Cover; Magnificent Cosmos; Scientific American Presents; by Staff Editor; 1 Page(s)

Table of Contents; Magnificent Cosmos; Scientific American Presents; by Staff Editor; 2 Page(s)

From the Editors; Magnificent Cosmos; Scientific American Presents; by Rennie; 1 Page(s)

Exploration of space has sprinted forward over the past two decades, even though no human has ventured outside the lunar orbit. Thanks to strings of probes with names like Voyager, Pioneer, Galileo, Magellan and SOHO, planetary and solar science thrived. We have seen all the planets but Pluto from close by, visited Mars and Venus by proxy, and even witnessed the collision of Comet Shoemaker-Levy with Jupiter. The moons graduated from minor players to varied, exotic worlds in their own right and possibly to abodes for life. The sun revealed its complex internal anatomy. Whole new classes of frozen bodies beyond Neptune's orbit came into view.

Meanwhile the magnificent Hubble Space Telescope, other orbiting instruments and their Earth-bound cousins peered clearly into deeper space. They showed us new types of galaxies and stars, spotted planets around other suns and took the temperature of the big bang. We better appreciated our own solar system after seeing how fiercely bright some corners of the universe burn.

Discovering Worlds; Magnificent Cosmos; Scientific American Presents; by Staff Editor; 1 Page(s)

1.)GIANT PLANETS ORBITING FARAWAY STARS 2.)SEARCHING FOR LIFE IN OUR SOLAR SYSTEM 3.)SEARCHING FOR LIFE IN OTHER SOLAR SYSTEMS 4.)PLANETARY TOUR

Giant Planets Orbiting Faraway Stars; Magnificent Cosmos; Scientific American Presents; by Marcy, Butler; 6 Page(s)

No doubt humans have struggled with the question of whether we are alone in the universe since the beginning of consciousness. Today, armed with evidence that planets do indeed orbit other stars, astronomers wonder more specifically: What are those planets like? Of the 100 billion stars in our Milky Way galaxy, how many harbor planets? Among those planets, how many constitute arid deserts or frigid hydrogen balls? Do some contain lush forests or oceans fertile with life?

For the first time in history, astronomers can now address these questions concretely. During the past two and a half years, researchers have detected eight planets orbiting sunlike stars. In October 1995 Michel Mayor and Didier Queloz of Geneva Observatory in Switzerland reported finding the first planet. Observing the star 51 Pegasi in the constellation Pegasus, they noticed a telltale wobble, a cyclical shifting of its light toward the blue and red ends of the spectrum. The timing of this Doppler shift suggests that the star wobbles because of a closely orbiting planet, which revolves around the star fully every 4.2 days-at a whopping speed of 482,000 kilometers (299,000 miles) an hour, more than four times faster than Earth orbits the sun.

Searching for Life in Our Solar System; Magnificent Cosmos; Scientific American Presents; by Jakosky, sidebar by Lipkin; 6 Page(s)

Since antiquity, human beings have imagined life spread far and wide in the universe. Only recently has science caught up, as we have come to understand the nature of life on Earth and the possibility that life exists elsewhere. Recent discoveries of planets orbiting other stars and of possible fossil evidence in Martian meteorites have gained considerable public acclaim. And the scientific case for life elsewhere has grown stronger during the past decade. There is now a sense that we are verging on the discovery of life on other planets.

To search for life in our solar system, we need to start at home. Because Earth is our only example of a planet endowed with life, we can use it to understand the conditions needed to spawn life elsewhere. As we define these conditions, though, we need to consider whether they are specific to life on Earth or general enough to apply anywhere.

Searching for Life in Other Solar Systems; Magnificent Cosmos; Scientific American Presents; by Angel, Woolf; 4 Page(s)

The search for extraterrestrial life can now be extended to planets outside our solar system. After years of looking, astronomers have turned up evidence of giant planets orbiting several distant stars similar to our sun. Smaller planets around these and other stars may have evolved living organisms. Finding extraterrestrial life may seem a Herculean task, but a space telescope mission called the Terrestrial Planet Finder, which the National Aeronautics and Space Administration plans to start in 2005, aims to locate such planets and search for evidence of life-forms, such as the primitive ones on Earth.

