Hubble Space Telescope

5.4 Radio Astronomy In addition to the visible radiation that penetrates Earth’s atmosphere on a clear day, radio radiation also reaches the ground. Indeed, as shown in Figure 3.9, the radio window in the electromagnetic spectrum is much wider than the optical window. (Sec. 3.3) Because the atmosphere is no hindrance to long-wavelength radiation, radio astronomers have built many ground-based radio telescopes capable of detecting cosmic radio waves. These devices have all been constructed since the 1950s—radio astronomy is a much younger subject than optical astronomy. The field originated with the work of Karl Jansky at Bell Labs in 1931, but only after the technological push of World War II did it grow into a distinct branch of astronomy. Jansky was engaged in a study of shortwave-radio interference when he discovered a faint static “hiss” that had no apparent terrestrial source. He noticed that the strength of the hiss varied in time and that its peak occurred about four minutes earlier each day. He soon realized that the peaks were coming exactly one sidereal day apart and correctly inferred that the hiss was not of terrestrial origin but came from a definite direction in space. That direction is now known to correspond to the center of our Galaxy. It took over a decade, and the realization by astronomers that interstellar gas could actually be observed at radio wavelengths, for the full importance of his work to be appreciated, but today Jansky is regarded as the father of radio astronomy. ESSENTIALS OF RADIO TELESCOPES Figure 5.21 Radio Telescope This is the world's largest fully steerable radio telescope, the 105-m-diameter device at the National Radio Astronomy Observatory in Green Bank, West Virginia. It is 150 meters tall—taller than the Statue of Liberty and nearly as tall as the Washington Monument. (NRAO) Figure 5.21 shows the world’s largest steerable radio telescope, the large 105-m- (340-foot-) diameter telescope located at the National Radio Astronomy Observatory in West Virginia. Although much larger than reflecting optical telescopes, most radio telescopes are built in basically the same way. They have a large, horseshoe-shaped mount supporting a huge curved metal dish that serves as the collecting area. The dish captures cosmic radio waves and reflects them to the focus, where a receiver detects the signals and channels them to a computer. Conceptually, the operation of a radio telescope is similar to the operation of an optical reflector with the detecting instruments placed at the prime focus (Figure 5.7a). However, unlike optical instruments, which can detect all visible wavelengths simultaneously, radio detectors normally register only a narrow band of wavelengths at any one time. To observe radiation at another radio frequency, we must retune the equipment, much as we tune a television set to a different channel. Radio telescopes must be built large partly because cosmic radio sources are extremely faint. In fact, the total amount of radio energy received by Earth’s entire surface is less than a trillionth of a watt. Compare this with the roughly 10 million watts our planet’s surface receives in the form of infrared and visible light from any of the bright stars visible in the night sky. To capture enough radio energy to allow detailed measurements to be made, a large collecting area is essential. Because of diffraction, the angular resolution of radio telescopes is generally quite poor compared with that of their optical counterparts. Typical wavelengths of radio waves are about a million times longer than those of visible light, and these longer wavelengths impose a corresponding crudeness in angular resolution. (Recall from Section 5.2 that the longer the wavelength, the greater the amount of diffraction.) Even the enormous sizes of radio dishes only partly offset this effect. The large radio telescope shown in Figure 5.21 can achieve resolution of about 1' when receiving radio waves having wavelengths of around 3 cm. However, it was designed to operate most efficiently (that is, it is most sensitive to radio signals) at wavelengths closer to 1 cm, where the resolution is approximately 20". The best angular resolution obtainable with a single radio telescope is about 10" (for the largest instruments operating at millimeter wavelengths)—at least 100 times coarser than the capabilities of some large optical systems. Radio telescopes can be built so much larger than their optical counterparts because their reflecting surface need not be as smooth as is necessary for shorter-wavelength light waves. Provided that surface irregularities (dents, bumps, and the like) are much smaller than the wavelength of the waves to be detected, the surface will reflect them without distortion. Because the wavelength of visible radiation is short (less than 10-6 m), very smooth mirrors are needed to reflect the waves properly, and it is difficult to construct very large mirrors to such exacting tolerances. However, even rough metal surfaces can accurately focus 1-cm waves, and radio waves of wavelength a meter or more can be reflected and focused perfectly well by surfaces having irregularities even as large as your fist. Figure 5.22 Arecibo Observatory An aerial photograph of the 300-m-diameter dish at the National Astronomy and Ionospheric Center near Arecibo, Puerto Rico. The receivers that detect the focused radiation are suspended nearly 150 m (about 45 stories) above the center of the dish. The left insert shows a close-up of the radio receivers hanging high above the dish. The right insert shows technicians adjusting the dish surface. (D. Parker; T. Acevedo/NAIC; Cornell University) Figure 5.23 Haystack Observatory Photograph of the Haystack dish, inside its protective radome. For scale, note the engineer standing at the bottom. Also note the dull shine on the telescope surface, indicating its smooth construction. Haystack is a poor optical mirror but a superb radio telescope. Accordingly, it can be used to reflect and accurately focus radiation having short radio wavelengths, even as small as a fraction of a centimeter. (MIT) Figure 5.22 shows the world’s largest and most sensitive radio