Death By Black Hole Part 3

An optometrist from Saint Paul, Minnesota, named Sherman Schultz wrote a letter in response to an article in the July 2002 issue of Sky and Telescope Sky and Telescope magazine. Schultz pointed out that the optical setup Lowell preferred for viewing the Venutian surface was similar to the gizmo used to examine the interior of patients' eyes. After seeking a couple of second opinions, the author established that what Lowell saw on Venus was actually the network of shadows cast on Lowell's own retina by his ocular blood vessels. When you compare Lowell's diagram of the spokes with a diagram of the eye, the two match up, ca.n.a.l for blood vessel. And when you combine the unfortunate fact that Lowell suffered from hypertension-which shows up clearly in the vessels of the eyeb.a.l.l.s-with his will to believe, it's no surprise that he pegged Venus as well as Mars with teeming with intelligent, technologically capable inhabitants. magazine. Schultz pointed out that the optical setup Lowell preferred for viewing the Venutian surface was similar to the gizmo used to examine the interior of patients' eyes. After seeking a couple of second opinions, the author established that what Lowell saw on Venus was actually the network of shadows cast on Lowell's own retina by his ocular blood vessels. When you compare Lowell's diagram of the spokes with a diagram of the eye, the two match up, ca.n.a.l for blood vessel. And when you combine the unfortunate fact that Lowell suffered from hypertension-which shows up clearly in the vessels of the eyeb.a.l.l.s-with his will to believe, it's no surprise that he pegged Venus as well as Mars with teeming with intelligent, technologically capable inhabitants.

Alas, Lowell fared only slightly better with his search for Planet X, a planet thought to lie beyond Neptune. Planet X does not exist, as the astronomer E. Myles Standish Jr. demonstrated decisively in the mid-1990s. But Pluto, discovered at the Lowell Observatory in February 1930, some 13 years after Lowell's death, did serve as a fair approximation for a while. Within weeks of the observatory's big announcement, though, some astronomers had begun debating whether it should be cla.s.sified as the ninth planet. Given our decision to display Pluto as a comet rather than as a planet in the Rose Center for Earth and s.p.a.ce, I've become an unwitting part of that debate myself, and I can a.s.sure you, it hasn't let up yet. Asteroid, planetoid, planetesimal, large planetesimal, icy planetesimal, minor planet, dwarf planet, giant comet, Kuiper Belt object, trans-Neptunian object, methane s...o...b..ll, Mickey's dim-witted bloodhound-anything but number nine, we naysayers argue. Pluto is just too small, too lightweight, too icy, too eccentric in its...o...b..t, too misbehaved. And by the way, we say the same about the recent high-profile contenders including the three or four objects discovered beyond Pluto that rival Pluto in size and in table manners.

TIME AND TECHNOLOGY moved on. Come the 1950s, radio-wave observations and better photography revealed fascinating facts about the planets. By the 1960s, people and robots had left Earth to take family photos of the planets. And with each new fact and photograph the curtain of ignorance lifted a bit higher. moved on. Come the 1950s, radio-wave observations and better photography revealed fascinating facts about the planets. By the 1960s, people and robots had left Earth to take family photos of the planets. And with each new fact and photograph the curtain of ignorance lifted a bit higher.

Venus, named after the G.o.ddess of beauty and love, turns out to have a thick, almost opaque atmosphere, made up mostly of carbon dioxide, bearing down at nearly 100 times the sea level pressure on Earth. Worse yet, the surface air temperature nears 900 degrees Fahrenheit. On Venus you could cook a 16-inch pepperoni pizza in seven seconds, just by holding it out to the air. (Yes, I did the math.) Such extreme conditions pose great challenges to s.p.a.ce exploration, because practically anything you can imagine sending to Venus will, within a moment or two, get crushed, melted, or vaporized. So you must be heatproof or just plain quick if you're collecting data from the surface of this forsaken place.

It's no accident, by the way, that Venus is hot. It suffers from a runaway greenhouse effect, induced by the carbon dioxide in its atmosphere, which traps infrared energy. So even though the tops of Venus's clouds reflect most of the Sun's incoming visible light, rocks and soils on the ground absorb the little bit that makes its way through. This same terrain then reradiates the visible light as infrared, which builds and builds in the air, eventually creating-and now sustaining-a remarkable pizza oven.

By the way, were we to find life-forms on Venus, we would probably call them Venutians, just as people from Mars would be Martians. But according to rules of Latin genitives, to be "of Venus" ought to make you a Venereal. Unfortunately, medical doctors reached that word before astronomers did. Can't blame them, I suppose. Venereal disease long predates astronomy, which itself stands as only the second second oldest profession. oldest profession.

The rest of the solar system continues to become more familiar by the day. The first s.p.a.cecraft to fly past Mars was Mariner 4, Mariner 4, in 1965, and it sent back the first-ever close-ups of the Red Planet. Lowell's lunacies notwithstanding, before 1965 n.o.body knew what the Martian surface looked like, other than that it was reddish, had polar ice caps, and showed darker and lighter patches. n.o.body knew it had mountains, or a canyon system vastly wider, deeper, and longer than the Grand Canyon. n.o.body knew it had volcanoes vastly bigger than the largest volcano on Earth-Mauna Kea in Hawaii-even when you measure its height from the bottom of the ocean. in 1965, and it sent back the first-ever close-ups of the Red Planet. Lowell's lunacies notwithstanding, before 1965 n.o.body knew what the Martian surface looked like, other than that it was reddish, had polar ice caps, and showed darker and lighter patches. n.o.body knew it had mountains, or a canyon system vastly wider, deeper, and longer than the Grand Canyon. n.o.body knew it had volcanoes vastly bigger than the largest volcano on Earth-Mauna Kea in Hawaii-even when you measure its height from the bottom of the ocean.

Nor is there any shortage of evidence that liquid water once flowed on the Martian surface: the planet has (dry) meandering riverbeds as long and wide as the Amazon, webs of (dry) tributaries, (dry) river deltas, and (dry) floodplains. The Mars exploration rovers, inching their way across the dusty rock-strewn surface, confirmed the presence of surface minerals that form only in the presence of water. Yes, signs of water everywhere, but not a drop to drink.

