Natural History magazine

Human ingenuity seldom fails to improve on the fruits of human invention. Whatever may have dazzled everyone on its debut is almost guaranteed to be superseded and, someday, to look quaint.

In 2000 B.C. a pair of ice skates made of polished animal bone and leather thongs was a transportation breakthrough. In 1610 Galileo's eight-power telescope was an astonishing tool of detection, capable of giving the senators of Venice a sneak peek at hostile ships before they could enter the lagoon. In 1887 the one-horsepower Benz Patent Motorwagen was the first commercially produced car powered by an internal combustion engine. In 1946 the thirty-ton, showroom-size ENIAC, with its 18,000 vacuum tubes and 6,000 manual switches, pioneered electronic computing. Today you can glide across roadways on in-line skates, gaze at images of faraway galaxies brought to you by the Hubble Space Telescope, cruise the autobahn in a 600-horsepower roadster, and carry your three-pound laptop to an outdoor cafe.

Of course, such advances don't just fall from the sky. Clever people think them up. Problem is, to turn a clever idea into reality, somebody has to write the check. And when market forces shift, those somebodies may lose interest and the checks may stop coming. If computer companies had stopped innovating in 1978, your desk might still sport a hundred-pound IBM 5110. If communications companies had stopped innovating in 1973, you might still be schlepping a two-pound, nine-inch-long cell phone. And if in 1968 the U.S. space industry had stopped developing bigger and better rockets to launch humans beyond the Moon, we'd never have surpassed the Saturn V rocket.

Oops!

Sorry about that. We haven't surpassed the Saturn V. The largest, most powerful rocket ever flown by anybody, ever, the thirty-six-story-tall Saturn V was the first and only rocket to launch people from Earth to someplace else in the universe. It enabled every Apollo mission to the Moon from 1969 through 1972, as well as the 1973 launch of Skylab 1, the first U.S. space station.

Inspired in part by the successes of the Saturn V and the momentum of the Apollo program, visionaries of the day foretold a future that never came to be: space habitats, Moon bases, and Mars colonies up and running by the 1990s. But funding for the Saturn V evaporated as the Moon missions wound down. Additional production runs were canceled, the manufacturers' specialized machine tools were destroyed, and skilled personnel had to find work on other projects. Today U.S. engineers can't even build a Saturn V clone.

What cultural forces froze the Saturn V rocket in time and space? What misconceptions led to the gap between expectation and reality?

Soothsaying tends to come in two flavors: doubt and delirium. It was doubt that led skeptics to declare that the atom would never be split, the sound barrier would never be broken, and people would never want or need computers in their homes. But in the case of the Saturn V rocket, it was delirium that misled futurists into assuming the Saturn V was an auspicious beginning—never considering that it could, instead, be an end.

On December 30, 1900, for its last Sunday paper of the nineteenth century, the Brooklyn Daily Eagle published a sixteen-page supplement headlined THINGS WILL BE SO DIFFERENT A HUNDRED YEARS HENCE. The contributors—business leaders, military men, pastors, politicians, and experts of every persuasion—imagined what housework, poverty, religion, sanitation, and war would be like in the year 2000. They enthused about the potential of electricity and the automobile. There was even a map of the world-to-be, showing an American Federation comprising most of the Western Hemisphere from the lands above the Arctic Circle down to the archipelago of Tierra del Fuego—plus sub-Saharan Africa, the southern half of Australia, and all of New Zealand.

Most of the writers portrayed an expansive future. But not George H. Daniels, a man of authority at the New York Central and Hudson River Railroad, who peered into his crystal ball and boneheadedly predicted:

It is scarcely possible that the twentieth century will witness improvements in transportation that will be as great as were those of the nineteenth century.

Elsewhere in his article, Daniels envisioned affordable global tourism and the diffusion of white bread to China and Japan. Yet he simply couldn't imagine what might replace steam as the power source for ground transportation, let alone a vehicle moving through the air. Even though he stood on the doorstep of the twentieth century, this manager of the world's biggest railroad system could not see beyond the automobile, the locomotive, and the steamship.

Three years later, almost to the day, Wilbur and Orville Wright made the first-ever series of powered, controlled, heavier-than-air flights. By 1957 the U.S.S.R. launched the first satellite into Earth orbit. And in 1969 two Americans became the first human beings to walk on the Moon.

Daniels is hardly the only person to have misread the technological future. Even experts who aren't totally deluded can have tunnel vision. On page 13 of the Eagle's Sunday supplement, the principal examiner at the U.S. Patent Office, W. W. Townsend, wrote, The automobile may be the vehicle of the decade, but the air ship is the conveyance of the century. Sounds visionary, until you read further. What he was talking about were blimps and zeppelins. Both Daniels and Townsend, otherwise well-informed citizens of a changing world, were clueless about what tomorrow's technology would bring.

Even the Wrights were guilty of doubt about the future of aviation. In 1901, discouraged by a summer's worth of unsuccessful tests with a glider, Wilbur told Orville it would take another fifty years for someone to fly. Nope: the birth of aviation was just two years away. On the windy, chilly morning of December 17, 1903, starting from a North Carolina sand dune called Kill Devil Hill, Orville was the first to fly the brothers' 600-pound plane through the air. His epochal journey lasted twelve seconds and covered 120 feet—a distance just shy of the wingspan of a Boeing 757.

Judging by what the mathematician, astronomer, and Royal Society gold medalist Simon Newcomb had published just two months earlier, the flights from Kill Devil Hill should never have taken place when they did:

Quite likely the twentieth century is destined to see the natural forces which will enable us to fly from continent to continent with a speed far exceeding that of the bird.

But when we inquire whether aerial flight is possible in the present state of our knowledge; whether, with such materials as we possess, a combination of steel, cloth and wire can be made which, moved by the power of electricity or steam, shall form a successful flying machine, the outlook may be altogether different.

Some representatives of informed public opinion went even further. The New York Times was steeped in doubt just one week before the Wright brothers went aloft in the original Wright Flyer. Writing on December 10, 1903—not about the Wrights but about their illustrious and publicly funded competitor, Samuel P. Langley, an astronomer, physicist, and chief administrator of the Smithsonian Institution—the Times declared:

We hope that Professor Langley will not put his substantial greatness as a scientist in further peril by continuing to waste his time, and the money involved, in further airship experiments. Life is short, and he is capable of services to humanity incomparably greater than can be expected to result from trying to fly.

You might think attitudes would have changed as soon as people from several countries had made their first flights. But no. Wilbur Wright wrote in 1909 that no flying machine would ever make the journey from New York to Paris. Richard Burdon Haldane, the British secretary of war, told Parliament in 1909 that even though the airplane might one day be capable of great things, from the war point of view, it is not so at present. Ferdinand Foch, a highly regarded French military strategist and the supreme commander of the Allied forces near the end of the First World War, opined in 1911 that airplanes were interesting toys but had no military value. Late that same year, near Tripoli, an Italian plane became the first to drop a bomb.

Early attitudes about flight beyond Earth's atmosphere followed a similar trajectory. True, plenty of philosophers, scientists, and sci-fi writers had thought long and hard about outer space. The sixteenth-century philosopher-friar Giordano Bruno proposed that intelligent beings in habited an infinitude of worlds. The seventeenth-century soldier-writer Savinien de Cyrano de Bergerac portrayed the Moon as a world with forests, violets, and people.

