The Interstellar Age (21 page)

At least, not until March of 1781. That’s when the German-born English musician and astronomer William Herschel (1738–1822) single-handedly doubled the size of the solar system. Herschel is one of those almost mythic characters from the early history of Western astronomy. Like Isaac Newton in the century before him, Herschel was a prominent polymath—a person with many different kinds of technical and academic skills. He focused much of his energy on, and made his living by, composing and performing music (twenty-four symphonies, fourteen concertos, and many other pieces) and also dabbled in astronomy, optics, and other areas of science. This dabbling eventually led him to design large mirrors and telescopes (based on Newton’s design) and to begin making systematic observations of the night sky. Soon enough, using a 6.2-inch-diameter, 7-foot-long telescope from his home observatory in Bath, he noticed an object with a “nonstellar disk” moving ever so slowly through the stars. Instead of appearing as a point of light like the stars, this object had a circular nature to it. Collaborating with others, Herschel was able to show that the object moved on
a planetary path, well beyond the orbit of Saturn.

Herschel’s first instinct was to name the new planet Georgium Sidus (George’s Star), after his king and patron, George III. In that regard he followed in the well-established footsteps of previous astronomical discoverers who wanted to (for lack of a more fitting term) kiss ass by naming their findings after the people who paid their bills. For example, in 1610 Galileo wanted to name the four
bright dots that he discovered orbiting Jupiter the Medician stars, after his patron and funding source Cosimo II de’ Medici (Grand Duke of Tuscany) and Cosimo’s three brothers. Thankfully, astronomers decided on Io, Europa, Ganymede, and Callisto instead. Anyway, no one (except perhaps King George) liked Herschel’s proposed new name, especially the rival French, who referred to the new world simply as Herschel. Astronomers eventually settled on Uranus, the Greek god of the sky. A few years later, using an even larger (and more cumbersome) telescope of 18.5 inches in diameter, he discovered two moons orbiting Uranus (later named Oberon and Titania by Herschel’s son, John, who also became a prominent astronomer), and shortly thereafter he discovered two more moons around Saturn—Mimas and Enceladus. All this while he continued to compose and perform music and while, “on the side,” he discovered infrared radiation, contributed to the study of Mars and the other planets, and made biologic observations with his microscopes.

Herschel was motivated and accomplished for sure, but like many others with such broad interests and talents, his didn’t do it all alone. Indeed, one of his most capable assistants was his sister, Caroline Herschel (1750–1848). She aided with his music and toiled at the telescope, contributing to new telescope designs, polishing the mirrors, and performing observations of her own. At a time when women had few opportunities in science and academia, Caroline became an accomplished astronomer in her own right, with an outstanding record of instrument building, comet discoveries, and
important work cataloguing faint stars. While she never became a full member of the all-boys club that was the Royal Astronomical Society back then, she came as close as any woman had (or would for almost a century more), and though she deserved more, she
was elected to “honorary” membership in recognition of her contributions.

The way that the Herschels—father, sister, son—had to work together and with other science and engineering/technology colleagues of the day to make their groundbreaking discoveries seems like a great example to me of one of the earliest cases of the slowly growing phenomenon of “Big Science.” Science, perhaps especially astronomical science, generally begins with highly motivated individuals making careful observations, or working out interactive theories, essentially on their own or with just a few select others. Notable examples from the history of Western astronomy include pioneers like the sixteenth-century Polish astronomer Nicolaus Copernicus; Danish observer Tycho Brahe and German astronomer Johannes Kepler working together in the late sixteenth century; the late-seventeenth-century English physicist Isaac Newton; and of course the first loner at the telescope, Galileo Galilei, in the early 1600s. But the history of individual, or “small,” science pushed forward mostly by key individuals goes much farther back in time and crosses many cultures, including notable Greek, Arab, Persian, Chinese, Indian, and other thinkers. The idea of collaborative science was generally more rare, although there were some important and profound early advances in math, physics, and astronomy made by larger groups working together, such as the astronomer and mathematician Nasir al-Din al-Tusi and his thirteenth-century research team studying planetary motion at the Maragheh Observatory in Iran. Or the academics working together on early forms of calculus to propose some of the first models for a sun-centered universe in the sixteenth-century Kerala school of mathematics in India (influencing, in their writings, another loner astronomer in Poland, the
previously mentioned Nicolaus Copernicus). Or Harvard astronomer Edward Pickering’s early-twentieth-century group of mostly female “computers” toiling through enormous telescopic data sets to work out the modern basis for the classification of stars.