Something bad happened on both Mars and Venus. Could something bad happen on Earth too? Our species currently turns row upon row of environmental k.n.o.bs, without much regard to long-term consequences. Who even knew to ask these questions of Earth before the study of Mars and Venus, our nearest neighbors in s.p.a.ce, forced us to look back on ourselves?

TO GET A better view of the more distant planets requires s.p.a.ce probes. The first s.p.a.cecraft to leave the solar system were better view of the more distant planets requires s.p.a.ce probes. The first s.p.a.cecraft to leave the solar system were Pioneer 10 Pioneer 10, launched in 1972, and its twin Pioneer 11, Pioneer 11, launched in 1973. Both pa.s.sed by Jupiter two years later, executing a grand tour along the way. They'll soon pa.s.s 10 billion miles from Earth, more than twice the distance to Pluto. launched in 1973. Both pa.s.sed by Jupiter two years later, executing a grand tour along the way. They'll soon pa.s.s 10 billion miles from Earth, more than twice the distance to Pluto.

When they were launched, however, Pioneer 10 Pioneer 10 and and 11 11 weren't supplied with enough energy to go much beyond Jupiter. How do you get a s.p.a.cecraft to go farther than its energy supply will carry it? You aim it, fire the rockets, and then just let it coast to its destination, falling along the streams of gravitational forces set up by everything in the solar system. And because astrophysicists map trajectories with precision, probes can gain energy from multiple slingshot-style maneuvers that rob orbital energy from the planets they visit. Orbital dynamicists have gotten so good at these gravity a.s.sists that they make pool sharks jealous. weren't supplied with enough energy to go much beyond Jupiter. How do you get a s.p.a.cecraft to go farther than its energy supply will carry it? You aim it, fire the rockets, and then just let it coast to its destination, falling along the streams of gravitational forces set up by everything in the solar system. And because astrophysicists map trajectories with precision, probes can gain energy from multiple slingshot-style maneuvers that rob orbital energy from the planets they visit. Orbital dynamicists have gotten so good at these gravity a.s.sists that they make pool sharks jealous.

Pioneer 10 and and 11 11 sent back better pictures of Jupiter and Saturn than had ever been possible from Earth's surfce. But it was the twin s.p.a.cecraft sent back better pictures of Jupiter and Saturn than had ever been possible from Earth's surfce. But it was the twin s.p.a.cecraft Voyager 1 Voyager 1 and and 2 2-launched in 1977 and equipped with a suite of scientific experiments and imagers-that turned the outer planets into icons. Voyager 1 Voyager 1 and and 2 2 brought the solar system into the living rooms of an entire generation of world citizens. One of the windfalls of those journeys was the revelation that the moons of the outer planets are just as different from one another, and just as fascinating, as the planets themselves. Hence those planetary satellites graduated from boring points of light to worlds worthy of our attention and affection. brought the solar system into the living rooms of an entire generation of world citizens. One of the windfalls of those journeys was the revelation that the moons of the outer planets are just as different from one another, and just as fascinating, as the planets themselves. Hence those planetary satellites graduated from boring points of light to worlds worthy of our attention and affection.

As I write, NASA's Ca.s.sini Ca.s.sini orbiter continues to orbit Saturn, in deep study of the planet itself, its striking ring system, and its many moons. Having reached Saturn's neighborhood after a "four-cushion" gravity a.s.sist, orbiter continues to orbit Saturn, in deep study of the planet itself, its striking ring system, and its many moons. Having reached Saturn's neighborhood after a "four-cushion" gravity a.s.sist, Ca.s.sini Ca.s.sini successfully deployed a daughter probe named successfully deployed a daughter probe named Huygens Huygens, designed by the European s.p.a.ce Agency and named for Christiaan Huygens the Dutch astronomer who first identified Saturn's rings. The probe descended into the atmosphere of Saturn's largest satellite, t.i.tan-the only moon in the solar system known to have a dense atmosphere. t.i.tan's surface chemistry, rich in organic molecules, may be the best a.n.a.log we have for the early prebiotic Earth. Other complex NASA missions are now being planned that will do the same for Jupiter, allowing a sustained study of the planet and its 70-plus moons.

IN 1584, in his book 1584, in his book On the Infinite Universe and Worlds, On the Infinite Universe and Worlds, the Italian monk and philosopher Giordano Bruno proposed the existence of "innumerable suns" and "innumerable Earths [that] revolve about these suns." Moreover, he claimed, working from the premise of a Creator both glorious and omnipotent, that each of those Earths has living inhabitants. For these and related blasphemous transgressions, the Catholic Church had Bruno burned at the stake. the Italian monk and philosopher Giordano Bruno proposed the existence of "innumerable suns" and "innumerable Earths [that] revolve about these suns." Moreover, he claimed, working from the premise of a Creator both glorious and omnipotent, that each of those Earths has living inhabitants. For these and related blasphemous transgressions, the Catholic Church had Bruno burned at the stake.

Yet Bruno was neither the first nor the last person to posit some version of those ideas. His predecessors range from the fifth-century B.C B.C. Greek philosopher Democritus to the fifteenth-century cardinal Nicholas of Cusa. His successors include such personages as the eighteenth-century philosopher Immanuel Kant and the nineteenth-century novelist Honore de Balzac. Bruno was just unlucky to be born at a time when you could get executed for such thoughts.

During the twentieth century, astronomers figured that life could exist on other planets, as it does on Earth, only if those planets...o...b..ted their host star within the "habitable zone"-a swath of s.p.a.ce neither too close, because water would evaporate, nor too far, because water would freeze. No doubt that life as we know it requires liquid water, but everyone had just a.s.sumed that life also required starlight as its ultimate source of energy.

Then came the discovery that Jupiter's moons Io and Europa, among other objects in the outer solar system, are heated by energy sources other than the Sun. Io is the most volcanically active place in the solar system, belching sulfurous gases into its atmosphere and spilling lava left and right. Europa almost surely has a deep billion-year-old ocean of liquid water beneath its icy crust. In both cases, the stress of Jupiter's tides on the solid moons pumps energy to their interiors, melting ice and giving rise to environments that might sustain life independent of solar energy.

