Category Dreams of Other Worlds

LIVING WITH A RESTLESS STAR

Imagine you woke up one morning and your planet had been engulfed by the atmosphere of a star. High-energy particles slam into the atmo­sphere, creating shimmering auroras. Sunspots flicker, each one re­leasing more energy than the largest atomic bomb. Great loops of hot gas uncoil from the star’s surface, extending millions of miles. Each whip crack of activity causes mayhem on orbiting satellites, frying their circuit boards and wreaking havoc on their guidance systems. Your planet is assailed by high-energy particles traveling at nearly the speed of light. Looking at the star, its emission doesn’t change detectably from day to day or year to year, but its invisible short wavelength radiation fluctuates wildly and unpredictably.

Where is this place? It’s the Earth, and the star is the Sun. This activity isn’t because the Sun has run out of nuclear fuel and has turned into a red giant; our star is in the dull middle age of its life. But the clean, crisp edge the Sun seems to have in the daytime sky is an illusion. That curved surface—the edge of the photosphere— corresponds to a place where the gas of the Sun has thinned out to the point where photons no longer careen into particles and they can travel freely.1 From that point they travel in straight lines with­out interruption, streaking to the Earth in eight minutes. Inside the photosphere, which is a slender sheath thinner than the skin on an apple compared to its radius, the Sun is opaque and our view is blocked. We see the boundary layer as having a sharp edge, just like a cloud has a well-defined edge.2

The edge is an illusion because there’s no physical surface or barrier. If you could penetrate the Sun in a spaceship, you’d feel no bump. The material of the Sun extends much further into space than its visible edge. In its physical quantities of temperature, pres­sure, and density, the Sun varies continuously and smoothly from its fusion core outward. It possesses a surprising multi-million – degree corona, the influence of which extends past the Earth’s orbit almost to the edge of the Solar System. Invisible forms of high-energy radiation and subatomic particles—ultraviolet waves, X-rays, gamma rays, and relativistic cosmic rays—streak toward us and past us at the speed of light. In a very real sense, we live “inside” the atmosphere of an active star, with profound and sur­prising consequences for life on Earth.

Hipparcos Touches All of Astronomy

Astrometry may be the “Cinderella” of modern astronomy, but astronomers in all fields are continually reminded that everything starts with mapping brightness and position. Hipparcos leveraged historical measurements by providing the most accurate reference frame.46 With the invention of the photographic plate in the mid­nineteenth century, comparing photographs of star positions from different eras could in principle reveal star motions, but it’s usually unclear which set of plates has the largest errors. With Hipparcos as the rock-steady “gold standard,” astronomers gleaned new in­sights from century-old data.

The hundred or more observations that the satellite made of each star allowed it to detect variability. Over 12,000 variable stars were found in the database, about 10 percent of all the stars studied, two-thirds of which were previously unknown.47 The observation of the variables was coordinated with a network of amateur astronomers, who filled in with data when a star was tem­porarily out of the satellite’s viewing zone. Hipparcos was also able to resolve or distinguish over 24,000 double and multiple star systems.48 Binary stars were actually a headache for the science team, because they mimicked problems in the photometry and a faint companion could throw off the position of the brighter star in a pair if the two images were not well separated.

A sampling of projects will give a sense of the dizzying range of investigations enabled by the Hipparcos data. Galactic archaeol­ogy is a good example. Hipparcos data showed that some of the stars in the neighborhood of the Sun are part of a disk that’s ten times thicker than the disk where most of the Milky Way’s star formation takes place.49 Differences in the heavy element abun­dance in the two components are consistent with a model where the Milky Way was assembled from smaller galaxies over billions of years.50 Ten percent of the stars in the spherical halo as well as some in the “thick” disk seem to come from a single “invader” gal­axy that was disrupted soon after the Milky Way formed. Also, the fine view of stellar motions provided by Hipparcos allows astrono­mers to turn back the clock and trace the Sun’s passage around the galaxy and in and out of the galactic disk over the past 500 million years. During that time the Sun has passed through spiral arms four times, each corresponding to an extended cold spell in the climate history of the Earth. It is speculated that exposure to high cosmic ray flux in the spiral arms leads to more cloud cover and longer Ice Ages.51

Hipparcos data were used to show that the dim companions in some stellar systems are brown dwarfs. These elusive objects are gas balls less than 8 percent of the mass of the Sun; too cool to shine by nuclear fusion, they emit a feeble infrared glow and slowly contract as they leak their energy into space. In 1991, a star being observed by Hipparcos dimmed slightly on five occasions due to the shadow of a giant planet passing in front of it. This was four years before Mayor and Quleoz stunned the world with their discovery of the first planet beyond the Solar System. But nobody was looking for such signals in the Hipparcos data, so the eclipses remained undetected until 1999.52 Since then additional exoplan­ets have been dug out of the database.

Hipparcos also produced a beautiful confirmation of general relativity (we earlier described a test of relativity by Cassini). Ein­stein’s theory states that mass bends light, and it was first con­firmed in 1919 by observations of the deflection of starlight as it grazed the limb of the Sun while observed during an eclipse. Gen­eral relativistic bending is 1.7 arc seconds at the limb of the Sun, and it declines with the projected distance from the Sun’s gravity but is still a detectable 0.004 arc seconds at right angles to the sight line toward the Sun.53 This subtle measurement shows that the paradigm of curved space applies everywhere. The curvature is so slight that it doesn’t negate the use of Euclidean triangles to measure distances. To test general relativity, while firming up mea­surements of the size and expansion rate of the universe, is quite an achievement for a small and often-overlooked space mission.

