by Dava Sobel
Unlike dim, distant Neptune, Jupiter makes such a naked-eye spectacle of golden light in the night sky that its presence has been known since antiquity, and therefore its discovery cannot be dated. Although the time of Jupiter’s birth has been deduced, the place of birth may lie far beyond the region where the planet now resides.
Planetary astronomers say that Jupiter formed 4.5 billion years ago from a seed of rock at a fortuitous location that predisposed it to gigantism. Far from the proto-Sun, the proto-planet rolled through the cold reaches of the primordial nebula, gathering icy tufts of hydrogen-rich compounds such as methane, ammonia, and water. Upon quickly attaining ten or twenty times Earth’s mass, young Jupiter drew in the nebula’s still plentiful light gas, and grew fat on hydrogen and helium.
No small world could have retained such a large envelope of gas, but Jupiter succeeded because of its superior mass, and consequently stronger gravity. Jupiter’s attractive power, the strongest of all the planets, also detoured passing comets from their elongated paths around the Sun, and forced them into Jovian orbit instead. It was most likely by consuming some number of these comets that Jupiter augmented its stores of carbon, nitrogen, and sulfur.
The whole world witnessed one such comet capture when Periodic Comet Shoemaker-Levy 9 crashed through the Jovian cloud banks. In 1992, this comet brushed so close to Jupiter that the planet tore it into twenty-one chunks as big as icebergs, plus many more as small as snowballs. The pieces then circled Jupiter for two years in single file, like a flying string of pearls, before diving to their destruction, one by one, over a week’s time in mid-July of 1994. As they fell through Jupiter’s atmosphere, they exploded in fireballs and thousand-miles-high plumes of debris.
Each detonation left a huge bruise on the clouds, until a whole necklace of black pearls hung around Jupiter, just south of the Great Red Spot. Although every comet fragment had struck the planet’s far side, out of telescope range, rapid rotation soon carried each new impact into view. The dark stains then spread thin on shock waves and winds, dispersed from day to day, and all but disappeared by late August, before scientists could distinguish between the cometary material and the inventory of elements dredged up from the planet.
Following the comet’s natural, unintentional probing of the Jovian atmosphere, the Galileo spacecraft reached Jupiter seventeen months later, in December 1995, and dropped a robot probe carrying seven scientific instruments through the clouds.
In the single hour it operated before being wrecked by heat and pressure, the Galileo probe radioed back eyewitness reports. It found that the high winds seen at high altitudes blew far more forcefully lower down, reinforcing the idea that the winds draw their energy from deep within the planet. The probe also measured relatively large quantities of the noble gases argon, krypton, and xenon on Jupiter. The abundance of these substances was what forced astronomers to consider a Jovian birth place far from the planet’s present home—out where frozen caches of noble gases could be incorporated into the infant planet. Later on, they reasoned, Jupiter drifted closer in as a result of countless gravitational interactions with other Solar System bodies.
The uniqueness of the on-site vantage point empowered the Galileo probe to overturn long-accepted theories by its every discovery. Likewise the things it failed to find caused consternation and conjecture throughout the planetary science community, as when water turned up missing from the returned data.
Astronomers had predicted that the probe, after piercing the visible, colorful ammonia cloud level, would fall through a thick lower layer of ice- and water-laden clouds, where it could be rained on, even struck by lightning. Classical astrologers had also characterized Jupiter as “moist,” in a medieval medical system that claimed the various planets’ hot, cold, moist, and dry qualities influenced human health by shifting the balance among the four bodily humors—blood, phlegm, and black and yellow bile. Moist Jupiter, holding sway over the blood, also inspired the “sanguine” temperament in individuals, making Jupiterians generally cheerful, or “jovial,” as opposed to mercurial, martial, or saturnine.
Contrary to all expectations, the Galileo probe had by chance encountered a dry area, entering a rare hot spot—one of those breaks in the clouds where Jupiter’s heat escapes into space. In time, however, the Galileo orbiter, mother ship to the probe, photographed titanic lightning bolts a thousand times brighter than Earthly discharges, and confirmed the presence of atmospheric water vapor. Indeed, outside the hot-spot “deserts” that continually shift their locations around Jupiter, many parts of the atmosphere appear saturated with water.
