Lunar Farside Radio Astronomy and Other Potential Astronomical Facilities

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Radio astronomy is the branch of astronomy that collects and evaluates radio signals from extraterrestrial radio sources. Radio astronomy is a relatively young branch of astronomy. It was started in the 1930s, when Karl Guthe Jansky (1905-50), an American radio engineer, detected the first extraterrestrial radio signals. Until Jansky's discovery, astronomers had used only the visible portion of the electromagnetic spectrum to view the universe.

The detailed observation of cosmic radio sources is difficult, however, because these sources shed so little energy on Earth. But starting in the mid-1940s with the pioneering work of the British astronomer Sir Alfred Charles Bernard Lovell (b. 1913), at the United Kingdom's Nuffield Radio Astronomy Laboratories at Jodrell Bank, the radio telescope has been used to discover some extraterrestrial radio sources so unusual that their very existence had not even been imagined or predicted by scientists.

One of the strangest of these cosmic radio sources is the pulsar—a collapsed giant star that has become a neutron star and now emits pulsating radio signals as it spins. When the first pulsar was detected in 1967, it created quite a stir in the scientific community. Because of the regularity of its signal, scientists thought they had just detected the first interstellar signals from an intelligent alien civilization.

Another interesting celestial object is the quasar, or quasi-stellar radio source. Discovered in 1964, quasars are now considered to be entire galaxies in which a very small part (perhaps only a few light-days across) releases enormous amounts of energy—equivalent to the total annihilation of millions of stars. Quasars are the most distant-known objects in the universe; some of them are receding from Earth at over 90 percent of the speed of light.

Farside Scientists

This artist's rendering depicts a very large radio telescope facility that has been constructed within an impact crater basin on the farside of the Moon (circa 2030). Use of the Moon's farside shields the giant radio telescope (which as shown here is much larger than the Arecibo Observatory) from the unwanted human-produced radio-frequency signals that now flood the terrestrial environment. (NASA)

This artist's rendering depicts a very large radio telescope facility that has been constructed within an impact crater basin on the farside of the Moon (circa 2030). Use of the Moon's farside shields the giant radio telescope (which as shown here is much larger than the Arecibo Observatory) from the unwanted human-produced radio-frequency signals that now flood the terrestrial environment. (NASA)

The Arecibo Observatory, a 1,000-foot- (305-m-) diameter giant metal dish, is the largest radio/radar telescope on Earth. The facility is located in a large, bowl-shaped natural depression in the tropical jungle of Puerto Rico. The Arecibo Observatory is the main observing instrument of the National Astronomy and Ionosphere Center (NAIC), a national center for radio and radar astronomy and ionospheric physics operated by Cornell University under contract with the National Science Foundation. The observatory operates on a continuous basis, 24 hours a day every day, providing observing time and logistic support to visiting scientists. When the giant telescope operates as a radio-wave receiver, it can listen for signals from celestial objects at the farthest reaches of the universe. As a radar transmitter/receiver, it assists astronomers and planetary scientists by bouncing signals off the Moon, off nearby planets and their satellites, asteroids, and even off layers of Earth's ionosphere.

The Arecibo Observatory has made many contributions to astronomy and astrophysics. In 1965 the facility (operating as a radar transmitter/ receiver) determined that the rotation rate of the planet Mercury was 59 days rather than the previously estimated value of 88 days. In 1974 the facility (operating as a radio-wave receiver) supported the discovery of the first binary pulsar system. This discovery led to an important confirmation of Albert Einstein's theory of general relativity and earned the American physicists Russell A. Hulse (b. 1950) and Joseph Hooten Taylor Jr. (b. 1941) the 1993 Nobel Prize in physics. In the early 1990s, astronomers used the facility to discover extrasolar planets in orbit around the rapidly rotating pulsar B1257+12.

