The Interstellar Age (22 page)

First, they taught the spacecraft to move from target to target—to slew and change its “attitude” or orientation—much more slowly and smoothly than it had done previously, using tiny puffs of well-timed thrusts from the hydrazine attitude-control system. Then they taught the spacecraft to anticipate, and to compensate for, the tape-recorder jolts using the same attitude-control system. The most elegant new mechanical trick that they taught
Voyager
, however, was something called
image motion compensation.
Even though the team had several years to diagnose and correct for the problems that
Voyager 2
had with moving its science instrument scan platform too
quickly during the most rapid-fire part of the Saturn flyby, mission planners didn’t want to take the chance of the same thing happening again during the similarly rapid Uranus bull’s-eye flyby sequences. So they restricted the planned motion of the scan platform and instead taught the
entire spacecraft
to gently pirouette (or at least partially pirouette) in the direction
opposite
its motion relative to Uranus and its moons when it was imaging them. From the viewpoint of the camera onboard the spacecraft, this would make those bodies appear to zip by more slowly. It was a ballet.

Voyager
’s brains—its main computer and its backup computer—also got major overhauls to prepare for the Uranus flyby. The main computer was reprogrammed to send its images immediately to the backup computer to process and compress them there instead of in the main memory, speeding up the rate that
Voyager
could take pictures by 70 percent. The computer was also taught how to use an experimental new data-encoding box that was on the spacecraft but that hadn’t been used at Jupiter or Saturn. “Data encoding” is the process of converting the images and other data into the compressed string of ones and zeros that would be broadcast by radio back to Earth. The new encoding scheme would be more efficient, more robust, to the weaker signal levels from Uranus in that it would enable JPL communications engineers to better reconstruct the original data even if some “packets” of the radio signal were lost or corrupted. It was also predicted to work better than the default encoding routine for the low light levels and dark moons and rings expected at Uranus.

Even the JPL Deep Space Network team got in on the game, increasing the sensitivity of their receivers to deal with
Voyager 2
’s weaker and weaker radio signals as the spacecraft sped farther away, and adding the capability to communicate with
Voyager 2
from the
Parkes Radio Telescope in Australia (the one made famous by successfully broadcasting Neil Armstrong and Buzz Aldrin’s
Apollo 11
lunar landing live to the world in 1969) during the most critical part of the Uranus flyby—which would be best visible from the Australian DSN stations. As Ed Stone wrote in one of the early scientific papers describing the
Voyager 2
results at Uranus, “
That all of this worked so well testifies to the high level of expertise and the spirit of teamwork within the
Voyager
project and supporting organizations.” Big Science.

GEOGRAPHY REINVENTED

Voyager
began observations of Uranus and its surroundings in November of 1985, while still more than 6 million miles from the planet. Every so often I would swing by Ed Danielson’s office at Caltech, or by one of the conference rooms in Millikan Library on campus, to catch a glimpse of the planet on one of the monitors there that was echoing the steady stream of images being sent to JPL. Students and staff would often gather around the small black-and-white displays, leaning in and squinting to read the telemetry text information also displayed in tiny type along with each picture. We could monitor the increasing apparent size of the planet by comparing it to the distance between the black dots called
reseau marks
that were etched onto the camera’s lens and which thus appeared in every image. In some images, we could even start to see the faint doughnutlike ghost from out-of-focus dust specks on the lens. Heading toward the end-of-year holidays, this new greenish-blue world was starting to get big in the headlights.

I went back to Rhode Island to visit my family over that holiday season and felt acutely, almost completely, detached from
Voyager.
Computer science and technology might have been slowly burgeoning at NASA, but the Internet was still just a small academic network among universities and select government labs—a far cry from the publicly available web of infinite knowledge and connectivity that we take for granted today. TV and newspaper media coverage of the flyby was rare, right up until the few days around the closest approach in late January. Once I got back to Pasadena, I used my magic
Voyager 2
team badge that Ed Danielson had gotten for me to get myself over to JPL as often I could, sometimes driving my 1963 Ford Galaxie 500 convertible (white, “three on the tree”) the seven miles between campus and JPL, sneaking into the visitors’ daytime parking lot, and a few times skipping out of class to bum a ride with Ed when he was making the trip. As it got closer to encounter day, parking became scarcer, with big TV vans and buses taking up space in the visitors’ lot and outside Von Kármán Auditorium, where the press conferences were being held. Even when I had to park more than a mile away, it didn’t matter. I couldn’t keep myself from running all the way to the lab.