But those writings were fantasies, not blueprints for action. By the early twentieth century, electricity, telephones, automobiles, radios, airplanes, and countless other engineering marvels were all becoming basic features of modern life. So couldn't earthlings build machines capable of space travel? Many people who should have known better said it couldn't be done, even after the successful 1942 test launch of the world's first long-range ballistic missile: Germany's deadly V-2 rocket. Capable of punching through Earth's atmosphere, it was a crucial step toward reaching the Moon.

Richard van der Riet Woolley, the eleventh British Astronomer Royal, is the source of a particularly woolly remark. When he landed in London after a thirty-six-hour flight from Australia, some reporters asked him about space travel. It's utter bilge, he answered. That was in early 1956. In early 1957 Lee De Forest, a prolific American inventor who helped birth the age of electronics, declared, Man will never reach the moon, regardless of all future scientific advances. Remember what happened in late 1957? Not just one but two Soviet Sputniks entered Earth orbit. The space race had begun.

Whenever someone says an idea is bilge (which, I suppose, is British for baloney), you must first ask whether it violates any well-tested laws of physics. If so, the idea is likely to be bilge. If not, the only challenge is to find a clever engineer—and, of course, a committed source of funding.

The day the Soviet Union launched Sputnik 1, a chapter of science fiction became science fact, and the future became the present. All of a sudden, futurists went overboard with their enthusiasm. The delerium that technology would advance at lightning speed replaced the delusion that it would barely advance at all. Experts went from having much too little confidence in the pace of technology to having much too much. And the guiltiest people of all were the space enthusiasts.

Commentators became fond of twenty-year intervals, within which some previously inconceivable goal would supposedly be accomplished. On January 6, 1967, in a front-page story, The Wall Street Journal announced: The most ambitious U.S. space endeavor in the years ahead will be the campaign to land men on neighboring Mars. Most experts estimate the task can be accomplished by 1985. The very next month, in its debut issue, The Futurist magazine announced that according to long-range forecasts by the RAND Corporation, a pioneer think-tank, there was a 60 percent probability that a manned lunar base would exist by 1986. In The Book of Predictions, published in 1980, the rocket pioneer Robert C. Truax forecast that 50,000 people would be living and working in space by the year 2000. When that benchmark year arrived, people were indeed living and working in space. But the tally was not 50,000. It was three. The first crew of the International Space Station.

All those visionaries (and countless others) never really grasped the forces that drive technological progress. In Wilbur and Orville's day, you could tinker your way into major engineering advances. Their first airplane did not require a grant from the National Science Foundation: they funded it through their bicycle business. The brothers constructed the wings and fuselage themselves, with tools they already owned, and got their resourceful bicycle mechanic, Charles E. Taylor, to design and hand-build the engine. The operation was basically two guys and a garage.

Space exploration unfolds on an entirely different scale. The first moonwalkers were two guys, too—Neil Armstrong and Buzz Aldrin—but behind them loomed the force of a mandate from an assassinated president, 10,000 engineers, $100 billion, and a Saturn V rocket.

Notwithstanding the sanitized memories so many of us have of the Apollo era, Americans were not first on the Moon because we're explorers by nature or because our country is committed to the pursuit of knowledge. We got to the Moon first because the United States was out to beat the Soviet Union, to win the Cold War any way we could. John F. Kennedy made that clear when he complained to top NASA officials in November 1962:

I'm not that interested in space. I think it's good, I think we ought to know about it, we're ready to spend reasonable amounts of money. But we're talking about these fantastic expenditures which wreck our budget and all these other domestic programs and the only justification for it in my opinion to do it in this time or fashion is because we hope to beat them [the Soviet Union] and demonstrate that starting behind, as we did by a couple of years, by God, we passed them.

Like it or not, war (cold or hot) is the most powerful funding driver in the public arsenal. When a country wages war, money flows like floodwaters. Lofty goals—such as curiosity, discovery, exploration, and science—can get you money for modest-size projects, provided they resonate with the political and cultural views of the moment. But big, expensive activities are inherently long term, and require sustained investment that must survive economic fluctuations and changes in the political winds.

In all eras, across time and culture, only three drivers have fulfilled that funding requirement: war, greed, and the celebration of royal or religious power. The Great Wall of China; the pyramids of Egypt; the Gothic cathedrals of Europe; the U.S. interstate highway system; the voyages of Columbus and Cook—nearly every major undertaking owes its existence to one or more of those three drivers. Today, as the power of kings is supplanted by elected governments, and the power of religion is often expressed in non-architectural undertakings, that third driver has lost much of its sway, leaving war and greed to run the show. Sometimes those two drivers work hand in hand, as in the art of profiteering from the art of war. But war itself remains the ultimate and most compelling rationale.

Having been born the same week NASA was founded, I was eleven years old during the voyage of Apollo 11, and had already identified the universe as my life's passion. Unlike so many other people who watched Neil Armstrong's first steps on the Moon, I wasn't jubilant. I was simply relieved that someone was finally exploring another world. To me, Apollo 11 was clearly the beginning of an era.

But I, too, was delirious. The lunar landings continued for three and a half years. Then they stopped. The Apollo program became the end of an era, not the beginning. And as the Moon voyages receded in time and memory, they seemed ever more unreal in the history of human projects.

Unlike the first ice skates or the first airplane or the first desktop computer—artifacts that make us all chuckle when we see them today—the first rocket to the Moon, the 364-foot-tall Saturn V, elicits awe, even reverence. Three Saturn V relics lie in state at the Johnson Space Center in Texas, the Kennedy Space Center in Florida, and the U.S. Space and Rocket Center in Alabama. Streams of worshippers walk the length of each rocket. They touch the mighty rocket nozzles at the base, like the apes who touched the Monolith in the 1968 film 2001: A Space Oddysey, and wonder how something so large could ever have bested Earth's gravity. To transform their awe into chuckles, our country will have to resume the effort to boldly go where no man has gone before. Only then will the Saturn V look as quaint as every other invention that human ingenuity has paid the compliment of improving upon.

See the latest information on Manhattanhenge.

What will future civilizations think of Manhattan Island when they dig it up and find a carefully laid out network of streets and avenues? Surely the grid would be presumed to have astronomical significance, just as we have found for the pre-historic circle of large vertical rocks known as Stonehenge, in the Salisbury Plain of England. For Stonehenge, the special day is the summer solstice, when the Sun rises in perfect alignment with several of the stones signaling the change of season.

For Manhattan, a place where the evening matters more than the morning, that special day comes on May 28; one of only two days in the year when the Sun sets in exact alignment with the Manhattan grid, fully illuminating every single cross-street for the last fifteen minutes of daylight. The other day is July 11th. Had Manhattan's grid been perfectly aligned with the geographic north-south line, then our special day would be the spring equinox, and if we so designated, the autumn equinox—the only two days on the calendar when the Sun rises due east and sets due west. But Manhattan is rotated 30 degrees east from geographic north, shifting the days of alignment elsewhere into the calendar. Upon studying American culture, and what is important to it, future anthropologists might credit the Manhattan alignments to cosmic signs of Memorial Day and, of course, the All-Star break. War and baseball.

Because Manhattan is so small (13 miles long) compared with Earth's distance to the Sun (about 93 million miles), the Sun's rays are essentially parallel by the time they reach Manhattan, allowing the Sun to be seen on all cross streets simultaneously, provided you have a clear view to the New Jersey horizon. Some major streets cross the entire island from river to river without obstruction, including 14th, 34th, and 42nd Streets. While the May 28 sunset qualifies as the exact day for this auspicious moment, the surrounding days will also work, as the sunset point migrates slowly north from day to day along the horizon, bringing with it ever-lengthening daylight hours.

