The Interstellar Age (19 page)

If
Voyager 1
hadn’t been targeted so closely to Titan and had instead been able to be diverted by Saturn’s gravity toward a flyby of Pluto in the 1980s, as NASA had envisioned for some of the original Grand Tour missions proposed in 1969–1970, we would have discovered much sooner that Pluto is a small world with a thin atmosphere, a surface mostly made of nitrogen ice, and orbited by at least
five
moons rather than just the one large one discovered from Earth-based telescopes in 1978. Maybe active plumes or nitrogen-powered volcanoes would have been discovered.

Charley Kohlhase is less sanguine. “I don’t think a far-off little, now–Kuiper Belt Object like Pluto, and the trip time and whether you could make it there or not, would ever have beaten out Titan.
Most of the people I knew did not regret giving up on Pluto. We were happy to go with Jupiter-Saturn-Titan for
Voyager 1
, and the smaller Grand Tour Jupiter-Saturn-Uranus-Neptune with
Voyager 2.
”

Ed Stone is similarly pragmatic. “Giving up something that you know you can do—Titan—for something where you’re not sure you could get there . . . there was no real science controversy about that decision,” he recalled. “And we didn’t really
know
Titan at all. We probably wouldn’t have had a probe landing on Titan if we hadn’t focused on it with
Voyager 1
,” he added, referring to the successful
Cassini-Huygens
Titan landing mission in 2005. Regardless, we will, hopefully, know for sure what Pluto is really like up close after the July 2015 Pluto flyby of NASA’s
New Horizons
spacecraft. Will our biases against the possibilities of life on small, distant worlds continue to be shattered by
New Horizons
as they have been shattered by
Voyager
?

CLOSE CALL

After
Voyager 1’
s successful November 1980 Saturn flyby, all of the team’s efforts became focused on planning for
Voyager 2
’s pass through the system in August 1981. With
Voyager 1
’s successful Titan flyby in the can, and no hope of gaining useful additional close-up imaging coverage of Titan from
Voyager 2
because of the thick haze layer, the path was cleared for
Voyager 2
to attempt the Grand Tour. By directing the spacecraft to a very close pass by Saturn, the team could use Saturn’s gravity to give the spacecraft a 90-degree turn to aim it toward a 1986 encounter with Uranus, and then, if all went well, possibly on to Neptune in 1989. But getting the
required course change meant getting very close to Saturn, close to the region where the bright A, B, and C rings orbit around the planet.
Pioneer 11
and
Voyager 1
had shown that the ring particles were (generally) sparsely separated, and that the ring plane could be traversed that close-in to the planet. Still, it had to happen in order to bring about the eventual encounters with Uranus and Neptune, plus it offered close encounters with the enigmatic moons Enceladus and Tethys. The successes of
Pioneer 11
and
Voyager 1
emboldened the team to take the risk—Saturn’s rings or bust! So the course was set.

Despite the overall feeling that success at Saturn had already been generally achieved by
Voyager 1
, for some of my colleagues who were involved in the mission
,
the days surrounding the closest approach of
Voyager 2
to Saturn were among the most harrowing on the mission. Team members who studied the rings, in particular, were nervous.
Voyager 1
had made a rather distant flyby past the rings, but the images still revealed strange waves, ripples, and twisting patterns in the rings that defied explanation (“Rings don’t do that!” they said). This was partly because the resolution of the images was too low to show adequate detail. It seemed like some fundamental piece of information about how rings form and evolve, and maybe how waves and wakes form within them, was missing. So they needed to get even closer to the rings with
Voyager 2
to try to understand what was going on. They were nervous, though, because they knew that the closer the spacecraft got to the main rings, the closer and denser the ring particles became, increasing the risk of ring particles impacting the spacecraft.

Even a tiny ring particle, no larger than a speck of silt or a grain
of sand, could cripple
Voyager
, which was traveling faster than 31,000 miles per hour relative to the rings. It’s the same principle of relative velocity and kinetic energy that explains how you can stop a tractor trailer traveling at 50 miles per hour with a common housefly—as long as the fly is going a million miles per hour! Tiny particles might not seem dangerous, but if they are moving extremely fast, they can carry an enormous amount of energy. Thus, getting closer to the rings offered great scientific potential but also presented great risk—a double-edged sword.