As technology has advanced, and the breadth of knowledge required to understand, utilize, and improve that technology has expanded, it has become more difficult for individuals, or even small groups of people, to define the cutting edge of science, and especially space science. Projects like the
Apollo
moon landings, the
Voyager
missions, the Hubble Space Telescope, or rovers on Mars require detailed theoretical calculations (to determine orbits, or to estimate camera exposure times in preplanned sequences, for example), advanced engineering technologies (such as new materials, new kinds of instruments, new software for communications and commanding), and clear scientific goals based on the most recent laboratory, telescopic, and computational discoveries. It is simply not possible for small groups of people to pull off projects of this scale, and so big teams with wide ranges of expertise are needed to design, build, and operate experiments, and to process and interpret the results. Big Science.

ROLLING WITH THE PUNCHES

When William and Caroline Herschel discovered Oberon and Titania, they and others couldn’t help but notice that those moons (and others found later) are spinning around Uranus in a vertical plane, like the wheels on a car rather than like a record on a turntable. The spin axis of Uranus is tilted on its side, at an angle close to
90 degrees relative to the rest of the planets. Uranus rolls around the sun, rather than spins. What’s up with that? One of the main goals of the
Voyager 2
flyby of Uranus was to try to find some clues to answer that question.

To try to come up with some ideas ahead of time, astronomers tried to understand why all the other planets spin like tops with their poles pointed roughly (within 20 to 30 degrees or so) perpendicular to the equator of the sun. The prevailing idea is that the sun and all the planets formed some 4.65 billion years ago from a condensing, spinning cloud of gas and dust. The cloud must have been spinning counterclockwise as viewed from above the north pole of the sun, because that’s the direction that the sun spins on its axis and that all the planets orbit around the sun. The notion that the planets formed from a
disk
of gas and dust helps to explain why their poles are pointed north-south: their equators are all forming within the plane of that relatively flat disk. But Uranus is an oddball: its equator is tipped over by 90 degrees. For part of its eighty-four-Earth-year trip around the sun, the north pole of Uranus is pointed right at the sun, and the entire southern hemisphere is dark; forty-two Earth years later the situation is reversed in southern summer, with the northern hemisphere in the complete darkness of polar night. In between, near the spring and fall equinoxes, sunlight falls on both hemispheres. A planet’s tilt determines the intensity of its seasons: Earth’s 23.5-degree tilt produces extreme seasons with constant sunlight or constant darkness for people or animals that live above the Arctic (or Antarctic) circles; Jupiter’s tilt is near 0 degrees, and so despite being superlative in many things, it has no seasons. Uranus has the most extreme tilt and thus the most extreme seasons, with its own Arctic circle falling very close to its equator.

How did Uranus get this way? No one knows for sure, but one popular hypothesis is that Uranus formed “normally” like the rest of the planets, but early in the history of the solar system it was knocked on its end, tipped over, by a giant grazing impact with another large terrestrial planet or a small gas giant. That impact would have essentially melted both bodies, but if a newly forming condensing cloud of post-impact gas and dust had started to spin vertically because of the force of the impact, that orientation could end up being the
new tilt for a newly formed (potentially merged) planet.