During the days right around closest approach on January 24, security was ramped up considerably at JPL. Guards were posted in the science work areas of Building 264. Limits were placed on the number of people allowed at once in the operations rooms, for fire code reasons, I’m sure, and also because of the need to preserve a relatively peaceful environment for performing tactical calculations and making weighty decisions about the flyby. For example, in late December, a faint new moon was found orbiting Uranus in some of the
Voyager 2
approach images. It was imaginatively dubbed 1985U1
(and later officially named Puck after the airy sprite in Shakespeare’s
A Midsummer Night’s Dream
). Mission planners realized that one of the preplanned images of Miranda could be reprogrammed to get a decent, though distant, view of this small new world. There was work to be done.

I wasn’t really doing any of the important work. Rather, I was a gofer (go for this, go for that) for Ed or other team members who needed copies made, a print retrieved from the image-processing lab, a cup of coffee, or a pizza delivery. I tried to avoid being tossed out for bumping into something, or even for breathing too loudly, or too much. I’m sure I had a dopey grin on my face most of the time.

We all knew that Uranus didn’t have strong, high-contrast cloud bands like Jupiter, or bright rings like Saturn, but I think many of us ended up being quite surprised at just how
bland
the atmosphere turned out to be, even at high resolution, when
Voyager 2
sped past. The blue-green color was strikingly different, to be sure—caused by methane in the upper atmosphere that absorbs mostly red light from the sun and reflects the rest. Weak, broad bands of slightly brighter or darker tones could be seen at some latitudes, and some occasional small white clouds—almost like smeared-out thunderheads—would pop into view and zip past now and then. But this giant planet was definitely a different flavor, a different beast, from the ones we had seen before.

Ed Stone and his “fields and particles” colleagues were delighted (though, he confessed, “not surprised”) to discover that Uranus does indeed have a strong magnetic field, and were even more delighted when that magnetic field turned out to be pretty weird. The fields that
Voyager
had found around Jupiter and Saturn were much stronger, and sort of what the science team had expected. Deep
within the interiors of those giant planets are mega-Earth-sized cores of hydrogen compressed to such high pressures and temperatures that the gas acts more like a metal, easily conducting electricity. We know from our own planet’s interior that spinning, electrically conducting cores of planets (such as Earth’s partially molten iron core) can generate a magnetic field. And the shape of that field is sort of like the shape that iron filings take when they are exposed to a regular bar magnet—the field lines orient along north-south magnetic “poles” that come close to lining up with the north-south spin axis of the planet. That’s why a compass can tell you which way is north.

At Jupiter and Saturn, the magnetic fields looked basically as expected—like the fields that would come from giant bar magnets placed at their centers. The field lines get warped and bent and “swept back” by the solar wind, streaming out almost cometlike into a long magnetic “tail” (called a
magnetotail
) that points away from the sun. In fact, Jupiter’s magnetic field and magnetotail are so enormous that if we could see it with our naked eye it would be five times larger than the full moon in our night sky—making it the largest single structure in the solar system except for the sun’s own magnetic field. Saturn’s field is not as large but is similarly impressive. By comparison,
Voyager
measurements showed that the magnetic field of Uranus is quite different, probably because of the crazy tilted way the planet spins. It’s almost like there’s a giant bar magnet down below the clouds—meaning that there is a rapidly spinning, conducting material of some kind—but Uranus’s is not located in the center of the planet. Instead, it appears to be offset to one side about one-third of the way to the cloud tops. And unlike Earth’s, the magnetic-field axis is not even close to being lined up with the planet’s edge-on spin axis; instead, it’s tilted at an angle of about 60 degrees from the
orientation of the planet’s spin. Your compass won’t do you much good there.