Even right here on Earth, new categories of organisms, collectively called extremophiles, thrive in conditions inimical to human beings. The concept of a habitable zone incorporated an initial bias that room temperature is just right for life. But some organisms just love several-hundred-degree hot tubs and find room temperature downright hostile. To them, we are the extremophiles. Many places on Earth, previously presumed to be unlivable, such creatures call home: the bottom of Death Valley, the mouths of hot vents at the bottom of the ocean, and nuclear waste sites, to name just a few.

Armed with the knowledge that life can appear in places vastly more diverse than previously imagined, astrobiologists have broadened the earlier, and more restricted, concept of a habitable zone. Today we know that such a zone must encompa.s.s the newfound hardiness of microbial life as well as the range of energy sources that can sustain it. And, just as Bruno and others had suspected, the roster of confirmed exosolar planets continues to grow by leaps and bounds. That number has now risen past 150-all discovered in the past decade or so.

Once again we resurrect the idea that life might be everywhere, just as our ancestors had imagined. But today, we do so without risk of being immolated, and with the newfound knowledge that life is hardy and that the habitable zone may be as large as the universe itself.

EIGHT.

VAGABONDS OF THE SOLAR SYSTEM.

For hundreds of years, the inventory of our celestial neighborhood was quite stable. It included the Sun, the stars, the planets, a handful of planetary moons, and the comets. Even the addition of a planet or two to the roster didn't change the basic organization of the system.

But on New Year's Day of 1801 a new category arose: the asteroids, so named in 1802 by the English astronomer Sir John Herschel, son of Sir William, the discoverer of Ura.n.u.s. During the next two centuries, the family alb.u.m of the solar system became crammed with the data, photographs, and life histories of asteroids, as astronomers located vast numbers of these vagabonds, identified their home turf, a.s.sessed their ingredients, estimated their sizes, mapped their shapes, calculated their orbits, and crash-landed probes on them. Some investigators have also suggested that the asteroids are kinfolk to comets and even to planetary moons. And at this very moment, some astrophysicists and engineers are plotting methods to deflect any big ones that may be planning an uninvited visit.

TO UNDERSTAND THE small objects in our solar system, one should look first at the large ones, specifically the planets. One curious fact about the planets is captured in a fairly simple mathematical rule proposed in 1766 by a Prussian astronomer named Johann Daniel t.i.tius. A few years later, t.i.tius's colleague Johann Elert Bode, giving no credit to t.i.tius, began to spread the word about the rule, and to this day it's often called the t.i.tius-Bode law or even, erasing t.i.tius's contribution altogether, Bode's law. Their handy-dandy formula yielded pretty good estimates for the distances between the planets and the Sun, at least for the ones known at the time: Mercury, Venus, Earth, Mars, Jupiter, and Saturn. In 1781, widespread knowledge of the t.i.tius-Bode law actually helped lead to the discovery of Neptune, the eighth planet from the Sun. Impressive. So either the law is just a coincidence, or it embodies some fundamental fact about how solar systems form. small objects in our solar system, one should look first at the large ones, specifically the planets. One curious fact about the planets is captured in a fairly simple mathematical rule proposed in 1766 by a Prussian astronomer named Johann Daniel t.i.tius. A few years later, t.i.tius's colleague Johann Elert Bode, giving no credit to t.i.tius, began to spread the word about the rule, and to this day it's often called the t.i.tius-Bode law or even, erasing t.i.tius's contribution altogether, Bode's law. Their handy-dandy formula yielded pretty good estimates for the distances between the planets and the Sun, at least for the ones known at the time: Mercury, Venus, Earth, Mars, Jupiter, and Saturn. In 1781, widespread knowledge of the t.i.tius-Bode law actually helped lead to the discovery of Neptune, the eighth planet from the Sun. Impressive. So either the law is just a coincidence, or it embodies some fundamental fact about how solar systems form.

It's not quite perfect, though.

Problem number 1: You have to cheat a little to get the right distance for Mercury, by inserting a zero where the formula calls for 1.5. Problem number 2: Neptune, the eighth planet, turns out to be much farther out than the formula predicts, orbiting more or less where a ninth planet should be. Problem no. 3: Pluto, which some people persist in calling the ninth planet* falls way off the arithmetic scale, like so much else about the place. falls way off the arithmetic scale, like so much else about the place.

The law would also put a planet orbiting in the s.p.a.ce between Mars and Jupiter-at about 2.8 astronomical units from the Sun. Encouraged by the discovery of Ura.n.u.s at more or less the distance t.i.tius-Bode said it would be, astronomers in the late eighteenth century thought it would be a good idea to check out the zone around 2.8 AUs. And sure enough, on New Year's Day 1801, the Italian astronomer Giuseppe Piazzi, founder of the Observatory of Palermo, discovered something there. Subsequently it disappeared behind the glare of the Sun, but exactly one year later, with the help of brilliant computations by the German mathematician Carl Friedrich Gauss, the new object was rediscovered in a different part of the sky. Everybody was excited: a triumph of mathematics and a triumph of telescopes had led to the discovery of a new planet. Piazzi himself named it Ceres (as in "cereal"), for the Roman G.o.ddess of agriculture, in keeping with the tradition of naming planets after ancient Roman deities. from the Sun. Encouraged by the discovery of Ura.n.u.s at more or less the distance t.i.tius-Bode said it would be, astronomers in the late eighteenth century thought it would be a good idea to check out the zone around 2.8 AUs. And sure enough, on New Year's Day 1801, the Italian astronomer Giuseppe Piazzi, founder of the Observatory of Palermo, discovered something there. Subsequently it disappeared behind the glare of the Sun, but exactly one year later, with the help of brilliant computations by the German mathematician Carl Friedrich Gauss, the new object was rediscovered in a different part of the sky. Everybody was excited: a triumph of mathematics and a triumph of telescopes had led to the discovery of a new planet. Piazzi himself named it Ceres (as in "cereal"), for the Roman G.o.ddess of agriculture, in keeping with the tradition of naming planets after ancient Roman deities.