Sunset on Manhattanhenge begins at 8:10PM, at a cross-street near you.

I am often asked by students whether their toilet bowl will flush clockwise or counterclockwise in the southern hemisphere, or whether it will flush straight down in Ecuador. This would, of course, be important information if you were ever kidnapped and blindfolded and dropped off in a strange land. If we assume a commode of conventional size, then this toilet bowl test will fail because the answer lies in the manufacturer's design. But if your northern hemisphere toilet bowl were a few hundred miles in diameter then the Coriolis force of the rotating Earth would easily overcome the random water currents, and force the bowl to empty its contents in a counter clockwise swirl. If you have southern hemisphere friends with an equally large toilet, then theirs would indeed empty in the opposite (clockwise) direction.

The circulation within oversized flush toilets is a natural consequence of motion on the surface of an object that rotates. We owe our detailed understanding of the effect to the work of Gaspard Gustave de Coriolis who, in 1831, presented details of the laws of mechanics in a rotating reference frame to the Academie des Sciences in Paris. Earth's surface provides an excellent place to demonstrate why the origin of the Coriolis force is relatively simple. Our planet rotates on its axis approximately once every 24 hours. In that 24 hour period, objects on Earth's equator travel a circle with a circumference of nearly 25,000 miles, which corresponds to a speed of a more than 1,000 miles per hour. By 41 degrees north, the latitude of New York City and the American Museum of Natural History, the circumference traveled is only about 19,000 miles. The west-to-east speed is now approximately 800 miles per hour. As you continue to increase in Earth latitude (north or south of the equator) your west-to-east speed decreases until it hits exactly zero miles per hour at the poles. (For this reason, most satellites are launched as close to the equator as possible, which enables them to get a good running start in their eastward orbits.

Imagine a puffy cloud in the northern hemisphere and a meteorological low pressure system directly to its north. The cloud will tend to move toward the low. But during the journey its greater eastward speed will enable the cloud to overtake the low, which is itself in motion, and end up east of its destination. Another puffy cloud that is north of the low will also tend to move toward the low, but will naturally lag behind and end up west of its destination. To an unsuspecting person on Earth's surface, these curved north-south paths would appear to be the effects of a mysterious force (the Coriolis force) yet no true force was ever at work.

When many puffy clouds approach a low pressure system from all directions you get a merry-go-round of counter-clockwise motion, which is better known as a cyclone. In extreme cases you get a monstrous hurricane with wind speeds upwards of a hundred miles per hour. For the southern hemisphere the same arguments will create a cyclone that spirals clockwise. The military normally knows all about the Coriolis force and thus introduces the appropriate correction to all missile trajectories. But in 1914, from the annals of embarrassing military moments, there was a World War I naval battle between the English and the Germans near the Falklands Islands off Argentina (52 degrees south latitude). The English battle cruisers Invincible and Inflexible engaged the German war ships Gneisenau and Scharnhorst at a range of nearly ten miles. Among other gunnery problems encountered, the English forgot to reverse the direction of their Coriolis correction. Their tables had been calculated for northern hemisphere projectiles, so they missed their targets by even more than if no correction had been applied. They ultimately won the battle against the Germans with about sixty direct hits, but it was not before over a thousand missile shells had fallen in the ocean.

In high school I knew all about the Coriolis force, but I never had the opportunity to test it on something as large as a swimming pool until the summer after my junior year when I worked as a lifeguard. At the mid-summer cleaning, I opened the drain valve to the pool and carefully observed the circulation. The water funneled in the wrong direction—clockwise. The last I had checked, I was life-guarding in Earth's northern hemisphere so I was tempted to declare Coriolis forces to be a hoax. But a fast back of the envelope calculation verified that the difference in Coriolis velocity across the pool was a mere 1/2 inch per minute. This is slow. The water currents from somebody just climbing out of the pool, or even a gentle breeze across the water's surface would easily swamp the effect and I would end up with clockwise one half the time and counterclockwise the other half of the time. A proper experiment to demonstrate the insignificance of the Coriolis forces would require that I empty and refill the pool dozens of times. But each try would dump 15,000 cubic feet of water and diminish my job security. So I didn't.

The air circulation near a high pressure systems, which are inelegantly known as anticyclones, is a reverse picture of our cyclone. On Earth, these high pressure systems are the astronomer's best friend because they are typically devoid of clouds. The surrounding air still circulates, but it does so without the benefit of clouds as tracers of the air flow. The circulation around low and high pressure systems, known as geostrophic winds presents us with the paradox that Coriolis forces tend to move air along lines of constant pressure (isobars) rather than across them.

Now imagine, if you will, a place that is not only fourteen hundred times larger than Earth, but has an equatorial speed that is about twenty-five times as fast, and has a deep, thick, colorful atmosphere. That place is the planet Jupiter, where a day lasts just 9 hours and 56 minutes. It is a cosmic garden of atmospheric dynamics where all rotationally induced cloud and weather patterns are correspondingly enhanced. In the most striking display of the Coriolis force in the entire solar system, Jupiter lays claim to the largest, most energetic, and longest-lived storm ever observed. It is an anticyclone that looks like a great red spot in Jupiter's upper atmosphere. We call it Jupiter's Great Red Spot. Discovered in the mid 1660s by the English physicist Robert Hooke and separately by the Italian astronomer Giovanni Cassini, the feature has persisted for over 300 years. It was not until the twentieth century when the Dutch-born, American astronomer Gerard Kuiper was the first to supply the modern interpretation of the Spot as a raging storm.

The Great Red Spot, by the way, is bigger than Earth, although its size and shape has varied over the years. It lives in Jupiter's southern hemisphere and rotates counterclockwise, which immediately tells us we have a high pressure system. The coloration, from orange-red to a barely visible pale cream, is generally attributed to various concentrations of phosphorus and sulfur compounds. Close-up images from the Voyager flyby missions of the late 1970s revealed a maelstrom of colorful curlicues at the interface of the Great Red Spot and the surrounding atmosphere. There were also strikingly resolved horizontal belts and zones interlaced with countless smaller cyclones and anticyclones that give Jupiter the appearance of an archaeological cross section of a Big Mac hamburger from McDonald's, bun included. Above all else, however, the Voyager data posed renewed theoretical challenges. It resolved Jovian features down to twenty miles in diameter—astonishingly small when one remembers Jupiter's size relative to Earth. Models of cosmic phenomena are often clean and tidy until they are tested outside of the limits in which they were formulated. Higher image resolution is one such example. When this happens, many models are discarded, others are modified, while some are freshly invented, but jumps in resolution have always been followed by a deeper understanding of the universe.

Whatever else a model of Jupiter's atmosphere is designed to explain, it should as a minimum, account for basic properties of the Great Red Spot such as its longevity, and perhaps its distinguished size, and that it is an anticyclone. An ideal model would be able to account for all atmospheric motion on Jupiter. The tools available to the theorist are Newton's laws of motion as adapted to the properties of gases and liquids—otherwise known as fluid mechanics.

