Scale and Survey of Our Solar System

SCALE OF THE SOLAR SYSTEM: 
PLANETARY ORBITS 
The primary, standard, measuring scale of the solar system is based on the earth's average distance from the sun, which is known as the astronomical unit (1 astronomical unit is about 150 million kilometers).' In deriving the scale of the solar system, astronomers have employed several independent techniques, of which the most accurate is that of timing the round trip of a pulsed radio signal reflected from a planet. Combining this information with the planets' distances in astronomical units, from Kepler's third law, leads to the absolute size of the solar system in kilometers.
A German astronomer, Johann Bode (1747-1826), called attention in 1772 to a numerical scheme, or rule (originally discovered by Johann Titius (1729-1796) in 1766), that seemed to predict the mean distances of the then-known planets from the sun. Al­though not a physical law in the same sense as Newton's laws, it is known as Bode's law; both Ura­nus, discovered in 1781, and the first asteroid, Ceres, found in 1801, adhered fairly well to this rule, but it broke down later when Neptune and Pluto were dis­covered. Despite the rule's having no unique physical basis, a similar rule relating the separations between planets seems to be characteristic of the formation of bodies in the gravitational field of a star.
All the planets are much alike in orbital character­istics. They revolve around the sun in the same direc­tion in roughly circular orbits that lie nearly in the same plane. Mercury, the innermost planet, and Pluto, the outermost planet, depart most from this regularity. Between the terrestrial planets (Mercury, Venus, the earth, and Mars) the average spacing is much smaller than that separating the Jo­vian planets, Jupiter, Saturn, Uranus, and Neptune. The planets orbit at mean distances ranging from 40 percent of the earth's distance from the sun to 40 times earth's distance, with orbital periods between a quarter of a year and 248 years.
THE SUN 
What is it about the sun that forces planets, asteroids, comets, and other bodies of the solar system to orbit around it? It is its immense gravitational reach since the sun contains 99.86 percent of the solar system's mass. (The natural satellites of the planets, on the other hand, are gravitationally bound to their parent planets because of their closeness to them.)
Like other stars, the sun is a gas from center to surface and generates its radiant energy deep within its hot interior. This giant gaseous sphere has a radius 109 times greater and a mass 333,000 times greater than that of the earth. The sun's family of planets thus intercepts only a minute amount of the radiation that streams from the sun, flooding the solar system.
In addition to the steady emission of radiant energy there are numerous transient phenomena occurring in the sun's outer layers, such as sunspots, plages, flares, prominences, and coronal holes. Associated with these is a flow of protons, electrons, other atomic particles, and magnetic fields out through the orbital planes of the planets. These so-called solar-wind par- ticles and magnetic fields impinge upon the planets and their magnetic fields, producing a variety of phe­nomena, such as the earth's aurora (northern and southern lights).
THE PLANETS 
Outside the sun the nine planets contain the next important share of the mass of the solar system. Although the planets had a common ori­gin, they have significant chemical, physical, and geo­logical differences. Such diversity stems mostly from their different masses and distances from the sun. At the time of the planets' formation these factors deter­mined the ability to retain matter and defined the chemical composition of that matter.
The matter composing the planets, their satellites, and the minor members of the solar system can be roughly divided into three broad classes on the basis of the ease with which it will vaporize. Those materials that are solids for temperatures less than about 2000 K are the rocky materials, such as iron, magnesium, and their oxides and silicates. The second class is the icy materials, which can remain solid only up to tem­peratures of a few hundred degrees Kelvin. Examples are the ices of water, carbon dioxide, ammonia, and methane. The remaining class consists of those mole­cules that are gases down to almost absolute zero, such as hydrogen and helium.
On the other hand, we know of sufficient chemical and physical similarities among the planets to enable us to divide them into two well-defined categories.
One category is an inner group composed mostly of rocky material, the terrestrial planets: Mercury, Ve­nus, the earth, and Mars. Since the moon is not significantly smaller than Mercury, many astronomers include the moon among the terrestrial planets. Though like the moon in size, Pluto should have an icy composition more like that of comets. Thus it does not belong to the terrestrials, and it is not like the second group either.
The second group, the Jovian planets-Jupiter, Saturn, Uranus, and Neptune-are farther from the sun; these planets are larger and consist mainly of the lighter elements, primarily hydrogen and helium, the most abundant elements in the universe. Jupiter and Saturn apparently have the same chemical com­position as the sun. Uranus and Neptune seem to have less hydrogen and helium and presumably more of the icy materials (frozen gases such as water, am­monia, methane, and carbon dioxide). Table 6.4 con­tains a summary of the specific physical properties of the planets.
THE SATElLITES 
Of the 49 or so satellites all but four belong to the Jovian planets. It seems likely that more will be discov­ered in the future; and in fact many more may even­tually be found since Jupiter and Saturn could grav­itationally bind a lot of small bodies. Very small and faint satellites of the Jovian planets cou Id easily escape detection by our present technology.  Two of Jupiter's satellites, Ganymede and Cal­listo, and one of Saturn's satellites, Titan, are as large as, or larger than, Mercu ry.
Those satellites that are reasonably near their par­ent planet move in nearly circular orbits in the plane of their planet's equator and in the same direction as their planet rotates. The outer satellites usually have more eccentric orbits, which are more highly inclined to the equatorial plane of their planet. The four outer satellites of Jupiter, the most distant satellite of Saturn, and the inner satellite of Neptune have orbits that are reversed from the direction of their planet's rotation. It is possible that the reason for these differ­ences is that the outer satellites were captured by the primaries after the planets and their inner satellite systems were formed.
RING SYSTEMS 
One of the most exciting developments in planetary research in recent years has been the discovery of ring systems for Uranus and jupiter. (Saturn's rings had been discovered with the introduction of the tele­scope into astronomy.) Rings are actually individual, small solid bodies in orbit about a planet in its equa­torial plane. They are thus very small satellites of the planet. Uranus's and jupiter's rings do not contain so many tiny satellites as do Saturn's; so they are much fainter than the ring system about Satu rn and have managed to escape detection until recently. Although no ring system has yet been found for Neptune, it is possible that it also has a faint set of rings similar to those around the other three Jovian planets.
MINOR MEMBERS
The asteroids, or minor planets, are rocky bodies whose diameter vary from a few between 150 and 1000 kilometers down to thousands less tha a kilomter across. Most asteroids are found between Mars and Jupiter, traveling around the sun in the same direction as the planets. However, many of them orbit the sun in the vicinity of the earth's orbit, with some in fairly elliptic orbits. In recent years the term "asteroid" has been expanded to include small ob­jects, which are presumably not comets, located in the outer portion of the solar system. It is unlikely that their physical makeup is like that of those in the inner part of the solar system.
The meteoroids range in size from irregular solid bodies, called meteorites when they strike the ground, to tiny particles, called meteors if they merely flash through the atmosphere. As we go down the scale in size, the number of meteoroids increases very rapidly. They are composed of rocky material and are apparently related to asteroids and comets. All the meteoroids are satellites of the sun and are apparently moving in a wide variety of orbits, as best as we can determine.
Unlike the planetary bodies, most comets move around the sun in highly eccentric orbits with very long periods of revolution and at all angles of inclina­tion. Some comets with short periods are regular vis­itors to the vicinity of the earth. The small masses of comets mean that it is possible for the larger planets, jupiter in particular, to alter their orbits. Astronomers believe a comet to be a "dirty iceball" that is a con­glomerate of icy materials mixed with rocky matter, while most of the asteroids and meteoroids are com­posed of a rocky material. The cometary composition is apparently characteristic of many bodies in the outer solar system. We shall have more to say in later chapters about the relationships of asteroids, mete­oroids, comets, satellites, and planets to each other.
The interplanetary medium is primarily gas particles-mostly protons and electrons-that are ejected from the sun's atmosphere at several hundred kilometers per second. These subatomic particles form the solar wind. Some dust is there too, most of it being cometary debris. Despite huge numbers of gas and dust particles, interplanetary space has fewer bits of matter and is a better vacuum than can be made in a terrestrial laboratory.