But when the astronomers looked a bit harder, and calculated an orbit and a distance and a brightness for Ceres, they discovered that their new "planet" was teeny. Within a few more years three more teeny planets-Pallas, Juno, and Vesta-were discovered in the same zone. It took a few decades, but Herschel's term "asteroids" (literally "starlike" bodies) eventually caught on, because, unlike planets, which showed up in the telescopes of the day as disks, the newfound objects could not be distinguished from stars except by their motion. Further observations revealed a proliferation of asteroids, and by the end of the nineteenth century, 464 of them had been discovered in and around the swath of celestial real estate at 2.8 AU. And because the swath turned out to be a relatively flat band and did not scatter around the Sun in every direction, like bees around a hive, the zone became known as the asteroid belt.

By now, many tens of thousands of asteroids have been catalogued, with hundreds more discovered every year. Altogether, by some estimates, more than a million measure a half-mile across and up. As far as anyone can tell, even though Roman G.o.ds and G.o.ddesses did lead complicated social lives, they didn't have 10,000 friends, and so astronomers had to give up on that source of names long ago. So asteroids can now be named after actors, painters, philosophers, and playwrights; cities, countries, dinosaurs, flowers, seasons, and all manner of miscellany. Even regular people have asteroids named after them. Harriet, Jo-Ann, and Ralph each have one: they are called 1744 Harriet, 2316 Jo-Ann, and 5051 Ralph, with the number indicating the sequence in which each asteroid's...o...b..t became firmly established. David H. Levy, a Canadian-born amateur astronomer who is the patron saint of comet hunters but has discovered plenty of asteroids as well, was kind enough to pull an asteroid from his stash and name it after me, 13123 Tyson. He did this shortly after we opened our $240-million Rose Center for Earth and s.p.a.ce, designed solely to bring the universe down to Earth. I was deeply moved by David's gesture, and quickly learned from 13123 Tyson's...o...b..tal data that it travels among most of the others, in the main belt of asteroids, and does not cross Earth's...o...b..t, putting life on Earth at risk of extinction. It's just good to check this sort of thing.

ONLY CERES-the largest of the asteroids, at about 580 miles in diameter-is spherical. The others are much smaller, craggy fragments shaped like doggy bones or Idaho potatoes. Curiously, Ceres alone accounts for about a quarter of the total asteroidal ma.s.s. And if you add up the ma.s.ses of all the asteroids big enough to see, plus all the smaller asteroids whose existence can be extrapolated from the data, you don't get anywhere near a planet's worth of ma.s.s. You get about 5 percent the ma.s.s of Earth's moon. So the prediction from t.i.tius-Bode, that a red-blooded planet lurks at 2.8 AU, was a bit exaggerated.

Most asteroids are made entirely of rock, though some are entirely metal and some are a mixture of both; most inhabit what's often called the main belt, a zone between Mars and Jupiter. Asteroids are usually described as being formed of material left over from the earliest days of the solar system-material that never got incorporated into a planet. But that explanation is incomplete at best and does not account for the fact that some asteroids are pure metal. To understand what's going on, one should first consider how the larger objects in the solar system formed.

The planets coalesced from a cloud of gas and dust enriched by the scattered remains of element-rich exploding stars. The collapsing cloud forms a protoplanet-a solid blob that gets hot as it accretes more and more material. Two things happen with the larger protoplanets. One, the blob tends to take on the shape of a sphere. Two, its inner heat keeps the protoplanet molten long enough for the heavy stuff-primarily iron, with some nickel and a splash of such metals as cobalt, gold, and uranium mixed in-to sink to the center of the growing ma.s.s. Meanwhile, the much more common, light stuff-hydrogen, carbon, oxygen, and silicon-floats upward toward the surface. Geologists (who are fearless of sesquipedalian words) call the process "differentiation." Thus the core of a differentiated planet such as Earth, Mars, or Venus is metal; its mantle and crust are mostly rock, and occupy a far greater volume than the core.

Once it has cooled, if such a planet is then destroyed-say, by smashing into one of its fellow planets-the fragments of both will continue orbiting the Sun in more or less the same trajectories that the original, intact objects had. Most of those fragments will be rocky, because they come from the thick, outer, rocky layers of the two differentiated objects, and a small fraction will be purely metallic. Indeed, that's exactly what's observed with real asteroids. Moreover, a hunk of iron could not have formed in the middle of interstellar s.p.a.ce, because the individual iron atoms of which it's made would have been scattered throughout the gas clouds that formed the planets, and gas clouds are mostly hydrogen and helium. To concentrate the iron atoms, a fluid body must first have differentiated.

BUT HOW DO solar system astronomers know that most main-belt asteroids are rocky? Or how do they know anything at all? The chief indicator is an asteroid's ability to reflect light, its albedo. Asteroids don't emit light of their own; they only absorb and reflect the Sun's rays. Does 1744 Harriet reflect or absorb infrared? What about visible light? Ultraviolet? Different materials absorb and reflect the various bands of light differently. If you're thoroughly familiar with the spectrum of sunlight (as astrophysicists are), and if you carefully observe the spectra of the sunlight reflected from an individual asteroid (as astrophysicists do), then you can figure out just how the original sunlight has been altered and thus identify the materials that comprise the asteroid's surface. And from the material, you can know how much light gets reflected. From that figure and from the distance, you can then estimate the asteroid's size. Ultimately you're trying to account for how bright an asteroid looks on the sky: it might be either really dull and big, or highly reflective and small, or something in between, and without knowing the composition, you can't know the answer simply by looking at how bright it is. solar system astronomers know that most main-belt asteroids are rocky? Or how do they know anything at all? The chief indicator is an asteroid's ability to reflect light, its albedo. Asteroids don't emit light of their own; they only absorb and reflect the Sun's rays. Does 1744 Harriet reflect or absorb infrared? What about visible light? Ultraviolet? Different materials absorb and reflect the various bands of light differently. If you're thoroughly familiar with the spectrum of sunlight (as astrophysicists are), and if you carefully observe the spectra of the sunlight reflected from an individual asteroid (as astrophysicists do), then you can figure out just how the original sunlight has been altered and thus identify the materials that comprise the asteroid's surface. And from the material, you can know how much light gets reflected. From that figure and from the distance, you can then estimate the asteroid's size. Ultimately you're trying to account for how bright an asteroid looks on the sky: it might be either really dull and big, or highly reflective and small, or something in between, and without knowing the composition, you can't know the answer simply by looking at how bright it is.