The first half, or inbound, part of
Voyager 2
’s flyby past Saturn was as routine as flying a tight trajectory past a gas giant planet can ever be, and made it possible to capture more of Andy Ingersoll’s giant planet atmosphere movies (covering Saturn’s northern hemisphere), and distant flybys of Titan and the icy moons Mimas, Dione, and Rhea—all of which had been photographed at higher resolution by
Voyager 1.
It was then that things got . . . interesting.

As the spacecraft fell deeper into Saturn’s gravity well and started to speed up (
Voyager 2
was eventually accelerated from 36,000 miles per hour to nearly 54,000 miles per hour by flying a gravity-assist trajectory behind Saturn) and get closer to the planet and the rings, the kinds of maneuvers that the sequencing team had to build into the instrument observations became more and more complex. Specifically, the cameras and other instruments on the spacecraft’s scan platform had to be pointed around more rapidly, and over a wider range, than they had been pointed in a very long time. This was a simple result of parallax—the fact that the closer you are to something, the larger it appears to be. Imagine trying to take a picture of the Statue of Liberty, for example, from a mile away on the Staten Island Ferry. The ferry is moving slowly relative to
Lady Liberty, who is far enough away to be leisurely photographed without much need for panning your camera. Now imagine trying to get that same photo from the passenger’s seat of an F-15 fighter jet, passing within a hundred yards of the statue and at 1,000 miles per hour! You would have to work fast and avoid craning or whiplashing your neck to try to catch Lady Liberty in a quick shot without blurring her out. It’s the same kind of challenge faced by the
Voyager 2
sequencing team during the closest, fastest part of the Saturn flyby.

All was going well as the spacecraft neared its closest approach to Saturn and—as planned—went into eclipse behind the planet as seen from the Earth. This was the start of a series of carefully orchestrated observations from a spectacular vantage point inside the Saturnian system that had never before been witnessed. Critical observations would be made of Enceladus and Tethys. Saturn’s rings would be viewed edge-on. And the clouds and storms in Saturn’s southern hemisphere would be observed up close and personal. The spacecraft would also go into a solar eclipse while behind Saturn, with sunlight shining through its outer atmosphere and scattering through the dense gases as it made its way back to the cameras and instruments on
Voyager 2
. In this way, the spacecraft could measure the chemistry of Saturn’s clouds as never before. On its journey, the spacecraft would navigate its way through the plane of the rings, not far from the main rings themselves.
Voyager 2
was on its own for most of this time, hidden behind Saturn, obscuring all possible communications. The intricate series of scan platform slews and camera exposures had therefore been preplanned and uploaded ahead of time. The data would be stored on the spacecraft’s 8-track tape recorder and then played back later, when there was time again
for dedicated communications back to Earth. A similar scheme had worked flawlessly for both
Voyagers
at Jupiter, and while it would get colder than normal during the eclipse, crossing through the ring plane was a risky maneuver. It didn’t help that for about ninety minutes the spacecraft would be out of contact with the Earth. Despite all these dangers,
the team was confident that everything would go as planned. Unfortunately, it didn’t.

It was after ten p.m. back in Pasadena, and the team knew that
Voyager 2
would be out of communications with the Earth until about midnight. Some team members went home to catch some sleep. Imaging team member Candy Hansen, exhausted from a hectic day of last-minute planning and first-minute data analysis, made it as far as her car in the JPL parking lot and just fell asleep there for the night. Candy says it was pretty common for her and other team members to catch some sleep there at work during the height of the planning and during the encounters themselves. “It was really hard to leave JPL, because you didn’t want to miss anything!” she told me. “At each of the encounters I just went out and slept in my car for a few hours. At the Jupiter flybys it was the backseat of my ’55 Chevy. By the time of Saturn, I had a little Toyota pickup truck with a camper shell to sleep in. At Neptune I had a van, so sleeping in my car was really comfy by then.”