The Solar System

Throughout the first five chapters of this book, we have primarily been laying a foundation for our ex­ploration of the cosmos. The first unit in the cosmos ior us to study is logically the solar system, ior it is the sun and its attendant planets among which our home, the earth, is located. Yet for purposes of study there is an awkward mixture of bodies in the solar system:
For example, the sun is a star-a very different kind of body from planets, or satellites, or asteroids, or com­ets; therefore we shall delay a detailed discussion of the sun and instead use it to introduce our discussion of stars.
Words define common human experience, which does not include direct contact with such entities as other planets or stars. So scientists use numbers to compare objects, and in this chapter we try to give you some sense of the scale of the solar system and the things in it.

Electromagnetic Radiations and Optical Telescopes and their Accessory Instruments

Optical telescopes. Optical telescopes collect and fo­cus electromagnetic radiation in the range of 3000 to 10,000 angstroms reaching the surface of the earth (optical window). In optical astronomy, astronomers work with an image of the celestial light source. The image may be formed by refracting lenses or by reflecting mirrors. In either case, important properties of the image are size, brightness, and resolution of adjacent points. The properties depend in different ways on the aperture and focal length of the objective. Turbulence in the earth's lower atmosphere blurs and distorts a star's image, causing it to scintillate. Other atmospheric conditions hamper our view of the heav­ens: weather conditions, atmospheric pollution, air· glow, and the refraction of light passing through the atmosphere.
Reflecting and refracting telescopes. Telescopes that use lenses for the objective are known as refrac· ting telescopes. Telescopes that use mirrors are known as reflecting telescopes. Modern reflectors are arranged so that images can be formed at three differ· entfocal positions: the prime, the Cassegrain, and the coude foci. Each provides certain advantages de­pending on the brightness of the celestial object and the type of data to be obtained.
Large modern telescopes are all reflecting tele·scopes They are free from chromatic aberration. They may be supported by their edges and from the back unlike lenses. Furthermore, reflection occurs from the front surface, decreasing the quality requirements on the mirror material will allow astronomers to move from survey activities.
The universe is highly transparent to gamma rays because of their great penetrating power. One would therefore expect them to be capable of carrying the imprint of their origin from far-distant places because they pass so easily through interstellar matter. Thus the unique penetration of gamma rays reveals direc­tional and temporal information about their origin in regions that are too dense for visible photons and even X rays to penetrate. Gamma rays serve as a probe to provide us with new insights into the structure of the cosmos.
Accessory instruments. Several types of radiation detectors, such as photographic emulsions and pho· toelectric devices, are attached to optical telescopes to analyze light in particular wavelengths. The most important properties of the radiation detector are its wavelength sensitivity, response over the wavelength interval, and the nature and range of response. Acces­sory instruments are used to analyze the wavelength composition of the radiation (i.e., type of spectrum) orthe amount of energy in the radiation (i.e., photom­etry).
Radio telescopes. Radio astronomy developed rapidly after the late 1940s. Devices to detect and lo­cate precisely sources of radio radiation disclosed that celestial objects radiate energy in the radio portion of the electromagnetic spectrum as well as in the visible light portion. The collecting element of a radio tele­scope is a parabolic metal dish. The "image" is a plot of the brightness of radio radiation at particular wave­lengths as the telescope scans the source of radio radiation.
Ultraviolet, X·ray, and gamma-ray radiation Tele­scopes, analyzing instruments, and radiation de­tectors are available to study cosmic radiation in the ultraviolet, X-ray, and gamma-ray portions of the elec­tromagnetic spectrum. Because earth's atmosphere absorbs radiation in these wavelengths, such radiation studies must be carried outside the atmosphere b balloons, high flying aircraft, rockets, or satellites.

What are Gamma Ray devices?

GAMMA-RAY DEVICES 

This new field of astronomy has burgeoned in the last decade or so from modest beginnings, employing balloons and rockets, to satellites (European and Ameri· can) carrying highly sensitive gamma-ray detectors. Like that of X rays, the great penetrating power of gamma·ray photons makes observation and detection different from those for visible photons. Gamma·ray photons carry the highest energy of any photon. The primary gamma·ray detector used for space astron­omy is a crystalline material that absorbs the gamma­ray photon, converting its energy to a flash of visible light. The visible photons can then be detected by a photoelectric device. 

In 1978 came the first evidence of several gamma­ray spectral lines. The search for additional discrete lines will be greatly extended with the launch of the Gamma Ray Observatory in 1985. 

What are X-ray Devices?

X-RAY DEVICES 
X-ray astronomers divide "their" portion of the elec­tromagnetic spectrum into two categories: soft X rays, from about 10 to 100 angstroms, and the more pene­trating hard X rays, from approximately 1 to 10 ang­stroms. Both X rays and gamma rays are emitted by regions of space characterized by very high tem­peratures, low density, and high-speed subatomic particles-that is, wherever there are extreme condi­tions involving nuclear and atomic reactions. The ob­served radiation is in part thermal radiation but mostly nonthermal radiation.
Most people are aware that X rays are more pene­trating than visible light since they pass through the human body when making X-ray pictures for medical diagnosis. In this great penetrating power lies the difficulty in making telescopes to focus X rays, analyz­ing instruments, and radiation detectors for X rays; for glass lenses and mirrors do not refract or reflect X rays impinging directly on them. If X rays strike a smooth surface at a very shallow angle, less than a couple of degrees, they will reflect off the surface. This phe­nomenon has been used successfully to design an all-grazing-incidence reflector, which focuses X rays as an optical telescope focuses visible light. Such an X-ray telescope was flown as the heart of the second High Energy Orbiting Observatory, known as the Ein­stein Observatory
At the focus of the X-ray telescope is the radiation detector, just as in the case of an optical telescope. Photographic emulsions can be made that are sensi­tive to X rays and can record an X-ray-picture. For hard X rays special crystalline materials will absorb X rays, converting their energy into photons of visible wave­lengths that can be detected with photoelectric de­vices. And there are solid silicon detectors whose abil­ity to conduct electrical charges is influenced by their absorption of X-ray photons.
Astronomers first began using X-ray detectors in balloons and rockets and in a few unmanned satellites during the 1960s. By 1967 they had discovered some 30 discrete X-ray sources. Then in December 1970 NASA's Explorer 42 satellite (Uhuru) was launched off the coast of Kenya, Africa. By the end of its useful life in 1973 it had scanned nearly the whole sky and had located nearly 200 X-ray sources. The newly discov­ered X-ray objects were named after the constellation in which they appeared, followed by X-1, X-2, and so on, in the order of discovery. For example, Taurus X-1, the first X-ray object discovered, is the Crab Nebula (see Section 14.3). Today, with a growing number of X-ray discoveries, it is convenient to designate the source by a catalog number.
A second generation of NASA satellites (the High Energy Astronomy Observatories), designated HEAO­1, HEAO-2, and HEAO-3, was launched in August 1977, November 1978, and September 1979, respectively. The three HEAO satellites are designed specifically to study X rays, gamma rays, and subatomic particles (called cosmic rays). The satellites shown in Figure 5.18 are each about 6 meters long and weigh approxi­mately 3000 kilograms. Instruments aboard these sat­ellites were designed to search the sky for discrete and diffuse background sources of X rays and gamma rays, to measure their total energy output and how that varies with wavelength, and to measure the ranges of energy, the composition, and the numbers of cosmic rays.
Proposed for launching in 1987 is a follow-up mis­sion to Einstein Observatory (HEAO-2), the Advanced X-ray Astrophysics Facility (AXAF). The satellite will be put into orbit from the Space Shuttle, which will also service it and eventually re­trieve it for modernization. AXAF's 1.2-meter grazing­incidence telescope will have 4 times the spatial reso­lution and at least 20 times the X-ray photon-collecting power of the X-ray telescope in the Einstein Obser­vatory. This space vehicle represents as big an advance for the X-ray region of the electromagnetic spectrum as Space Telescope does for the visible and ultraviolet. Beyond AXAF an even larger telescope facility is in the planning process. To be known as the Large Area Mod­ular Array (LAMAR) X-ray telescope, it will have 10 times the sensitivity to X rays that ASAF does.

