The Interstellar Age (7 page)

At its core, every spacecraft, whether on a flyby, orbiting, landing, or roving mission, is
built around a basic chassis called a
bus
. The basic starting design of the
Voyagers
was directly inherited from that of the
Mariners.
Voyager
’s bus is a ten-sided (decahedral) ringlike aluminum structure a little over one foot tall and six feet wide
with ten compartments that house most of the spacecraft’s electronics and computers. Louvers on some of the ten faces of the bus open and close automatically to help keep the temperature inside relatively constant. In the center of the ring, a pressurized tank is loaded up with 220 pounds of hydrazine (N
₂
H
₄
), a common low-thrust propellant used in spacecraft thrusters. The bus design is not exactly the same among all of the
Mariner
spacecraft—some are hexagonal and some are octahedral, but all serve the same basic function of housing and thermally controlling the spacecraft’s main electronics components. Similar bus designs were used for
Pioneer
,
Magellan
,
Galileo
,
Cassini
, and even the Hubble Space Telescope
.
Once engineers find a design that works in the space business, they tend to stick with it.

Building a spacecraft and outfitting it with modern instruments is as complex an undertaking as building an Egyptian pyramid or a Gothic cathedral—and in the 1970s we didn’t yet have robots to help fabricate these wonders of human technology. Thousands of people with a huge range of expertise were required in many diverse areas, including mechanical, thermal, electrical, systems, and software engineering; materials science; physics; planetary and space science; fiscal and human resource management; and even in basic hands-on skills such as welding, soldering, sewing, wire-wrapping, and using machine-shop hand tools. People were involved at JPL as well as at other NASA Centers, such as the Cape Canaveral launch facility; at subcontractors and vendors across the country who were providing parts and services and expertise to the core JPL team; and at universities around the world whose faculty, staff, and students were building instruments and preparing to conduct their scientific investigations.

Spacecraft Systems, Ground Data Systems, and Mission Operations Systems subteams were designed to focus specifically on
issues like power, thermal control, communications, propulsion, navigation, software, mission operations, instruments, and science. But key managers and bigger-picture thinkers had to be embedded within each subteam to help them work well with other teams. It was known that missions that fail do so usually because the different subteams didn’t understand one another’s functions or how they must work together. Conversely, successful missions always have outstanding, often paranoid, systems engineers and managers as liaisons.

At the heart of the
Voyagers
, protected inside the main bus from the cold and radiation of space, are three computer systems that control the spacecraft and its instruments. These include (1) the primary computer, known as the Command Control Subsystem; (2) the Attitude and Articulation Control Subsystem (which despite the name is not a mind-control device—it handles propulsion and spacecraft and instrument orientation); and (3) the Flight Data System, whose most important function is to transmit data from the instruments back to Earth.
Voyager
’s computer can process 80,000 instructions per second—cutting-edge space technology in the mid-1970s, but about a million times slower than the laptop that I am using to write this text.

“In everyone’s pocket right now is a computer far more powerful than the one we flew on
Voyager,
” notes imaging team member and JPL scientist Rich Terrile, “and I don’t mean your cell phone—I mean the key fob that unlocks your car.”

Still, by splitting the computational functionality into three computer systems, each of which was
reprogrammable
(a feature that was relatively new) and had its own fully redundant backup system,
Voyager
engineers had much more software and operational
flexibility than any previous spacecraft. That flexibility would be especially useful when
Voyager 2
’s mission changed dramatically after the Saturn flyby.

The
Voyager
Flight Data System had two ways to get the data back to us humans.
Voyager
could be commanded to radio the data back in real time, essentially as soon as it was captured, and much of
Voyager
’s interplanetary cruise “fields and particles” (nonimaging) data are transmitted this way. Because there were times that the spacecraft were blocked from communication with the Earth, however (as when passing behind a planet or moon), real-time transmissions weren’t always possible. So the second way to transmit data relies on using
Voyager
’s onboard 8-track tape player-recorder to quickly record about 100 images and other data from the instruments, and then to play them back later, when the spacecraft was back in real-time contact with the Earth and not busy doing other things. Being able to record and then play back data later made operations and communications more efficient, but it came at the expense of relying on yet another moving-parts system that had to work well for more than a decade in deep space.