This method of spectral a.n.a.lysis led initially to a simplified three-way cla.s.sification scheme, with carbon-rich C-type asteroids, silicate-rich S-type asteroids, and metal-rich M-type asteroids. But higher precision measurements have since sp.a.w.ned an alphabet soup of a dozen cla.s.ses, each identifying an important nuance of the asteroid's composition and betraying multiple parent bodies rather than a single mother planet that had been smashed to smithereens.

If you know an asteroid's composition then you have some confidence that you know its density. Curiously, some measurements of the sizes of asteroids and their ma.s.ses yielded densities that were less than that of rock. One logical explanation was that those asteroids weren't solid. What else could be mixed in? Ice, perhaps? Not likely. The asteroid belt sits close enough to the Sun that any species of ice (water, ammonia, carbon dioxide)-all of whose density falls below that of rock-would have evaporated long ago due to the Sun's heat. Perhaps all that's mixed in is empty s.p.a.ce, with rocks and debris all moving in tandem.

The first bit of observational support for that hypothesis appeared in images of the 35-mile-long asteroid Ida, photographed by the s.p.a.ce probe Galileo Galileo during its flyby on August 28, 1993. Half a year later a speck was spotted about 60 miles from Ida's center that proved to be a mile-wide, pebble-shaped moon! Dubbed Dactyl, it was the first satellite ever seen orbiting an asteroid. Are satellites a rare thing? If an asteroid can have a satellite orbiting it, could it have two or ten or a hundred? In other words, could some asteroids turn out to be heaps of rocks? during its flyby on August 28, 1993. Half a year later a speck was spotted about 60 miles from Ida's center that proved to be a mile-wide, pebble-shaped moon! Dubbed Dactyl, it was the first satellite ever seen orbiting an asteroid. Are satellites a rare thing? If an asteroid can have a satellite orbiting it, could it have two or ten or a hundred? In other words, could some asteroids turn out to be heaps of rocks?

The answer is a resounding yes. Some astrophysicists would even say that these "rubble piles" as they are now officially named (astrophysicists once again preferred pith over polysyllabic prolixity) are probably common. One of the most extreme examples of the type may be Psyche, which measures about 150 miles in overall diameter and is reflective, suggesting its surface is metallic. From estimates of its overall density, however, its interior may well be more than 70 percent empty s.p.a.ce.

WHEN YOU STUDY objects that live somewhere other than the main asteroid belt, you're soon tangling with the rest of the solar system's vagabonds: Earth-crossing killer asteroids, comets, and myriad planetary moons. Comets are the s...o...b..a.l.l.s of the cosmos. Usually no more than a couple of miles across, they're composed of a mixture of frozen gases, frozen water, dust, and miscellaneous particles. In fact, they may simply be asteroids with a cloak of ice that never fully evaporated. The question of whether a given fragment is an asteroid or a comet might boil down to where it formed and where it's been. Before Newton published his objects that live somewhere other than the main asteroid belt, you're soon tangling with the rest of the solar system's vagabonds: Earth-crossing killer asteroids, comets, and myriad planetary moons. Comets are the s...o...b..a.l.l.s of the cosmos. Usually no more than a couple of miles across, they're composed of a mixture of frozen gases, frozen water, dust, and miscellaneous particles. In fact, they may simply be asteroids with a cloak of ice that never fully evaporated. The question of whether a given fragment is an asteroid or a comet might boil down to where it formed and where it's been. Before Newton published his Principia Principia in 1687, in which he laid out the universal laws of gravitation, no one had any idea that comets lived and traveled among the planets, making their rounds in and out of the solar system in highly elongated orbits. Icy fragments that formed in the far reaches of the solar system, whether in the Kuiper Belt or beyond, remain shrouded in ice and, if found on a characteristic elongated path toward the Sun, will show a rarefied but highly visible trail of water vapor and other volatile gases when it swings inside the orbit of Jupiter. Eventually, after enough visits to the inner solar system (could be hundreds or even thousands) such a comet can lose all its ice, ending up as bare rock. Indeed, some, if not all, the asteroids whose orbits cross that of Earth may be "spent" comets, whose solid core remains to haunt us. in 1687, in which he laid out the universal laws of gravitation, no one had any idea that comets lived and traveled among the planets, making their rounds in and out of the solar system in highly elongated orbits. Icy fragments that formed in the far reaches of the solar system, whether in the Kuiper Belt or beyond, remain shrouded in ice and, if found on a characteristic elongated path toward the Sun, will show a rarefied but highly visible trail of water vapor and other volatile gases when it swings inside the orbit of Jupiter. Eventually, after enough visits to the inner solar system (could be hundreds or even thousands) such a comet can lose all its ice, ending up as bare rock. Indeed, some, if not all, the asteroids whose orbits cross that of Earth may be "spent" comets, whose solid core remains to haunt us.

Then there are the meteorites, flying cosmic fragments that land on Earth. The fact that, like asteroids, most meteorites are made of rock and occasionally metal suggests strongly that the asteroid belt is their country of origin. To the planetary geologists who studied the growing number of known asteroids, it became clear that not all orbits hailed from the main asteroid belt.

As Hollywood loves to remind us, someday an asteroid (or comet) might collide with Earth, but that likelihood was not recognized as real until 1963, when the astrogeologist Eugene M. Shoemaker demonstrated conclusively that the vast 50,000-year-old Barringer Meteorite Crater near Winslow, Arizona, could have resulted only from a meteorite impact, and not from volcanism, or some other Earth-based geologic forces.

As we will see futher in Section 6, Shoemaker's discovery triggered a new wave of curiosity about the intersection of Earth's...o...b..t with that of the asteroids. In the 1990s, s.p.a.ce agencies began to track near-earth objects-comets and asteroids whose orbits, as NASA politely puts it, "allow them to enter Earth's neighborhood."