What are Ultravoilet Telescopes?

ULTRAVIOLET TELESCOPES 
The ultraviolet portion of the electromagnetic spec­trum has been divided by astronomers into three seg­ments, more or less derived from the time in which serious research into them began. First there is the ground-based ultraviolet, from 4000 angstroms to the atmospheric cutoff at 3000 angstroms; next the far ultraviolet from 3000 to 1000 angstroms; and last the extreme ultraviolet from 1000 to 100 angstroms.
Ultraviolet observations began after World War II, in October 1946, when a captured German V-2 rocket carried a small ultraviolet spectrograph to a height of 100 kilometers. During the ascent it recorded the ul­traviolet portion of the solar spectrum down to 2200 angstroms. Telescopes, analyzing instruments, and radiation detectors for ultraviolet research are basic­ally the same kinds of instrument used in visible and infrared observations. The principle difference is that a number of types of glass are not transparent to ultra­violet photons but are highly absorbing. Therefore, special materials must be used for lenses and entrance windows into the instrument. The principles of oper­ation are the same as those for visible radiation. Since ultraviolet-sensitive film cannot be retrieved from an orbiting satellite, photoelectric devices have been the primary radiation detectors, so that data could be ra­dioed back to ground stations.
Between 1962 and 1975 eight Orbiting Solar Obser­vatories (050-1 through 050-8) were launched for study of the sun in ultraviolet wavelengths arid X-ray and gamma-ray radiation. In December 1968 the first Orbiting Astronomical Observatory (OAO-2) began sampling the ultraviolet and far-ultraviolet radiation. By the time OAO-2 ended its useful life in February 1973, it had carried out photometry on more than 1000 objects from planets to galaxies. Its successor, illus­trated in Figure 5.18, OAO-Copernicus, launched in August 1972, carried an 0.8-meter ultraviolet telescope and three small X-ray telescopes, and was even more active than OAO-2.
In january 1978 the International Ultraviolet Ex­plorer, an orbiting observatory, shown in Figure 5.18, was launched by NASA. This was a joint undertaking by NASA and several western European countries. Its facilities have been used for studies of planets, stars, galaxies, and the interstellar medium in the wave­length range from 1150 to 3200 angstroms. Astrono­mers conduct their experiments from an elaborate console of controls located at the Goddard Space Flight Center.

Space and Space Vehicles for Outer Space

SPACE VEHICLES 
High-altitude aircraft and balloons are the least ex­pensive way of investigating invisible extraterrestrial radiation. Jet aircraft can ascend to about 15 kilome­ters, while balloons are useful up to about 30 kilome­ters, above which only 5 percent of the atmosphere remains. Rockets, though their flights are short com­pared to balloon flights, lasting for minutes instead of hours, can climb five times higher than balloons can. Artificial satellites cost much more than rocket flights, but satellites can continuously monitor events over different regions of the electromagnetic spectrum for long periods of time, an advantage that outweighs their additional cost. Since 1958 the United States has launched hundreds of instrumented satellites that have either orbited the earth or been sent to search the solar system.
Up until about 1983 the means of getting satellites off the su rface of the earth have been rockets. The advent of Space Shuttle has now provided another way to launch satellites. Space Shuttle is a true aero­space launch vehicle in that it takes off like a rocket, maneuvers in earth orbit as do other spacecraft, but lands like an airplane. This launch vehicle was de­signed to carry heavy loads into space and to be, re­usable. Satellites can be carried into orbit in the Shuttle's cargo bay; when the Shuttle is in orbit, the satellites can be lifted out by a retractable arm and placed in their orbits. This also means that satellites can be retrieved from orbit to be brought back to the earth's surface or serviced and returned to orbit. Space Shuttle gives us the capability of carrying pieces of immense spacecraft, including manned space sta­tions, into orbit to be assembled there. Its versatility signals a new generation of space exploration.
An important group of space vehicles has been the observatory satellites placed in orbits several hundred kilometers above the earth. Two of the most sophisti­cated and costly observatory satellites have been the Orbiting Astronomical Observatories; OAO-2 and OAO-Copernicus.
The Skylab program cost $6 billion. Three crews of three men each spent 171 days in Skylab between May and November, 1973. These astronauts carried out dozens of astronomical, biomedical, and technolog­ical experiments. The abandoned station was to re­main in orbit for several years; it was hoped that with Space Shuttle astronauts would be able to reuse the station in the future. However, Skylab was dragged down by the atmosphere to a fiery demise over the Indian Ocean, scattering pieces over western Austra­lia on july 11, 1979.
In early 1985 NASA expects to place a 2.4-meter unmanned reflecting telescope named Space Tele­scope in orbit at an altitude of 500 kilometers. Its optics and instrumentation will be enclosed in a cylindrical tube 13 meters long and 4.3 meters wide. Auxiliary apparatus includes two imaging cam­eras, faint-object and high-resolution spectrographs, a photometer, and other specialized devices. These analyzing instruments are designed to cover the wave­length range from about 1000 angstroms in the ultra­violet to 8000 angstroms in the near infrared. Data from the telescope will be radioed in digital (number) form through the Goddard Space Flight Center in Greenbelt, Maryland, to the Space Telescope Sci­ences Institute on the johns Hopkins University cam­pus for processing.
Out in space no atmospheric absorption or tur­bulence will distort the images produced by the tele­scope. Thus the telescope should see astronomical sources up to 50 times fainter than those visible from the earth's surface; in terms of distance a faint object can be seven times farther away than could be seen from the surface of the earth. Space Telescope's spa­tial resolution will be 10 times better than the best earth-based reflectors. With proper maintenance from Space Shuttle, the telescope could operate for at least a decade. Unlike its ground-based counterparts, Space Telescope will scan the electromagnetic spec­trum from the deep ultraviolet to the infrared.
Other space observatories are the planetary probes, which are literally the most exotic. Their role is to go to a planet to photograph and ana­lyze from a close flyby, to orbit the planet, or in some cases to land. As examples, the Viking 1 and Viking 2 spacecraft landed on the surface of Mars (we shall discuss them later). Other examples are the Voyager 1 and Voyager 2 spacecraft launched to encounter ju­piter, Saturn, and perhaps Uranus and Neptune. Much of our understanding of the nature of the uni­verse is changing-rapidly and dramatically-because of these space observatories.

What are Ultraviolet, X-Ray, and Gamma-Ray Telescopes?