Contemporary models do capture the basic features of the Great Red Spot, but very little is known about the structure of Jupiter's under-layers. Jupiter radiates more heat than it receives from the Sun, and there are enormous thermal reservoirs in Jupiter's interior that can drive atmospheric flow patterns. One source is the radioactive decay of trace elements while another is the left-over heat form Jupiter's initial contraction from a proto-planetary cloud to a planet in the early solar system. The sustaining source of energy for the Spot could also (or instead) be tapped from other sources. On Earth, hurricanes are partially driven by the latent heat released to the atmosphere when rain drops condenses out of the air. A similar mechanism may dominate in Jupiter's atmosphere as its gases condense toward its liquid interior. The Spot has also been observed (and successfully modeled) to dine upon smaller turbulent eddies in its vicinity. This cannibalistic behavior is yet another source of energy. Clues to the deeper cloud layers will almost certainly be gained when the spacecraft Galileo passes Jupiter (in December 1995) and parachutes a mini-probe that will measure temperature, density, composition, wind speeds, and lightning events as it descends through the outer atmosphere.

For now, there is no reasonable hope of describing every one of Jupiter's surface features in detail. A more realistic approach is to construct an atmospheric model that provides a statistically equivalent picture of Jupiter's surface features. In other words, a model of a Big Mac can approximate all Big Macs even though it may not look like any one in particular.

One nagging problem with models that always produce a single, sustained anticyclone is the blunt reality that Jupiter's northern hemisphere is devoid of a twin Great Red Spot. Clearly, if models show that big spots are inevitable, then the north ought to have one too. Elsewhere in the solar system, the Coriolis force has given rise to a great dark spot on Neptune. We call it Neptune's Great Dark Spot. Like Jupiter's Great Red Spot, it is an anticyclone of epic proportions in Neptune's southern hemisphere that appears without a twin in the north. This is a problem that may require an as yet unexplored north-south asymmetry in both Jupiter's and Neptune's internal structure. One way to induce such an asymmetry would be to survive a cosmic collision in one of your two hemispheres. The July 1994 encounter between Jupiter and the dozens of crumbled comet parts from Shoemaker-Levy 9 left visible and sustained scars on Jupiter's outer gaseous surface. The long-term effects of this impulse of deposited energy remains to be seen. Will the scars form stable new structures among the cloud-tops? Or will the scars dissipate completely into the atmosphere? For the moment, feel free to consider the new blemishes to be extra ingredients in your hamburger.

Science consists in discovering the frame and operations of Nature, and reducing them, as far as may be, to general rules or laws—establishing these rules by observations and experiments, and thence deducing the causes and effects of things.

Sir Isaac Newton, The Principia 1687

In scientific inquiry, often the answer to one simple question fortuitously explains the answers to many others; they may even answer questions that have yet to be conceived. Powerful ideas unify concepts or phenomena that were previously thought to be unrelated. For example, Sir Isaac Newton identified a falling apple and Earth's orbiting moon as different effects of a single law of universal gravitation represented by a simple equation. (The falling apple did not actually hit Sir Isaac on the head. He saw it fall from afar.)

Newton's famous equation is a recipe to compute the force of gravity between any two objects in the universe. With a basic application of Newton's equation you can show that the force of gravity is greatest where an object is nearest another object and least at the point where it is farthest. As you stand on Earth, for example, Earth's gravity is slightly stronger at your feet than at your head. The differential is small, so don't blame your light-headedness on this phenomenon. Earth pulls on your feet with a force that is only one ten thousandths of one percent stronger than that at your head.

This simple difference in gravity, officially known as the tidal force, is felt by all objects as they are pulled by the gravity of all other objects in the universe. Tidal forces are the direct cause of a diverse array of cosmic phenomena that otherwise seem to have nothing to do with one another. Some of my favorites: the daily rise and fall of Earth's oceanic tides; Earth's gradually slowing rotation rate, which is making the days longer and longer; the Moon's slow spiral away from Earth; the Moon showing only one face toward Earth at all times; Pluto, and its lone moon Charon, showing each other only one face during their mutual orbit; the geological (or is it iological?) activity of Io, one of Jupiter's moons; the breaking apart of comet Shoemaker Levy-9 in its close encounter with Jupiter; the long tails of colliding galaxies in collision; and the spectacularly gory death to which you would succumb if you approached the center of a black hole (as detailed in last month's Universe essay Death by Black Hole).

Tidal forces are strongly dependent on distance. A mild increase in distance between two objects can make a large difference in the strength of the tidal force. For example, if the Moon were just twice its current distance from us, then its tidal force on Earth would decrease by a factor of eight. At its current average distance of 240,000 miles from Earth, the Moon manages to create sizable atmospheric, oceanic, and crustal tides by attracting the part of Earth nearest the Moon more strongly than the part of Earth that is farthest. (The Sun is so far away that in spite of its generally strong gravity, its tidal force on Earth amounts to less than half that of the Moon.) The oceans respond most visibly in being stretched toward the direction of the Moon. Meanwhile, as the solid Earth continues to rotate, the continental shelves are constantly trying to push forward the1.5 quintillion tons of bulging ocean water.

In this force-war, the oceanic bulge is always found slightly ahead of the Moon's location in its monthly orbit. Rotating within the bulge, Earth suffers an enormous source of friction between the sloshing oceanic water and the continental shelves and shores. (Tidal energy is lost to friction at a slightly higher rate than the rate of consumption of electrical energy by all quarter billion residents of the United States.) The consequence? Earth rotates more and more slowly—the days are getting longer at a rate of about 1/500 of a second per day per century. It doesn't sound like much, until you stop and think about it: every century, the duration of every day increases by 1/500 of a second. While not reported in the newspapers next to the table of coastal tides, it ought to be, because at this rate, full seconds add up fast. Since the 1970s, we have been officially adjusting our daily time-reckoning with leap seconds that are added every few years at the end of June or December. Don't tell anybody, but I have actually attended one or two leap second parties, where everybody counts down from 61 beginning at 11:59 p.m.

The best evidence for the slowing down of Earth's rotation comes from detailed records of total solar eclipses that date back many centuries. If Earth's rotation rate were faster in the past, then a total solar eclipse as seen on Earth's surface would miss the expected spot and occur west of where we thought it would, which is precisely what the records show — the earliest recorded eclipses were offset along Earth's surface by nearly a thousand miles.

Meanwhile, Earth's bulged gravity field, positioned slightly ahead of the Moon in its orbit, acts in return as an energy pump. Like the effect of rocking your legs in rhythm with a playground swing, the Moon slowly ascends into larger and larger orbits. You want proof? In 1969, when the Apollo 11 astronauts Neil Armstrong and Buzz Aldrin visited the Moon's Sea of Tranquillity, they left behind (among other things) a series of corner reflectors that are designed to reflect light in exactly the same direction that it arrives. Starting shortly after the Moon landing, and continuing today in places such as the McDonald Observatory Laser Ranging Station in west Texas, high-powered lasers on Earth are beamed to the Moon, and the return signal is carefully timed.

Knowing the speed of light, one can compute the Moon's distance with unprecedented accuracy: with a twenty-five year baseline of measurements we know that the Moon is spiraling away from us at a rate of about two inches per year, just as predicted by tidal theory. Earth's rotation will continue to slow down, and the Moon will continue to spiral away until the Earth day exactly equals the lunar month. At that time, one Earth rotation will last over 1000 hours, which would require 4 million leap seconds per day. No need to panic just yet. You have over a trillion years to think about it.

Earth's tidal force upon the Moon has completed its job long ago: the Moon's rotation has slowed so that its period of rotation exactly equals its period of revolution around Earth. Whenever this happens, an orbiting object will always show the same face to the body it orbits—it becomes tidally locked. In other words, as seen from Earth, the Moon has a permanent near side and far side, and when viewed from the near side of the Moon, Earth never sets. At one time or another during a lunar month, however, all sides of the Moon receive sunlight. So contrary to common parlance, folklore, and the title of Pink Floyd's best-selling 1973 rock album, there is not now, nor was there ever, a dark side of the moon.