THE LITTLE ROVERS THAT COULD

The essay is short and very simple; the words are almost heartbreaking: “I used to live in an orphanage. It was dark and cold and lonely. At night, I looked up at the sparkly sky and felt better. I dreamed I could fly there. In America, I can make all my dreams come true. . . . Thank you for the Spirit and the Opportunity.”1 Sofi Collis was abandoned at birth into a Siberian orphanage and brought by her adoptive parents to live in Scottsdale, Arizona. In 2003, at age nine, she was writing in response to a call from NASA for names for its upcoming Mars rovers. A team of judges selected by the Lego Company and the nonprofit Planetary Society painstakingly whit­tled 10,000 entries down to thirty-three, and NASA made the final selection. Sofi unveiled the names at a pre-launch press conference hosted by Sean O’Keefe, NASA’s administrator, who noted that her story twinned the two original spacefaring countries. It’s safe to say her dreams are as boundless as space itself.

Six years later, in language that was formal, cumbersome, and numbingly prosaic relative to the lofty sentiment it conveyed, the U. S. House of Representatives gave a formal nod to these remark­able robotic emissaries with the following resolution, number 67 from the first session of the 111th session of Congress, adopted unanimously:

Whereas the Mars Exploration Rovers Spirit and Opportunity suc­cessfully landed on Mars on January 3, 2004, and January 24, 2004, respectively, on missions to search for evidence indicating that Mars

once held conditions hospitable to life; whereas NASA’s Jet Propul­sion Laboratory (JPL), managed by the California Institute of Technol­ogy (Caltech), designed and built the Rovers, Spirit and Opportunity; whereas Cornell University led the development of advanced scien­tific instruments carried by the 2 Rovers, and continues to play a leading role in the operation of the 2 Rovers and the processing and analysis of the images and other data sent back to Earth; whereas the Rovers relayed over a quarter million images taken from the surface of Mars; whereas studies conducted by the Rovers have indicated that early Mars was characterized by impacts, explosive volcanoes, and subsurface water; whereas each Rover has discovered geologi­cal evidence of ancient Martian environments where habitable condi­tions may have existed; whereas the Rovers have explored over 21 kilometers of Martian terrain, climbed Martian hills, descended deep into large craters, survived dust storms, and endured three cold, dark Martian winters; and whereas Spirit and Opportunity will have passed 5 years of successful operation on the surface of Mars on January 3, 2009, and January 24, 2009, respectively, far exceeding the original 90-Martian day mission requirement by a factor of 20, and are con­tinuing their missions of surface exploration and scientific discovery: Now therefore be it resolved, that the House of Representatives com­mends the engineers, scientists, and technicians of the Jet Propul­sion Laboratory and Cornell University for their successful execution and continued operation of the Mars Exploration Rovers, Spirit and Opportunity; and recognizes the success and significant scientific contributions of NASA’s Mars Exploration Rovers.2

A Family Portrait

The Voyagers took a part of the Solar System that had been studied briefly and in little detail, and fleshed it out into a family portrait of four giant planets, their ring systems and magnetic fields, plus forty-eight of their moons. The additions and revisions have been enough to cause textbooks on astronomy to be rewritten.13 Let’s see what was learned about major members of the family.

First up was Jupiter. This mighty gas giant is three times more massive than any other planet and 320 times more massive than the Earth (figure 4.2). The Voyagers reached Jupiter in 1979, with a separation of four months. Even after centuries of telescopic ob­servations, there were surprises. The Great Red Spot was revealed to be a huge anticyclone, large enough to swallow the Earth, with eddies and smaller storms around its periphery. It had changed color from orange to dark brown in the six years since the Pio­neer flybys. As they passed behind the planet the Voyagers saw lightning illuminating the darkness of the night-side atmosphere.14 Voyager 1 discovered a very faint ring system around Jupiter made of dust ejected from the inner moons after high-velocity impacts.15 The rings are far less dramatic than Saturn’s but are equally inter­esting scientifically. The main ring circles from 122,000 to 129,000 kilometers away from the center of the planet.

Voyager 1 discovered two substantial new moons of Jupiter: Thebe and Metis. Both are irregular in shape; Thebe is 70 miles in its largest dimension and Metis is only 35 miles long.16 Voyager 2 got into the act by discovering Adrastea, which is no bigger than a small town, 15 miles across. However, the real excitement came from Io, Jupiter’s closest moon and the fourth largest moon in the Solar System. This strange-looking rock, with its mottled yellow-

A Family Portrait

Figure 4.2. The Earth and Jupiter compared. The biggest atmospheric phenom­ena on Jupiter, like the Great Red Spot pictured here, rival the Earth in size and persist for centuries. The Voyagers provided new and unprecedented detail on the atmospheric properties of all four gas giants, and indirect evidence that they possess rocky cores (NASA Planetary Photojournal).

brown surface, looking like a moldy orange, is the most geolog­ically active world in the Solar System. The Voyagers saw nine erupting volcanoes between them, marking the first time active volcanism had been seen anywhere other than the Earth. Plumes shoot out of the volcanoes at up to 2,000 mph and rise 300 miles above Io’s surface.17 There are more than 400 volcanoes dotting the moon’s surface, not all of which are active.

Why is Io so lively? Normally, moons are geologically dead be­cause there’s not enough radioactive heating from their interior rocks to drive tectonic activity. But Io is in a gravitational “tug of war” with nearby Jupiter and the other three Galilean moons: Ganymede, Callisto, and Europa. This incessant pulling and push­ing heats up the interior of the moon, like a racquetball heats up when it’s flexed repeatedly. Io is distorted by the stretching force of Jupiter’s gravity, and this departure from a sphere is called a tidal bulge. It bulges by up to 300 feet, which is enormous compared to the roughly one-foot stretching of the Earth’s solid mass by the Moon. Sulfur and oxygen atoms ejected by the volcanoes create a torus of plasma around Jupiter, and these ionized atoms have been detected millions of miles away, right to the edge of Jupiter’s mag­netosphere. Lava flows episodically paint the surface red, orange, and yellow, and fizz off enough material to coat the entire surface with an inch of sulfur every year.