The largest and most powerful telescope now in space, the Hubble Space Telescope, can just make out mountains on Mars at 30 kilometers (19 miles). Pictures sharp enough to display geologic features of planets around other stars would require an array of space telescopes the size of the U.S. But pictures of Earth do not reveal the presence of life unless they are taken at very high resolution. Such images could be obtained with unmanned spacecraft sent to other solar systems, but the huge distance between Earth and any other planet makes this approach impractical.

Planetary Tour; Magnificent Cosmos; Scientific American Presents; by Staff Editors; 2 Page(s)

Some four and a half billion years ago, and for reasons that scientists have yet to agree upon, a flat, round cloud of gas and dust began to contract in the interstellar space of our Milky Way galaxy, itself already at least five billion years old. As this cloud collapsed toward its center, its relatively small initial rate of spin increased. This spinning, in turn, hurled agglomerations of dust outward, enabling them to resist the gravitational pull of a massive nebula at the center of the cloud.

As this giant central nebula-the precursor of our sun-collapsed in on itself, the temperature at its center soared. Eventually, the heat and pressure were enough to ignite the thermonuclear furnace that would make life possible and that will probably burn for another five billion years.

Mercury; Magnificent Cosmos; Scientific American Presents; by Staff Editor; 2 Page(s)

The innermost planet in the solar system, Mercury has the most extreme characteristics of the terrestrial bodies. Daytime temperatures on the planet reach 427 degrees Celsius (801 degrees Fahrenheit)-hot enough to melt zinc. At night, however, the lack of an atmosphere lets the temperature plunge to -183 degrees C, which is cold enough to freeze krypton.

Mercury is also unusually dense. To account for its density of 5.44 grams per cubic centimeter (0.20 pound per cubic inch), astronomers believe the planet must have a relatively huge core that is unusually iron-rich. The core probably takes up 42 percent of Mercury's volume; in comparison, Earth's core is only about 16 percent, and Mars's, about 9 percent.

Venus; Magnificent Cosmos; Scientific American Presents; by Staff Editor; 2 Page(s)

Though named for the goddess of love, Venus is more like Earth's ugly sister. The two planets formed from the same general region of the solar nebula, suggesting that their compositions are basically similar. They are of roughly the same size, mass and density, and Venus orbits the sun at an average distance abou 70 percent that of Earth's.

But where Earth has temperatures and conditions conducive to life, a variety of environments and a robust magnetic field, Venus is a dry, hellish, high-pressure furnace whose magnetic field is not even strong enough to keep the solar wind from stripping away the upper atmosphere. Below ever present clouds of sulfuric acid and a thick carbon dioxide atmosphere, the Venusian surface hits temperatures up to 450 degrees Celsius (842 degreea Fahrenheit).

Earth; Magnificent Cosmos; Scientific American Presents; by Staff Editor; 2 Page(s)

That it teems with life makes Earth a precious oddity among planets-although just how odd, scientists cannot say. Certainly the conditions that made life possible were sensitive to the planet's surface temperature and therefore to its distance from the sun.

Abundant liquid water was critical to the planet's evolution. This water moderated temperatures, eroded rocks, dissolved minerals and supported complex chemical reactions, some of which yielded single-celled life close to four billion years ago. Macroscopic animals started proliferating only around 600 million years ago, eons after photosynthesis enriched the atmosphere with oxygen.

Mars; Magnificent Cosmos; Scientific American Presents; by Staff Editor; 2 Page(s)

Mars's relative nearness, mythological connotations and even its hue have made it the favored planet of popular culture. Countless works of science fiction and science have explored the possibility of life on Mars. In 1976, however, the two U.S. Viking probes found no evidence of life at their landing sites.