telescope, located in Arecibo, Puerto Rico. Approximately 300 m (1000 feet) in diameter, the surface of the Arecibo telescope spans nearly 20 acres. Constructed in 1963 in a natural depression in the hillside, the dish was originally surfaced with chicken wire, which was lightweight and cheap. Although fairly rough, the chicken wire was adequate for proper reflection because the openings between adjacent strands of wire were much smaller than the long-wavelength radio waves to be detected. The entire Arecibo dish was resurfaced in 1974 with thin metal panels, and upgraded in 1997 so that it can now be used to study shorter-wavelength radio radiation. Since the 1997 upgrade, the panels can be adjusted to maintain a precise spherical shape to an accuracy of about 3 mm over the entire surface. At a frequency of 5 GHz (corresponding to a wavelength of 6 cm—the shortest wavelength that can be studied given the properties of the dish surface), the telescope’s angular resolution is about 1'. The huge size of the dish creates one distinct disadvantage, however. The Arecibo telescope cannot be pointed very well to follow cosmic objects across the sky. The detectors can move roughly 10º on either side of the focus, restricting the telescope’s observations to those objects that happen to pass within about 20º of overhead as Earth rotates. Arecibo is an example of a rough-surfaced telescope capable of detecting long-wavelength radio radiation. At the other extreme, Figure 5.23 shows the 36-m-diameter Haystack dish in northeastern Massachusetts. It is constructed of polished aluminum and maintains a parabolic curve to an accuracy of about a millimeter all the way across its solid surface. It can reflect and accurately focus radio radiation with wavelengths as short as a few millimeters. The telescope is contained within a protective shell, or radome, that protects the surface from the harsh New England weather. It acts much like the protective dome of an optical telescope, except that there is no slit through which the telescope “sees.” Incoming cosmic radio signals pass virtually unimpeded through the radome’s fiberglass construction. THE VALUE OF RADIO ASTRONOMY Despite the inherent disadvantage of relatively poor angular resolution, radio astronomy enjoys many advantages. Radio telescopes can observe 24 hours a day. Darkness is not needed for receiving radio signals because the Sun is a relatively weak source of radio energy, so its emission does not swamp radio signals arriving at Earth from elsewhere in the sky. In addition, radio observations can often be made through cloudy skies, and radio telescopes can detect the longest-wavelength radio waves even during rain or snowstorms. Poor weather causes few problems because the wavelength of most radio waves is much larger than the typical size of atmospheric raindrops or snowflakes. Optical astronomy cannot be done under these conditions because the wavelength of visible light is smaller than a raindrop, a snowflake, or even a minute water droplet in a cloud. Figure 5.24 Orion Nebula in Radio and Visible The Orion Nebula is a star-forming region about 1500 light-years from Earth. (The nebula is located in the constellation Orion and can be seen in Figure 1.6.) The bright regions in this photograph are stars and clouds of glowing gas. The dark regions are not empty, but their visible emission is obscured by interstellar matter. Superimposed on the optical image is a radio contour map (white lines) of the same region. Each curve of the contour map represents a different intensity of radio emission. The resolution of the optical image is about 1"; that of the radio map is 1'. (background photo: AURA) However, perhaps the greatest value of radio astronomy (and, in fact, of all invisible astronomies) is that it opens up a whole new window on the universe. There are two main reasons for this. First, just as objects that are bright in the visible part of the spectrum (the Sun, for example) are not necessarily strong radio emitters, many of the strongest radio sources in the universe emit little or no visible light. Second, visible light may be strongly absorbed by interstellar dust along the line of sight to a source. Radio waves, on the other hand, are generally unaffected by intervening matter. Many parts of the universe cannot be seen at all by optical means but are easily detectable at longer wavelengths. The center of the Milky Way Galaxy is a prime example of such a totally invisible region—our knowledge of the Galactic center is based almost entirely on radio and infrared observations. Thus, these observations not only afford us the opportunity to study the same objects at different wavelengths, but also allow us to see whole new classes of objects that would otherwise be completely unknown. Figure 5.24 shows an optical photograph of the Orion Nebula (a huge cloud of interstellar gas) taken with the 4-m telescope on Kitt Peak. Superimposed on the optical image is a radio map of the same region, obtained by scanning a radio telescope back and forth across the nebula and taking many measurements of radio intensity. The map is drawn as a series of contour lines connecting locations of equal radio brightness, similar to pressure contours drawn by meteorologists on weather maps or height contours drawn by cartographers on topographic maps. The inner contours represent stronger radio signals, the outside contours weaker signals. The radio map in Figure 5.24 has many similarities to the visible-light image of the nebula. For instance, the radio emission is strongest near the center of the optical image and declines toward the nebular edge. But there are also subtle differences between the radio and optical images. The two differ mainly toward the upper left of the main cloud, where visible light seems to be absent, despite the existence of radio waves. How can radio waves be detected from locations not showing any light emission? The answer is that this particular nebular region is known to be especially dusty in its top left quadrant. The dust obscures the short-wavelength visible light but not the long-wavelength radio radiation. Our radio map allows us to see the true extent of this cosmic source.

