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Endangered Night Skies

ON THE COVER IS A FIRST. It shows nearly the entire Earth as seen at night from orbiting spacecraft. Viewed by day from hundreds of miles in space, our planet shows no clear signs of intelligent life. But at night, lights from cities and rural fires visibly trace the activities of humankind.

Initially I became interested in the appearance of the Earth from "outside" through my work related to the search for extraterrestrial intelligence. I worked out what the radio Earth looks like from interstellar distances and what might be deduced about our civilization from an analysis of the most prominent signals (without the ability actually to decode the video images or the audio).

This then led to the present work on the appearance of the entire Earth at visual wavelengths. I had always been fascinated by the night-time satellite photographs of the U.S. and what it said about urbanization, highways, and light pollution, and I always wondered why no one had bothered to put together a mosaic of the entire "Earth at Night." So I did. As I got into the project, I found it yet more revealing about us, for not only is there the strong contribution from urban lighting, but also other signs of our global society: gas burn-off flares in oil fields and fires for heating and cooking and slash-and-burn agriculture. And nature also contributes through ever-changing aurorae.

The mosaic of nighttime views presented here was obtained by the Air Force's Defense Meteorological Satellite Program. The DMSP spacecraft are designed to provide global weather coverage on a continual basis. They travel in nearly polar orbits, 500 miles up, following the globe's midnight-noon line. Every 0.4 seconds each satellite scans a 2000-by-2-mile swath on the ground, using a 5-inch telescope. Successive scans are then built up to produce an image. Although the terrain below is scrutinized at both visual and infrared wavelengths, the image here comes only from the visual band.

A DMSP telescope can detect a surprisingly small amount of light: an unshielded fluorescent lamp of only 100 watts is detectable on the original survey, although more power is necessary to show on the reproduction here. DMSP has coarser detail, but is far more sensitive than the more familiar Landsat images; for example, Landsat cannot even detect New York City at night!

What do the DMSP images reveal of the Earth at night? As expected, a large fraction of the light leakage to space corresponds to heavily urbanized regions. Streetlights account for the prominence of high-income population centers such as Europe, North America, Japan, and other major metropolitan regions. Routes such as the Trans-Siberian Railroad and Interstate 5 along the U.S. West Coast are nicely etched, while the delimiting effects of geographical features such as the Himalayas and the Urals are also apparent.

A closer look at the image reveals many other sources apart from cities. The most prominent natural sources are aurorae a small one is seen here over northern Scandinavia. Other natural contributions arise from reflected moonlight, lightning, forest fires, high-altitude airglow, and even erupting volcanoes.

Human activity gives rise to a number of light sources. Surprisingly, many controlled fires in the tropics show up the results of cooking and heating, slash-and-burn agriculture, and widespread clearing of forests. While the number of these fires at any location depends on the season, the yearly average is high. In the image shown here, they are prominent throughout Southeast Asia and southern India. Many recent studies have argued that this deforestation is leading towards ecological disaster.

In the more remote of the world's oil fields, the burn-off of natural gas, which comes up with the oil in a frothy mixture, is another light source. If it is not economical to pipe or to liquefy the gas, it is treated as garbage and burned in rows of standpipes. (Flares are not associated with those regions solely producing natural gas, for there the wells are properly capped.) Gas flares show clearly in Indonesia, the Persian Gulf states, the Tashkent region of the Soviet Union, Libya, Algeria, and northeastern South America (Lake Maracaibo).

Finally, Japanese and Korean squid fishermen use hundreds of megawatts of lights, strung along the decks of their boats, to lure squid to the surface. When the fleets are out squidding (but not on the present image), the Sea of Japan appears as bright as Japan itself!

For the astronomer, all this activity has created a grave situation. Even the slightest contamination from city lights can be disastrous when observing faint objects. With the amount of "light pollution" ever growing, observatories face severe limitations on the types of projects they can undertake.

For instance, in California over the past decade the brightness of outdoor lights has grown about 20 percent annually, a total increase of a factor of six. This means that the glare from the city of San Jose and the Santa Clara Valley now greatly hampers the projects of astronomers using the nearby 120-inch diameter telescope at Lick Observatory on Mt. Hamilton the faintest stars and galaxies which we can see today must be four times brighter than was formerly the case. Even worse is the case of the Mt. Wilson Observatory 100-inch telescope, just outside Pasadena, which has been completely vitiated by the lights of the Los Angeles Basin.