The orbiter portion of the Galileo spacecraft went on to explore the Jovian system for seven years. Unlike the probe, which made only a quick diagnostic descent into Jupiter, the orbiter became a long-lived artificial companion to the Galilean satellites.
Galileo took commands from mission controllers at the Jet Propulsion Laboratory in southern California, who periodically fired the spacecraft’s rocket engine to adjust its orbit, sending it now close in toward Jupiter to visit Europa, now out on a wide loop to fly by distant Callisto. As Galileo navigated among the Galilean moons, it discerned the defining characteristic of each: Nearby Io, the reddest, most volcanic body known; Europa, host to an ice-capped salt water ocean; Ganymede, the Solar System’s largest satellite; Callisto, one of the most primitive and pummeled ones.*
Just as the planetary alignments in a horoscope limn the possibilities of a life, so the relative positions of these moons have determined their destiny. Io, the nearest, exhibits the trauma of a too-close attachment. Jupiter’s gravitational pull has racked Io with tidal stress, keeping its interior permanently melted, so that fire fountains of lava spew unceasingly from some one hundred fifty active volcanoes.
Europa, the next nearest to Jupiter and the smallest of the Galilean satellites, also shows signs of internal heating by tidal stress. But the material melted on Europa has apparently been ice, not rock. Thanks to Galileo, many scientists now believe a salty sea, more voluminous than the Atlantic and Pacific together, lies sandwiched between Europa’s frozen surface and its rocky depths, and moreover that its waters might support some form of extraterrestrial life.
Ganymede, though larger than the planet Mercury and farther from Jupiter than Io or Europa, also endures tidal stress. Internal heat keeps Ganymede’s iron core partially molten, and this conductive, convective interior sustains the moon’s own magnetic field, similar to Jupiter’s field, albeit much smaller and weaker.
Only Callisto, beaten and scarred by large ancient impacts, stands aloof from tidal effects. Callisto lies so far from Jupiter that it requires more than two weeks to orbit the planet, while Io makes its way around in less than two days, Europa in three, and Ganymede in seven. Meanwhile the mammoth invisible bubble of the Jovian magnetosphere, which extends millions of miles into space and engulfs all the planet’s many moons, spins in synch with Jupiter every ten hours.
As the magnetosphere races past the moons, it bombards them with charged particles, and makes off with fresh particles lifted from their surfaces. The volcanoes of Io pour a constant stream of ions and electrons into the magnetosphere, inducing tremendous currents between Io and Jupiter, several million amperes strong. Indeed, the orbit of Io seethes with so much electrical activity and lethal radiation that it poses a threat even to unmanned spacecraft. Galileo had to wait until quite late in its study of the Jovian satellites to risk any close flybys of Io. And every time Galileo did pass near Io, one or another of its instruments would shut down, or act up, or take a particle hit that at least partially disabled it. In the end, however, Galileo proved so resilient that it once flew through the plume of an erupting volcano and survived to recount its experience.
This valiant spacecraft, beset from the outset by numerous difficulties that delayed its launch and threatened its performance, developed a distinct personality that endeared it to the engineers who built it and the astronomers it served. Sometime between
1982 (the intended launch date) and 1989 (the year of the actual launch), Galileo suffered damage that went undetected until the craft was well en route to Jupiter. First its umbrella-like main antenna, designed to beam hundreds of thousands of digital images and instrument readings back to Earth, refused to open all the way; then the spacecraft’s tape recorder, meant to store data between broadcasts, jammed. Desperate mission controllers worked from the ground for four years to repair and reprogram the star-crossed Galileo in space, before it got to Jupiter in 1995. Their efforts not only salvaged the spacecraft, but also prolonged its expected life in orbit, so that the mission was deemed a triumph, even though the communications setbacks reduced the anticipated flood of information to a trickle.
Had astronomy and astrology not parted ways so long ago, some of the Galileo mission’s problems might have been foreseen. A natal chart drawn for Galileo, “born” at Cape Canaveral on the day of its launch, October, 18, 1989, reflects a strong, even aggressive spacecraft, with the Sun in Libra for balance, and Mars conjunct with the Sun at the midheaven, adding ambition. At the ascendant, Saturn, Uranus, and Neptune cluster together, which lends a sense of seriousness and importance to the venture. Mercury, however, the planet of communication, makes the worst possible angle—a square, or negative aspect—with Jupiter’s position. Another unfortunate Mercury square opposes the powerful triad of Saturn, Uranus, and Neptune.