Using the Arecibo Observatory as an example, several lunar-base planners have suggested selecting appropriate natural impact craters on the Moon in which to construct even larger radio telescopes. Radio astronomy from the lunar surface offers the distinct advantages of a low radio-noise environment and a stable platform in a low-gravity environment. The farside of the Moon is permanently shielded from direct terrestrial radio emissions. As future radio telescope designs approach their ultimate (theoretical) performance limits, this uniquely quiet lunar environment

An artist's rendering of an advanced robotic optical telescope on the lunar surface that uses a self-guided "walking" mobility platform. In one lunar-based astronomy scenario, several such identical telescopes communicate with each other and arrange themselves at appropriate locations and distances across the Moon's surface in order to function as a very large interferometer. (NASA/JSC; artist, Pat Rawlings)

An artist's rendering of an advanced robotic optical telescope on the lunar surface that uses a self-guided "walking" mobility platform. In one lunar-based astronomy scenario, several such identical telescopes communicate with each other and arrange themselves at appropriate locations and distances across the Moon's surface in order to function as a very large interferometer. (NASA/JSC; artist, Pat Rawlings)

may be the only location in all cislunar space where sensitive, radio-wave-detection instruments can be used to full advantage, both in radio astronomy and in the search for extraterrestrial intelligence (SETI). In fact, radio astronomy, including extensive SETI efforts, should represent one of the main lunar industries by the end of this century. In a certain sense, SETI performed by lunar-based scientists will be extraterrestrials searching for other extraterrestrials.

The Moon also provides a solid, seismically stable, low-gravity, high-vacuum platform for conducting precise interferometric and astrometric observations. An interferometer is an instrument that achieves high angular resolution by combining signals from at least two widely separated telescopes (optical interferometer) or a widely separated antenna array (radio interferometer). Radio interferometers are one of the basic instruments of radio astronomy. In principle, the interferometer produces and measures interference fringes from two or more coherent wave trains from the same source. These instruments are used to measure wavelengths, to measure the angular width of sources, to determine the angular position of sources, and for many other scientific purposes.

The Very Large Array (VLA) is an example of the type of extended radio-astronomy facility that can also be constructed in the lunar surface. The VLA is a spatially extended radio-telescope facility at Socorro, New Mexico. This facility consists of 27 antennae, each 82 feet (25 m) in diameter, that are configured in a giant "Y" arrangement on railroad tracks over a 12.4-mile (20-km) distance. The VLA is operated by the National Radio Astronomy Observatory and sponsored by the National Science Foundation.

The VLA has four major antenna configurations: A array, with a maximum antenna separation of 22.4 miles (36 km); B array, with a maximum antenna separation of 6.2 miles (10 km); C array, with a maximum antenna separation of 2.2 miles (3.6 km); and D array, with a maximum antenna separation of 0.6 mile (1 km). The operating resolution of the VLA is determined by the size of the array. At its highest, the facility has a resolution of 0.04 arc seconds. This corresponds, for example, to an ability to "see" a 43-gigahertz (GHz) radio-frequency source the size of a golf ball at a distance of 93 miles (150 km). The facility collects the faint radio waves emitted by a variety of interesting celestial objects and produces radio images of these objects with as much clarity and resolution as the photographs from some of the world's largest optical telescopes.

The technique of focusing and combining the signals from a distributed array of smaller telescopes to simulate the resolution of a single, much-larger telescope is called aperture synthesis. In astronomy a radio telescope is used to measure the intensity of the weak, static-like cosmic-radio waves coming from some particular direction in the universe.

orbiting Quarantine Facility

The Orbiting Quarantine Facility is a proposed Earth-orbiting laboratory in which soil and rock samples from other worlds—for example, Martian soil and rock specimens—could first be tested and examined for potentially harmful alien microorganisms, before the specimens are allowed to enter Earth's biosphere. A space-based quarantine facility, either in stable orbit around Earth or on the surface of the Moon (in association with a permanent lunar base) provides several distinct advantages: (1) It eliminates the possibility of a sample-return spacecraft crashing and accidentally releasing its potentially deadly cargo of alien microorganisms; (2) it guarantees that any alien organisms that might escape from the orbiting laboratory's confinement facilities cannot immediately enter Earth's biosphere; and (3) it ensures that all quarantine workers remain in total isolation during protocol testing of the alien soil and rock samples.

