systematically search through the terrestrial microwave window for signals originating from an extraterrestrial source of intelligence. The plan calls for the use of existing radio telescopes, mature microwave technology, and very large special-purpose multichannel spectrum analyzers and signal-processing systems to carry out a promising set of exploratory search strategies.

    The entire sky is to be scanned with moderate sensitivity over the frequency range of 1 to 10 GHz. A set of about nearby solar-type stars will be targeted for much more sensitive searches over the 1-to 3-GHz frequency range. The types of signals sought are those believed never produced by any natural process; they are compressed in frequency and perhaps in time as well.

    If implemented soon, this plan which was recommended by the Astronomy Survey Committee in Astronomy and Astrophysics for the s , Volume 1, National Research Council, should occupy most of the decade to be covered by this document. The measurement accuracy appropriate for such a search strategy may be determined by establishing the sensitivity required to detect the artificial microwave signals generated by current terrestrial technology, if the planet Earth was assumed to be located across the Milky Way galaxy from us.

    Since we do not deliberately transmit signals intended for communication with another technology, the Earth model must consist of signals generated for our own purposes that leak into interstellar space. The weakest, but by far the most numerous, signals are the narrowband carriers for radio and television broadcasts; these are rated at 10 6 to 10 7 W of effective isotropic radiated power EIRP.

    Successful detection by this planned search will require the existence of extraterrestrial transmitters that are more powerful perhaps intentional or closer than the other side of the Milky Way. Although the planned microwave search will not conclude until nearly the end of the next decade, advanced planning for follow-on searches should be conducted concurrently. The planned microwave search may fail to detect any signals, either because the strategy is flawed or because the sensitivity and coverage of parameter space were inadequate.

    The microwave region of the spectrum is ''preferred'' for such signal detection because the natural astrophysical background radiation is least at those frequencies. The naturally quiet microwave window extends to at least GHz , and signals from orbital transmitters may occupy the upper end of the window. The planned microwave search is restricted to the lower portion of the window by the increased atmospheric noise inherent to any terrestrial ground-based search. Future searches will require access to space to extend the search to higher frequencies and to escape the increasing interference generated by terrestrial communication technology.

    Conclusions concerning the best possible signal-to-natural-noise ratio as a function of frequency if space-based transmitters and receivers are assumed depend upon scaling laws for the construction of orbital structures.

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    Under certain scaling laws, the infrared appears to provide the best signal-to-noise ratio; under others, the microwave region is still preferred. Experience with on-orbit construction in the coming decades will allow an empirical determination of how the size of an antenna scales with wavelength.

    enter Therefore, it is essential to consider and review other plausible strategies for the detection of electromagnetic signals from technologies that generate them and to support development of those that are worthwhile, should they be required. Even a highly technological civilization might not deliberately generate signals that are detectable over interstellar distances.

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    The question is then whether the technology of such a civilization could be detected in some other way. It is impossible to extrapolate the likely progress of technology, here or elsewhere, with any degree of certainty. Although speculative, however, it is appropriate to consider those technological activities in which we now engage and ask how they might appear from afar if they were increased in scale and intensity as some scientists and engineers have proposed.

    Energy production or transformation will probably be an important concern for any advanced technology.