Galileo’s other “children of Jupiter” turned out to have distinct personalities as well (plate 5). Ganymede is the largest moon in the Solar System, even larger than Mercury and more than twice as big as Pluto, which has seemed poor and misbegotten since astrono­mers demoted it to the status of a dwarf planet in 2006. Gany­mede has terrain that’s partially cratered and partially grooved, which is thought to indicate tectonic processes. Rocks mixed with ice comprise the top layer. More recent observations point to a liquid ocean under the icy crust.18 The Voyagers saw huge impact craters on Callisto, with a couple indicating impacts almost large enough to blast the moon into fragments. However, the craters were strangely smooth, with almost no topographic relief, indicat­ing that water ice at one point had flowed over them and filled them in. Europa attracted keen attention from the mission sci­entists when the first images came back. Low-resolution pictures from Voyager 1 showed linear features crisscrossing the surface. Higher resolution pictures from Voyager 2 increased the puzzle be­cause the features were so flat they couldn’t have been created by the familiar terrestrial process of slabs of crust sliding and collid­ing with each other. The only plausible explanation was that they were ice floes. Other observations pointed to a liquid ocean tens of kilometers deep under the few kilometers of ice. Europa vaulted into its current position as one of the most compelling targets for a future lander.19

The Voyager flybys of Saturn took place in 1980 and 1981. They got four times closer to Saturn’s cloud-tops than they did to

Jupiter’s upper atmosphere.20 Saturn’s magnificent rings were the source of many puzzles and a few surprises. Viewed up close, the rings had incredibly detailed structure. Not only were there con­centric rings and gaps ranging from large and fuzzy to razor-sharp, but the cameras revealed radial spokes, kinks, and delicate braids. Time-lapse photography proved that some of these features came and went. The rings were made of mixed icy and rocky particles ranging from microscopic to house-sized, shepherded by Saturn’s many small and irregularly shaped moons.

The rings of Saturn and the other gas giants are the result of an amazingly subtle gravitational dance. With no choreographer other than Newton’s Law of Gravity, a disk of rocky and dusty material can naturally develop very complex structure. A reso­nance occurs for any pair of orbits where the periods are related by two small integers. In that case the two objects have a boosted interaction that can cause them to rearrange their position, cause one to be ejected from the system, or cause either one to clear out small particles in a ring at a particular radius. Some resonances are stable, such as the orbits of Jupiter’s moons Ganymede, Europa, and Io, which have orbital periods related by the ratio 1:2:4. Think of a child on a swing; you can sustain or increase their motion not only by pushing them once per cycle of their motion, but also every second cycle or every third cycle, and so on. Although the principles of orbital resonance are understood, not all the subtle features in Saturn’s rings have yet been explained. Voyager also earmarked Saturn’s large moon Titan as a place to return to. Titan has a thick atmosphere of nitrogen and was inferred to have bod­ies of liquid ethane and methane on its surface. As we’ll see in the next chapter, Cassini did give Titan the attention it deserved twenty years later.

Very little was known about dark and shadowy family members Uranus and Neptune before Voyager. The highlights of Voyager 2’s solo flybys of the two outermost planets included the discovery that the magnetic fields are tilted far from their rotation axes. The new data trebled the number of moons of Uranus from 5 to 15 (27 are now known, most named after characters in the plays of Wil­liam Shakespeare and a few named after characters in Alexander Pope’s poem “The Rape of the Lock”), and it doubled the num­ber of moons of Neptune from three to seven (thirteen are now known, named after Greek and Roman water gods). Miranda, the innermost of Uranus’s five large moons, is a bizarre object. Even though it’s only 300 miles in diameter, it has huge canyons and terraces ten miles high and mixed young and old surfaces. Voyager scientists thought it might have been the pieces of a smashed moon that came back together, but now it’s thought that Miranda’s to­pography arose from tidal heating at a time when it was in a much more eccentric orbit than it is now. Neptune rounds out the fam­ily portrait. This frigid and gloomy planet has howling winds of 1,200 mph and a Great Dark Spot similar in size to Jupiter’s Great Red Spot.21 Its large moon Triton might have been kept molten for a billion years after its capture by Neptune; geysers on its surface spew soot and nitrogen gas into its sparse atmosphere. Triton is the coldest place in the Solar System: -391 °F, a temperature at which the air we breathe would freeze solid.

What Cassini Discovered

Cassini has rewritten the book on Saturn, making many scientific discoveries; the first six years of study were summarized in 2010 in two papers in the journal Science.19 Two of the greatest advances were conceptual. The first was to paint a sharply etched portrait of a gas giant planet and its moons as intriguing worlds with “per­sonalities” and quirks that made them noteworthy. Voyager paved the way, but Cassini got closer to its targets and spent a larger amount of time in their neighborhood. Another realization was the plausibility of the outer Solar System for biology. Although far from the Sun’s warming rays, the largest of Saturn’s moons get internal energy from radioactive decay of their rocky material, and many of the moons get extra heat by being tidally “squeezed” by the gravity of the massive planet. If life’s minimum requirements are energy, liquid water, and organic material, those conditions may be met on half a dozen of the moons of Saturn. This animates the search for life elsewhere in the universe, since giant planets with attendant moon systems are expected to be commonplace.

Not all scientific data are equally digestible to public audiences. Spectra or measures of magnetic field strengths and charged par­ticle fluxes are abstract and esoteric without a lot of background information and context. So the general awareness of Cassini has been based on its imaging. In that respect the Imaging Science Sub­system is preeminent, and ISS team leader Carolyn Porco has been an eloquent spokesperson for why we should care about the outer planets and their moons. The portraits made of the Saturn system, best seen at a glance in a poster mosaic of sixty-four scenes from Saturn on the ciclops. org website, are an eloquent testimony to the interplay of shadows and light in the realm where there’s no air to diffuse and scatter light. The shadows of moons fall on rings, or on each other, and shallow-angled light casts deep shadows on the hills and pock-marked surfaces of the moons. The chiaroscuro is worthy of Caravaggio. In fact, there’s an unbroken lineage in the natural depiction of strong light and shadow that connects Leon­ardo and Galileo’s sketches of the Moon, Bonestell’s paintings, and the actual images from Cassini.