The main disadvantage of radio astronomy compared with optical work is its relatively poor angular resolution. However, in some circumstances, radio astronomers can overcome this problem by using a technique known as interferometry. This technique makes it possible to produce radio images of angular resolution higher than can be achieved with even the best optical telescopes, on Earth or in space.

Figure 5.25 VLA Interferometer This large interferometer is made up of 27 separate dishes spread along a Y-shaped pattern about 30 km across on the Plain of San Augustin near Socorro, New Mexico. The most sensitive radio device in the world, it is called the Very Large Array or VLA, for short. (b) A close-up view from ground level of some of the VLA antennas. Notice that the dishes are mounted on railroad tracks so that they can be repositioned easily. (NRAO)

Figure 5.26 Radio–Optical Comparison  (a) VLA radio “photograph” (or radiograph) of the spiral galaxy M51, observed at radio frequencies with an angular resolution of a few arc seconds; (a) shows nearly as much detail as (b), an actual (light) photograph of that same galaxy made with the 4-m Kitt Peak optical telescope.(NRAO/AURA)
	Figure 5.27 Interferometer Array Some of the telescopes of the COAST interferometer can be seen here. (b) This extremely high-resolution image of the double-star Capella was made in the near infrared at 830 nm. (MRAO)

Although the technique was originally developed by radio astronomers, interferometry is no longer restricted to the radio domain. Radio interferometry became feasible when electronic equipment and computers achieved speeds great enough to combine and analyze radio signals from separate radio detectors without loss of data. As the technology has improved, it has become possible to apply the same methods to higher-frequency radiation. Millimeter-wavelength interferometry has already become an established and important observational technique, and both the Keck telescopes and the VLT are expected to be used for infrared interferometry within the next few years.

Optical interferometry is currently the subject of intensive research. In 1997 a group of astronomers in Cambridge, England, succeeded in combining the light from three small optical telescopes to produce a single, remarkably clear, image. Each telescope of the Cambridge Optical Aperture Synthesis Telescope (COAST) was only 0.4 m in diameter, but with the mirrors positioned 6 m apart, the resulting resolution was a stunning 0.01"—better by far than the resolution of the best adaptive-optics systems on the ground, or of HSToperating above Earth’s atmosphere. This enabled astronomers to “split” the binary star Capella, whose two member stars are separated by only 0.05" and is therefore normally seen from the ground only as a slightly oblong blur. With the COAST array, the two stars are cleanly and individually separated. Figure 5.27 shows some of the telescopes of this interferometer and some sample results. NASA plans to place an optical interferometer—theSpace Interferometry Mission, or SIM—in Earth orbit by 2010.