There is hope, however, to save observatories from the steady encroachment of light pollution. Several municipalities have cooperated with neighboring observatories in adopting lighting policies satisfactory to astronomers, and in doing so have usually found that they also save considerable sums. Injurious effects can be minimized by regulating the use of outdoor lighting, especially in the late night hours, and by using better shields above lights. But given that a certain amount of lighting is necessary, what else can be done?

It turns out that the various types of streetlights are not at all equivalent in terms of their damage to our views of the universe. Some concentrate their energy output in a few very narrow wavelength bands, thus allowing largely unimpeded observations over most of the visual range from the shortest blue wavelengths to the longest red ones. But other lights spread their energy over the entire visual band, in effect blinding the astronomer no matter where he operates. To be specific, desirable lights include the standard bluish mercury-vapor lights, largely being replaced these days because of energy inefficiency, and yellowish low-pressure sodium lights. The latter are energy efficient and in wide use in Europe, but have largely lost out in the General Electric-dominated U.S. market in competition with peach-colored high-pressure sodium lights, a harmful type which spreads its energy over a broad wavelength range and is demonstrably less efficient than its low-pressure cousin.

Nevertheless, in several recent cases astronomers have convinced local authorities to adopt favorable policies. Arizona, in particular Tucson, with its concentration of world-class telescopes, has been a leader in this field Governor Bruce Babbit even favors making the entire state a "dark zone."

The most recent battle has involved the city of San Diego and 50-mile-distant Palomar Observatory, whose renowned 200-inch telescope and other major instruments have become much less sensitive due to light pollution the capabilities of the 200-inch are now only equivalent to those of a 140-inch telescope at a dark site. Last February the San Diego City Council overturned an earlier decision to convert all of its street lighting to the high-pressure sodium type and acquiesced to astronomers' arguments for much less damaging and more cost-effective low-pressure sodium lights. A further, celestial benefit then accrued to the city when, in gratitude, the Observatory proposed that newly discovered asteroid 3043 be named Asteroid San Diego!

While optical astronomers battle the onslaught of light pollution, their colleagues observing with the radio portion of the electromagnetic spectrum similarly curse interference from the world's communications and radar transmitters. But unlike the optical spectrum, the radio spectrum is carefully managed by international and national agencies. This has allowed radio astronomers to gain exclusive rights to certain bands of frequencies so that, in principle, they can use these bands with little interference. The reality, however, is often otherwise. For example, a TV satellite transmitting to its ground station need only inadvertently emit one-millionth of its power at a frequency reserved for astronomy to render useless a radio telescope "dish" studying a distant, faint quasar. Other common causes of interference are aviation, weather and military radars, radio and television broadcasts, industrial activities such as welding, and even automobile engines. Radio astronomers increasingly must seek remote sites and take costly measures to counter the manmade interference.

Light and radio pollution is clearly a serious problem for the modern astronomer. Observatories must now be located not only where natural conditions are ideal, but where people are absent as well. The number of possible sites is rapidly dwindling and their costs of operation increasing. If anything like the present rate of growth continues, it seems certain that astronomy will eventually be forced into space, where the expense for comparable capabilities typically runs a hundred times greater.

But the greatest damage to our global culture from light pollution has already arrived. It is sad to note that hundreds of millions of people, perhaps even a majority of the inhabitants of North America and Europe, are now regularly denied the nighttime universe. No longer do they know the exquisite thrill of a meteor shooting across the sky or the deep introspection brought on by the resplendent tapestry of two thousand stars banding the Milky Way. At a time when the very survival of our species depends on such a common vision, we have instead wrapped Earth in an electromagnetic fog.

For further information on model ordinances, technical specifications, etc., write to Light Pollution, Dr. David Crawford, Kitt Peak National Observatory, Box 26732, Tucson, AZ 85726.

Detailed discussions and examples of DMSP imagery by T. A. Croft can be found in the July 1978 issue of Scientific American, page 86, and in the report, "Nocturnal Images of the Earth from Space," produced for the U.S. Geological Survey in 1977. The general problem of light pollution is discussed by K. W. Riegel in Science for March 30, 1973, page 1285. A full analysis by C. Sagan and D. Wallace of the much more difficult task of detecting human activities from daytime satellite images of Earth was published in Icarus, Vol. 15, 1971, page 515. Radio leakage from Earth (and its possible importance in searching for extraterrestrial intelligence) is analyzed by S. Brown, C. Wetherill, and the author in Science for January 27, 1978, page 377, and on a popular level in Mercury for March 1979.

For their assistance I thank T. Gregory and K. Gordon of the DMSP Library, now located at the World Data Center, Box 449, University of Colorado, Boulder, Colorado 80309, and D. Azose for photographic work. Negatives and prints of DMSP images are available to anyone, through the World Data Center. ■