The science started flowing long before Cassini got to Saturn. As it passed Jupiter to get a gravity assist, the spacecraft took 26,000 pictures and studied the circulation patterns that produce counter­rotating atmospheric bands on the giant planet. It also provided evidence that Jupiter’s faint ring originates from micro-meteorite impacts on the smaller moons.30 Cassini gave a nod to Albert Ein­stein with a new test of his general theory of relativity. Gravity very slightly bends and slows down light and any other form of radiation passing a massive object. Cassini sent radio signals past the Sun to the Earth so this delay could be measured. The results agreed with the predictions of general relativity theory with a pre­cision of one part in 50,000, improving on the precision of previ­ous tests by a factor of fifty.31

In 2004, Cassini discovered three new moons of Saturn, bring­ing the total to sixty-one. They’re small, between three and four kilometers across, like free-floating mountains in space. A year later it found a slightly larger moon in one of the gaps in the rings. The moons and rings engage in a complex gravitational waltz. Some gaps in the rings are caused by a moon clearing out particles at that distance, but others are caused by a more distant moon driving a resonance. If orbits have periods that are related by the ratio of two small integers, then the outer object can influence the inner one, and in the case of an outer moon, it clears out a gap in the rings, as described in the last chapter. Since there are many moons, there are many ratios that can cause resonance. Harmonic effects like this cause much of the complexity of the ring system.32

They orbit silently in the vacuum of space, but ring and moon systems have the timbre of a beautiful musical instrument, each one unique.

Cassini’s arrival at Saturn in July 2004 involved a daring and risky maneuver. It shot the gap between the F and G rings, equiva­lent to threading the eye of a needle. The High-Gain Antenna had to be pivoted away from Earth and along the flight path to protect it from hits by small particles, and then the rockets were used for a very precise deceleration that allowed Saturn to capture the space­craft. At its closest approach it skimmed just 13,000 miles above Saturn’s cloud tops. Imagine the majestic view if humans had been along for the ride, watching the towering cloudscapes of a planet so big 760 Earths would fit inside. Cassini provided fascinating new details on the ring system.33 To a casual observer it seems miraculous that such subtle and complex patterns could arise by unguided natural processes, but the gravitational mechanisms be­hind the rings have been known for decades. Cassini measured the size distribution of ring particles more accurately than before, and its data showed how ring particles can join into loose aggregations or “piles of rubble.” And, it found instances of moons stealing par­ticles from rings as well as times when moons eject particles. New data showed that the rings are 90 percent water, so they’re most similar to a huge bumper car ride with careening chunks of ice of all sizes—from microscopic to the size of a house. Surprisingly, when seen up close the rings are tinged red, which scientists specu­late is due to rust or small organic molecules mixed in with the water ice (plate 7). To the connoisseur of gravitational dynamics, there’s a life’s work in understanding the shifting spokes, the spiral density waves, the embedded moonlets, and the features that look like waves and straw and rope.34

Much of the time thereafter has been spent planning and execut­ing flybys. An early passage near Phoebe was the only one that will be possible for this curious little moon. Phoebe is one of the out­ermost moons and only 150 miles across, but parts of its heavily cratered surface appear very bright and scientists think there’s ice under the surface. The lion’s share of the flybys have been used to explore Titan. Titan is the second largest moon in the Solar System.

Only Jupiter’s Ganymede is larger. Titan is 50 percent larger than the Earth’s Moon and nearly twice the mass. It is unprecedented in having an atmosphere thicker than Earth, made of the same pri­mary ingredient, nitrogen. Setting aside episodic lava flows on Io, it’s the only object beyond Earth where stable bodies of liquid have been seen on the surface. Cassini has used radar to penetrate the thick murk of the atmosphere and reveal surface details as small as a kilometer.35

Titan’s orange haze is naturally produced photochemical smog. Methane and ethane are mixed in with the nitrogen and they form clouds and rain, which falls on the surface.36 There’s weather here but it’s a completely alien chemical environment. Other trace in­gredients include propane, acetylene, argon, and hydrogen cyanide. Add some oxygen and you’d have the recipe for a real conflagra­tion, and it’s not anything humans would want to breathe. Hydro­carbons break up and recombine in the upper atmosphere under the action of sunlight. The methane present would be converted into more complex molecules in only 50 million years, suggesting that it must be replenished from Titan’s interior. Robert Zubrin pointed out that the base of Titan’s atmosphere is so dense and its gravity is so gentle that astronauts could potentially fly by wearing powered wings attached to their arms.37

The real prize on Titan is not the chemical haze in the atmo­sphere but the glittering liquid on the surface. In late 2009, NASA released a gorgeous picture of the northern polar region, back-lit by the Sun, with a glint of specular reflection from a body of liq – uid.38 Other images show that the liquid levels do not vary by more than 3 millimeters—there’s little surface wind on Titan. The north and south polar regions are dotted with lakes varying from a mile across to larger than the largest lake on Earth. The main ingredi­ents of the lakes are likely to be methane and ethane, with smaller contributions from ammonia and water.39 Evaporation from the lakes is not enough to supply the methane seen in the atmosphere, implying even larger reservoirs of liquid methane underground. Titan is literally swimming in organic materials. There’s hundreds of times more mass of liquid hydrocarbons on Titan than the sum of all the oil and gas reserves on Earth.