Two events thrust Mars back into the public consciousness lately. In 1996 a team from the National Aeronautics and Space Administration Johnson Space Center and Stanford University announced that unusual characteristics in a meteorite known to have come from Mars could be best interpreted as the vestiges of ancient bacterial life. In the summer of 1997 the Mars Pathfinder lander and its diminutive roving vehicle, Sojourner, analyzed and imaged Martian rocks, atmosphere and soil. Investigators concluded that many of the rocks were deposited by a massive flood at least two billion years ago and that some of them were surprisingly similar to a class of Earth rocks known as andesites.

Jupiter; Magnificent Cosmos; Scientific American Presents; by Staff Editor; 2 Page(s)

Jupiter represents a departure from the four relatively tiny rock planets that precede it as we travel away from the sun. It is the first of the four "gas giants," planets that dwarf Earth and that have no solid surfaces. Jupiter does everything on a grand scale. It is larger than all the other planets combined, and its moon Ganymede is bigger than Mercury.

Jupiter's hydrogen and helium content once led astronomers to think that the planet formed out of the same gas cloud that gave rise to the sun. More recent analysis of the subtleties in Jupiter's chemistry point to a solid core, with perhaps the mass of 10 Earths, about which the rest of the planet formed. Jupiter also differs in kind from the terrestrial planets by radiating more energy than it receives from the sun. In 1994 fragments of Comet Shoemaker-Levy 9 slammed into Jupiter, thrilling observers.

Saturn; Magnificent Cosmos; Scientific American Presents; by Staff Editor; 2 Page(s)

Saturn's rings make it one of the most familiar, and spectacular, images of astronomy, not to mention science fiction. When Galileo trained a primitive telescope on the planet for the first time in 1610, he was misled. From the poorly resolved image in his viewfinder, he believed Saturn to be a triple-system, with a large body in the center and smaller ones on each side. The rings may be much younger than the planet itself, and great mathematicians have found them worthy of contemplation. Laplace and James Clerk Maxwell calculated that Saturn's rings must consist of many smaller objects. Although the planet is almost the size of Jupiter, its mass is but one third as great, giving Saturn the lowest mean density of any solar system object.

As a gas giant, the planet has no single rotation period but rather a variety depending on latitude. Upper atmosphere clouds travel around the equator in as little as 10 hours and 10 minutes; clouds in high latitudes may take half an hour longer to pass across the planet. Based on gravitational field data, Saturn appears to have a solid core with a mass equivalent to up to 20 Earths. As the most oblate planet, the pull of gravity at its equator is less than three quarters of that at the poles.

Uranus; Magnificent Cosmos; Scientific American Presents; by Staff Editor; 2 Page(s)

Strange even by the standards of the far reaches of the solar system, Uranus is an almost featureless, bluegreen planet that has the distinction of being knocked on its side. Its axis of rotation points 98 degrees away from its orbital axis. This unique tilt most likely testifies to a massive collision while the planet was still forming. Adding to its peculiarity, Uranus's magnetic field is also tilted, 59 degrees from the rotation axis. Finally, the planet rotates in the opposite direction that Earth does. Although greatly enhanced images from Voyager 2's visit in 1986 reveal bands like those on Saturn and Jupiter, the planet seems to be far more placid than its stormy gas giant comrades. Uranus maintains their custom, however, of accompaniment by rings and numerous satellites.

Ten small moons were discovered by Voyager in 1986. Nine rings were found in 1977 during stellar occultations; two more have been found since.

Neptune; Magnificent Cosmos; Scientific American Presents; by Staff Editor; 2 Page(s)

Astronomers searched for an eighth planet when Uranus's observed orbit disagreed with its calculated one, leading to suspicions of a large body exerting gravitational forces. In 1846 they confirmed the existence of Neptune, a planet so far from the sun that it will take another 13 years before it completes its first full orbit since discovery. The planet is the eighth from the sun in average distance, but it ends a twodecade tenure as the outermost planet in 1999, when Pluto again moves beyond it. The atmosphere of deep-blue Neptune is raked by winds moving at up to 700 meters (2,300 feet) per second, the fastest found on any planet. Denser than the other gas giants, Neptune probably has ice and molten rock in its interior, although rotational data imply that these heavy materials are spread out rather than concentrated in a tidy core.