While she and others slept, a skeleton crew, including Rich Terrile, kept watch for
Voyager 2
’s signal to come out from behind Saturn. Rich had invested significant time in planning for the ring-plane-crossing images that would be taken during the spacecraft’s pass behind Saturn, as well as the close-up images of Tethys and Enceladus. He couldn’t sleep without knowing how those had gone. It is a common theme among the Voyager group: “You didn’t want to
leave work because something new was just around the corner. The next camera move was going to show something unique. And you wanted to be there to see it, to interpret it. It was an electrifying experience.”

When
Voyager 2
did emerge from behind Saturn, there were some cheers and perhaps even a few unconfirmed Champagne corks popped among the small group still on shift. But soon it was clear that something had gone wrong. “All of a sudden, the pictures stopped coming. They were sort of frozen,” recalls Rich Terrile. “Oh my gosh, we’ve got a big problem here.” Telemetry showed that a series of hardware and software errors had occurred while the spacecraft was behind the planet, and that the spacecraft was not what the engineers call “healthy.” Something had happened to cause the preplanned sequence to stop taking data.

“It was an amazingly frustrating, shocking, kind of ‘What do I do? I’m totally helpless’ kind of experience,” Rich Terrile recalls, “where suddenly the spacecraft is
not
doing something when it’s
supposed
to be doing something. You don’t have any idea what’s going on, and it’s a billion miles away.” He remembers feeling as if he went through the various stages of grief (shock, denial, anger, and so on) compressed into a few minutes. “This can’t be happening!” he felt.

Ed Stone had been catching up on a little sleep at home that night as well, but got up early to head back to JPL to be in a morning East Coast media interview about
Voyager
’s results and status. “I think it was probably four in the morning,” he recalls, “and I showed up and they said”—he whispers—“‘Scan platform—it’s stopped!’ So I had to go on this live TV interview and talk about it, even though I’d just heard, and didn’t know anything!” No one else knew much yet either. Ed remembers that the team’s primary feelings
were of real concern for the spacecraft and of course the future of the mission. “We’d had a wonderful Saturn encounter, so Saturn wasn’t really the issue. The issue was, is this a spacecraft that’s going to get to Uranus and Neptune?”

At first, many people’s thoughts turned to the ring-plane crossing. Maybe
Voyager 2
had rammed into a chunk of ice or dust floating among the outer rings? The Plasma Wave Subsystem instrument’s PI, the late Fred Scarf of TRW, reported to the
Voyager
science team the day after the scan platform anomaly that his instrument had detected activity “
a million times the normal energy level very close to the time of the ring-plane crossing.” Scarf played a cassette tape recording of the “sounds of the ring plane crossing” and hypothesized that they were detecting bursts of energy from the high-speed impacts of small dust grains with the spacecraft. As reported by
Voyager
imaging team member David Morrison in his
almost minute-by-minute account of the Saturn flybys, “
The quantity of such impacts was truly staggering—thousands upon thousands, not just at the moment of ring-plane crossing, but extending for several minutes on either side. The roaring sound of these impacts on the tape that he played, sounding almost like a hailstorm striking a tin roof, sent chills down the spines of the seventy-five scientists attending the meeting. But did this unexpected plasma activity really have anything to do with the scan platform failure? No one could tell.”

Had it been too much of a risk to take? If
Voyager
had crashed into a larger-than-normal (a micrometer or two in size) ring particle, it must nevertheless have been an extremely small particle, because the spacecraft’s trajectory when it came out from behind Saturn was still exactly on target for where it was supposed to be. Still, even tiny impacts with ring particles could cause localized
problems with some of the spacecraft’s instruments and subsystems, so the possibility was taken seriously as a potential culprit.

Another possibility was that the scan platform mechanism had failed because of the larger temperature contrast encountered when
Voyager 2
went behind Saturn. As JPL’s testing of the flight scan platform showed, as its mechanisms and components were used, they heated up. Meanwhile, the outside temperature of the spacecraft itself got colder than it had ever been while it was passing though the eclipsed darkness of Saturn’s shadow. It was possible that wear and tear and “normal” aging of the scan platform’s gears and drive shafts and lubrication in the deep-space environment could respond to sudden, more extreme temperature changes in unpredictable ways—so that hypothesis had to be taken seriously too.