5.6 Space-Based Astronomy

Optical and radio astronomy are the oldest branches of astronomy, but since the 1970s there has been a virtual explosion of observational techniques covering the rest of the electromagnetic spectrum. Today, all portions of the spectrum are studied, from radio waves to gamma rays, to maximize the amount of information available about astronomical objects. As noted earlier, the types of astronomical objects that can be observed differ quite markedly from one wavelength range to another. Full-spectrum coverage is essential not only to see things more clearly but even to see some things at all. Because of the transmission characteristics of Earth’s atmosphere, astronomers must study most wavelengths other than optical and radio from space. The rise of these “other astronomies” has therefore been closely tied to the development of the space program.

INFRARED ASTRONOMY

Infrared studies are a very important component of modern observational astronomy. Generally, infrared telescopes resemble optical telescopes, but their detectors are designed to be sensitive to longer-wavelength radiation. Indeed, as we have seen, many ground-based “optical” telescopes are also used for infrared work, and some of the most useful infrared observing is done from the ground (for example, from Mauna Kea—see Figure 5.11), even though the radiation is somewhat diminished in intensity by our atmosphere.

Figure 5.28 Smog Revealed (a) An optical photograph (left) taken near San Jose, California, and an infrared photo (right) of the same area taken at the same time. Longer-wavelength infrared radiation can penetrate smog much better than short-wavelength visible light. (b) Two views of an especially dusty part of our Galaxy, in optical (left) and infrared (right) light. This is the central region of the Orion Nebula, seen much more clearly in the infrared band. (Harvard Observatory; NASA)
	Figure 5.29 Infrared Telescopes (a) A gondola containing a 1-m infrared telescope (lower left) is readied for its balloon-borne ascent to an altitude of about 30 km, where it will capture infrared radiation that cannot penetrate Earth’s atmosphere. (b) An artist’s conception of the Infrared Astronomy Satellite (IRAS), placed in orbit in 1983. This 0.6-m telescope surveyed the infrared sky at wavelengths ranging from 10–100 µm. During its 10 months of operation it greatly increased astronomers’ understanding of many different aspects of the universe—from the formation of stars and planets to the evolution of galaxies. (SAO; NASA)

During its 10-month lifetime (and long afterward—the data archives are still heavily used even today), IRAS contributed greatly to our knowledge of the clouds of Galactic matter destined to become stars and, possibly, planets. These regions of interstellar gas are composed of warm gas that cannot be seen with optical telescopes or adequately studied with radio telescopes. Because much of the material between the stars has a temperature between a few tens and a few hundreds of kelvins, Wien’s law tells us that the infrared domain is the natural portion of the electromagnetic spectrum in which to study it. (Sec. 3.4)Throughout the text we will encounter many findings made by this satellite about comets, stars, galaxies, and the scattered dust and rocky debris found among the stars.

Figure 5.30(a) shows an IRAS image of the Orion region. The image is represented in false color, a technique commonly used for displaying images taken in nonvisible light. The colors do not represent the actual wavelength of the radiation emitted, but instead some other property of the source, in this case temperature, descending from white to red to black. The whiter regions thus denote higher temperatures and hence greater strength of infrared radiation. Figure 5.30(b) shows the same region photographed in (true color) visible light. At about 1' angular resolution, the fine details of the Orion nebula evident in the visible portion of the earlier image (Figure 5.24) cannot be perceived. Nonetheless, astronomers can extract much useful information about this object and others like it from infrared observations. Star-forming clouds of warm dust and gas, and extensive groups of bright young stars, completely obscured at visible wavelengths, are seen.

Figure 5.30 Infrared Image (a) This infrared image of the Orion Nebula and its surrounding environment was made by the Infrared Astronomy Satellite. In this false-color image, colors denote different temperatures, descending from white to red to black. The whiter regions denote greater intensity of infrared radiation. (b) The same region photographed in visible light. The labels a and b refer, respectively, to Betelgeuse and Rigel, the two brightest stars in the constellation. Note how the red star Betelgeuse is easily seen in the infrared (part a), while the blue star Rigel is very faint. (NASA; J. Sanford)

Only a few long-duration infrared satellites have followed the IRAS mission. In November 1995 the European Space Agency launched the 0.6-m Infrared Space Observatory (ISO).The mission ended in May 1998. During its lifetime, ISO’s imaging and spectroscopic observations refined and extended the pioneering work of IRAS. Today, the instrument package aboard the Hubble Space Telescope includes a near-infrared camera and spectroscope. NASA plans to deploy the 0.85-m Space Infrared Telescope Facility (SIRTF)in 2002. Like ISO, SIRTF’s detectors will be designed to operate at wavelengths in the 3-µm to 200-µm range.

