The Interstellar Age (17 page)

Indeed, just in the last few years, support for a NASA mission back to the Jupiter system that would focus on Europa (and that would complement the Europeans’
JUICE
mission to Ganymede) has grown substantially. This support comes partly from the scientific community, which ranked a return mission to Europa as an extremely high priority for NASA in the most recent National Academies of Sciences “
Decadal Survey of Planetary Science”; partly from extraordinary public and media interest in astrobiology and the search for life elsewhere in the universe; and partly (and unexpectedly) from the individual, personal interest of US
congressman John Culberson, who represents Texas’s Seventh District in the suburbs west of Houston. For reasons that I can describe only as remarkable and supremely fortunate for my line of work, Mr. Culberson just loves Europa. He is fascinated by the possibility of life on this ocean world. I’ve visited him in his office in Washington (he is perhaps the only person out of 535 members of Congress with photos of Europa on his office walls!) and, along with such
Voyager
team members as Candy Hansen (who recently served a term as the chair of the American Astronomical Society’s Division for Planetary Sciences), have helped keep him updated on the latest findings about Europa from NASA’s missions and telescopes. He is an educated and engaged advocate for space exploration—a rare and delightful occurrence in Congress, to be sure. But perhaps most important, he is also a high-ranking member of the US House of Representatives Committee on Science, and so he takes it as a personal goal and passion to try to convince his colleagues in Congress to authorize funds for NASA to go back to Europa and find out what it’s really like. Mr. Culberson has been successful in the past few years in getting some funds allocated for the development of new technologies needed to operate spacecraft for long periods of time in Jupiter’s high-radiation environment. Sometimes, even in rocket science, a personal touch makes all the difference.

5

Drama within the Rings

E
VEN WITH TWO
spectacular flybys of Jupiter in the bag, and a trove of planetary science discoveries and puzzles opened in the process, the
Voyager
team didn’t feel like they had much time to rest and reflect on their good fortune. Both probes were speeding on to encounters with the giant ringed planet Saturn, with
Voyager 1
’s Saturn flyby set to occur only sixteen months after
Voyager 2
’s at Jupiter. That might seem like a lot of time, but every working hour was spent on the planning that needed to be done to optimize the trajectories of the spacecraft past the planet’s moons and rings.

I remember the first time I saw Saturn through a real astronomical telescope. I must have been around ten years old, and I think it was during a Boy Scout field trip to a small rural observatory run by a local amateur astronomy club (I think it was the SkyScrapers, the
club I would later join). It was a clunky old refractor—the kind of long telescope that uses lenses instead of the more modern shorter kind that uses mirrors—with a tube like an iron water main and giant counterweights and rivets that could have come right out of a 1930s WPA construction project. Still, it was a
real
telescope, capable of large magnification and good image quality, especially from a rural site on a clear summer night. The telescope operator was hopping around the sky, manually slewing the tube to focus on many of the greatest hits (which he knew by heart) for our viewing pleasure. Double stars, nebulae, star clusters, a faint, fuzzy comet . . . Each of us had a few seconds of viewing time, perched up on a step stool so we could reach the eyepiece. When it was my turn to see Saturn, highly magnified, in steady skies, with rings tilted gloriously in view, I remember going into an almost trancelike state, hypnotized by the elegance of what I was seeing. The next kid in line started pushing me to step away so he could have his turn, which I did only reluctantly.

What I didn’t realize at the time was that professional astronomers in the 1970s weren’t really
that
far ahead of the amateurs in terms of knowing what Saturn was really like. It’s funny how we always think that the experts are leagues beyond the rest of us, members of a secret club that we can never enter. Knowledge of the chemistry of Saturn’s atmosphere at the time was based on some good measurements from big telescopes, some assumed similarities with the chemistry of Jupiter, and the assumption that this gas giant, too, was likely mostly made of hydrogen and helium, like the sun (after all, it formed from the same cloud of gas and dust that formed the sun itself). Even less was known about the planet’s famous rings and its collection of ten then-known moons. It was
understood that the rings could not be a solid ring of material (the British physicist James Clerk Maxwell showed in the 1850s that such a ring would break apart from the stresses of the
inner and outer parts orbiting at different speeds). It was also known that some of the moons were quite large, perhaps even planet-sized bodies. The first hints of the icy composition of these worlds were only just starting to come in from the latest high-tech spectrometers mounted on large Earth-based telescopes.

While the
Pioneer 11
flyby of the Saturn system in September 1979 helped us understand the gravity and magnetic fields of Saturn in much more detail, even that flyby encounter, with its limited imaging capabilities, didn’t dramatically increase our knowledge of what Saturn and its rings and moons looked like in detail. Besides showing that Saturn’s largest moon, Titan, is an extremely cold world—at only maybe 100 degrees or so above absolute zero, probably too cold for life as we know it—a key facet of
Pioneer 11
’s trip through the Saturn system was to simply demonstrate that it could be done.