Ultraviolet, X-Ray, and Gamma-Ray Telescopes 
Although much useful and important observational work remains to be done from ground-based obser­vatories, an increasing portion of future astronomical research will be carried out from platforms outside most of the veil that is the earth's atmosphere. Up to the middle of this century nearly all our knowledge about the cosmos had come from studying the visible light of astronomical objects, and the visible and radio windows still constitute our most readily accessible sources of information. Yet much of today's research is centered on the invisible regions of the electro­magnetic spectrum, which do not penetrate the earth's atmosphere. To ex­plore these regions-the infrared, the ultraviolet, the X-ray, and' the gamma-ray wavelengths-new tech­niques and equipment are being developed, which must be flown above the atmosphere in some type of space vehicle.
SPACE VEHICLES 
High-altitude aircraft and balloons are the least ex­pensive way of investigating invisible extraterrestrial radiation. Jet aircraft can ascend to about 15 kilome­ters, while balloons are useful up to about 30 kilome­ters, above which only 5 percent of the atmosphere remains. Rockets, though their flights are short com­pared to balloon flights, lasting for minutes instead of hours, can climb five times higher than balloons can. Artificial satellites cost much more than rocket flights, but satellites can continuously monitor events over different regions of the electromagnetic spectrum for long periods of time, an advantage that outweighs their additional cost. Since 1958 the United States has launched hundreds of instrumented satellites that have either orbited the earth or been sent to search the solar system.
Up until about 1983 the means of getting satellites off the su rface of the earth have been rockets. The advent of Space Shuttle has now provided another way to launch satellites. Space Shuttle is a true aero­space launch vehicle in that it takes off like a rocket, maneuvers in earth orbit as do other spacecraft, but lands like an airplane. This launch vehicle was de­signed to carry heavy loads into space and to be, re­usable. Satellites can be carried into orbit in the Shuttle's cargo bay; when the Shuttle is in orbit, the satellites can be lifted out by a retractable arm and placed in their orbits. This also means that satellites can be retrieved from orbit to be brought back to the earth's surface or serviced and returned to orbit. Space Shuttle gives us the capability of carrying pieces of immense spacecraft, including manned space sta­tions, into orbit to be assembled there. Its versatility signals a new generation of space exploration.
An important group of space vehicles has been the observatory satellites placed in orbits several hundred kilometers above the earth. Two of the most sophisti­cated and costly observatory satellites have been the Orbiting Astronomical Observatories; OAO-2 and OAO-Copernicus.
The Skylab program cost $6 billion. Three crews of three men each spent 171 days in Skylab between May and November, 1973. These astronauts carried out dozens of astronomical, biomedical, and technolog­ical experiments. The abandoned station was to re­main in orbit for several years; it was hoped that with Space Shuttle astronauts would be able to reuse the station in the future. However, Skylab was dragged down by the atmosphere to a fiery demise over the Indian Ocean, scattering pieces over western Austra­lia on july 11, 1979.
In early 1985 NASA expects to place a 2.4-meter unmanned reflecting telescope named Space Tele­scope in orbit at an altitude of 500 kilometers. Its optics and instrumentation will be enclosed in a cylindrical tube 13 meters long and 4.3 meters wide. Auxiliary apparatus includes two imaging cam­eras, faint-object and high-resolution spectrographs, a photometer, and other specialized devices. These analyzing instruments are designed to cover the wave­length range from about 1000 angstroms in the ultra­violet to 8000 angstroms in the near infrared. Data from the telescope will be radioed in digital (number) form through the Goddard Space Flight Center in Greenbelt, Maryland, to the Space Telescope Sci­ences Institute on the johns Hopkins University cam­pus for processing.
Out in space no atmospheric absorption or tur­bulence will distort the images produced by the tele­scope. Thus the telescope should see astronomical sources up to 50 times fainter than those visible from the earth's surface; in terms of distance a faint object can be seven times farther away than could be seen from the surface of the earth. Space Telescope's spa­tial resolution will be 10 times better than the best earth-based reflectors. With proper maintenance from Space Shuttle, the telescope could operate for at least a decade. Unlike its ground-based counterparts, Space Telescope will scan the electromagnetic spec­trum from the deep ultraviolet to the infrared.
Other space observatories are the planetary probes, which are literally the most exotic. Their role is to go to a planet to photograph and ana­lyze from a close flyby, to orbit the planet, or in some cases to land. As examples, the Viking 1 and Viking 2 spacecraft landed on the surface of Mars (we shall discuss them later). Other examples are the Voyager 1 and Voyager 2 spacecraft launched to encounter ju­piter, Saturn, and perhaps Uranus and Neptune. Much of our understanding of the nature of the uni­verse is changing-rapidly and dramatically-because of these space observatories.
ULTRAVIOLET TElESCOPES 
The ultraviolet portion of the electromagnetic spec­trum has been divided by astronomers into three seg­ments, more or less derived from the time in which serious research into them began. First there is the ground-based ultraviolet, from 4000 angstroms to the atmospheric cutoff at 3000 angstroms; next the far ultraviolet from 3000 to 1000 angstroms; and last the extreme ultraviolet from 1000 to 100 angstroms.
Ultraviolet observations began after World War II, in October 1946, when a captured German V-2 rocket carried a small ultraviolet spectrograph to a height of 100 kilometers. During the ascent it recorded the ul­traviolet portion of the solar spectrum down to 2200 angstroms. Telescopes, analyzing instruments, and radiation detectors for ultraviolet research are basic­ally the same kinds of instrument used in visible and infrared observations. The principle difference is that a number of types of glass are not transparent to ultra­violet photons but are highly absorbing. Therefore, special materials must be used for lenses and entrance windows into the instrument. The principles of oper­ation are the same as those for visible radiation. Since ultraviolet-sensitive film cannot be retrieved from an orbiting satellite, photoelectric devices have been the primary radiation detectors, so that data could be ra­dioed back to ground stations.
Between 1962 and 1975 eight Orbiting Solar Obser­vatories (050-1 through 050-8) were launched for study of the sun in ultraviolet wavelengths arid X-ray and gamma-ray radiation. In December 1968 the first Orbiting Astronomical Observatory (OAO-2) began sampling the ultraviolet and far-ultraviolet radiation. By the time OAO-2 ended its useful life in February 1973, it had carried out photometry on more than 1000 objects from planets to galaxies. Its successor, illus­trated in Figure 5.18, OAO-Copernicus, launched in August 1972, carried an 0.8-meter ultraviolet telescope and three small X-ray telescopes, and was even more active than OAO-2.
In january 1978 the International Ultraviolet Ex­plorer, an orbiting observatory, shown in Figure 5.18, was launched by NASA. This was a joint undertaking by NASA and several western European countries. Its facilities have been used for studies of planets, stars, galaxies, and the interstellar medium in the wave­length range from 1150 to 3200 angstroms. Astrono­mers conduct their experiments from an elaborate console of controls located at the Goddard Space Flight Center.
X-RAY DEVICES 
X-ray astronomers divide "their" portion of the elec­tromagnetic spectrum into two categories: soft X rays, from about 10 to 100 angstroms, and the more pene­trating hard X rays, from approximately 1 to 10 ang­stroms. Both X rays and gamma rays are emitted by regions of space characterized by very high tem­peratures, low density, and high-speed subatomic particles-that is, wherever there are extreme condi­tions involving nuclear and atomic reactions. The ob­served radiation is in part thermal radiation but mostly nonthermal radiation.
Most people are aware that X rays are more pene­trating than visible light since they pass through the human body when making X-ray pictures for medical diagnosis. In this great penetrating power lies the difficulty in making telescopes to focus X rays, analyz­ing instruments, and radiation detectors for X rays; for glass lenses and mirrors do not refract or reflect X rays impinging directly on them. If X rays strike a smooth surface at a very shallow angle, less than a couple of degrees, they will reflect off the surface. This phe­nomenon has been used successfully to design an all-grazing-incidence reflector, which focuses X rays as an optical telescope focuses visible light. Such an X-ray telescope was flown as the heart of the second High Energy Orbiting Observatory, known as the Ein­stein Observatory
At the focus of the X-ray telescope is the radiation detector, just as in the case of an optical telescope. Photographic emulsions can be made that are sensi­tive to X rays and can record an X-ray-picture. For hard X rays special crystalline materials will absorb X rays, converting their energy into photons of visible wave­lengths that can be detected with photoelectric de­vices. And there are solid silicon detectors whose abil­ity to conduct electrical charges is influenced by their absorption of X-ray photons.
Astronomers first began using X-ray detectors in balloons and rockets and in a few unmanned satellites during the 1960s. By 1967 they had discovered some 30 discrete X-ray sources. Then in December 1970 NASA's Explorer 42 satellite (Uhuru) was launched off the coast of Kenya, Africa. By the end of its useful life in 1973 it had scanned nearly the whole sky and had located nearly 200 X-ray sources. The newly discov­ered X-ray objects were named after the constellation in which they appeared, followed by X-1, X-2, and so on, in the order of discovery. For example, Taurus X-1, the first X-ray object discovered, is the Crab Nebula. Today, with a growing number of X-ray discoveries, it is convenient to designate the source by a catalog number.
A second generation of NASA satellites (the High Energy Astronomy Observatories), designated HEAO­1, HEAO-2, and HEAO-3, was launched in August 1977, November 1978, and September 1979, respectively. The three HEAO satellites are designed specifically to study X rays, gamma rays, and subatomic particles (called cosmic rays). The satellites shown in Figure 5.18 are each about 6 meters long and weigh approxi­mately 3000 kilograms. Instruments aboard these sat­ellites were designed to search the sky for discrete and diffuse background sources of X rays and gamma rays, to measure their total energy output and how that varies with wavelength, and to measure the ranges of energy, the composition, and the numbers of cosmic rays.
Proposed for launching in 1987 is a follow-up mis­sion to Einstein Observatory (HEAO-2). The satellite will be put into orbit from the Space Shuttle, which will also service it and eventually re­trieve it for modernization. AXAF's 1.2-meter grazing­incidence telescope will have 4 times the spatial reso­lution and at least 20 times the X-ray photon-collecting power of the X-ray telescope in the Einstein Obser­vatory. This space vehicle represents as big an advance for the X-ray region of the electromagnetic spectrum as Space Telescope does for the visible and ultraviolet. Beyond AXAF an even larger telescope facility is in the planning process. To be known as the Large Area Mod­ular Array (LAMAR) X-ray telescope, it will have 10 times the sensitivity to X rays that ASAF does.
GAMMA-RAY DEVICES 
This new field of astronomy has burgeoned in the last decade or so from modest beginnings, employing balloons and rockets, to satellites (European and Ameri· can) carrying highly sensitive gamma-ray detectors. Like that of X rays, the great penetrating power of gamma·ray photons makes observation and detection different from those for visible photons. Gamma·ray photons carry the highest energy of any photon. The primary gamma·ray detector used for space astron­omy is a crystalline material that absorbs the gamma­ray photon, converting its energy to a flash of visible light. The visible photons can then be detected by a photoelectric device.
In 1978 came the first evidence of several gamma­ray spectral lines. The search for additional discrete lines will be greatly extended with the launch of the Gamma Ray Observatory in 1985. The GRO satellite