“The combination of the planet’s spin axis tilt, and the offset, tilted nature of the magnetic field, combined to make the angle of the field relative to the solar wind really not much different than all the other magnetic fields that we’ve encountered,” says Ed Stone. “But what was different was that, unlike the others, the tail end of the Uranian magnetic field is a spiral, because it’s being wound up by the planet’s tilted-over spin.” The solar wind warps and sweeps this bizarre and unexpected structure downwind in a unique way.

One of the possible implications of the planet’s strange magnetic field is that the inner core of Uranus, which
Voyager
gravity data showed is rocky and icy and about the size of the Earth, is not hot and dense enough to be electrically conductive. Instead, the magnetic field might be offset from the center because the electrically conductive layer that is causing the magnetic field might be one of the layers above the core, in the planet’s mantle.

“The electrical current system inside the planet, the circulation of ionized particles, is clearly not a simple, global thing,” offered Ed Stone. “That may well have to do with the way the interior is differentiated.”
Voyager
results and theoretical models appear to show that the mantle of Uranus is rich in water ice and other kinds of “volatile” molecules that started out as ices. At high enough pressures and temperatures, many of these ices, especially if they have hydrogen in their structure (such as H
₂
O), can become electrically conductive when vaporized and compressed. Some planetary scientists thought that the strange, offset, tilted magnetic field discovered by
Voyager
observations could be telling us that Uranus is really not like Jupiter and Saturn but is instead a completely different kind of
giant planet. But it was hard to know, partly because the physical characteristics of Uranus are so different from the other giant planets.

Tilted on its side, not generating its own internal heat . . . “Clearly, something strange had happened to that planet,” says Heidi Hammel. “It was hard to generalize about how Uranus-like planets could be so different from Jupiter and Saturn, because Uranus itself was just personally so screwed up.”

During the days leading up to
Voyager 2
’s eventual closest approach, we all witnessed the five large Uranian moons go from mere specks of light to small disks to fully resolved worlds of their own. This was an especially exciting process for me. I hadn’t been through the Jupiter and Saturn flybys in this way; I hadn’t seen those dots slowly revealed as distinct worlds. Rather, like most people, the first time I saw Io or Europa or Titan revealed for what it truly is, was via one of the “greatest hits” close-approach photos that was published in the newspaper or shown on the evening news after each flyby. It was just—
bam!
Io has volcanoes! Or—
bam!
Mimas has a giant crater that makes it look like the Death Star! During
Voyager 2
’s Uranus approach, however, there were no such moments. Rather, the reality of these new worlds came into view slowly, over many weeks, with a grace and air of anticipation that more accurately reflected the gentle gravity assist that we were all actually going through, riding along with the spacecraft.

During those last ten hours, though, as
Voyager
plunged deeper into the gravity well of the seventh planet, we all experienced plenty of breathtaking moments. One by one the five large icy moons were revealed to us from
Voyager
’s high-resolution images, and a total of
ten new, smaller moons would eventually be discovered lurking in the images. All the large moons are heavily cratered, attesting to
their generally ancient ages. Oberon and Umbriel are the most cratered, suggesting that they have changed little during their more than 4-billion-year histories. It was a mystery to us why Umbriel is so dark compared to the other four large moons—indeed, it’s still a mystery today. Perhaps its surface contained a higher fraction of carbon-bearing ices that have been darkened more over time by the constant irradiation from the solar wind. The other moons show more geologic diversity. Fractures of 1 to 3 miles deep as well as cliffs on Titania suggest a past active interior. Similarly large rifts on Ariel, as well as evidence for some sort of icy, perhaps cryovolcanic, flows, suggest past tectonic activity and internal heating on that world. But the most diversity, and the most vexing mysteries, came from
Voyager
’s high-resolution images of tiny Miranda, the innermost of the large Uranian moons.