THE PLANET JUPITER plays a mighty role in the lives of the more distant asteroids and their brethren. A gravitational balancing act between Jupiter and the Sun has collected families of asteroids 60 degrees ahead of Jupiter in its solar orbit, and 60 degrees behind it, each making an equilateral triangle with Jupiter and the Sun. If you do the geometry, it places the asteroids 5.2 AU from both Jupiter and the Sun. These trapped bodies are known as the Trojan asteroids, and formally occupy what's called Lagrangian points in s.p.a.ce. As we will see in the next chapter, these regions act like tractor beams, holding fast to asteroids that drift their way. plays a mighty role in the lives of the more distant asteroids and their brethren. A gravitational balancing act between Jupiter and the Sun has collected families of asteroids 60 degrees ahead of Jupiter in its solar orbit, and 60 degrees behind it, each making an equilateral triangle with Jupiter and the Sun. If you do the geometry, it places the asteroids 5.2 AU from both Jupiter and the Sun. These trapped bodies are known as the Trojan asteroids, and formally occupy what's called Lagrangian points in s.p.a.ce. As we will see in the next chapter, these regions act like tractor beams, holding fast to asteroids that drift their way.

Jupiter also deflects plenty of comets that head toward Earth. Most comets live in the Kuiper Belt, beginning with and extending far beyond the orbit of Pluto. But any comet daring enough to pa.s.s close to Jupiter will get flung into a new direction. Were it not for Jupiter as guardian of the moat, Earth would have been pummeled by comets far more often than it has. In fact, the Oort Cloud, which is a vast population of comets in the extreme outer solar system, named for Jan Oort, the Danish astronomer who first proposed its existence, is widely thought to be composed of Kuiper Belt comets that Jupiter flung hither and yon. Indeed, the orbits of Oort Cloud comets extend halfway to the nearest stars.

What about the planetary moons? Some look like captured asteroids, such as Phobos and Deimos, the small, dim, potato-shaped moons of Mars. But Jupiter owns several icy moons. Should those be cla.s.sified as comets? And one of Pluto's moons, Charon, is not much smaller than Pluto itself. Meanwhile, both of them are icy. So perhaps they should be regarded instead as a double comet. I'm sure Pluto wouldn't mind that one either.

s.p.a.cECRAFT HAVE EXPLORED a dozen or so comets and asteroids. The first to do so was the car-sized robotic U.S. craft a dozen or so comets and asteroids. The first to do so was the car-sized robotic U.S. craft NEAR Shoemaker NEAR Shoemaker (NEAR is the clever acronym of Near Earth Asteroid Rendezvous), which visited the nearby asteroid Eros, not accidentally just before Valentine's Day in 2001. It touched down at just four miles an hour and, instruments intact, unexpectedly continued to send back data for two weeks after landing, enabling planetary geologists to say with some confidence that 21-mile-long Eros is an undifferentiated, consolidated object rather than a rubble pile. (NEAR is the clever acronym of Near Earth Asteroid Rendezvous), which visited the nearby asteroid Eros, not accidentally just before Valentine's Day in 2001. It touched down at just four miles an hour and, instruments intact, unexpectedly continued to send back data for two weeks after landing, enabling planetary geologists to say with some confidence that 21-mile-long Eros is an undifferentiated, consolidated object rather than a rubble pile.

Subsequent ambitious missions include Stardust, Stardust, which flew through the coma, or dust cloud, surrounding the nucleus of a comet so that it could capture a swarm of minuscule particles in its aerogel collector grid. The goal of the mission was, quite simply, to find out what kinds of s.p.a.ce dust are out there and to collect the particles without damaging them. To accomplish this, NASA used a wacky and wonderful substance called aerogel, the closest thing to a ghost that's ever been invented. It's a dried-out, spongelike tangle of silicon that's 99.8 percent thin air. When a particle slams in at hypersonic speeds, the particle bores its way in and gradually comes to a stop, intact. If you tried to stop the same dust grain with a catcher's mitt, or with practically anything else, the high-speed dust would slam into the surface and vaporize as it stopped abruptly. which flew through the coma, or dust cloud, surrounding the nucleus of a comet so that it could capture a swarm of minuscule particles in its aerogel collector grid. The goal of the mission was, quite simply, to find out what kinds of s.p.a.ce dust are out there and to collect the particles without damaging them. To accomplish this, NASA used a wacky and wonderful substance called aerogel, the closest thing to a ghost that's ever been invented. It's a dried-out, spongelike tangle of silicon that's 99.8 percent thin air. When a particle slams in at hypersonic speeds, the particle bores its way in and gradually comes to a stop, intact. If you tried to stop the same dust grain with a catcher's mitt, or with practically anything else, the high-speed dust would slam into the surface and vaporize as it stopped abruptly.

The European s.p.a.ce Agency is also out there exploring comets and asteroids. The Rosetta Rosetta s.p.a.cecraft, on a 12-year mission, will explore a single comet for two years, ama.s.sing more information at close range than ever before, and will then move on to take in a couple of asteroids in the main belt. s.p.a.cecraft, on a 12-year mission, will explore a single comet for two years, ama.s.sing more information at close range than ever before, and will then move on to take in a couple of asteroids in the main belt.

Each of these vagabond encounters seeks to gather highly specific information that may tell us about the formation and evolution of the solar system, about the kinds of objects that populate it, about the possibility that organic molecules were transferred to Earth during impacts, or about the size, shape, and solidity of near-earth objects. And, as always, deep understanding comes not from how well you describe an object, but from how that object connects with the larger body of acquired knowledge and its moving frontier. For the solar system, that moving frontier is the search for other solar systems. What scientists want next is a thorough comparison of what we and exosolar planets and vagabonds look like. Only in this way will we know whether our home life is normal or whether we live in a dysfunctional solar family.

NINE.

THE FIVE POINTS OF LAGRANGE.

The first manned s.p.a.cecraft ever to leave Earth's...o...b..t was Apollo 8 Apollo 8. This achievement remains one of the most remarkable, yet unheralded firsts of the twentieth century. When that moment arrived, the astronauts fired the third and final stage of their mighty Saturn V Saturn V rocket, rapidly thrusting the command module and its three occupants up to a speed of nearly seven miles per second. Half the energy to reach the Moon had been expended just to reach Earth's...o...b..t. rocket, rapidly thrusting the command module and its three occupants up to a speed of nearly seven miles per second. Half the energy to reach the Moon had been expended just to reach Earth's...o...b..t.