When Earth's rotation slows down until it exactly matches the orbital period of the Moon, then Earth will no longer be rotating within its oceanic tidal bulge and the Earth-Moon system will have achieved a double tidal lock. In what sounds like an undiscovered wrestling hold, double tidal locks are energetically favorable (like a ball coming to rest at the bottom of a hill), and are thus common in the universe. The planet Pluto and its lone moon Charon, have achieved it in a 6.4 day cosmic waltz. A related phenomenon will unfold before your eyes when you spin one of the mobiles of the American sculptor Alexander Calder. If any pair of the dangling parts are elongated, then they will eventually align with each other and, in effect, become tidally locked, although energy, not gravity is the active ingredient here.

The Earth-Moon, and Pluto-Charon systems are orbiting pairs in which the satellite is nearby and relatively large when compared with the host. One could accurately describe them as double planets. Such configurations lead to strong tidal forces and are also found among all closely orbiting double star systems, which themselves become doubly tidally locked. After learning about the general strength and prevalence of lunar tides, students often asked me whether the Moon's tidal forces can affect human behavior. Yes, provided you had a very, very big head. For example, if your brain were, say, 7,000 miles in diameter (the size of Earth), then the Moon's tidal forces would indeed give you an oblong-shaped cranium and impart untold consequences on your mental faculties. For normal Homo sapiens, however, the Moon's difference in gravity from one side of the head to the other is immeasurably small. The weight of an understuffed down pillow imparts a squeezing force that is over seven trillion times larger than the Moon's tidal force on your head—a fact not shared with you by those who write about werewolves and other moon-based dysfunctional behavior.

No discussion about tidal forces would be complete without due respect to the planet Jupiter. Packing more mass than all other planets combined, Jupiter has tidally locked all of its inner satellites, including Galileo's famous four: Io, Europa, Ganymede, and Callisto. To be tidally locked should mean that there is no energy being lost to friction, but a careful study of Io shows that the exact shape of its orbit is noticeably affected by the combined gravity of other nearby satellites. In other words, Io's distance from Jupiter varies, which also means it predictably speeds up as it orbits closer to Jupiter and slows down as it orbits farther. Now consider that Io's rotation rate exactly equals the time it takes for one complete trip around Jupiter and you have a satellite that shows only one face to Jupiter—but its face appears to jiggle to and fro as Io's Jupiter-facing tidal bulge continually flexes the satellite.

When the Moon flexes Earth's oceans, they simply slosh back and forth. But when a Jupiter-sized tidal force acts upon a nearby solid body, then the internal stress can become a prodigious source of heat. In one of the more timely and impressive predictions in the history of space probes, Stanton Peale of the University of California and collaborators published a paper in 1979 titled, Melting of Io by Tidal Dissipation. Later that year, images sent by the Voyager 1 space craft revealed extraordinary volcanic activity, complete with mountain calderas and plumes.

Jupiter's tidal forces also wreak havoc on comets that wander too close. The late comet Shoemaker-Levy 9 was minding its own business in orbit around the Sun when during one of its trips it came too close to Jupiter and was captured into a greatly elongated orbit. In 1992 it came so close to the giant planet that tidal forces ripped apart the comet into dozens of pieces. On the next pass, in July 1994, none of the two dozen comet parts cleared the cloud-tops. As if to exact a kamikaze-style revenge, they all blazed into Jupiter's thick and colorful atmosphere at a speed of nearly 40 miles per second, and exploded with the equivalent energy of hundreds of billions of tons of TNT.

On a cosmic scale, the tidal forces between two colliding galaxies can create spectacular photo-opportunities. Whole galaxies are often re-shaped and ripped apart in such an encounter. As confirmed by computer simulations, tell-tale evidence includes long, often distorted tidal tails of stars that are created during the encounter. One such system is a pair of colliding galaxies 50 million light years away, NGC4038 and NGC4039. They are nick-named Antennae but they really look like two procreating mice. Another galactic wreckage looks like the old-fashioned model of the atom (complete with orbiting electrons), superimposed upon a peace symbol. Astronomers affectionately refer to this one as Atoms for Peace.

The next time you find yourself on a shoreline watching the tide come in, remember that the frame and operations of Nature extend to the farthest galaxies, and causes and effects of things are, fortunately, remarkably few in number.

Sometimes it seems that everybody is trying to tell you when and how the world is supposed to end. Some scenarios are more familiar than others. Those that are widely discussed in the media include rampant infectious disease, nuclear war, collisions with asteroids or comets, and environmental decay. While different in origin, each can induce the end of human species (and perhaps selected other life forms) on Earth. Indeed implicit in clichéd slogans such as Save the Earth is the egocentric call to save life on Earth, not the planet itself.

In fact, humans cannot really kill Earth. Earth will remain in orbit around the Sun, along with its planetary brethren, long after Homo sapiens has become extinct by whatever cause. But there are less familiar, though just as real, end-of-world scenarios that jeopardize our temperate planet in its stable orbit around the Sun. I offer these prognostications not because humans are likely to live long enough to observe them, but because the tools of astrophysics enable me to calculate them. Three that come to mind are the death of the Sun, the impending collision between our Milky Way galaxy and the Andromeda galaxy, and the death of the universe, about which the community of astrophysicists has recently achieved consensus.

Computer models of stellar evolution are akin to actuarial tables. They indicate a healthy 10 billion year life expectancy for our Sun. At an estimated age of 5 billion years, it has another 5 billion years of relatively stable energy output. By then, if we have not figured out a way to leave Earth, then we will bear witness to a remarkable evolutionary change in the Sun as it runs out of fuel.

The Sun owes its stability to the controlled fusion of hydrogen into helium in its 15 million degree core. The gravity that wants to collapse the star is held in balance by the outward gas pressure that is sustained by the fusion. While more than 90 percent of the Sun's atoms are hydrogen, the ones that matter are those that reside in the core. When the core is exhausted of its hydrogen, the Sun is left with a central ball of helium atoms that require a higher temperature than does hydrogen to fuse into heavier elements. Now out of balance, gravity wins, the inner regions of the star collapse, and the central temperature rises through 100 million degrees, which triggers the fusion of helium into carbon.

In the process, the Sun's luminosity grows astronomically, which forces its outer layers to expand to bulbous proportions, engulfing the orbits of Mercury and Venus. Eventually, the Sun will swell to occupy the entire sky as its expansion subsumes the orbit of Earth. This would be bad. The temperature on Earth will rise until it equals the 3,000 degree rarefied outer layers of the expanded Sun. Our atmosphere will evaporate away into interplanetary space and the oceans will boil off as Earth becomes a red-hot, charred ember orbiting deep within the Sun. Eventually, the Sun will cease all nuclear fusion, loose its spherical, tenuous, gaseous envelope, and expose its dying central core. Scenarios such as these will one day force manned space travel to become a global priority.