The chart shows Jupiter occupying Galileo’s seventh house, the mansion of marriage and partnership. Surely the spacecraft partnered with Jupiter through its life work, and also united with Jupiter in its ultimate fate. As the aging Galileo ran out of rocket fuel for steering control, it obeyed one last command that directed it on a collision course for the giant planet. If Galileo, with its onboard store of plutonium, were left a derelict in orbit, NASA officials feared, it might one day stray into Europa, contaminating the pristine seas there, or even killing some nascent life form.
On September 21, 2003, the day of its demise, Galileo descended into Jupiter’s clouds, disintegrated, and scattered its atoms to the Jovian winds. “It has rejoined the probe,” some project scientists said, as though mourning a friend laid to rest. “They are both part of Jupiter now.”
By the final hour of Galileo’s odyssey, the spacecraft’s horoscope showed Saturn, the planet of endings, well inside the eighth house, the mansion of death.
*Two horoscopes drawn for Galileo during his lifetime (1564–1642) show his Sun near six degrees in Pisces. While his birth in Pisa on February 15 would seem to make him an Aquarian (since Aquarius is the Sun sign of those born January 20–February 18), the calendar reforms of 1582 moved his birthday to the 25th.
*Johannes Kepler (1571–1630), court astronomer and astrologer in Prague, first referred to the “Medicean stars” as “Galilean satellites” in 1610. Simon Marius, a contemporary of Galileo and Kepler, gave the moons their enduring individual names by selecting four favored lovers of the mythological Zeus/Jupiter.
MUSIC
of the
SPHERES
Between 1914 and 1916, the English composer Gustav Holst created the only known example of a symphonic tribute to the Solar System, his Opus 32, The Planets, Suite for Orchestra. Neither Haydn’s “Mercury” (Symphony no. 43 in E flat major) nor Mozart’s “Jupiter” (no. 41 in C; K. 551) had attempted as much. In fact, the title “Jupiter” did not attach itself to Mozart’s work until decades after his death. Similarly, Beethoven’s “Moonlight” Sonata was known for thirty years as Opus 27, no. 2, before a poet likened its melody to moonlight shining on a lake.
The Planets suite contains seven movements, as opposed to nine. Pluto had not yet been discovered at the time Holst was writing, and he excluded Earth. Nevertheless the piece persists as musical accompaniment to the Space Age, partly because people still like it, and partly because nothing else has supplanted it. To make up for its lacks, contemporary composers have augmented it with occasional new movements, such as “Pluto,” “The Sun,” and “Planet X.”
Holst grew interested in planets through astrology. In 1913, after a burst of reading on the subject, he began casting friends’ horoscopes and thinking of the planets in terms of their astrological significance, such as “Jupiter, the Bringer of Jollity,” “Uranus, the Magician,” and “Neptune, The Mystic.” His daughter and biographer, Imogen, also a composer, recalled that her father’s “pet vice” of astrology led him on to study astronomy, “and the excitement of it would send up his temperature whenever he tried to understand too much at once. He was perpetually chasing the idea of the Space-Time continuum.”
A natural affinity between music and astronomy has prevailed since at least the sixth century B.C., when the Greek mathematician Pythagoras perceived “geometry in the humming of the strings” and “music in the spacing of the spheres.” Pythagoras believed the cosmic order obeyed the same mathematical rules and proportions as the tones on a musical scale. Plato reprised the idea two centuries later, in The Republic, introducing the memorable phrase “music of the spheres” to describe the melodious perfection of the heavens. Plato spoke also of “celestial harmony” and “the most magnificent choir”—terms that imply the songs of angels, though they referred specifically to the unheard polyphony of the planets in their gyrations.
Copernicus cited the “ballet of the planets” when he choreographed his heliocentric universe, and Kepler built on the work of Copernicus by returning repeatedly to the major and minor scales. In 1599 Kepler derived a C major chord by equating the relative velocities of the planets with the intervals playable on a stringed instrument. Saturn, the farthest and slowest planet, issued the lowest of the six notes in this chord, Mercury the highest.