Living with a Star

Humans have always been aware that the Sun’s rays sustain life on Earth. But until the invention of the telescope, the Sun was thought to be smooth and pristine. There’s some evidence of naked-eye ob­servations of sunspots in China as early as 28 BC, and possibly sev­eral centuries earlier.3 Aristotle’s strong supposition that the heav­ens were perfect meant that sunspots probably went undiscovered in Europe for nearly two thousand years. Galileo saw sunspots in 1610 and noticed that they rotated with the Sun. Jesuit astronomer Christoph Scheiner also observed them soon after, and he believed them to be orbiting satellites.4 But by 1630, Scheiner had aban­doned his Aristotelian view and accepted that sunspots rotated at different rates, faster at the equator and slower at the poles. This proved that the Sun couldn’t be a solid object but was more prob­ably composed of gas or liquid. Through the seventeenth century, the number of sunspots declined almost to zero, giving a first hint of the Sun’s effect on our weather, as a period of low sunspot num­bers called the Maunder minimum corresponded to decades when Europe went into a deep freeze, with southern England ice-bound and people in Holland skating on the frozen canals all summer. It wasn’t until the 1840s that Heinrich Schwabe discovered that sun­spot numbers varied with an eleven-year cycle (figure 7.1).

Подпись: Sunspot number

Figure 7.1. Sunspots have been systematically counted since the invention of the telescope. There was an extended “Maunder minimum” of sunspot numbers soon thereafter, which corresponded to a period of very cold climate in northern Europe. This was an early indication of the possible connection between Sun variation and climate (NASA/David Hathaway).

 

Living with a Star

Around the same time that Galileo was doing his pioneering work with the telescope, William Gilbert recognized that the Earth had a magnetic field similar to the dipole field of magnetized rocks called lodestones. Over the next several hundred years, scientists explored the Earth’s invisible magnetic field and showed that it fluctuated in ways that seemed to depend on the Sun. In 1777, Jo­hann Wilcke noticed that auroral rays in the polar regions lay along the field lines of Gilbert’s dipole. Then, in 1859, two scientists ob­served a white light flare on the Sun, followed two days later by a geomagnetic storm on the Earth.5 This once-in-a-millennium event caused aurorae all around the world, including places that had never experienced such a phenomenon in human memory, such as the Caribbean. The northern lights were so bright in the North American Rockies that miners ate breakfast, convinced that the day had started. Telegraph systems failed worldwide and operators reported that they received severe shocks by sparks leaping from the equipment.

Coincidences like this were provocative, but they didn’t amount to an explanation. In 1896, Norwegian scientist Kristian Birkeland devised an elegant experiment where the Sun was represented by a cathode and the Earth by a spherical, magnetized anode. With both situated in a large vacuum chamber, he watched as the appa­
ratus behaved like a miniature Earth-Sun system, with an electrical discharge like an aurora occurring at high geomagnetic latitudes. Not long afterward, George Ellery Hale built the first instrument to study solar flares in detail, on Mount Wilson in California, and he confirmed the two-day lag between flares and geomagnetic storms on Earth. Hale proved that sunspots were magnetized—the first detection of magnetic fields beyond the Earth. He also showed that the polarity of sunspot pairs is the same within a cycle, but reverses from one sunspot cycle to the next.6

Many researchers have looked for correlations between sun­spots and weather on the Earth. Sunspots are actually cooler than the area around them, because the tight bundle of magnetic field lines that they contain restricts the convection that carries heat up from the solar interior. How, therefore, could a sunspot minimum correspond to colder temperatures on the Earth? In part it’s be­cause the area around a sunspot is brighter than an average patch of the Sun’s surface (figure 7.2). So the Sun is brighter when there are more sunspots. But that still leaves a mystery. When scientists developed digital detectors in the 1970s, they found that the full variation of the Sun’s brightness over a sunspot cycle was only one part in a thousand, or 0.1 percent, not nearly big enough to explain significant temperature variations on Earth. The Maunder Minimum aside, there’s no convincing evidence that the sunspot cycle drives non-s easonal climate change, especially the rise in global temperatures over the past half century.7

However, the variations of the Sun in invisible forms of high – energy radiation are much more extreme than its variation in vis­ible light. Moreover, space isn’t the simple and absolute vacuum that most scientists once imagined. Fifty years ago it was shown that the Sun is surrounded by a vast region of plasma, a diffuse, magnetized gas so hot that the particles are traveling near light speed. The stream of particles emerging from the Sun—the solar wind—travels millions of miles to us, where it hits and distorts the Earth’s magnetic field.8 Even though the nature of the ef­fects is complex and subtle, the Sun and Earth are one connected system.

Living with a Star

Figure 7.2. The cells and granulation seen in this image of the surface of the Sun reflect seething, turbulent motions of high temperature plasma. Sunspots occur in pairs and are slightly cooler areas where the photosphere is threaded by an organized magnetic field and high-energy particles and energy can escape. This one is about the size of the Earth (NASA/SOHO).

Passing the Baton to Gaia

In a sense, not much has changed since ancient humans first looked toward the skies. Astronomers today are equally compelled to in­terpret the patterns in the stars within our galaxy and beyond. In late 2013, ESA plans to launch its next astrometry mission with the Gaia spacecraft (Global Astrometric Interferometer for Astrophys­ics), which will expand the work of Hipparcos.54 The Gaia mis­sion, however, will be far more extensive and accurate in its survey of stars, taking in data on a thousand million stars in the Milky Way, charting the position, distance, brightness, and movement of each one (figure 8.5). It will achieve this by using much larger mir­rors than Hipparcos and much more sensitive CCD detectors. Its goal is a precision of 20 micro-arcseconds for stars at magnitude 15, or 10,000 times fainter than the eye can see, and 200 micro – arcseconds for stars at magnitude 20, or a million times fainter than the eye can see. To extend an earlier analogy, the smaller of these tiny angles would be formed by drawing lines from the top and bottom of Lincoln’s eye on a penny in New York and having them come to an apex not in Paris but on the surface of the Moon!