“
Pioneer 11
was targeted to fly through Saturn’s ring plane near where
Voyager 2
had to fly through the ring plane to go on to Uranus,” says Ed Stone.
Pioneer
was only about a year or so ahead of the
Voyagers
, and so Ed and Charley Kohlhase and the rest of the
Voyager
team were paying careful attention to
Pioneer
’s mission, which was run by colleagues from NASA’s Ames Research Center, just south of San Francisco. Among the questions scientists had for
Pioneer
: Could a spacecraft pass unscathed through the plane of Saturn’s rings, not that far from the dense rings themselves? Would there be unanticipated radiation or magnetic field effects close to Saturn that could be more dangerous than what was encountered at Jupiter?

From below and beyond the rings,
Pioneer 11
’s farewell pictures of Saturn foretold of the even more spectacular sights and
perspectives to come from the
Voyagers
that would follow.
Pioneer
had passed through the ring plane of Saturn without incident—but perhaps only just barely. Later analysis of the
Pioneer 11
trajectory showed that the spacecraft may have
just
missed (by about 2,500 miles—a near miss in astronomy) crashing into a small moon only later discovered orbiting near Saturn’s rings. The experience caused some concern for
Voyager 2
, but mission planners were not particularly concerned.

“We thought, So it passed within 2,500 miles of something. So what?” Charley Kohlhase says. “Space is big!”

Saturn Swingbys.
Voyager 1
(
top
) and
Voyager 2
(
bottom
) flyby trajectories past Saturn.
(NASA/JPL)

Regardless, because of the importance of the Titan flyby,
Voyager 1
had to be targeted to cross the plane of Saturn’s rings much farther from the planet itself than
Pioneer
had.
Voyager 2
, however, would have to take a deeper plunge through the ring plane—closer-in to the planet—if it was going to use Saturn’s gravity to slingshot ahead to Uranus and Neptune. Just like at Jupiter,
Voyager
mission planners tried to time each spacecraft’s two-day trip through the heart of the Saturn system to get as close to the planet, rings, and as many moons as possible, albeit with some important constraints.

An important constraint was imposed by Ed Stone and the
Voyager
science team: the trajectory had to take the spacecraft
behind
and
into the shadow of
the planet as seen from both the Earth and the sun, so that both sunlight traveling to the spacecraft, and the spacecraft’s radio signal traveling to Earth, would pass through Saturn’s upper atmosphere. Such an event is called an
occultation
, because the planet blocks or
occults
(obscures) the sun (or the Earth) from the viewpoint of the observer, which in this case is the
Voyager
spacecraft. An eclipse is a kind of occultation. Occultations of sunlight passing through a planetary atmosphere (which
Voyager
could observe) or
Voyager
’s radio signals passing through a planetary atmosphere (which the DSN could observe from Earth) provide a way for scientists to probe the details of the pressure, temperature, and chemistry of gases in that planet’s atmosphere. As sunlight passes through the upper atmosphere, it is absorbed by gases deeper and deeper in the atmosphere until it is blocked entirely by the increasing density of the gas (for a giant planet), or by the surface itself (for a moon or terrestrial planet). Instruments on
Voyager
could measure the patterns of that absorbed sunlight and use those patterns as fingerprinting tools for the identification of specific atoms and molecules. The same is true of the DSN antennas watching the pattern of
Voyager
’s radio signal slowly fade from view as it went behind the planet from the Earth’s perspective. It’s a powerful scientific trick to exploit and so it couldn’t be passed up when designing the optimum orbit trajectory. I can imagine that it was just another source of headaches sometimes for Charley Kohlhase and his mission-design team, however.

Even more important, perhaps, was the flyby of the moon Titan. What little information there was about Titan up until that point seemed to suggest that its environment could be similar to that of the early Earth. The flyby was a possible way to get in touch with our
primordial past! That called for close-up imaging and other measurements of Titan, including a pass behind the moon as seen from the Earth and the sun. This requirement would almost single-handedly define the eventual trajectory and fate of
Voyager 1
past Saturn. Moreover, the possibility of
Voyager 2
continuing on to Uranus and Neptune would depend entirely on whether
Voyager 1
’s Titan encounter was successful.

Prior to the space age, Titan had been discovered to have an atmosphere (the only moon known to have one), consisting of at least methane and
perhaps some other complex hydrocarbons.
Pioneer 11
’s low-resolution flyby images showed just an orangey sphere, bland but suggestive that the atmosphere was likely thick and hazy. Despite having extremely low temperatures, the evidence that Titan was a model of what the early Earth’s atmosphere may have been like was significant. Before life began adding oxygen to our planet’s atmosphere, Earth’s atmosphere was also rich in hydrocarbon (and nitrogen), what chemists call a
reducing environment
(as opposed to an
oxidizing environment
).

In some pioneering experiments in the 1950s, biochemists Stanley Miller and Harold Urey did a famous set of experiments to demonstrate how adding water and energy (like lightning) to a reducing environment containing simple hydrocarbon gases could lead to the formation of even more complex organic molecules, including some simple amino acids. Biologists, and now astrobiologists as well, believe that this kind of chemistry
could have led to the formation of life on Earth.
Voyager
scientists wondered if it could have led to similar kinds of biogeochemical magic on Titan. Maybe the oldest single-celled organisms in the solar system were swimming around in Titan’s primordial soup.