The engines were no longer necessary after the third stage fired, except for any midcourse tuning the trajectory might require to ensure the astronauts did not miss the Moon entirely. For 90 percent of its nearly quarter-million-mile journey, the command module gradually slowed as Earth's gravity continued to tug, but ever more weakly, in the opposite direction. Meanwhile, as the astronauts neared the Moon, the Moon's force of gravity grew stronger and stronger. A spot must therefore exist, en route, where the Moon's and Earth's opposing forces of gravity balance precisely. When the command module drifted across that point in s.p.a.ce, its speed increased once again as it accelerated toward the Moon.

If gravity were the only force to be reckoned, then this spot would be the only place in the Earth-Moon system where the opposing forces canceled each other out. But Earth and the Moon also orbit a common center of gravity, which resides about a thousand miles beneath Earth's surface, along an imaginary line connecting the centers of the Moon and Earth. When objects move in circles of any size and at any speed, they create a new force that pushes outward, away from the center of rotation. Your body feels this "centrifugal" force when you make a sharp turn in your car or when you survive amus.e.m.e.nt park attractions that turn in circles. In a cla.s.sic example of these nausea-inducing rides, you stand along the edge of a large circular platter, with your back against a perimeter wall. As the contraption spins up, rotating faster and faster, you feel a stronger and stronger force pinning you against the wall. At top speeds, you can barely move against the force. That's just when they drop the floor from beneath your feet and twist the thing sideways and upside down. When I rode one of these as a kid, the force was so great that I could barely move my fingers, they being stuck to the wall along with the rest of me.

If you actually got sick on such a ride, and turned your head to the side, the vomit would fly off at a tangent. Or it might get stuck to the wall. Worse yet, if you didn't turn your head, it might not make it out of your mouth due to the extreme centrifugal forces acting in the opposite direction. (Come to think of it, I haven't seen this particular ride anywhere lately. I wonder if they've been outlawed.) Centrifugal forces arise as the simple consequence of an object's tendency to travel in a straight line after being set in motion, and so are not true forces at all. But you can calculate with them as though they are. When you do, as did the brilliant eighteenth-century French mathematician Joseph-Louis Lagrange (17361813), you discover spots in the rotating Earth-Moon system where the gravity of Earth, the gravity of the Moon, and the centrifugal forces of the rotating system balance. These special locations are known as the points of Lagrange. And there are five of them.

The first point of Lagrange (affectionately called L1) falls between Earth and the Moon, slightly closer to Earth than the point of pure gravitational balance. Any object placed there can orbit the Earth-Moon center of gravity with the same monthly period as the Moon and will appear to be locked in place along the Earth-Moon line. Although all forces cancel there, this first Lagrangian point is a precarious equilibrium. If the object drifts sideways in any direction, the combined effect of the three forces will return it to its former position. But if the object drifts directly toward or away from Earth, ever so slightly, it will irreversibly fall either toward Earth or the Moon, like a barely balanced marble atop a steep hill, a hair's-width away from rolling down one side or the other.

The second and third Lagrangian points (L2 and L3) also lie on the Earth-Moon line, but this time L2 lies far beyond the far side of the Moon, while L3 lies far beyond Earth in the opposite direction. Once again, the three forces-Earth's gravity, the Moon's gravity, and the centrifugal force of the rotating system-cancel in concert. And once again, an object placed in either spot can orbit the Earth-Moon center of gravity with the same monthly period as the Moon.

The gravitational hilltops represented by L2 and L3 are much broader than the one represented at L1. So if you find yourself drifting down to Earth or the Moon, only a tiny investment in fuel will bring you right back to where you were.

While L1, L2, and L3 are respectable s.p.a.ce places, the award for best Lagrangian points must go to L4 and L5. One of them lives far off to the left of the Earth-Moon centerline while the other is far off to the right, each representing a vertex of an equilateral triangle, with Earth and Moon serving as the other vertices.

At L4 and L5, as with their first three siblings, all forces balance. But unlike the other Lagrangian points, which enjoy only unstable equilibrium, the equilibria at L4 and L5 are stable; no matter which direction you lean, no matter which direction you drift, the forces prevent you from leaning farther, as though you were in a valley surrounded by hills.

For each of the Lagrangian points, if your object is not located exactly where all forces cancel, then its position will oscillate around the point of balance in paths called librations. (Not to be confused with the particular spots on Earth's surface where one's mind oscillates from ingested libations.) These librations are equivalent to the back-and-forth rocking a ball would undergo after rolling down a hill and overshooting the bottom.

More than just orbital curiosities, L4 and L5 represent special places where one might build and establish s.p.a.ce colonies. All you need do is ship raw construction materials to the area (mined not only from Earth, but perhaps from the Moon or an asteroid), leave them there with no risk of drifting away, and return later with more supplies. After all the raw materials were collected in this zero-gravity environment, you could build an enormous s.p.a.ce station-tens of miles across-with very little stress on the construction materials. And by rotating the station, the induced centrifugal forces could simulate gravity for its hundreds (or thousands) of residents. The s.p.a.ce enthusiasts Keith and Carolyn Henson founded the "L5 Society" in August 1975 for just that purpose, although the society is best remembered for its resonance with the ideas of Princeton physics professor and s.p.a.ce visionary Gerard K. O'Neill, who promoted s.p.a.ce habitation in his writings such as the 1976 cla.s.sic The High Frontier: Human Colonies in s.p.a.ce The High Frontier: Human Colonies in s.p.a.ce. The L5 Society was founded on one guiding principle: "to disband the Society in a ma.s.s meeting at L5," presumably inside a s.p.a.ce habitat, thereby declaring "mission accomplished." In April 1987, the L5 Society merged with the National s.p.a.ce Inst.i.tute to become the National s.p.a.ce Society, which continues today.

The idea of locating a large structure at libration points appeared as early as 1961 in Arthur C. Clarke's novel A Fall of Moondust A Fall of Moondust. Clarke was no stranger to special orbits. In 1945, he was the first to calculate, in a four-page, hand-typed memorandum, the location above Earth's surface where a satellite's period exactly matches the 24-hour rotation period of Earth. A satellite with that orbit would appear to "hover" over Earth's surface and serve as an ideal relay station for radio communications from one nation to another. Today, hundreds of communication satellites do just that.