It is anticipated that Gaia will measure and characterize several thousand exoplanets, detecting them by the subtle motion they in­duce in their parent star. Among these distant worlds, some that are Earth-like must surely exist. However, Gaia’s primary mission will be to produce an extraordinarily precise three-dimensional

Passing the Baton to Gaia

Figure 8.5. The protean work of measuring the distances to stars facilitates a wide range of astronomy projects. Where Hipparcos reached several million stars, ESA’s Gaia mission will reach and measure nearly a billion, enabling everything from a local test of general relativity to a better understanding of the archaeology of the Milky Way (ESA/Gaia).

map of the Milky Way galaxy and sharpen the crispness of our 3D view.55 As explained on the Gaia website, “In the process, Gaia will also map the motions of stars, which encode their origin and subsequent evolution. Through comprehensive photometric classi­fication, Gaia will provide the detailed physical properties of each of the billion stars observed.” Such information will include data on luminosity, temperature, gravity, and elemental composition, which can be used to unravel information on the origin, structure, and evolution of our galaxy. “Gaia’s expected scientific harvest is of almost inconceivable extent and implication. . . . Amongst other results relevant to fundamental physics, Gaia will follow the bend­ing of star light by the Sun, over the entire celestial sphere, and therefore directly observe the structure of space-time.” As a means of comparison with its predecessor, Gaia planners note: “The 16 volumes of Hipparcos would instead be 160,000 volumes and in­stead of filling one normal bookshelf, that bookshelf would have to stretch the equivalent distance of Paris to Amsterdam.”56

Perhaps most important, Gaia will dramatically alter our sense of humankind’s place in the Milky Way and in the universe. Far exceeding the Apollo 8 image of “Earthrise from the Moon,” or the Apollo 17 image of the “Whole Earth,” the Gaia mission has the potential to entirely overhaul our thinking about our grain of sand, the Earth, in the chasm of interstellar space. Astronomers have already discussed the extent to which the mission will pow­erfully impact nonspecialist audiences. In the coming decade, our children will have the opportunity to see in exquisite detail where we are in relation to our nearest star neighbors as educators plan virtual astronomy courses for educational institutions and plan­etaria. Based on new and rich data from Gaia, virtual flights into a 3D version of a larger portion of the Milky Way galaxy are just one of the expected educational outcomes.57 With Gaia, and the virtual galaxy its data will help build, the scientific and public un­derstanding regarding our place in the Milky Way is poised to be radically reconfigured.

Roughly half the world’s population live in urban areas. Light pollution inherent to cities means that 50 percent of humankind has to a large extent lost access to, and knowledge built upon, the night sky. However, scientific missions like Hipparcos and Gaia are restoring our relationship to the stars and generating remarkable new knowledge regarding Earth’s place in our galaxy. Hipparcos has already rewritten the narratives we will tell our children about the past and present morphology of the Milky Way and the exo­planets we are currently finding orbiting nearby stars. What Gaia promises to contribute will rewrite the textbooks again. The story of our galaxy that we hand on to future generations will disclose the number and locations of many other worlds like ours and will inevitably be vital to the human narrative of future ages.

Geographies of Other Worlds

By the late twentieth century, Mars emerged as the familiar land­scape we recognize and cherish. Unlike Mercury, Venus, or even our own Moon, Mars has come into full relief. It is as if a Mercator’s projection of Mars has lifted from its paper and rounded into a globe with tangible polar ice caps, soaring volcanoes, immense rift valleys, recognizable craters, and plains of crescent barchan dunes. Our robotic partners have added these features of Martian terrain to the iconic landscapes that preoccupy, and persist in, the human imagination. Images sent back by Mariner, the Viking Orbiters, Mars Global Surveyor, and Mars Reconnaissance Orbiter allow us to turn the globe of Mars in our hands, so to speak, and trace the planet’s volcanoes, its broken and unzipped canyons, and its ancient and desiccated river beds.3 With the Mars Exploration Rovers Spirit and Opportunity, we’ve explored in fine detail Victoria, Gusev, and Endeavor craters and captured our closest view yet of the red planet. Asked to name a landmark on the Moon, most people would prob­ably answer the Sea of Tranquility, the landing site of Apollo 11. A few might mention the lunar Apennines, landing site of Apollo 15. But ask most fifth graders about Olympus Mons, Tharsis Bulge, or Valles Marineris and they not only readily reply, but also immedi­ately envision these remarkable topographic features.

With a fleet of NASA planetary missions perusing the Solar Sys­tem, by the late twentieth century “near space” emerged as a set of familiar landscapes. Serge Brunier comments that “the deserts of Mars and the rings of Saturn are as familiar to us today as the most awe-inspiring landscapes of our own planet.”4 Cultural geog­rapher Denis Cosgrove similarly has observed that the “differenti­ated surfaces” of the Moon, Mercury, and Mars “are increasingly present” to us and have become a valued aesthetic to scientists and general audiences alike. As the planetary landscapes of our Solar System settle into the imagination, Cosgrove explains that this terrain has evolved into places of “detailed human understand­ing and care.”5 He equates our familiarity with lunar or Martian panoramas or the surfaces of Saturnian and Jovian moons with the crude maps and charted rocks that seafarers used to cross the open ocean. For the earliest mariners, horizon markers, if only the for­bidding and jagged edge of a dangerous coast, would have been a welcome sight, waypoints indicating location and direction. Even a handful of recognizable land or sea features made the seemingly blank and formidable marine expanse less threatening and more navigable. Passes and shorelines, mapped and remembered, altered our relation to Earth’s globe. The currently unfolding planetary geographies are again reconfiguring our sense of place, this time in relation to the Solar System itself.