ULTRAVIOLET ASTRONOMY

On the short-wavelength side of the visible spectrum lies the ultraviolet domain. This region of the spectrum, extending in wavelength from 400 nm (blue light) down to a few nanometers (“soft” X-rays), has only recently begun to be explored. Because Earth’s atmosphere is partially opaque to radiation below 400 nm and is totally opaque below about 300 nm (in part because of the ozone layer), astronomers cannot conduct any useful ultraviolet observations from the ground, not even from the highest mountaintop. Rockets, balloons, or satellites are therefore essential to any ultraviolet telescope—a device designed to capture and analyze this high-frequency radiation.

One of the most successful ultraviolet space missions was the International Ultraviolet Explorer (IUE), placed in Earth orbit in 1978 and shut down for budgetary reasons in late 1996 (see Discovery 18-1). Like all ultraviolet telescopes, its basic appearance and construction were quite similar to optical and infrared devices. Several hundred astronomers from all over the world used IUE’s near-ultraviolet spectroscopes to explore a variety of phenomena in planets, stars, and galaxies. In subsequent chapters we will learn what this relatively new window on the universe has shown us about the activity and even the violence that seems to pervade the cosmos. Today, the Far Ultraviolet Spectrographic Explorer (FUSE) satellite, launched in 1999, is extending IUE’s work into the “far” ultraviolet (around 100 nm) regime. In addition, the Hubble Space Telescope (Discovery 5-1), best known as an optical telescope, is also a superb imaging and spectroscopic ultraviolet instrument.

Figure 5.31(a) shows an image of a supernova remnant—the remains of a violent stellar explosion that occurred some 150,000 years ago—obtained by the Extreme Ultraviolet Explorer (EUVE) satellite. Launched in 1992 and terminated (again for budgetary reasons) in 2000, EUVE operated at the short-wavelength (1–50 nm) end of the ultraviolet range, making it sensitive to phenomena involving high temperatures (hundreds of thousands to millions of kelvins) or other energetic events. (Sec. 3.4) EUVE mapped out our local cosmic neighborhood as it presents itself in the far ultraviolet, and radically changed astronomers’ conception of interstellar space in the vicinity of the Sun.

Figure 5.31 Ultraviolet Images (a) A camera on board the Extreme Ultraviolet Explorer satellite captured this image of the Cygnus Loop supernova remnant, the result of a massive star blowing itself virtually to smithereens. The release of energy was prodigious, and the afterglow lingered for millennia. The glowing field of debris shown here within the telescope's circular field of view lies some 2700 light-years from Earth. Based on the velocity of the outflowing debris, astronomers estimate that the explosion itself must have occurred about 150,000 years ago. (b) This false-color image of the spiral galaxy M74 was made by an ultraviolet telescope aboard the Astropayload carried by a space shuttle in 1995. (NASA)

HIGH-ENERGY ASTRONOMY

High-energy astronomy studies the universe as it presents itself to us in X rays and gamma rays—the types of radiation whose photons have the highest frequencies and hence the greatest energies. How do we detect radiation of such short wavelengths? First, it must be captured high above Earth’s atmosphere because none of it reaches the ground. Second, its detection requires the use of equipment fundamentally different in design from that used to capture the relatively low energy radiation discussed up to this point.

The difference in the design of high-energy telescopes comes about because X-rays and gamma rays cannot be reflected easily by any kind of surface. Rather, these rays tend to pass straight through, or be absorbed by, any material they strike. When X-rays barely graze a surface, however, they can be reflected from it in a way that yields an image, although the mirror design is fairly complex (see Figure 5.32). For gamma rays, no such method of producing an image has yet been devised. Present-day gamma-ray telescopes simply point in a specified direction and count photons received.

Figure 5.32 X-Ray Telescope The arrangement of mirrors in an X-ray telescope allows X-rays to be reflected at grazing angles and focused to form an image.