Three hypothetical results of such protocol testing are: (1) no replicating alien organisms are discovered; (2) replicating alien organisms are discovered, but they are also found not to be a threat to terrestrial life-forms; or (3) hazardous, replicating alien life-forms are discovered. If potentially harmful replicating alien organisms were discovered during these protocol tests, then orbiting quarantine-facility workers would either render the sample harmless (for example, through heat- and chemical-sterilization procedures); retain it under very carefully controlled conditions in the orbiting complex and perform more detailed studies on the alien life-forms; or properly dispose of the sample before the alien life-forms could enter Earth's biosphere and infect terrestrial life-forms.

In addition to protocol testing, a quarantine facility associated with a future lunar base could provide scientists the unique opportunity of performing detailed studies on alien samples—perhaps even simulating alien-world environments in a type of extraterrestrial zoo. Exobiologists at the lunar base and on Earth (through telescience) could examine the alien life-forms (most likely microscopic organisms) without the fear of having a containment breach that then endangers the terrestrial biosphere. For further protection, the Moon-based quarantine facility could be located a significant distance away from the main base, with only authorized human personnel (or teleoperated robots) allowed inside the actual alien-world chambers.

Sensitivity is defined as the radio telescope's ability to detect these very weak radio signals, while resolution is defined as the telescope's ability to locate the source of these signals. The sensitivity of a distributed array of telescopes, such as the VLA, is proportional to the sum of the collecting areas of all the individual elements, while the array's resolution is determined by the distance (baseline) over which the array elements can be spread. Each of the 82-foot- (25-m-) diameter dish antennae in the VLA was specially designed with aluminum panels formed into a parabolic sur-

face accurate to 0.02 inch (0.5 mm)—a design condition that enables the antennae to focus radio signals as short as one-centimeter wavelength.

The VLA is used to produce radio images with as much detail as those made by an optical telescope. To accomplish this, the VLA's 27 dish-shaped antennae are arranged in a giant Y-pattern, with the southeast and southwest arms of the Y-pattern each 13 miles (21 km) long, and the north arm 11.8 miles (19 km) long. The resolution of this radio telescope array is varied by changing the separation and spacing of its 27 antenna elements. The VLA is generally found in one of four standard-array configurations. In the smallest antenna-dispersion configuration (D array—the low-resolution configuration), the 27 individual antennae are clustered together and form an equivalent radio antenna with a baseline of just 0.62 mile (1 km). In the largest antenna-dispersion configuration (A array—the high-resolution configuration), the individual antennae stretch out in a giant Y-pattern that produces a maximum baseline of 22.4 miles (36 km).

Astrometry is the branch of astronomy that involves very precise measurements of the motion and position of celestial objects. For example, the availability of ultra-high-resolution (microarcsecond) optical, infrared, and radio observatories will allow astronomers to carefully search for extrasolar planets encircling nearby stars. (The search for extrasolar planets is discussed in chapter 11.)

With respect to planetary science and exobiology, the lunar base would also be an ideal location for an extraterrestrial quarantine facility. Astro-biology (also called exobiology) is the scientific discipline involving the search for and study of living organisms found on celestial bodies beyond Earth. Samples of possible life-bearing materials from Mars, Europa, or other solar system bodies, which are suspected of containing microorganisms potentially harmful to the terrestrial biosphere, could be analyzed, tested, challenged, and (if necessary) fully contained on the Moon. Under these circumstances, there would be a thorough scientific investigation of the samples by human scientists without exposing the terrestrial biosphere to any undo risk.

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