It sounds outrageous and ad hoc . . . because it is. In general, scientists don’t like to invoke special one-time events like this to explain the world(s) around us, but sometimes, to paraphrase the famous Sherlock Holmes, if you’ve ruled out the impossible, and all you’re left with is the improbable, that’s probably the right answer. Only recently in planetary science has the idea of giant impacts as major agents of planetary change become more widely accepted. Indeed, despite the crazy sound of it, the idea of a giant grazing impact between the very young Earth and a Mars-sized protoplanet is the best explanation of the formation of our moon, based on
Apollo
samples and analysis of the Earth’s interior composition.

So maybe giant impacts aren’t that crazy after all. Maybe there would be something about the planet’s magnetic field lines (if the planet had a magnetic field—
Voyager 2
would find out!) or interior structure that would prove to be the smoking gun that supported
some
model for why the planet is tilted over. Maybe, maybe not.

“We thought a lot about that,” Ed Stone says. “But we had a terra-centric view that was limiting our considerations: We
assumed
that like all other magnetic fields that we’d seen at that time that the magnetic pole would be near the rotational pole. So we were expecting to see a unique situation where the solar wind was directly impinging on the planet’s south magnetic pole. On the Earth, that’s the ‘funnel’ where particles come in, and it’s a really interesting place.” The “funnel” Ed is talking about is the convergence of the Earth’s magnetic field lines near the north and south poles. The field lines coming together act to concentrate the high-energy solar wind particles that are streaming along those lines, increasing their density and the intensity of their interactions with the Earth’s atmosphere. This is part of the reason that Alaskans and Canadians and Scandinavians (and Antarctic penguins) see such intense and beautiful auroral displays—the funnel concentrates the energy, and the aurora is one of the ways that that energy is dissipated. I can imagine Ed and others wondering if they would witness similarly spectacular auroral displays at Uranus. The reality, however, turned out to be quite different.

The crazy tipped-over geometry of Uranus meant that instead of a relatively leisurely, multiday tour past the planet’s moons and rings like at Jupiter and Saturn,
Voyager 2
would instead be flying a banzai-like bull’s-eye trajectory, piercing through the Uranus system like an arrow flying through a target at over 51,000 miles per hour, with only about ten hours to conduct all the needed close-up observations. The
Voyager
navigation team needed to aim for a specific closest-approach point within just a few diameters of the planet in order to make the spacecraft pass through the shadow of Uranus (to measure the atmospheric composition and structure) and to give
Voyager
the needed gravity-assist tweak to send it on to
Neptune and complete the hoped-for Grand Tour. Passing that close to the planet and its bull’s-eye pattern of moons meant that mission planners could only try to tweak the timing of the flyby so that
Voyager
would pass close to the innermost moon, tiny Miranda. It was unfortunate that the other moons couldn’t be studied so closely—just bad luck because of the geometry of the flyby. On the other hand, unbeknownst to the team, Miranda would turn out to be the most interesting of the five large icy satellites of Uranus.

Bull’s-Eye.
Voyager 2
flyby trajectory past Uranus.
(NASA/JPL)

There was another, more serious, problem that the
Voyager
team had to solve in order to make the Uranus flyby a success: image smear. Four times less sunlight at Uranus than at Saturn meant that the cameras and other instruments would have to expose their pictures four times as long to get the same image quality. But this was a fast flyby—less than half a day, and the spacecraft had picked up an extra 18,000 miles per hour of speed by the gravity assist at Saturn. The team had already noticed some small amount of image smear in the Saturn images compared to the Jupiter images (where the sun was brightest and exposures shortest among the entire Grand Tour). Some of the smear seemed to be caused by the jolting starts and stops of the tape recorder, which shook the spacecraft a little bit during long-exposure photos. Leaving the shutter open even longer would mean that the images would be smeared out even more. Instead of seeing crisp views of new worlds, blurry, streaky photos would be taken. The team had to find a solution, and indeed they did:
Voyager 2
was almost completely reconfigured and reprogrammed between Saturn and Uranus, becoming a faster, smoother photographer and a more high-tech spacecraft.