Like Uranus, Neptune has a magnetic field off kilter with its rotational axis, the latter's being tilted by 47 percent. The source of the field seems to be well outward from the planet's center. Its rings may have formed long after the planet itself, and the outermost ring's odd assortment of particle sizes may be the result of a satellite breakup within the past few thousand years. Neptune's defiant moons include Nereid, with the most eccentric orbit of any planetary satellite, seven times as distant from the planet at its farthest compared with its closest approach; and Triton, whose orbit opposes Neptune's rotation and is tilted 157 degrees from the planet's equator.

Pluto; Magnificent Cosmos; Scientific American Presents; by Staff Editor; 2 Page(s)

Is Pluto really a planet? Until about six years ago, the question would have seemed silly. But in the early 1990s, astronomers found a region of orbiting bodies just beyond Neptune. The region, which was dubbed the Kuiper belt, is populated mostly by bodies too small to be planets and also by comets with relatively short periods, meaning that they approach the sun at least once every couple of centuries.

Most astronomers still consider Pluto a planet. Although its mass is only 1/400 that of Earth, it is still easily the largest Kuiperlike object. Also, Pluto seems to be more reflective than the other bodies in the Kuiper belt. Tradition may also have something to do with it; Pluto has been regarded as a planet since Clyde Tombaugh discovered it in 1930.

Asteroids; Magnificent Cosmos; Scientific American Presents; by staff Editor; 2 Page(s)

Concentrated between the orbits of Mars and Jupiter float thousands of what astronomers often call minor planets, or asteroids. These might have coalesced to form a small planet had they not been under the immense gravitational influence of Jupiter, which accelerated them. Low-velocity collisions of small bodies can build a planet, but bodies moving at five kilometers per second, the average for asteroids, collide violently. Such collisions can send chunks of asteroids out of their typical orbit between Mars and Jupiter. Some fragments take up stable orbits, part of which brings them closer to Earth or, on occasion, to the surface of our planet as meteorites. Our knowledge of asteroids should increase significantly early in 1999, when a probe called Near Earth Asteroid Rendezvous approaches within 48 kilometers of the asteroid Eros.

Comets; Magnificent Cosmos; Scientific American Presents; by Staff Editor; 2 Page(s)

The word "comet," from the Greek, means "long--haired," an apt description for what may appear to be a blur or smudge in the heavens. Visitors from the farthest reaches of the solar system, comets consist of a solid nucleus of dust and ice, which has led them to be called "dirty snowballs." Interactions with the sun produce the nebulous coma and one or more tails that smear the comet against the sky. It was most likely a comet (although an asteroid remains a candidate) that smashed into Earth 65 million years ago, causing the mass extinction that killed the dinosaurs and paved the way for our own evolution.

Fire and Light; Magnificent Cosmos; Scientific American Presents; by Staff Editor; 1 Page(s)

1.)SOHO REVEALS THE SECRETS OF THE SUN 2.)V1974 CYGNI 1992: THE MOST IMPORTANT NOVA OF THE CENTURY 3.)COSMIC RAYS AT THE ENERGY FRONTIER 4.)GAMMA-RAY BURSTS 5.)COLOSSAL GALACTIC EXPLOSIONS 6.)THE GHOSTLIEST GALAXIES

SOHO Reveals the Secrets of the Sun; Magnificent Cosmos; Scientific American Presents; by Lang; 6 Page(s)

From afar, the sun does not look very complex. To the casual observer, it is just a smooth, uniform ball of gas. Close inspection, however, shows that the star is in constant turmoil-a fact that fuels many fundamental mysteries. For instance, scientists do not understand how the sun generates its magnetic fields, which are responsible for most solar activity, including unpredictable explosions that cause magnetic storms and power blackouts here on Earth. Nor do they know why this magnetism is concentrated into so-called sunspots, dark islands on the sun's surface that are as large as Earth and thousands of times more magnetic. Furthermore, physicists cannot explain why the sun's magnetic activity varies dramatically, waning and intensifying again every 11 years or so.