Of all the planetary geographies beyond those of Earth, Mars’s iconic features are perhaps the most familiar and deeply embed­ded. Viking 1’s initial glimpses of a rock-strewn plain rising to a salmon-colored sky surprised and delighted us, as did the frost – covered rubble of Utopia Planitia photographed by the Viking 2 Lander. With its Arizona-like desert terrain, and dust devils danc­ing lazily over seemingly endless rock fields, Mars has become a place we know, remember, and dream of exploring. Even now, teams of scientists practice working in Mars-analog environments such as the Mars Society’s Desert Research Station in Utah, or the barren and desiccated salt pans of Chile’s Atacama Desert. In a valley near the Atlas Mountains in Morocco, scientists are study­ing the landscape in preparation for ESA’s ExoMars mission.6 Re­search at these sites is focused on developing new technologies necessary for exploring Mars with a geologist’s hands and eyes.7 Until a mission manages to place astronauts on the Martian sur­face, we linger over coffee table volumes from which spill its alien terrain of jagged and scalloped craters, hanging canyons, and high clouds clinging to volcanic summits.

Jim Bell’s Postcards from Mars vividly illustrates why we are so captivated by the red planet. Multi-page panoramas tumble open to a sunset glimpsed by Spirit from the rim of Gusev Crater, and of Opportunity’s sweeping view of Endurance Crater, with its angu­lar dunes a meter in height. A day on Mars, or Martian sol, is not so different from that on Earth, about 24 hours and 37 minutes. Like Earth, Mars has seasons due to its similar axial tilt, polar ice caps, and a year of 687 days. Bell, however, clarifies that while its landscape in appearance resembles Earth’s desert terrain, atmo­spheric conditions on Mars are nothing like the world we inhabit. Having served as the landscape photographer and primary camera operator for the MER rovers, Bell was responsible for processing and interpreting images from Spirit and Opportunity’s panoramic cameras or pancams. Of the Mars he has so patiently and tirelessly photographed and rendered, Bell writes: “There’s an ‘I’ve seen that place before’ feel of looking out the window across a long drive in the desert somewhere. Rocks, hill, sky—it’s all very Earthlike and comforting, in a way. But it’s an illusion. It’s 30 to 50 degrees below zero (°C or °F, it doesn’t matter) on average out there; the air is almost entirely carbon dioxide, with only a trace of oxygen; and it hasn’t rained in something like 2 to 3 billion years, if ever.”8

Panoramas taken by the rovers’ pancams, specially designed to capture Mars in spectacular detail, called for hundreds of photos to be carefully stitched together for a single panoramic view.9 “I wanted them to be postcards—views showcasing the beauty of the natural environment that we now found ourselves in,” writes Bell. “We even set the first few mosaics up to be rectangular in shape, just like postcards.” Photocomposition requires proper interpreta­tion with regard to color under Martian sunlit conditions and is driven by both aesthetic and scientific objectives.10 For instance, in the first image Spirit took of its landing site, Bell noted scuff marks in the Martian soil made by cables that retracted the lander’s air­bags: “The scratches looked very strange, though, like someone had taken a carpenter’s plane and dragged it across the ground, so that pieces of soil were lifted and curled up like wood shavings. A place that at first glance seems commonplace turns out to be quite alien after all.” Large, sweeping vistas taken by Spirit and Opportunity have afforded greater understanding of water deposition, erosion, as well as aeolian processes, and further demonstrate that Mars is a planet of ever-evolving geomorphology.11 When, for instance, the rover team wondered about the frequency of dust devils, Bell ordered the rovers to lie in wait and record what might pass the camera lens: “Within days we started catching dust devils moving across the plains in the images. Over the course of months of oc­casional monitoring we saw hundreds of them. . . . As the plains got warmer with the advance of the seasons, it became clear that these mini-storms are a major way that dust gets moved around on Mars.”12 Fortuitously, the dust devils happened to clean the rovers’ solar arrays, inadvertently extending their lives for years.

Driving the rovers over broad stretches of terrain reminds us of the grave challenges of Martian exploration, for robotic or human explorers. During the fall of 2004, when the Sun had shifted far­ther North and its light was less direct, Spirit was trekking as far as possible each day, depending on battery reserves. Bell recalls, “When we drove or parked, we had to try to bask, lizard-like, with the panels tilted as much into the Sun as possible to maximize our power supply.” He recounts how the rover team targeted pre­selected “lily pads” on slopes that would catch the most sunlight. Late in 2005, while parked on Husband Hill, Spirit took some time away from its geological scrutiny to look upward and make a few astronomical observations. There, along with the rover, Bell recorded “curving star trails, potato-shaped moons moving in the night, shooting stars, the setting Sun, and the rising Earth [as] fa­miliar, yet alien and evocative.”13

It was detailed images of the Martian landscape sent back by the Viking orbiters and landers that enticed Steve Squyres to be­come a planetary scientist. Principal investigator for the MER mission, Squyres recounts how in 1977, as an undergraduate at Cornell University, he became entranced with Mars’s stark ter­rain. He signed up for a graduate seminar to be taught by one of the Viking project scientists and ended up gaining access to the “Mars Room” in Clark Hall that housed the pictures stream­ing in from Viking. While some of the images had been collected into notebooks, the majority, Squyres noted, “were on long rolls of photographic paper, stacked on the floor or still in their ship­ping cartons.” Looking to uncover in a matter of minutes some hook for a course assignment, Squyres quickly became captivated by the alien landscape unfolded before him in photos that he real­ized even scientists had not yet digested. He found himself poring over image after image—for the next four hours. “The planet that I saw in those pictures is a beautiful, terrible, desolate place,” wrote Squyres, who recalls stepping “out of that room knowing exactly what I wanted to do with the rest of my life.”14 Squyres’s journey to exploring Mars resulted in NASA’s twin rovers Spirit and Op­portunity that in turn have captivated the world’s next generation of planetary scientists and explorers.