It can be surprising just how well what now seems like old technology really worked. I had an 8-track tape player in my car for a time in the ’80s, and I remember having a heck of a time keeping that machine from eating my tapes. Sure, all I had were used yard-sale Bee Gees and Barry Manilow tapes, but it was still a hassle to deal with untangling them. Similarly, though not as catastrophically, the tape recorders on both
Voyagers
experienced a number of glitches over the years. For example, when the tape got to the end, it would rewind and start again at the beginning (like an old VCR tape). But this stop, fast rewind, then restart cycle made the entire
free-floating spacecraft jolt and wiggle. Even though the forces involved were tiny, the resulting jitter messed up images and other data being taken by the super-sensitive science instruments. As with all the other glitches and surprises, the engineers had to learn to work around these idiosyncrasies.

One of the biggest sources of anxiety that engineers had surrounding the mission, going back to Gary Flandro’s original Grand Tour design days, was the need to keep the spacecraft functioning optimally for such long and unprecedented periods of time in the very cold outer solar system. And to top it off, they would have to withstand brief excursions through such dangerous environments as the harsh radiation created by Jupiter’s magnetic field, or the probably very dusty and icy environment within the plane of Saturn’s rings. A variety of strategies were adopted to mitigate these risks. One was to use shielding made of heavy metals like tantalum as well as radiation-hardened parts to protect the spacecraft from high-energy cosmic ray particles (high-speed protons and other atomic nuclei) in the magnetic fields of the giant planets.

Another strategy was brute-force redundancy of such critical systems as computers, tape recorders, radio transmitters, and receivers. Indeed, shortly after launch,
Voyager 2
’s primary radio receiver failed. The computer automatically switched to the backup receiver, but it, too, began to fail. The
Voyager
team was able to figure out how to avoid failure of the backup, and over time was able to figure out how to communicate with the spacecraft despite its having only a partially working radio and no backup receiver. To assuage their fears of losing the receiver entirely, though, the mission planners had uploaded a small backup mission sequence to
Voyager
that “would hibernate, inside the central computer,” says JPL Mission
Planning Office Manager Charley Kohlhase, “such that if we ever lost the other receiver, and could not command any more, at least it had one sequence to carry out: it would get the approach pictures and data for the
next
planet.”

Charley was the JPL orbital dynamics engineer and mission architect who winnowed thousands of possible flight paths down to the best handful and who led the design of the critical flyby maneuvers that the spacecraft would use to get amazing images of each planet’s atmosphere, moons, and rings, and then slingshot on to the next destination. The flexibility that he helped build into the
Voyager
systems was an important part of overall risk mitigation.

Round-trip travel times for radio signals are many hours between Earth and the outer solar system, and to address this reality
Voyager
utilized a critical and relatively new risk-mitigation strategy called
autonomous fault protection. Voyager
’s programmers knew that the spacecraft would be too far away for real-time communication and diagnosis of problems, so they had to devise ways for the spacecraft to recognize problems on their own and to protect themselves from further damage or harm. The engineers who design software fault-protection routines are the kind of paranoid (in a good way) people you’d want with you while you are preparing for a camping trip. Did you pack the tent? How about rain gear? What if the tent starts to leak? And then it freezes? And then the wind picks up? And you run out of water? And there’s a bear—no, two! Problem after imagined problem has to be anticipated and a solution thought through. Practitioners of this art talk about exploring every possible branch of the “fault tree”—every conceivable, even unlikely, bad thing that could happen—and having a solution for each situation that saves the day. Having a backup system is one risk-reducing
step, but figuring out how to have it switch on by itself when needed was another.
Voyager 2
’s automatic switch to its backup receiver was one example of fault protection in action.