Not long after the Sun terrorizes Earth, the Milky Way will encounter some problems of its own. Of the hundreds of thousands of galaxies whose velocity relative to the Milky Way has been measured, only a few are moving toward us while all the rest are moving away at a speed directly related to their distances from us. Discovered in the 1920s by Edwin Hubble (after whom the Hubble Space Telescope was named), the general recession of galaxies is the observational signature of our expanding universe. The Milky Way and the three-hundred-billion-star Andromeda galaxy are close enough to each other that the effect of the expanding universe is negligible. We happen to be drifting toward each other at about 100 kilometers per second (a quarter million miles per hour). If our (unknown) sideways motion is small, then at this rate, the 2.2 million light-year distance that separates us will shrink to zero in about seven billion years.

Interstellar space is so vast that there is no need to fear whether stars in the Andromeda galaxy will accidentally slam into the Sun. During the galaxy-galaxy encounter, which would be a spectacular sight from a safe distance, stars are likely to pass each other by. But the event would not be worry-free. Some of Andromeda's stars are likely to swing close enough to our solar system to influence the orbit of the planets and of the hundreds of billions of resident comets. For example, close stellar flybys can throw one's gravitational allegiance into question. Computer simulations commonly show that the planets are either stolen by the interloper in a flyby looting or they become unbound and are flung forth into interplanetary space.

Remember how choosy Goldilocks was with other people's porridge? If we are stolen by the gravity of another star, there is no guarantee that our new-found orbit will be at the right distance to sustain liquid water on Earth's surface—a condition generally agreed to be a prerequisite to sustaining life as we know it. If Earth orbits too close, its water supply evaporates. And if Earth orbits too far, its water supply freezes solid.

By some miracle of future technology, if Earth inhabitants had managed to prolong the life of the Sun, then these efforts will be rendered irrelevant when Earth is flung in space. The absence of a nearby energy source will allow Earth's surface temperature to drop swiftly to hundreds of degrees below zero Fahrenheit. This would also be bad. Our cherished atmosphere of nitrogen and oxygen and other gases would first liquefy and then freeze solid, encrusting the Earth like icing on a cake. We would freeze to death before we had a chance to starve to death. The last surviving life on Earth would be those privileged organisms that had evolved to rely not on the Sun's energy but on (what will then be) weak geothermal sources, where the heat of Earth's interior emerges from the crust. At the moment, humans are not among them. There will be, of course, other planets that we can visit in orbit around healthy stars in other galaxies.

But the long-term fate of the cosmos cannot be postponed or avoided. No matter where you hide, you will be part of a universe that inexorably marches towards a peculiar oblivion. The latest and best evidence available on the space density of matter and the expansion rate of the universe suggest that we are on a one-way trip: the collective gravity of everything in the universe is insufficient to halt and reverse the cosmic expansion.

Currently, the most successful description of the universe and its origin combines the big bang with our modern understanding of gravity, derived from Einstein's general theory of relativity. The early universe was a trillion-degree maelstrom of matter mixed with energy, affectionately known as the primordial soup. During the fourteen billion year expansion that followed, the background temperature of the universe has dropped to a mere 3 degrees on the absolute (kelvin) temperature scale. As the universe continues to expand, this temperature will continue to approach zero.

Such a low background temperature does not directly affect us on Earth because our Sun (normally) grants us a cozy life. But as each generation of stars is born from the interstellar gas clouds of the galaxy, less and less gas remains to compose the next generation of stars. Eventually the gas supply will run out, as it already has in nearly half the galaxies in the universe. The small fraction of stars with the highest mass collapse completely, never to be seen again. Some stars end their lives by blowing their guts across the galaxy in a supernova explosion. This returned gas can then be tapped for the next generation. But the majority of stars—Sun included—ultimately exhaust the fuel at their cores and, after the bulbous giant phase, collapse to form a compact orb of matter that radiates its feeble leftover-heat to the frigid universe

The complete list of corpses may be familiar: black holes, neutron stars (pulsars), white dwarfs, and even brown dwarfs are each a dead end on the evolutionary tree of stars. What they each have in common is an eternal lock on cosmic construction materials. In other words, if stars burn out and no new ones are formed to replace them, then the universe will eventually contain no living stars.

How about Earth? We rely on the Sun for a daily infusion of energy to sustain life. If the Sun and the energy from all other stars were cut off from us then mechanical and chemical processes (life included) on and within Earth would wind down. Eventually, the energy of all motion gets lost to friction and the system reaches a single uniform temperature. This would really be bad. The starless Earth will lie naked in the presence of the frozen background of the expanding universe. The temperature on Earth will drop the way a freshly baked pie cools on a window sill. Yet Earth is not alone in this fate. Trillions of years into the future, when all stars are gone, and every process in every nook and cranny of the expanding universe has wound down, all parts of the cosmos will cool to the same temperature as the ever-cooling background. At that time, space travel will no longer provide refuge. Even Hell will have frozen over. We may then declare that the universe has died—not with a bang, but with a whimper.

Every now and then, a single photograph appears in the press that somehow forces you to take pause and reassess your place in the universe. In the 1960s, the first photograph of Earth from space reminded us that, as geologists had been telling us for some time, land masses do not have political boundaries drawn upon them—we were all together on spaceship Earth. Then there was the well-publicized photograph of Earth-rise over the barren lunar horizon taken by the astronauts of Apollo 8, the first manned mission to orbit the Moon. Earth looked small, fragile, and distant—just another orb out there in space.

For me, one of the Hubble Space Telescope's recently released photographs, now known as the Hubble deep field, ranks among the world's most profound images. It seems to have the right ingredients. It is unfamiliar. It is otherworldly. And it lures me someplace I have never been before.

What makes the Hubble deep field so special? The image grants the viewer a peek at a remarkably detailed subset of the billions and billions of galaxies in the universe, captured in a time line that spans from the first few billion years after the big bang, all the way to the present. Astrophysicists have been peeking at the universe with telescopes for 400 years, so the act of peeking itself is nothing new. But what the Hubble Telescope provides (by virtue of its above-the-atmosphere venue) is the highest resolution, and thus the clearest view, of the universe ever achieved in the history of optical telescopes.

Many ground-based pictures already exist of the seemingly countless galaxies in the outer universe, but in all cases the galaxies appear as undistinguished smudges. If you had bad vision you would encounter a similar problem when you looked at a lawn: you are told it is a lawn; you know in your mind it is a lawn; but all you notice is a sea of green, and you are not forced to think deep thoughts about what's there. With good vision, however, the green lawn is revealed to be composed of multitudes of blades of grass. There are even insects crawling about. You are now forced to recognize the lawn to be a world unto itself.

The Hubble deep field is a small, specially chosen, random, boring patch of sky that covers less than one one-hundredth the area of the full moon. To be specially chosen, yet random, simply means that a random field was selected among all fields that: 1) could be monitored continuously by the Hubble Space Telescope, without the Sun or the Earth getting in the way; 2) was away from the plane of the Milky Way galaxy, where densely packed stars, gas and dust clouds obscure our view of the rest of the universe; 3) was void of bright stars that might become over-exposed; and 4) was not coincident with clusters of galaxies that have already been cataloged.

The Hubble deep field, a full-color image created from 342 repeated exposures, was taken by the Hubble Space Telescope during a continuous stretch of orbits that spanned ten consecutive days, which represents far more observing time than is ever granted to an individual research project. In practice, long exposures are created by adding together many repeated, shorter exposures. With each added image, objects in the field of view become more and more pronounced against the background, which enables dimmer and dimmer objects come into view. One of several reasons for this tactic is that if a hardware or software problem arises within a single image, then you still have all the rest of the images to add together, which may still enable you to accomplish your scientific objectives.