Radio Telescopes and Devices

DISCOVERY OF CELESTIAL RADIO WAVES 
In 1931 a Bell Telephone engineer, Karl Jansky (1905-1950), was trying to find where the interference disrupting transatlantic radiophone circuits came from. He discovered that some of the radio noise was not from the earth; it was extraterrestrial. The primary source was the center of the Milky Way, in the constel­lation of Sagittarius. In 1936 an Illinois radio engineer, Grote Reber (1911- ), pursued the phenomenon far­ther. He built the first parabolic radio telescope, 9.5 meters in diameter, and made the first radio map of the sky. The strongest signals he found came from the star clouds in Sagittarius and from several discrete sources toward the center of our Galaxy. The next major discovery was in 1942, by British radar operators and scientists tracking down suspected radar jamming during World War II; they discovered that the inter­ference was radio emission from the sun.
At fjrst astronomers did not grasp just how significant Jansky's work was; they were preoccupied with their observations of the universe through the optical window of the earth's atmosphere. But after World War II radio astronomy came into its own when phys­icists, radio engineers, and astronomers joined forces to build larger and more efficient radio telescopes. Radio astronomy since then has led to startling discov­eries, such as interstellar molecules, pulsars, and the enigmatic quasars. Today our concept of any cosmic body is based upon its appearance all across the elec­tromagnetic spectrum, with the radio region an ex­tremely important component.
RADIO-TELESCOPE DESIGN 
Because the physical nature of a radio wave is exactly the same as that of a light wave, the problem of de­signing a radio telescope is similar to that of designing an optical telescope. There are some practical differ­ences. Radio waves pass through most materials with­out any interaction; thus it is not feasible to design a "lens" for radio waves that will focus them in a refract­ing telescope. But any metal will reflect radio waves; so a dish-shaped metal mirror will focus radio waves, just as a glass mirror focuses light waves. The reflect­ing surface of the dish can be an open, fine-wire mesh or a solid metal with a parabolic shape. Radio waves are reflected from the su rface and converge toward a focal point, where a small collector aerial absorbs the concentrated energy turning it into an electrical current. From there the current or signal is carried by an electrical cable to the receiving equipment, which processes the signal just as in your home radio receiver.
After amplification the signal variations are record­ed in one of several ways The signal changes can then be fed into a computer for analysis. When the computer has done its job, a formerly invis­ible part of the universe is revealed as in the radio map.
Astronomers can increase the sensitivity of radio telescopes, with better accuracy in pointing and with higher resolution, by expanding the collecting area of the dish or by improving the capabilities of the re­ceiver. With the largest radio telescopes we can obtain a resolution of about l' of arc, comparable to that of the eye. That is like seeing a penny 65 meters away. The most powerful radio telescopes can detect energy from sources whose power is comparable to that of a terrestrial FM broadcast station many light years away.
The radio telescope is remotely controlled by the astronomer from an electronic console. Moderate­sized radio dishes, up to about 100 meters or so in diameter, are steerable and have equatorial mount­ings that follow the rotating sky just as optical tele­scopes do. Larger and heavier dishes employ an at­tazimuth mounting-one that rotates about a vertical axis and a horizontal axis. This minimizes the dis­tortion in the shape of the dish due to changing the orientation of the dish in the earth's gravitational field while tracking an object. A computer directs the rota­tion about both axes.
Even larger and more unwieldy antennas are fixed pointing upward while the rotating earth sweeps much of the sky by the antenna's field of view. Are­cibo, Puerto Rico, has the biggest fixed antenna, 2. metal dish 305 meters across contoured out of a natu· ral bowl in the ground. It can survey the sky to within 20° of the zenith, allowing coverage of about 40 percent of the entire sky.
RADIO INTERFEROMETRY 
Astronomers have searched endlessly for better re­solving power. It can be achieved by building bigger telescopes or by observing at shorter wavelengths for a chosen aperture or by using the phenomenon of interference. Interferometry is a tech­nique involving two or more radio telescopes. Radio radiation from an astronomical source received at the individual telescopes is combined to obtain data that have a spatial resolving power equal to that of a single telescope as large as the distance between the individ­ual receivers. With interferometry an astronomer can obtain details about the spatial structure of a given celestial object that a single radio telescope could never reveal.
For many years the separation between the individ­ual telescopes of the interferometer was limited by the lengths of the cables connecting the antennas be­cause the technique depends upon combining, at the same instant, the signals received by the separate tele­scopes. With the advent of the atomic clock (a clock governed by the vibrations of certain atoms) it became possible to record the signals received by the different telescopes, along with the precise time, and to com­pare them later. This allowed the individual tele­scopes to be greatly separated, even on opposite sides of the earth. The technique is called Very Long Base­line Interferometry (VLBI). It has been used with a geosynchronous or geostationary satellite as the link in the communications channel between the tele­scopes.
In the Very Large Array (VLA) radio interferometer recently put into operation in New Mexico signals from each of 27 individual radio tele­scopes are combi ned by a computer. Each dish is 25 meters in diameter, and the 27 individual telescopes are moved along railroad tracks arranged in the shape of an enormous 21-kilometer Y. Nine dishes will be located on each branch of the Y, and the system can provide a total of 351 interferometer pairs of antennas. The energy-collecting power of the VLA is roughly equivalent to a single 122-meter telescope. The VLA will be able to achieve spatial resolution of about 1 second of arc in 10 hours of observing, or about that of the 5.1-meter Hale optical telescope. This ability makes it comparable to large optical telescopes in fer­reting out the structure of various cosmic bodies.