Beyond the Solar System

Mission “creep” is NASA jargon for the situation when new goals or capabilities (and usually extra associated costs) are added to a mission as it is being developed. The phrase has negative connota­tions, but the Voyager spacecraft wear it as a badge of honor. Their original two-planet mission was designed to last five years. With all the objectives for Jupiter and Saturn achieved, flybys of the two outermost planets—Uranus and Neptune—proved irresistible to mission planners. The five-year mission extended to twelve. As the spacecraft moved into uncharted territory, the time it took for a round-trip signal to be sent from JPL stretched to more than a day, so engineers figured out how to program the Voyagers for remote control, allowing them to be more autonomous. Now the mission has been running long enough that the Voyagers may outlive many of their designers. As the Voyagers recede from Earth, the mod­est power sent back with their high-gain antennas dilutes through an ever-larger volume of space. Currently, the deep-space tracking network detects a minuscule 10-16 Watts (equivalent to a billion billionth of a light bulb) from each spacecraft.22

What lies beyond the Solar System? Uranus is at 30 A. U., or thirty times the Earth-Sun distance. From 30 to 50 A. U. lies a vast ring of debris similar to the Asteroid Belt. Called the Kuiper Belt, it contains about 100,000 rocks larger than 30 miles across, in­cluding some dwarf planets.23 Where the Solar System ends and interstellar space begins is a matter of argument. The Sun sends high-energy charged particles out from its upper atmosphere in all directions. This solar “wind” streaks past us at a speed of a million miles per hour and creates an evacuated region or bubble called the heliosphere. The heliosphere extends past all the planets and ends when the solar wind runs into the rarified gas of hydrogen and helium that permeates the regions between stars in the Milky Way. A shock wave is created as the rapidly moving wind slows to subsonic speeds. The location of this boundary layer is one of the big unanswered questions in astrophysics.

The Voyagers have several ways to diagnose the invisible pro­cesses that take place in the near vacuum of the outer Solar Sys­tem. They can study the strength and orientation of the magnetic field of the Sun, the composition, direction, and energies of solar wind particles, and the strength of the radio emission from be­yond the heliosphere. At the boundary is the place where the solar wind slows suddenly from a speed of 1 million miles per hour and becomes a lot denser and hotter. In December 2004, Voyager 1 crossed this shock.24 A few years later, Voyager 2 followed its twin into the unknown. Recent results have been surprising. The edge of the Solar System isn’t smooth but is filled with chaotic mag­netic bubbles 100 million miles wide. These bubbles are formed when the Sun’s distant magnetic field lines reorganize and form separate structures. Until this observation, it had been expected that the space far beyond the Sun would be smooth and feature­less. In December 2010, Voyager 1 saw the outward speed of the solar wind particles slow to zero. Stagnation of the solar wind was unexpected and may mean that the spacecraft are on the verge of entering a new realm, the vast space between the stars.

Voyager is as ambitious as any project humans have under­taken. To date, the 13,000 work-years devoted to these spacecraft are half as much as all the labor summoned by King Cheops of

Egypt to build the Great Pyramid at Giza, with its slanted passage­way so that the dead pharaoh’s soul could reside with the stars.25 Now, 4,500 years later, we have the means to go there directly as our robotic emissaries glide toward other worlds.

Huygens Pays a Visit to Titan

The centerpiece of Cassini’s symphonic mission, a brief aria of great excitement and beauty, was the descent of the Huygens probe to the surface of Titan in 2005.40 Titan is the most Earth-like world in the Solar System. It has weather, erosion, active geology, and a complex topography of lakes and rivers and flood plains. Three billion years ago, when the Sun was dimmer and no oxygen had been pumped into Earth’s atmosphere by microbes, the two cold worlds had strong similarities.

Huygens represented the main contribution of the European Space Agency to the Cassini mission and even though it returned a modest amount of data, it was not a disappointment. On Decem­ber 25, 2004, the probe separated from the main craft and began its perilous descent. Buffeted by winds in the upper atmosphere and unable to get a navigational lock on the Sun due to the thick smog, it slowed by parachute and landed on January 14, 2005, on what appeared to be a flood plain, scattered with cobbles of water ice (plate 8). Since the surface conditions of Titan were unknown, Huygens was never designed to be a lander. Rather, it was designed to survive landing on any surface from rocks to ocean, and trans­mit a small amount of data before expiring. This was dictated by the limitations of the batteries, which only had three hours of life, much of which was taken up by the descent. There was one minor disaster in the mission, when a software error prevented some of the lander’s images from being uploaded. While 350 images were returned, a similar number were lost. There was also a major di­saster averted. Long after launch, some dedicated and persistent engineers discovered that Cassini’s communication equipment had a major design flaw which would have caused the loss of all Huy­gens’s data. The probe had to send its data by radio to Cassini’s 4- meter antenna and then on to Earth. However, the acceleration of the probe would have Doppler-shifted its data out of range of the Cassini hardware, and the hardware could not be reprogrammed. To salvage this situation, flight engineers changed the landing and flyby trajectory so that the Doppler shift was greatly reduced.

Huygens weighed 700 pounds and carried six scientific instru­ments, most of which had been designed to study the atmosphere.

A microphone on one instrument captured the first sounds ever recorded on any planetary body apart from Earth. Another instru­ment mapped wind speeds at all elevations down to the ground. A third carried a lamp to illuminate the surface, which was useful, since Titan is a very murky moon. Huygens team member Martin Tomasko recalled: “We had great difficulty obtaining these pic­tures. We had only one percent of the illumination from the Sun, we’re going into a very thick atmosphere with lots of haze that blocks light from penetrating to low levels, and we’re taking pic­tures of an asphalt parking lot at dusk.”41 A fourth instrument did the clever trick of heating itself up just before the impact so it could analyze the vapor that came off the surface. The surface was a frigid -290°F or -179°C, cold enough that ice is brittle and methane is a liquid. Scientists clustered around monitors in the mission control room got to stare at a bleak and remarkable scene for several hours, longer than the expected thirty minutes, before the batteries finally died.