To observe with the Hubble normally requires that an astrophysicist write a detailed proposal that states and defends the scientific motivations of a project, the target objectives, and why the project must be accomplished from orbit rather than from the many available ground-based telescopes. The proposal is then reviewed and critiqued by a committee of peers and awarded observing time on the basis of merit. Often, more than twice as much observing time is requested than is available, so most proposals are awarded no time at all. But thanks to something called director's discretionary time, Robert Williams, the Director of the Space Telescope Science Institute, was able to do what nobody else would have been permitted to do: point the telescope in a random place just to see what's there.

At about 1/100 the area of the full moon, the Hubble deep field sampled only about 1/15,000,000 of the 41,000 square degrees of the entire sky. Even so, this single image reveals thousands galaxies. If we diligently count every one of them—from the large, bright ones, down to the small, faint ones—and then multiply the result by 15,000,000, we get a fast estimate for the total number of observable galaxies in the universe. When the Hubble deep field image was released, media accounts (based on the NASA press release that accompanied it) widely reported that there are five times as many galaxies in the universe than previously estimated, raising the count from 10 to 50 billion. Actually, even from fuzzy ground-based images, the estimates had already ranged from a lower limit of about 10 billion to an upper limit of about 100 billion, depending on how thoroughly you believed that the population of under-luminous dwarf galaxies had been counted. What the headlines should have said was that data from the Hubble deep field allow us to confidently raise the previous lower limit from 10 to 50 billion galaxies.

As you watch astrophysicists bandy billions, it may look as though we are clueless about the galactic contents of the universe. But when you consider that all numbers above a trillion (of which there are many) and all numbers below a billion, are not in the running, then the range in our ignorance is quite small.

For me, what is most striking about the Hubble deep field is the richness in morphology revealed in even the tiniest of galaxies. The photogenic spirals, kindred forms to our own Milky Way, show the characteristic central bulges and the knots of freshly made stars that dot the spiral arms. Each of these galaxies, however small they appear in the image, is its own collection of hundreds of billion of stars. There are other galactic forms such as elliptical and irregular galaxies. Though less photogenic, they too are part of the cosmic census.

The colors of galaxies are dominated by the colors of the most luminous resident stars. Bluish galaxies tend to have active areas where stars are forged; assortments of freshly made stars typically contain extremely hot (at least 20,000 kelvins), ultra luminous blue giants. Reddish galaxies contain relatively cool (3,000 kelvins), yet ultra luminous, red giant stars. In any galaxy, the absence of blue reveals the absence of stellar nurseries, which generally implies that the gas content (from which stars are made) was exhausted long ago.

But long ago is looking straight at us. On average, we expect the smaller galaxies to be farther away than the larger galaxies. And their light has been traveling longest in time to reach us. In other words, we see them not as they are, but as they used to be. As sedimentary deposits on Earth indicate a geological time line, distant objects betray a time long passed in the history of the universe. No new concepts here: Light from your elbow, provided it is where it belongs (hinged from your shoulder) is about a nanosecond (a billionth of a second) away in light travel time from your body's light detector known as the retina. The Moon: about 1.5 seconds away. The Sun: 500 light-seconds. The nearest star: 4.1 light years. The beautiful spiral galaxy M100: 65 million light years. (Yes, voyeuristic residents of M100 could now be watching Earth's dinosaurs go extinct.) Most of the galaxies in the Hubble deep field are billions and billions of light years away. The light we now see left their stars before single-celled life began on Earth. And in some cases, before Earth, itself.

With images of the quality of the Hubble deep field, one can even begin to test for evolutionary trends in galaxy colors. As stars are forged out of interstellar gas, less and less gas remains to create subsequent generations of stars. Eventually, stars stop forming. We would thus, on average, expect the more distant galaxies to be bluer than the nearby galaxies. A first estimate of the distances to these galaxies can be made from the Hubble data, but more reliable distances must be obtained from follow-up measurements. Until that happens, our evolutionary interpretation retains a level of uncertainty: are the smallest galaxies on the image small because they are normal-sized galaxies that happen to be far away? Or are they small because they are, indeed, dwarf galaxies that happen to be right in front of our noses? The real answer is likely to be some combination of these scenarios, with the distant galaxy description being the dominant of the two.

Let there be no misunderstanding: large, ground-based telescopes (such as the 10-meter Keck telescope in Hawaii) have detected galaxies as far away as the farthest galaxies in the Hubble deep field. Telescopes of the ten-meter class have over twenty times the light gathering power of the Hubble Space Telescope. But somehow, the relatively fuzzy, ground based images do not act as powerfully on my imagination. I am captured intellectually, but not emotionally. Only with sharp images am I viscerally reminded me that there are other worlds out there. Billions and billions of them.

In spite of the quality and beauty of the Hubble data, many scientific questions remain unanswered: Do we have the right to extrapolate what we learn from one postage stamp-sized region of the sky to the entire universe? How many more galaxies might have revealed themselves with an even longer exposure? How far away is the farthest galaxy? How soon after the big bang did galaxies form?

Many questions also remain that do not lend themselves to immediate scientific inquiry: Is there some undiscovered law of physics that will completely change our modern understanding of the cosmos the way Einstein's theories of relativity redefined our understanding of the physical universe? On planets around stars in the galaxies of the Hubble deep field, are there life forms that are contemplating the universe the way we are? Or are they not paying attention because they are just looking for shelter, food, and sex, as does most life on Earth?

As I galaxy-gaze through time upon their diversity of colors, shapes, sizes, brightnesses, and structural detail, the boundary between knowledge and ignorance calls to me. When I reach for the edge of the universe, I do it knowing that along some paths of cosmic discovery, there are times when, at least for now, one must be content to love the questions themselves.

Not all scientific discoveries are made by lone, anti-social researchers. Nor are all discoveries accompanied by media headlines and best-selling books. Some involve many people, span many decades, require complicated mathematics, and are not easily summarized by the press. Such discoveries pass almost unnoticed by the general public.

My vote for the most under-appreciated discovery of the twentieth century is the realization that supernovae—the explosive death throes of high-mass stars—are the primary source for the origin and relative mix of heavy elements in the universe. This unheralded discovery took the form of an extensive research paper published in 1957 in the journal Reviews of Modern Physics titled The Synthesis of the Elements in Stars, by E. Margaret Burbidge, Geoffrey R. Burbidge, William Fowler, and Fred Hoyle. In the paper they built a theoretical and computational framework that freshly interpreted forty years of musings by others on such hot topics as the sources of stellar energy and the transmutation of elements.

Cosmic nuclear chemistry is a messy business. It was messy in 1957 and it is messy now. The relevant questions have always included: How do the various elements from the famed periodic table of elements behave when subjected to assorted temperatures and pressures? Do the elements fuse or do they split? How easily is this accomplished? Does the process liberate or absorb energy?

The periodic table is, of course, much more than just a mysterious chart of a hundred, or so, boxes with cryptic symbols in them. It is a sequence of every known element in the universe arranged by increasing number of protons in their nuclei. The two lightest are hydrogen, with one proton, and helium, with two protons. Under the right conditions of temperature, density, and pressure, you can use hydrogen and helium to synthesize every other element on the periodic table.

A perennial problem in nuclear chemistry involves calculating accurate collision cross-sections, which are simply measures of how close one particle must get to another particle before they interact significantly. Collision cross-sections are easy to calculate for things such as cement mixers or a houses moving down the street on flat-bed trucks, but it can be a challenge for elusive subatomic particles. A detailed understanding of collision cross-sections is what enables you to predict nuclear reaction rates and pathways. Often small uncertainties in tables of collision cross-sections can force you to draw wildly erroneous conclusions. The problem greatly resembles what would happen if you tried to navigate your way around one city's subway system while using another city's subway map as your guide.