Einstein's Relativity Concepts

EINSTEIN'S CONCEPTS OF MOTION, SPACE, AND TIME 
The Newtonian theory of gravitation has been very successful in explaining most gravitational problems involving the material objects of the macroscopic world. As we have pointed out, in another world, the submicroscopic universe of the atom, nuclear and electromagnetic forces dominate the very weak grav­itational force, and they determine form and behavior in that realm. But it is the movement of bodies at high velocities or in the presence of very strong grav­itational fields, where Newton's theory of gravitation breaks down, that we are interested in at this point. Here relativity has changed cosmology profoundly, helping to explain cosmic reality.
The special theory of relativity of 1905 was not orig­inally designed as a refinement of Newtonian grav­itational theory but was addressed by Einstein to some aspects of electromagnetic radiation, that is, light. Special relativity theory deals with physical phenom­ena in which gravitational forces are not involved. By 1916 Einstein had worked out another, more compre­hensive theory, which was an alternative to the gra\­itational theory of Newton. He incorporated in his general theory of relativity a description of nop­uniform, or accelerated, motion between observers
The general theory of relativity has important impli­ns for our ideas of motion, mass, and the geom­of space and time.
at is relative in relativity? Let us consider an example. If we walk down the aisle of a moving train car can be thought of as the frame of reference of our motion. Looking out of the train window, our frame of reference moving relative to the ground as another frame of reference. We know that ground moves relative to the center of the earth as er frame of reference. The center of the earth moves relative to the sun as still another frame of reference. And so it is on a larger scale-we cannot ascertais our possible absolute motion in the universe we can find no frame of reference that is absolutely still.
Einstein coupled the three dimensions of space together with that of time and taught us that when we measure space and time, we find no absolute results the answers are relative, depending on the observer. Two people who are moving relative to each other see the same events going on at different places erent times; each experiences the event in his own frame of reference. We need relativity in astronomy use we cannot drop anchor in the universe and pssively watch what happens around us: Each of us is observer and participant. The first truth in the relativistic universe is that all motion is relative; that is, there is no preferred frame of reference in which space and time are defined absolutely.
Newtonian space has three numbers, the spatial coordinates that describe where an object is: for­ward-backward, right-left, and up-down. Location has nothing to do with time. According to Newton, Absolute space, in its own nature, without relation to anything external, remains always similar and immov­able .... "Absolute, true and mathematical time, of itself, and from its own nature, flows equably without relation to anything external. ... "
Einstein's theory on the other hand coupled space and time, making time a fourth dimension. In his gen­eral theory of relativity Einstein described the geom­etry of four-dimensional space-time, in which we, planets, stars, and galaxies exist. The space-time con­tinuum shows deformities (that is, space curvature) in the vicinity of material bodies; the more massive the body, the stronger is this curvature. From Newton's viewpoint an object moves in a curved path in response to the gravitational force of a massive body. For Einstein an object moves in a curved path as a natural consequence of the space curvature produced by the massive body. Where no massive body exists, the space-time geometry is flat, showing no curvature, and the object moves uniformly in a straight line. In the Newtonian world it moves that way because no force is acting upon it.

How were Celestial Radio Waves discovered?

DISCOVERY OF CELESTIAL RADIO WAVES
In 1931 a Bell Telephone engineer, Karl Jansky (1905-1950), was trying to find where the interference disrupting transatlantic radiophone circuits came from. He discovered that some of the radio noise was not from the earth; it was extraterrestrial. The primary source was the center of the Milky Way, in the constel­lation of Sagittarius. In 1936 an Illinois radio engineer, Grote Reber (1911- ), pursued the phenomenon far­ther. He built the first parabolic radio telescope, 9.5 meters in diameter, and made the first radio map of the sky. The strongest signals he found came from the star clouds in Sagittarius and from several discrete sources toward the center of our Galaxy. The next major discovery was in 1942, by British radar operators and scientists tracking down suspected radar jamming during World War II; they discovered that the inter­ference was radio emission from the sun.
At fjrst astronomers did not grasp just how significant Jansky's work was; they were preoccupied with their observations of the universe through the optical window of the earth's atmosphere. But after World War II radio astronomy came into its own when phys­icists, radio engineers, and astronomers joined forces to build larger and more efficient radio telescopes. Radio astronomy since then has led to startling discov­eries, such as interstellar molecules, pulsars, and the enigmatic quasars. Today our concept of any cosmic body is based upon its appearance all across the elec­tromagnetic spectrum, with the radio region an ex­tremely important component.

What is Radio Interferometer?

Radio Interferometer
Astronomers have searched endlessly for better re­solving power. It can be achieved by building bigger telescopes or by observing at shorter wavelengths for a chosen aperture or by using the phenomenon of interference. Interferometry is a tech­nique involving two or more radio telescopes. Radio radiation from an astronomical source received at the individual telescopes is combined to obtain data that have a spatial resolving power equal to that of a single telescope as large as the distance between the individ­ual receivers. With interferometry an astronomer can obtain details about the spatial structure of a given celestial object that a single radio telescope could never reveal.
For many years the separation between the individ­ual telescopes of the interferometer was limited by the lengths of the cables connecting the antennas be­cause the technique depends upon combining, at the same instant, the signals received by the separate tele­scopes. With the advent of the atomic clock (a clock governed by the vibrations of certain atoms) it became possible to record the signals received by the different telescopes, along with the precise time, and to com­pare them later. This allowed the individual tele­scopes to be greatly separated, even on opposite sides of the earth. The technique is called Very Long Base­line Interferometry (VLBI). It has been used with a geosynchronous or geostationary satellite as the link in the communications channel between the tele­scopes.
In the Very Large Array (VLA) radio interferometer recently put into operation in New Mexico signals from each of 27 individual radio tele­scopes are combi ned by a computer. Each dish is 25 meters in diameter, and the 27 individual telescopes are moved along railroad tracks arranged in the shape of an enormous 21-kilometer Y. Nine dishes will be located on each branch of the Y, and the system can provide a total of 351 interferometer pairs of antennas. The energy-collecting power of the VLA is roughly equivalent to a single 122-meter telescope. The VLA will be able to achieve spatial resolution of about 1 second of arc in 10 hours of observing, or about that of the 5.1-meter Hale optical telescope. This ability makes it comparable to large optical telescopes in fer­reting out the structure of various cosmic bodies.