To solve such puzzles-and better predict the sun's impact on our planet-the European Space Agency (ESA) and the National Aeronautics and Space Administration launched the two-ton Solar and Heliospheric Observatory (SOHO, for short) on December 2, 1995. The spacecraft reached its permanent strategic position-which is called the inner Lagrangian point and is about 1 percent of the way to the sun-on February 14, 1996. There SOHO is balanced between the pull of Earth's gravity and the sun's gravity and so orbits the sun together with Earth. Earlier spacecraft studying the sun orbited Earth, which would regularly obstruct their view. In contrast, SOHO monitors the sun continuously: 12 instruments examine the sun in unprecedented detail. They downlink several thousand images a day through NASA's Deep Space Network antennae to SOHO's Experimenters' Operations Facility at the NASA Goddard Space Flight Center located in Greenbelt, Md.

V1974 Cygni 1992: Most Important Nova of the Century; Magnificent Cosmos; Scientific American Presents; by Starrfield, Shore; 5 Page(s)

Never has a nova been watched by so many astronomers with so many instruments. Since its discovery by Peter Collins, an amateur astronomer in Boulder, Colo., in the early morning of February 19, 1992, nova V1974 Cygni has been recorded in x-rays through radio waves and from the ground, the air, Earth orbit and beyond.

Within hours of his report, we looked at the nova with the International Ultraviolet Explorer (IUE) satellite. We caught it in the "fireball" stage-familiar from photographs of hydrogen bomb explosions, when the gases are first expanding. Before long, it became the only nova to be seen both in birth and in death. In late 1993 the low-energy x-rays coming from the nova's core ceased, indicating to us that the nuclear explosion had run out of fuel.

Cosmic Rays at the Energy Frontier; Magnificent Cosmos; Scientific American Presents; by Cronin, Gaisser, Swordy; 6 Page(s)

Roughly once a second, a subatomic particle enters Earth's atmosphere carrying as much energy as a well-thrown rock. Somewhere in the universe, that fact implies, there are forces that can impart to a single proton 100 million times the energy achievable by the most powerful Earthbound accelerators. Where and how?

Those questions have occupied physicists since cosmic rays were first discovered in 1912 (although the entities in question are now known to be particles, the name "ray" persists). The interstellar medium contains atomic nuclei of every element in the periodic table, all moving under the influence of electrical and magnetic fields. Without the screening effect of Earth's atmosphere, cosmic rays would pose a significant health threat; indeed, people living in mountainous regions or making frequent airplane trips pick up a measurable extra radiation dose.

Gamma-Ray Bursts; Magnificent Cosmos; Scientific American Presents; by Fishman, Hartmann; 6 Page(s)

About three times a day our sky flashes with a powerful pulse of gamma rays, invisible to human eyes but not to astronomers' instruments. The sources of this intense radiation are likely to be emitting, within the span of seconds or minutes, more energy than the sun will in its entire 10 billion years of life. Where these bursts originate, and how they come to have such incredible energies, is a mystery that scientists have been attacking for three decades. The phenomenon has resisted study-the flashes come from random directions in space and vanish without trace-until very recently.

On February 28, 1997, we were lucky. One such burst hit the Italian-Dutch Beppo-SAX satellite for about 80 seconds. Its gamma-ray monitor established the position of the burst-prosaically labeled GRB 970228-to within a few arc minutes in the Orion constellation, about halfway between the stars Alpha Tauri and Gamma Orionis. Within eight hours, operators in Rome had turned the spacecraft around to look in the same region with an x-ray telescope. They found a source of xrays (radiation of somewhat lower frequency than gamma rays) that was fading fast, and they fixed its location to within an arc minute.

