The big science event in August was the total eclipse of the Sun which will traverse the mainland US on August 21. It’s an amazing, awe-inspiring event, no doubt, and it gives astronomers and other scientists some well-deserved attention from the media.
But the eclipse also had an unintended consequence: it has figurately and literally overshadowed (pun intended) the 40th anniversary of the launch of the twin Voyager spacecraft on their “grand tour” of the outer planets – Jupiter Saturn, Uranus, and Neptune – due to a fortunate planetary alignment which occurs only once every 176 years.
Both spacecraft have travelled far beyond their original objectives. Voyager 2 was launched from Cape Canaveral on Aug. 20, 1977 and is now almost 11 billion miles (18 billion km) distant, while Voyager 1 followed a few weeks later and is now actually ahead of Voyager 2 at 13 billion miles (21 billion km). It is the world's only craft to reach interstellar space, although Voyager 2 is expected to cross that boundary during the next few years, Figure 1 .
The concept of the paired Voyager spacecraft mission was to undertake a grand tour of the four large planets, a trajectory made possible due to a rare planetary alignment worked out by complex astrophysics “mechanics.” (Image courtesy of NASA/JPL)
Both are still transmitting weakly (at low rates, of course) working surprisingly well far beyond their design life, Figure 2 . In addition to instrumentation, each Voyager carries a 12-inch, gold-plated copper phonograph record (remember, there were no CDs or MP3s back then) containing messages from Earth: Beethoven's Fifth, chirping crickets, a baby's cry, a kiss, wind and rain, a thunderous moon rocket launch, African pygmy songs, Solomon Island panpipes, a Peruvian wedding song and greetings in dozens of languages. There are also more than 100 electronic images on each record showing 20th -century life, traffic jams and all.
The Voyagers packed a lot of instrumentation into their limited payload, and the instrumentation of the period was far larger, more power hungry, and less sophisticated than what we now routinely use.(Image courtesy of NASA/JPL)
The mission has been managed by the Jet Propulsion Laboratory since its inception, and in some cases, three generations of staffers have been on the project. There are so many amazing aspects to the paired mission that it is hard to know where to start or which to cite. Obviously, the components on these spacecraft are ancient and crude by our standards and mostly analog, but they have certainly done well.
The list of what is still operating is impressive, Figure 3 . Many functions and tasks which we now routinely assign to low-power microcontrollers had to be implemented with discrete digital logic and components. Nor could designers use the “latest, greatest” just-released components, either; they had to restrict themselves only to ones with a solid, verified track record. There wasn’t much detailed understanding on the challenges of space travel either, such as solar-heating effects, long periods of extreme cold, or cosmic radiation.
Powerful design tools we assume available such as CAD, simulation, BOM management, and project tracking, simply didn’t exist or were also crude. It’s a tribute to the designers, engineers, and scientists that they did so much with what they had, and did not say “we can’t do it, we don’t have the tools yet.” They made it happen with what they had, the only environment they knew.
Even after 40 years, JPL reports that some of the instrumentation packages are still turned on and available – truly astounding.(Image courtesy of NASA/JPL)
There’s more to the Voyager story than the spacecraft themselves, of course. There’s the challenge of providing power, produced by thermoelectric (thermocouple) generators (TEGs) heated by the decay of radioactive material. Although not efficient (around 10 to 20 percent, depending on various factors) they have a long-life and are reliable, as there is no chemical aspect to break down.
There’s also the challenge of tracking ad communicating with the Voyagers at such distances, given their weak output and limited antenna size. Both the Earth-based and the spacecraft receivers are dealing with SNRs so low we can’t believe any sort of acceptable data rate and BER are possible, yet there is a link. And how many engineers can accept a working on project where the one-way propagation delay is measured in hours, now at about 20 hours for Voyager 1 and 16 hours for Voyager 2)?
There’s also a fascinating but little-known prequel story to the Voyager missions. Even with the rare planetary alignment, the journey would have not been possible without the benefit of a gravity slingshot, in which the gravitational pull of a planet is used to accelerate the spacecraft on its journey beyond that planet. This is now a routine undertaking, and is even occasionally used to slow a spacecraft down into a desired path while minimizing fuel consumption. The slingshot would not have been possible without the work of Michael Minovitch, a UCLA graduate student who had a summer job at JPL, Reference 1 . In 1961, he conceived of the slingshot and worked out the details as an after-hours, “on his own” project; you can read a scanned pdf of his project paper explaining the concept and the planetary mechanics at Reference 2 . It, too, is a throwback in time, as it is done on a typewriter (what’s that?), the drawings are done by hand, and the special characters (mostly Greek, plus others) are hand-written into the text and figures!
In addition to the interesting mission information and perspectives at the JPL web site (see References 3 through 6 ), there’s a well-written book proving a lengthy but fascinating look at the dual missions, “Voyager: seeking newer worlds in the third great age of discovery,” by Stephen J. Pyne. It’s worth checking out.
Have you followed the Voyager missions? Could you work on a project with such a long life to “completion”, so many periods of “nothingness”, and one-way path delays of tens of hours?
- IEEE Engineering360
- ”A Method for Determining Interplanetary Free-Fall Reconnaissance Trajectories”, Jet Propulsion Laboratory, Technical Memo #312-130, Michael Minovitch
- ”A Once-in-a-Lifetime Alignment”, Jet Propulsion Laboratory.
- Two Voyagers Taught Us How to Listen to Space, Jet Propulsion Laboratory
- “Mission Status”, Jet Propulsion Laboratory
- “Voyager at 40: Keep Reaching for the Stars”, Jet Propulsion Laboratory
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