As Kepler developed his three laws of planetary motion, he expanded the planets’ voices from single notes to short melodies, in which individual tones represented different speeds at given points along the various orbits. “With this symphony of voices,” he said, “man can play through the eternity of time in less than an hour and can taste in small measure the delight of the Supreme Artist by calling forth that very sweet pleasure of the music that imitates God.”
For his 1619 book, Harmonice Mundi (The Harmony of the World), Kepler drew the five-line musical staff with key-signatures for the several parts, and set down each planet’s theme in the hollow, lozenge-shaped tablature of his time. Mercury’s highly eccentric, high-speed, high-pitched refrain ranged seven octaves above Saturn’s bass-clef rumbling from low G to low B and back again.
“I feel carried away and possessed by an unutterable rapture over the divine spectacle of the heavenly harmony,” said Kepler. “Give air to the heaven, and truly and really there will be music.”*
The two Voyager spacecraft, launched in 1977 and currently headed for the outer boundaries of the Solar System, further this musical heritage. As potential envoys to extraterrestrials, both craft carry a specially engineered golden record (complete with its own playback equipment) that expresses the music of the spheres as computer-generated tones designating the velocities of the Sun’s planets. The Voyager Interstellar Record also says “Hello” in fifty-five languages and plays music selected from numerous cultures and composers, including Bach, Beethoven, Mozart, Stravinsky, Louis Armstrong, and Chuck Berry.
* * *
WHETHER BY intention or inspiration, Gustav Holst ignored the established order of the planets when he initiated his suite with “Mars, the Bringer of War” in July 1914. Real war, what Holst’s generation called the Great War, broke out that autumn, but the forty-year-old Holst, barred from active service by neuritis and nearsightedness, moved directly on to “Venus, the Bringer of Peace.” In performance, as in composition, the full suite invariably begins at Mars, travels inward to Venus and “Mercury, the Winged Messenger,” then out again to Jupiter and straight on through Saturn and Uranus to Neptune, where the voices of a female choir, sequestered in a room offstage, are made to fade out at the finale (with no sacrifice in pitch) by the slo
w, silent closing of a door.
The suite’s immediate popular success amazed Holst, and changed him from an accomplished musician to a famous one. Forced to comment publicly on The Planets, he let it be known that “Saturn, the Bringer of Old Age”—at nine minutes forty seconds the longest of the suite’s seven movements—was his favorite. “Saturn brings not only physical decay,” Holst said in the planet’s defense, “but also a vision of fulfillment.”
Seen for the first time through a backyard telescope, ringed Saturn, icon of the otherworldly, is the vision most likely to turn an unsuspecting viewer into an astronomer forever. The spectacular Saturnian ring system spans a disk 180,000 miles wide from one ring tip, or ansa, to the other. Its vast breadth approaches the distance from Earth to the Moon, yet the average ring depth scarcely exceeds the height of a thirty-story building. In Holst’s day, astronomers trying to describe the rings’ incomparable flatness grasped at pancakes and phonograph records as metaphors, before settling on a sheet of shirt cardboard the size of a football stadium. (Improved measurements have since replaced the cardboard with tissue paper.)
Saturn appears with Jupiter and Venus in a painting of the night sky over Holst’s beloved Cotswolds, given to him at the 1927 festival in his honor where he conducted The Planets for the last time. Artist Harold Cox said he had consulted the Astronomer Royal on the correct placement of the planets for this portrait of a May night in 1919—the year the public first heard The Planets in concert, and Holst won appointment as professor at the Royal College of Music. Saturn looks like a mere bright spot in the painting, duller than the lights of Jupiter or Venus, and ringless, of course, since the naked eye cannot discern the celebrated rings. This is not to say they are invisible or absent from the painting, however. On the contrary, the rings sparkle so with reflective ice and snow that they fairly triple Saturn’s luster. All the ring components, which range in size from dust grains to boulders big as houses, are thought to be at least ice-coated, if not wholly composed of frozen water. The body of Saturn, in contrast, is a gas giant much like Jupiter, made of hydrogen and helium, only smaller and paler and twice as far removed from the Sun. Without its surround of ice crystals, snowflakes, and snowballs of all sizes, Saturn would hardly dazzle viewers a billion miles distant.