Figure 5.33 X-Ray Image An X-ray image of the Orion region, taken by theROSAT X-ray satellite. (Compare with Figures 1.6, 5.24, and 5.30.) Objects that are most intensely emitting X-rays are colored blue, including the three stars of Orion’s belt and the glowing nebula below them at bottom center in the photograph. Note that the star Betelgeuse (marked as a) now appears relatively faint; compare Figure 5.28. (European Space Agency)

In July 1999, NASA launched the Advanced X-Ray Astrophysics Facility (AXAF , renamed Chandraafter launch, in honor of the Indian astrophysicist Subramanyan Chandrasekhar). With greater sensitivity, a wider field of view, and better resolution than either Einstein or ROSAT, Chandrais providing high-energy astronomers with new levels of observational detail. Discovery 5-2discusses the Chandra mission in more detail. The European X-ray Multi-Mirror satellite (now known as XMM-Newton) was launched in December 1999.XMM is more sensitive than Chandra (that is, it can detect fainter X-ray sources), but it has significantly poorer angular resolution (5" compared to 0.5" for Chandra), making the two missions complementary to one another.

Gamma-ray astronomy is the youngest entrant into the observational arena. As just mentioned, imaging gamma-ray telescopes do not exist, so only fairly coarse (1º resolution) observations can be made. Nevertheless, even at this resolution, there is much to be learned. Cosmic gamma rays were originally detected in the 1960s by the U.S. Vela series of satellites, whose primary mission was to monitor illegal nuclear detonations on Earth. Since then, several X-ray telescopes have also been equipped with gamma-ray detectors.

By far the most advanced instrument was the Compton Gamma-Ray Observatory (CGRO),launched by the space shuttle in 1991. At 17 tons, it was the largest astronomical payload ever launched by NASA at that time. CGRO scanned the sky and studied individual objects in much greater detail than had previously been attempted. Figure 5.34 shows the satellite in low Earth orbit, along with a false-color gamma-ray image of a highly energetic outburst in the nucleus of a distant galaxy. The mission ended on June 4, 2000, when, following a failure of one of the satellite’s three gyroscopes, NASA opted for a controlled reentry and dropped CGRO into the Pacific Ocean.

Figure 5.34 Gamma-Ray Observatory  (a) This photograph of the 17-ton Compton Gamma-Ray Observatory (CGRO; named after an American gamma-ray pioneer) was taken by an astronaut during the satellite’s deployment from the space shuttle Atlantis over the Pacific Coast of the United States. (b) A typical false-color gamma-ray image—this one showing a violent event in the distant galaxy 3C279, also known as a “gamma-ray blazar.” (NASA)

List some scientific benefits of placing telescopes in space. What are the drawbacks of space-based astronomy?

Table 5.1 lists the basic regions of the electromagnetic spectrum and describes objects typically studied in each frequency range. Bear in mind that the list is far from exhaustive and that many astronomical objects are now routinely observed at many different electromagnetic wavelengths. As we proceed through the text, we will discuss more fully the wealth of information that high-precision astronomical instruments can provide us.

It is reasonable to suppose that the future holds many further improvements in both the quality and the availability of astronomical data and that many new discoveries will be made. The current and proposed pace of technological progress presents us with the following exciting prospect: Early in the twenty-first century, if all goes according to plan, it will be possible, for the first time ever, to make simultaneous high-quality measurements of any astronomical object at all wavelengths, from radio to gamma ray. The consequences of this development for our understanding of the workings of the universe may be little short of revolutionary.

As a preview of the sort of comparison that full-spectrum coverage allows, Figure 5.35 shows a series of images of our own Milky Way Galaxy. They were made by several different instruments, at wavelengths ranging from radio to gamma ray, over a period of about five years. By comparing the features visible in each, we immediately see how multiwavelength observations can complement one another, greatly extending our perception of the dynamic universe around us.

Figure 5.35 Multiple Wavelengths The Milky Way Galaxy, as it appears at (a) radio, (b) infrared, (c) visible, (d) X-ray, and (e) gamma-ray wavelenghts. Each frame is a panoramic, view covering the entire sky. The center of the Galaxy, which lies in the direction of the constellation Sagittarius at the center of each map. (NRAO; NASA; Lund Observatory; K. Dennerl/ and W. Voges; NASA)