The Reg chats with Voyager Imaging Team member Dr Garry E Hunt

Register hack learns the benefits of brimming the tank before flight

Onward, to Saturn!

The Saturn encounters occurred in 1981 and 1982, with the observation of Titan being a top priority for the project. While some have described the encounter as disappointing, because the Voyager's cameras could not pierce the orange haze to observe the surface, Hunt's enthusiasm is infectious: "We've got the hydrocarbons. So we discovered it was the first nitrogen atmosphere outside the Earth, and we discovered the presence of acetylene and ethane and methane... we were trying to work out the atmospheric circulation. We couldn't see the surface, which was a great bind. We would love to have seen the surface."

The surface of Titan would be viewed close-up by the Huygens lander, more than two decades later.

There were other highlights for Hunt in the Saturn encounter. His favourite moon is Mimas, which he calls the "Death Star" owing to "a zonking great crater" in the side. He also listed the discovery of red spots on Saturn, lightning and active weather systems. And, of course, the spokes in the rings around the planet.

Fixing the platform

Following Saturn, and with the spacecraft performing well, funding was required in order to continue on to Uranus, as well a confession. "A group of us was sent off by JPL to Washington, and they said, 'You can't do that. You think you're going that far? You haven't got any fuel!' We owned up. We had far more than we need."

Unfortunately, the team would discover that all was not well aboard Voyager 2. The scan platform, which would be used to swing the cameras and other instruments to bear on Uranus, had jammed solid. The first hint of an issue came from a press enquiry.

"One night we came back from dinner and we got a phone call. And they said to us, 'All these pictures are dark space.' And we looked at each other and said, 'Oh we're calibrating. We're just taking calibration pictures.' We put the phone down and said, 'What the hell has happened?'"

The lubrication in the gears that move the platform had frozen. Hunt remembers the "three days of hell" required to recover the spacecraft well. "You can't call out the AA man to help you. You think, ah, we have the engineering model in von Kármán, and I checked all the ways it could work, and I found that if they missed out one of the speeds, I can't remember, it may be the fastest speed, they would just move it on one speed, they could actually move it again, and they got it back together again."

It was indeed the fastest slew rate that caused the gears of the scan platform to seize. The solution described by Hunt was tried out on Voyager 1, since it had no more scheduled targets, and then sent to Voyager 2.

Encounter at Uranus

The team had nearly five years before Voyager 2 was due to arrive at Uranus so took the opportunity to perform substantial updates to the on-board software as well as hardware upgrades on the ground. In addition to getting the Deep Space Network dishes expanded in order to pick up the weaker Voyager signal, data compression techniques were introduced to squeeze the most out of the limited bandwidth as possible.

The spacecraft also became more intelligent in what it actually sent back to Earth. "We modified what we sent back to send back real pictures and not masses of black sky. So if you got a little image of something and the rest is dark sky, stop doing it."

Hunt is also particularly proud of the image motion compensation software, added to the spacecraft to deal with the pitch and yaw of the probe as exposure times lengthened for the low light levels of the outer planets.

The stunning images sent back by the veteran spacecraft are testament to the upgrades. When asked if there are any images where, perhaps, it all went a bit wrong, Hunt is adamant: "Every one was good... but obviously some have more scientific value."

Uranus, of course, had not enjoyed much in the way of ground observations before Voyager's arrival. Hunt recalls: "We saw features there. Differences between the hemispheres. Odd little clouds. Extraordinary features on some of the satellites. Evidence of geysers and other sort of motions that said these satellites aren't... dead bodies. They're being hit."

Hunt also remembers a surprising use for the probe's imagery: "The interesting thing about Uranus was, we produced this ghastly UV picture using UV filter to show the structure, and it appeared on just about every chocolate box on the planet."

Running rings around Neptune

The final planetary encounter for the Voyager project took place in 1989, as the veteran probe sped past Neptune. The mission was to throw up one last scare for the imaging team. As Hunt puts it, an earthbound astronomer had "found rings around Neptune. And, oh my God, that's fine, except it's bang into the path that we're sending the spacecraft. Here was the trajectory, and lo and behold, it's aiming straight for these rings."

Thankfully, the probe sailed through, taking very, very long exposure imagery along the way.

Pale blue goodbye

In 1990, 18 years after its creation, the work of the Voyager imaging team came to an end. Hunt had already moved from academia to business but remained an active participant. The Voyager cameras were set a final task before being eventually, and permanently, turned off.

The "Family Portrait" taken by Voyager 1 shows all the planets of the solar system other than Mars and Mercury. The Earth, famously, is less than a pixel in size, appearing as a "Pale Blue Dot".

"It was one of Carl's [Sagan] suggestions. The Blue Dot was a great suggestion, because I think it just shows that we are no more than a speck amongst the stars," said Hunt. "We had been together for 18 years, and that was always our farewell thing. That was our Valentine's present for 1990."

As the members of the original imaging team fade away, the Voyagers continue their journey. Plans are afoot to keep communicating with the probes until 2025, or perhaps a little later, before the power finally gives out. As well as the famous golden records, the probes also carry the signatures of all the scientists and engineers involved in the grand project, many of whom will not see the final signal from their spacecraft disappear.

"My signature is on there," Hunt said. "It's going out of the solar system. How wonderful to think that was done." ®