What is Radio Telescope by Design?

Radio Telescope Design
Because the physical nature of a radio wave is exactly the same as that of a light wave, the problem of de­signing a radio telescope is similar to that of designing an optical telescope. There are some practical differ­ences. Radio waves pass through most materials with­out any interaction; thus it is not feasible to design a "lens" for radio waves that will focus them in a refract­ing telescope. But any metal will reflect radio waves; so a dish-shaped metal mirror will focus radio waves, just as a glass mirror focuses light waves. The reflect­ing surface of the dish can be an open, fine-wire mesh or a solid metal with a parabolic shape. Radio waves are reflected from the su rface and converge toward a focal point, where a small collector aerial absorbs the concentrated energy turning it into an electrical current. From there the current or signal is carried by an electrical cable to the receiving equipment, which processes the signal just as in your home radio receiver.
After amplification the signal variations are record­ed in one of several ways The signal changes can then be fed into a computer for analysis. When the computer has done its job, a formerly invis­ible part of the universe is revealed as in the radio map.
Astronomers can increase the sensitivity of radio telescopes, with better accuracy in pointing and with higher resolution, by expanding the collecting area of the dish or by improving the capabilities of the re­ceiver. With the largest radio telescopes we can obtain a resolution of about l' of arc, comparable to that of the eye. That is like seeing a penny 65 meters away. The most powerful radio telescopes can detect energy from sources whose power is comparable to that of a terrestrial FM broadcast station many light years away.
The radio telescope is remotely controlled by the astronomer from an electronic console. Moderate­sized radio dishes, up to about 100 meters or so in diameter, are steerable and have equatorial mount­ings that follow the rotating sky just as optical tele­scopes do. Larger and heavier dishes employ an at­tazimuth mounting-one that rotates about a vertical axis and a horizontal axis. This minimizes the dis­tortion in the shape of the dish due to changing the orientation of the dish in the earth's gravitational field while tracking an object. A computer directs the rota­tion about both axes.
Even larger and more unwieldy antennas are fixed pointing upward while the rotating earth sweeps much of the sky by the antenna's field of view. Are­cibo, Puerto Rico, has the biggest fixed antenna, 2. metal dish 305 meters across contoured out of a natu· ral bowl in the ground. It can survey the sky to within 20° of the zenith, allowing coverage of about 40 percent of the entire sky.

Celestial Radio Waves' Discovery

DISCOVERY OF CELESTIAL RADIO WAVES
In 1931 a Bell Telephone engineer, Karl Jansky (1905-1950), was trying to find where the interference disrupting transatlantic radiophone circuits came from. He discovered that some of the radio noise was not from the earth; it was extraterrestrial. The primary source was the center of the Milky Way, in the constel­lation of Sagittarius. In 1936 an Illinois radio engineer, Grote Reber (1911- ), pursued the phenomenon far­ther. He built the first parabolic radio telescope, 9.5 meters in diameter, and made the first radio map of the sky. The strongest signals he found came from the star clouds in Sagittarius and from several discrete sources toward the center of our Galaxy. The next major discovery was in 1942, by British radar operators and scientists tracking down suspected radar jamming during World War II; they discovered that the inter­ference was radio emission from the sun.
At fjrst astronomers did not grasp just how significant Jansky's work was; they were preoccupied with their observations of the universe through the optical window of the earth's atmosphere. But after World War II radio astronomy came into its own when phys­icists, radio engineers, and astronomers joined forces to build larger and more efficient radio telescopes. Radio astronomy since then has led to startling discov­eries, such as interstellar molecules, pulsars, and the enigmatic quasars. Today our concept of any cosmic body is based upon its appearance all across the elec­tromagnetic spectrum, with the radio region an ex­tremely important component.

Radio Waves' Discovery and Radio Telescopes

DISCOVERY OF CELESTIAL RADIO WAVES 
In 1931 a Bell Telephone engineer, Karl Jansky (1905-1950), was trying to find where the interference disrupting transatlantic radiophone circuits came from. He discovered that some of the radio noise was not from the earth; it was extraterrestrial. The primary source was the center of the Milky Way, in the constel­lation of Sagittarius. In 1936 an Illinois radio engineer, Grote Reber (1911- ), pursued the phenomenon far­ther. He built the first parabolic radio telescope, 9.5 meters in diameter, and made the first radio map of the sky. The strongest signals he found came from the star clouds in Sagittarius and from several discrete sources toward the center of our Galaxy. The next major discovery was in 1942, by British radar operators and scientists tracking down suspected radar jamming during World War II; they discovered that the inter­ference was radio emission from the sun.
At fjrst astronomers did not grasp just how significant Jansky's work was; they were preoccupied with their observations of the universe through the optical window of the earth's atmosphere. But after World War II radio astronomy came into its own when phys­icists, radio engineers, and astronomers joined forces to build larger and more efficient radio telescopes. Radio astronomy since then has led to startling discov­eries, such as interstellar molecules, pulsars, and the enigmatic quasars. Today our concept of any cosmic body is based upon its appearance all across the elec­tromagnetic spectrum, with the radio region an ex­tremely important component.
RADIO-TELESCOPE DESIGN 
Because the physical nature of a radio wave is exactly the same as that of a light wave, the problem of de­signing a radio telescope is similar to that of designing an optical telescope. There are some practical differ­ences. Radio waves pass through most materials with­out any interaction; thus it is not feasible to design a "lens" for radio waves that will focus them in a refract­ing telescope. But any metal will reflect radio waves; so a dish-shaped metal mirror will focus radio waves, just as a glass mirror focuses light waves. The reflect­ing surface of the dish can be an open, fine-wire mesh or a solid metal with a parabolic shape. Radio waves are reflected from the su rface and converge toward a focal point, where a small collector aerial absorbs the concentrated energy turning it into an electrical current. From there the current or signal is carried by an electrical cable to the receiving equipment, which processes the signal just as in your home radio receiver.
After amplification the signal variations are record­ed in one of several ways The signal changes can then be fed into a computer for analysis. When the computer has done its job, a formerly invis­ible part of the universe is revealed as in the radio map.
Astronomers can increase the sensitivity of radio telescopes, with better accuracy in pointing and with higher resolution, by expanding the collecting area of the dish or by improving the capabilities of the re­ceiver. With the largest radio telescopes we can obtain a resolution of about l' of arc, comparable to that of the eye. That is like seeing a penny 65 meters away. The most powerful radio telescopes can detect energy from sources whose power is comparable to that of a terrestrial FM broadcast station many light years away.
The radio telescope is remotely controlled by the astronomer from an electronic console. Moderate­sized radio dishes, up to about 100 meters or so in diameter, are steerable and have equatorial mount­ings that follow the rotating sky just as optical tele­scopes do. Larger and heavier dishes employ an at­tazimuth mounting-one that rotates about a vertical axis and a horizontal axis. This minimizes the dis­tortion in the shape of the dish due to changing the orientation of the dish in the earth's gravitational field while tracking an object. A computer directs the rota­tion about both axes.
Even larger and more unwieldy antennas are fixed pointing upward while the rotating earth sweeps much of the sky by the antenna's field of view. Are­cibo, Puerto Rico, has the biggest fixed antenna, 2. metal dish 305 meters across contoured out of a natu· ral bowl in the ground. It can survey the sky to within 20° of the zenith, allowing coverage of about 40 percent of the entire sky.
RADIO INTERFEROMETRY 
Astronomers have searched endlessly for better re­solving power. It can be achieved by building bigger telescopes or by observing at shorter wavelengths for a chosen aperture or by using the phenomenon of interference. Interferometry is a tech­nique involving two or more radio telescopes. Radio radiation from an astronomical source received at the individual telescopes is combined to obtain data that have a spatial resolving power equal to that of a single telescope as large as the distance between the individ­ual receivers. With interferometry an astronomer can obtain details about the spatial structure of a given celestial object that a single radio telescope could never reveal.
For many years the separation between the individ­ual telescopes of the interferometer was limited by the lengths of the cables connecting the antennas be­cause the technique depends upon combining, at the same instant, the signals received by the separate tele­scopes. With the advent of the atomic clock (a clock governed by the vibrations of certain atoms) it became possible to record the signals received by the different telescopes, along with the precise time, and to com­pare them later. This allowed the individual tele­scopes to be greatly separated, even on opposite sides of the earth. The technique is called Very Long Base­line Interferometry (VLBI). It has been used with a geosynchronous or geostationary satellite as the link in the communications channel between the tele­scopes.
In the Very Large Array (VLA) radio interferometer recently put into operation in New Mexico signals from each of 27 individual radio tele­scopes are combi ned by a computer. Each dish is 25 meters in diameter, and the 27 individual telescopes are moved along railroad tracks arranged in the shape of an enormous 21-kilometer Y. Nine dishes will be located on each branch of the Y, and the system can provide a total of 351 interferometer pairs of antennas. The energy-collecting power of the VLA is roughly equivalent to a single 122-meter telescope. The VLA will be able to achieve spatial resolution of about 1 second of arc in 10 hours of observing, or about that of the 5.1-meter Hale optical telescope. This ability makes it comparable to large optical telescopes in fer­reting out the structure of various cosmic bodies.