Apart from this ignorance, scientists had suspected for some time that if exotic nuclear process existed anywhere in the universe, then the centers of stars were as good a place as any to find it. In particular, the British theoretical astrophysicist Sir Arthur Eddington published a paper in 1920 titled the The Internal Constitution of the Stars where he argued that the Cavendish Laboratory in England, the most famous atomic and nuclear physics research center of the day, could not be the only place in the universe that managed to change some elements onto others:

But is it possible to admit that such a transmutation is occurring? It is difficult to assert, but perhaps more difficult to deny, that this is going onÉand what is possible in the Cavendish Laboratory may not be too difficult in the sun. I think that the suspicion has been generally entertained that the stars are the crucibles in which the lighter atoms which abound in the nebulæ are compounded into more complex elements.

Eddington's paper predates by several years the discovery of quantum mechanics, without which, our knowledge of the physics of atoms and nuclei was feeble, at best. With remarkable prescience, Eddington began to formulate a scenario for star-generated energy via the thermonuclear fusion of hydrogen to helium and beyond:

We need not bind ourselves to the formation of helium from hydrogen as the sole reaction which supplies the energy [to a star], although it would seem that the further stages in building up the elements involve much less liberation, and sometimes even absorption, of energy. The position may be summarised in these terms: the atoms of all elements are built of hydrogen atoms bound together, and presumably have at one time been formed from hydrogen; the interior of a star seems as likely a place as any for the evolution to have occurred.

The observed mix of elements on Earth and elsewhere in the universe was another desirable thing for a model of the transmutation of the elements to explain. But first a mechanism was required. By 1931, quantum mechanics was developed (although the neutron was not yet discovered) and the astrophysicist Robert d'Escourt Atkinson, published an extensive paper that he summarizes in his abstract as a synthesis theory of stellar energy and of the origin of the elements in which the various chemical elements are built up step by step from the lighter ones in stellar interiors, by the successive incorporation of protons and electrons one at a time.

At about the same time, the nuclear chemist William D. Harkins published a paper noting that elements of low atomic weight are more abundant than those of high atomic weight and that, on the average, the elements with even atomic numbers are about 10 times more abundant than those with odd atomic numbers of similar value. Harkins surmised that the relative abundances of the elements depend on nuclear rather than on conventional chemical processes and that the heavy elements must have been synthesized from the light ones.

The detailed mechanism of nuclear fusion in stars could ultimately explain the cosmic presence of many elements, especially those that you get each time you add the two-proton helium nucleus to your previously forged element. These constitute the abundant elements with even atomic numbers that Harkins refers to. But the existence and relative mix of many other elements remained unexplained. Another means of element buildup must have been at work.

The neutron, discovered in 1932 by the British physicist James Chadwick while working at the Cavendish Laboratories, plays a significant role in nuclear fusion that Eddington could not have imagined. To assemble protons requires hard work because they naturally repel each other. They must be brought close enough together (often by way of high temperatures, pressures, and densities) for the short-range strong nuclear force to overcome their repulsion and bind them. The chargeless neutron, however, repels no other particle, so it can just march into somebody else's nucleus and join the other assembled particles. This step has not yet created another element; by adding a neutron we have simply made an isotope of the original. But for some elements, the freshly captured neutron is unstable and it spontaneously converts itself into a proton (which stays put in the nucleus), and an electron (which escapes immediately). Like the Greek soldiers who managed to breach the walls of Troy by hiding inside the Trojan Horse, protons can effectively sneak into a nucleus under the guise of a neutron.

If the ambient flow of neutrons is high, then an atom's nucleus can absorb many in a row before the first one decays. These rapidly absorbed neutrons help to create an ensemble of elements that are identified with the process and differ from the assortment of elements that result from neutrons that are captured slowly.

The entire process is known as neutron capture and is responsible for creating many elements that are not otherwise formed by traditional thermonuclear fusion. The remaining elements in nature can be made by a few other means, including slamming high-energy light (gamma rays) into the nuclei of heavy atoms, which then break apart into smaller ones.

At the risk of oversimplifying the life cycle of a high-mass star, it is sufficient to recognize that a star is in the business of making and releasing energy, which helps to support the star against gravity. Without it, the big ball of gas would simply collapse under its own weight. A star's core, after having converted its hydrogen supply into helium, will next fuse helium into carbon, then carbon to oxygen, oxygen to neon, and so forth up to iron. To successively fuse this sequence of heavier and heavier elements requires higher and higher temperatures for the nuclei to overcome their natural repulsion. Fortunately this happens naturally because at the end of each intermediate stage, the star's energy source temporarily shuts off, the inner regions collapse, the temperature rises, and the next pathway of fusion kicks in. But there is just one problem. The fusion of iron absorbs energy rather than releases it. This is very bad for the star because it can now no longer support itself against gravity. The star immediately collapses without resistance, which forces the temperature to rise so rapidly that a titanic explosion ensues as the star blows its guts to smithereens. During the explosion, the star's luminosity can increase a billion-fold. We call them supernovae, although I always felt that the term super-duper-nova would be more appropriate.

Throughout the supernova explosion, the availability of neutrons, protons, and energy enable elements to be created in many different ways. By combining 1) the well-tested tenets of quantum mechanics, 2) the physics of explosions, 3) the latest collision cross sections, 4) the varied processes by which elements can transmutate into one another, and 5) the basics of stellar evolutionary theory, Burbidge, Burbidge, Fowler, & Hoyle decisively implicated supernova explosions as the primary source of all elements heavier than hydrogen and helium in the universe.

With supernovae as the smoking gun, they got to solve one other problem for free: when you forge elements heavier than hydrogen and helium inside of stars then it does the rest of the universe no good unless those elements are somehow cast forth to interstellar space and made available to form planets and people. Yes, we are stardust.

I do not mean to imply that all of our cosmic chemical questions are solved. A curious contemporary mystery involves the element technetium, which, in 1937, was the first element to be synthesized in the laboratory. (The name technetium, along with other words that use the root prefix tech- derives from the Greek word technetos, which translates to artificial.) The element has yet to be discovered naturally on Earth, but it has been found in the atmosphere of a small fraction of red giant stars in our galaxy. This alone would not be cause for alarm were it not for the fact that technetium has a half-life of a mere two-million years, which is much, much shorter than the age and life expectancy of the stars in which it is found. In other words, the star cannot have been born with the stuff, for if it were, there would be none left by now. There is also no known mechanism to create technetium in a star's core and have it dredge itself up to the surface where it is observed, which has led to exotic theories that have yet to achieve consensus in the astrophysics community.

Red giants with peculiar chemical properties are rare, but nonetheless common enough for there to be a cadre of astrophysicists (mostly spectroscopists) who specialize in the subject. In fact, my professional research interests sufficiently overlap the subject for me to be a regular recipient of the internationally distributed Newsletter of Chemically Peculiar Red Giant Stars, not available on the newsstand, it typically contains conference-news and updates on research in progress. To the interested scientist, these ongoing chemical mysteries are no less seductive than questions related to black holes, quasars, and the early universe. But you will hardly ever read about them. Why? Because once again, the media has predetermined what is not worthy of coverage, even when the news item is something as uninteresting as the cosmic origin of every element in your body.