Where is this magical place? It's not low Earth orbit. Occupants there, such as the Hubble s.p.a.ce Telescope Hubble s.p.a.ce Telescope and the and the International s.p.a.ce Station International s.p.a.ce Station, take about 90 minutes to circle Earth. Meanwhile, objects at the distance of the Moon take about a month. Logically, an intermediate distance must exist where an orbit of 24 hours can be sustained. That happens to lie 22,300 miles above Earth's surface.

ACTUALLY, THERE IS NOTHING unique about the rotating Earth-Moon system. Another set of five Lagrangian points exist for the rotating Sun-Earth system. The Sun-Earth L2 point in particular has become the darling of astrophysics satellites. The Sun-Earth Lagrangian points all orbit the Sun-Earth center of gravity once per Earth year. At a million miles from Earth, in the direction opposite that of the Sun, a telescope at L2 earns 24 hours of continuous view of the entire night sky because Earth has shrunk to insignificance. Conversely, from low Earth orbit, the location of the unique about the rotating Earth-Moon system. Another set of five Lagrangian points exist for the rotating Sun-Earth system. The Sun-Earth L2 point in particular has become the darling of astrophysics satellites. The Sun-Earth Lagrangian points all orbit the Sun-Earth center of gravity once per Earth year. At a million miles from Earth, in the direction opposite that of the Sun, a telescope at L2 earns 24 hours of continuous view of the entire night sky because Earth has shrunk to insignificance. Conversely, from low Earth orbit, the location of the Hubble Hubble telescope, Earth is so close and so big in the sky, that it blocks nearly half the total field of view. The telescope, Earth is so close and so big in the sky, that it blocks nearly half the total field of view. The Wilkinson Microwave Anisotropy Probe Wilkinson Microwave Anisotropy Probe (named for the late Princeton physicist David Wilkinson, a collaborator on the project) reached L2 for the Sun-Earth system in 2002, and has been busily taking data for several years on the cosmic microwave background-the omnipresent signature of the big bang itself. The hilltop for the Sun-Earth L2 region in s.p.a.ce is even broader and flatter than that for the Earth-Moon L2. By saving only 10 percent of its total fuel, the s.p.a.ce probe has enough to hang around this point of unstable equilibrium for nearly a century. (named for the late Princeton physicist David Wilkinson, a collaborator on the project) reached L2 for the Sun-Earth system in 2002, and has been busily taking data for several years on the cosmic microwave background-the omnipresent signature of the big bang itself. The hilltop for the Sun-Earth L2 region in s.p.a.ce is even broader and flatter than that for the Earth-Moon L2. By saving only 10 percent of its total fuel, the s.p.a.ce probe has enough to hang around this point of unstable equilibrium for nearly a century.

The James Webb Telescope James Webb Telescope, named for a former head of NASA from the 1960s, is now being planned by NASA as the follow-on to the Hubble Hubble. It too will live and work at the Sun-Earth L2 point. Even after it arrives, plenty of room will remain-tens of thousands of square miles-for more satellites to come.

Another Lagrangian-loving NASA satellite, known as Genesis Genesis, librates around the Sun-Earth L1 point. In this case, L1 lies a million miles toward the Sun. For two and a half years, Genesis Genesis faced the Sun and collected pristine solar matter, including atomic and molecular particles from the solar wind. The material was then returned to Earth via a midair recovery over Utah and studied for its composition, just like the sample return of the faced the Sun and collected pristine solar matter, including atomic and molecular particles from the solar wind. The material was then returned to Earth via a midair recovery over Utah and studied for its composition, just like the sample return of the Stardust Stardust mission, which had collected comet dust. mission, which had collected comet dust. Genesis Genesis will provide a window to the contents of the original solar nebula from which the Sun and planets formed. After leaving L1, the returned sample did a loop-the-loop around L2 and positioned its trajectory before it returned to Earth. will provide a window to the contents of the original solar nebula from which the Sun and planets formed. After leaving L1, the returned sample did a loop-the-loop around L2 and positioned its trajectory before it returned to Earth.

Given that L4 and L5 are stable points of equilibrium, one might suppose that s.p.a.ce junk would acc.u.mulate near them, making it quite hazardous to conduct business there. Lagrange, in fact, predicted that s.p.a.ce debris would be found at L4 and L5 for the gravitationally powerful Sun-Jupiter system. A century later, in 1905, the first of the "Trojan" family of asteroids was discovered. We now know that for L4 and L5 of the Sun-Jupiter system, thousands of asteroids lead and follow Jupiter around the Sun, with periods that equal that of Jupiter's. Behaving for all the world as though they were responding to tractor beams, these asteroids are eternally tethered by the gravitational and centrifugal forces of the Sun-Jupiter system. Of course, we expect s.p.a.ce junk to acc.u.mulate at L4 and L5 of the Sun-Earth system as well as the Earth-Moon system. It does. But not nearly to the extent of the Sun-Jupiter encounter.

As an important side benefit, interplanetary trajectories that begin at Lagrangian points require very little fuel to reach other Lagrangian points or even other planets. Unlike a launch from a planet's surface, where most of your fuel goes to lift you off the ground, launching from a Lagrangian point would resemble a ship leaving dry dock, gently cast adrift into the ocean with only a minimal investment of fuel. In modern times, instead of thinking about self-sustained Lagrangian colonies of people and farms, we can think of Lagrangian points as gateways to the rest of solar system. From the Sun-Earth Lagrangian points you are halfway to Mars; not in distance or time but in the all-important category of fuel consumption.

In one version of our s.p.a.ce-faring future, imagine fuel stations at every Lagrangian point in the solar system, where travelers fill up their rocket gas tanks en route to visit friends and relatives elsewhere among the planets. This travel model, however futuristic it reads, is not entirely far-fetched. Note that without fueling stations scattered liberally across the United States, your automobile would require the proportions of the Saturn V Saturn V rocket to drive coast to coast: most of your vehicle's size and ma.s.s would be fuel, used primarily to transport the yet-to-be-consumed fuel during your cross-country trip. We don't travel this way on Earth. Perhaps the time is overdue when we no longer travel that way through s.p.a.ce. rocket to drive coast to coast: most of your vehicle's size and ma.s.s would be fuel, used primarily to transport the yet-to-be-consumed fuel during your cross-country trip. We don't travel this way on Earth. Perhaps the time is overdue when we no longer travel that way through s.p.a.ce.

TEN.