Infrared Devices - Infrared Telescopes

Infrared Devices 
In 1800 William Herschel detected the infrared com­ponent of solar radiation by positioning thermome­ters beyond the red end of the sun's visible spectrum and thus foreshadowed the astronomy of invisible spectral regions. What we have seen over the last 15 years in these regions has revolutionized our concept of the universe.
INFRARED TELESCOPES 
We can subdivide the infrared spectrum into three segments. A large part of the infrared spectrum is not visible at ground level because of absorption by water vapor, carbon dioxide, and molecular oxygen, which lie between the ground and about 15 kilometers alti­tude. Consequently, airplanes, balloons, rockets, and satellites are extensively used to lift the infrared tele­scope above the veiling atmosphere. Astronomers can also locate infrared facilities on mountaintops, such as the one in the Hawaiian islands to make ground-based infrared observations.
The liquid-helium-cooled infrared detector can be used with the appropriate analyzing instruments on an ordinary optical telescope to study the cosmos. But because of the longer wavelength of infrared radi­ation, the image-producing quality of the telescope objective need not be so fine as it must be for the visible region. Thus a number of new telescopes have been designed and built for infrared astronomy only. A national observatory for infrared astronomy is built high on the 4200-meter inactive Hawaiian volcano Mauna Kea. A 3.0-meter infrared tele­scope constructed by NASA and the University of Hawaii is in operation there along with a 3.B-meter infrared telescope belonging to the United Kingdom. Other major infrared telescope facilities are the MMT in Arizona, the University of Wyoming facility, and Mexico's 2.1-meter reflector.

What are Photometers?

PHOTOMETERS 
The photometer is an accessory device that the astron­omer attaches to the telescope at the focal position of the objective to measure the amount of radiation com­ing from the astronomical object. Where the spec­trograph is used to examine the spectral composition of radiation, the photometer can be made to scan the spectrum formed by the spectrograph. It measures the amount of radiant energy, on either a relative or an absolute scale, at one wavelength or in a band of wavelengths. The photometer is much like an ex­posure meter on a camera: Incident light is converted into an electrical current. One can use a variety of techniques to define the wavelength region for the photometer, such as a spectrograph or color filters. And the radiation detector is generally today a photo­electric device.
The photoelectric photometer is usually limited to measuring only one light source, such as a star, at a time. But the limitation is compensated for by the photoelectric photometer's very great accuracy. Be­cause of its quick response to changes in amounts of light, the photoelectric photometer is particularly use­ful in continually monitoring the change in brightness of an object whose emission of radiant energy varies with time (for example, a number of stars are known to be variable light sources).

What are Spectrographs?

SPECTROGRAPHS 
The photographic plate and the photoelectric device enhance our ability to detect light from different as­tronomical sources, but they are not basically analyz­ing instruments. We can equip an accessory instrument with either of these detectors and attach it to the telescope to analyze light. The two basic types of ana­lyzing instrument are the spectrograph and the photo­meter.
The. spectrograph disperses the composite light from the source into its component wavelengths so that we can, for example, determine the elements that compose the light source. Spectroscopy, which is the study of the spectra of light sources, is astronomy's fundamental interpretive tool.
A prism or grating spectrograph receives the concentrated light coming from the telescope's objective on an entrance slit. The light diverging past the slit enters a collimator, which deliv­ers a beam of parallel rays to the dispersing device. Then these rays pass through either a prism or reflect off a grating, which separates the light into its constit­uent wavelengths. The dispersed light is focused by a camera system onto a radiation detector (a photo­graphic plate or a photoelectric device) as individual color images of the entrance slit. Each wavelength forms a distinct image of the slit. The images of the slit in the different wavelengths are arrayed in an orderly progression of colors from red to violet to create the observed spectrum of the composite light falling on the entrance slit.

What are Image Intensifiers?

IMAGE INTENSIFIERS 
Electronic image intensifiers do as their name implies, they intensify, or amplify, the light from weak sources of radiation. I n one such system photons from the telescope are focused onto a photocathode surface, which ejects electrons. The electrons are increased in number, accelerated, and focused by means of electric and magnetic fields onto a phosphorescent screen, which emits a spark of light for each electron that strikes it. Thus the faint light from the astrono­mical source is amplified by the device into light sufficient to record the image on a photographic plate. Alternatively a computer circuit can be used to count the electrons during the exposure. Still other image-intensifying techniques are in use or in devel­opmental stages; these techniques can reduce ex­posure times by factors of 50 to 100 over those for photographic systems.

What are Photoelectric devices

PHOTOELECTRIC DEVICES 
The photoelectric device is an application of the pho­toelectric effect. The basic principle is to liberate elec­trons from a metal surface by exposing it to photons in a light beam and then to measure the number of electrons with electronic circuitry. The photoelectric device, like the photographic emulsion, can·be made to respond to different wavelength regions by varying the metals used in making the surface of a device. The biggest advantage of the photoelectric device is that it can be manufactured to have a very large dynamic range of response; in addition its response is linear to the number of incident photons for a fictitious device. With modern electronics it is possible to adapt the photoelectric device to count individual photons or to use a mosaic of devices to form a picture much as a photographic plate does.
As an illustration of the photoelectric device's im­portance as a radiation detector, only about 15 per­cent of the nights of observing on the 5.1-meter Hale telescope are devoted to photographic work. On 85 percent of the nights some kind of photoelectric de­tecting device is being used.