A variety of additional subsystems and instruments were attached to
Voyager
’s bus. These included seven sets of “booms,” or appendages of various lengths, extending away from the bus. The longest is a boom made of fiberglass, known as the magnetometer boom; at forty-three feet in length it keeps the magnetic sensor at its tip as far away as possible from magnetic “contamination” by the spacecraft’s other metallic and electronic components. The next longest appendages are a pair of thirty-three-foot-long antennas used by the plasma wave and radio astronomy experiments, extending down and away from the rest of the spacecraft. Opposite the magnetometer boom, an eight-foot-long “science boom” holds the Plasma Wave, Cosmic Ray, and Low-Energy Charged Particle instruments along its length, and a steerable scan platform on its end that carries the imaging and spectroscopy remote sensing instruments. By turning the scan platform in different directions, the
Voyager
team could point the cameras and other instruments at targets of interest without having to slew the whole spacecraft. This was a substantial time-saving feature, but it introduced the risks of yet another set of moving parts that would have to continue to work well over more than a decade in deep space. Indeed, a problem with the scan platform on
Voyager 2
would cause some tense and dramatic moments for the team during the spacecraft’s passage through the ring plane of Saturn.

The shortest boom holds the spacecraft’s radioisotope thermoelectric generators (RTGs), small nuclear reactors that convert energy from the heat given off by the radioactive decay of a few dozen
golf-ball-sized spheres of plutonium-238 into electricity to run the spacecraft and instruments. Mounted on top of the bus is a parabolic radio telescope twelve feet in diameter called the high-gain antenna, used for communicating with Earth. And finally, dangling from the bottom of the bus are some triangular struts that look like odd, spindly legs because they’re not attached to anything. During launch, however, those struts were attached to an upper-stage propulsion module that helped the
Voyagers
reach their final departure velocity and were then jettisoned.

Before any spacecraft is sent into space, it has to be put through a bunch of tests that simulate the conditions and environments that it will face. These include vibration tables, where the individual instruments and the spacecraft as a whole are violently shaken in the same ways that they will be during launch—and, for good measure, they are shaken
much more
than they will be during launch. For the engineers involved, seeing their creations treated like this can be a painful experience.

The
Voyagers
were assembled from about 65,000 separate parts in JPL’s Building 179—the famous “High Bay” Spacecraft Assembly Facility where spacecraft like the
Rangers
,
Mariners
,
Vikings
,
Galileo
,
Cassini
, and the Mars rovers
Pathfinder
,
Spirit
,
Opportunity
, and
Curiosity
were also brought into the world. The High Bay is a Class 10,000 clean room (less than 10,000 particles of 0.5 micron or larger per cubic foot of air), making it a great place to work if you have allergies. Workers in the High Bay have to wear protective clothing (known affectionately as bunny suits) to keep bacteria and other particles (human beings generate millions of skin, hair, dirt, and dust particles every minute) from contaminating the spacecraft.

I’ve spent time in Building 179, mostly up in the visitors’ gallery,
but occasionally, luckily, inside the High Bay itself, and to me the place is the closest thing to a modern Gothic cathedral’s inner sanctum that I can imagine. Deep inside the High Bay, almost holy relics of our modern civilization are being carefully tended by illuminati who have gone through years of study and training for the privilege of being in that room. They wear ritualistic garb to ensure maximum purity, follow elaborate, carefully prescribed procedures, and when their novitiate work is done, the Chosen One (the spacecraft) emerges from the cathedral and is lofted to the heavens. In that building, and through those doors, pieces of this planet have been worked into structures and systems that are now parts of other planets. Or, in the case of the
Voyagers
, which passed through there too, they are now permanent wanderers among the stars. Building 179 is a factory for Earth’s cosmic artifacts, for the things that we cast off this world that will, ultimately, represent us and our time to our progeny, and perhaps even to other beings that we cannot yet begin to imagine. It’s no wonder I gravitate to the place whenever I’m at JPL.