Colossal Galactic Explosions; Magnificent Cosmos; Scientific American Presents; by Veilleux, Cecil, Bland-Hawthorn; 6 Page(s)

Millions of galaxies shine in the night sky, most made visible by the combined light of their billions of stars. In a few, however, a pointlike region in the central core dwarfs the brightness of the rest of the galaxy. The details of such galactic dynamos are too small to be resolved even with the Hubble Space Telescope. Fortunately, debris from these colossal explosions-in the form of hot gas glowing at temperatures well in excess of a million degrees-sometimes appears outside the compact core, on scales that can be seen directly from Earth.

The patterns that this superheated material traces through the interstellar gas and dust surrounding the site of the explosion provide important clues to the nature and history of the powerful forces at work inside the galactic nucleus. Astronomers can now determine what kind of engines drive these dynamos and the effects of their tremendous outpourings on the intergalactic medium.

The Ghostliest Galaxies; Magnificent Cosmos; Scientific American Presents; by Bothun; 4 Page(s)

Astronomers have known for decades that galaxies exist in three basic types: elliptical, spiral and irregular. The ellipticals are spheroidal, with highest light intensity at their centers. Spirals, which include our own Milky Way, have a pronounced bulge at their center, which is much like a mini-elliptical galaxy. Surrounding this bulge is a spiral-patterned disk populated with younger, bluish stars. And irregular galaxies have relatively low mass and, as their name implies, fit none of the other categories.

With only minor refinements, this system of galactic classification has changed little since astronomer Edwin Hubble originated it some 70 years ago. Technological advances, however, have significantly improved astronomers' ability to find objects outside the Milky Way galaxy that are extraordinarily hard to detect. Over the past decade my colleagues and I have used an ingenious method of photographic contrast enhancement invented by astronomer David J. Malin of the Anglo-Australian Observatory, as well as electronic imaging systems based on improved charge-coupled devices (CCDs).

A Universal View; Magnificent Cosmos; Scientific American Presents; by Staff Editor; 1 Page(s)

1.)THE EVOLUTION OF THE UNIVERSE 2.)THE EXPANSION RATE AND SIZE OF THE UNIVERSE 3.)THE SELF-REPRODUCING INFLATIONARY UNIVERSE 4.)DARK MATTER IN THE UNIVERSE 5.)A SCIENTIFIC ARMADA

The Evolution of the Universe; Magnificent Cosmos; Scientific American Presents; by Peebles, Schramm,Turner, Kron; 6 Page(s)

At a particular instant roughly 12 billion years ago, all the matter and energy we can observe, concentrated in a region smaller than a dime, began to expand and cool at an incredibly rapid rate. By the time the temperature had dropped to 100 million times that of the sun's core, the forces of nature assumed their present properties, and the elementary particles known as quarks roamed freely in a sea of energy. When the universe had expanded an additional 1,000 times, all the matter we can measure filled a region the size of the solar system.

At that time, the free quarks became confined in neutrons and protons. After the universe had grown by another factor of 1,000, protons and neutrons combined to form atomic nuclei, including most of the helium and deuterium present today. All of this occurred within the first minute of the expansion. Conditions were still too hot, however, for atomic nuclei to capture electrons. Neutral atoms appeared in abundance only after the expansion had continued for 300,000 years and the universe was 1,000 times smaller than it is now. The neutral atoms then began to coalesce into gas clouds, which later evolved into stars. By the time the universe had expanded to one fifth its present size, the stars had formed groups recognizable as young galaxies.

The Expansion Rate and the Size of the Universe; Magnificent Cosmos; Scientific American Presents; by Freedman; 6 Page(s)

Our Milky Way and all other galaxies are moving away from one another as a result of the big bang, the fiery birth of the universe. As we near the end of the millennium, it is interesting to reflect that during the 20th century, cosmologists discovered this expansion, detected the microwave background radiation from the original explosion, deduced the origin of chemical elements in the universe and mapped the largescale structure and motion of galaxies. Despite these advances, elementary questions remain. When did the colossal expansion begin? Will the universe expand forever, or will gravity eventually halt its expansion and cause it to collapse back on itself?

For decades, cosmologists have attempted to answer such questions by measuring the universe's size-scale and expansionrate. To accomplish this task, astronomers must determine both how fast galaxies are moving and how far away they are. Techniques for measuring the velocities of galaxies are well established, but estimating the distances to galaxies has proved far more difficult. During the past decade, several independent groups of astronomers have developed better methods for measuring the distances to galaxies, leading to completely new estimates of the expansion rate. Recently the superb resolution of the Hubble Space Telescope has extended and strengthened the calibration of the extragalactic distance scale, leading to new estimates of the expansion rate.

The Self-Reproducing Inflationary Universe; Magnificent Cosmos; Scientific American Presents; by Linde; 6 Page(s)

If my colleagues and I are right, we may soon be saying good-bye to the idea that our universe was a single fireball created in the big bang. We are exploring a new theory based on a 15-year-old notion that the universe went through a stage of inflation. During that time, the theory holds, the cosmos became exponentially large within an infinitesimal fraction of a second. At the end of this period, the universe continued its evolution according to the big bang model. As workers refined this inflationary scenario, they uncovered some surprising consequences. One of them constitutes a fundamental change in how the cosmos is seen. Recent versions of inflationary theory assert that instead of being an expanding ball of fire the universe is a huge, growing fractal. It consists of many inflating balls that produce new balls, which in turn produce more balls, ad infinitum.

Cosmologists did not arbitrarily invent this rather peculiar vision of the universe. Several workers, first in Russia and later in the U.S., proposed the inflationary hypothesis that is the basis of its foundation. We did so to solve some of the complications left by the old big bang idea. In its standard form, the big bang theory maintains that the universe was born about 15 billion years ago from a cosmological singularity-a state in which the temperature and density are infinitely high. Of course, one cannot really speak in physical terms about these quantities as being infinite. One usually assumes that the current laws of physics did not apply then. They took hold only after the density of the universe dropped below the so-called Planck density, which equals about 1094 grams per cubic centimeter.

Dark Matter in the Universe; Magnificent Cosmos; Scientific American Presents; by Rubin; 5 Page(s)

Imagine, for a moment, that one night you awaken abruptly from a dream. Coming to consciousness, blinking your eyes against the blackness, you find that, inexplicably, you are standing alone in a vast, pitch-black cavern. Befuddled by this predicament, you wonder: Where am I? What is this space? What are its dimensions?

Groping in the darkness, you stumble upon a book of damp matches. You strike one; it quickly flares, then fizzles out. Again, you try; again, a flash and fizzle. But in that moment, you realize that you can glimpse a bit of your surroundings. The next match strike lets you sense faint walls far away. Another flare reveals a strange shadow, suggesting the presence of a big object. Yet another suggests you are moving-or, instead, the room is moving relative to you. With each momentary flare, a bit more is learned.

A Scientific Armada; Magnificent Cosmos; Scientific American Presents; by Beardsley; 4 Page(s)

A decade from now humanity's understanding of the solar system, no less the universe beyond, will have grown vastly more focused and detailed. During the next 10 years, roughly 50 scientific expeditions will blast off from Earth-a veritable armada of missions to visit planets, comets and asteroids, as well as to make sensitive observations of deep space from above Earth's occluding atmosphere. Researchers will very likely resolve some longstanding questions at the same time that they grapple with as yet undreamed of conundrums.

As many as nine spacecraft will intensively survey Mars during the next 10 years, including the Mars Global Surveyor now in orbit. If plans under consideration get the go-ahead, samples from the red planet will return to Earth for analysis sometime after 2005. Missions to Pluto and, perhaps, Mercury and Venus (though not currently scheduled) may advance onto the National Aeronautics and Space Administration's timetable.