When the news broke recently that communications had finally been re-established with Voyager 2, I felt a momentary surge of panic. I’ve literally been following the Voyager missions since the twin space probes launched back in 1977, and I’ve been dreading the inevitable day when the last little bit of plutonium in their radioisotope thermal generators decays to the point that they’re no longer able to talk to us, and they go silent in the abyss of interstellar space. According to these headlines, Voyager 2 had stopped communicating for eight months — could this be a quick nap before the final sleep?
Thankfully, no. It turns out that the recent blackout to our most distant outpost of human engineering was completely expected, and completely Earth-side. Upgrades and maintenance were performed on the Deep Space Network antennas that are needed to talk to Voyager. But that left me with a question: What about the rest of the DSN? Could they have not picked up the slack and kept us in touch with Voyager as it sails through interstellar space? The answer to that is an interesting combination of RF engineering and orbital dynamics.
Below the Belt
To understand the outage, one needs to know a little about the Deep Space Network and how it works. I discussed this in detail in the past, but here’s a quick summary. The DSN is comprised of three sites: Madrid in Spain, Goldstone in California, and the site in Canberra, Australia. Each site has an array of dish antennas ranging from 26 meters in diameter to a whopping 70-meter dish. The three sites work together to provide a powerful communication infrastructure that has supported just about every spacecraft that has been launched in the last 50 years or so.
What’s interesting about the DSN sites is their geographic arrangement. Looking down on Earth from the north pole, the DSN sites are spaced almost exactly 120° apart. This means that each site’s view of the sky overlaps the other about 300,000 km into space, thus providing round-the-clock coverage for every space probe. But space travel is not necessarily only two-dimensional, and that’s where the geographic oddities of Earth and curiously, the birth of the solar system itself, play into the recent Voyager blackout.
Almost everything that orbits the Sun does so in a pretty well-defined plane called the ecliptic. The ecliptic plane is likely a remnant of the early disk of dust and debris that eventually congealed into our Sun and the planets. The only major body in the solar system that varies appreciably from the ecliptic is Pluto, whose orbit is inclined about 17° to the ecliptic. The Earth is pretty much always in the ecliptic, and therefore anything that leaves Earth is pretty much going to stay in that plane too, unless provisions are made to alter its orbit.
And that’s exactly what happened with the Voyager twins. Launched to take advantage of a quirk in orbital alignment of the outer planets that occurs only once every 175 years, the Voyager probes were able to complete their Grand Tour because each planetary encounter was planned to give the probes a gravitational assist, flinging them on to their next destination. Both probes remained very close to the plane of the ecliptic for the first part of their journey before intersecting the orbit of Jupiter and picking up speed for the trip to Saturn.
At Saturn, the twin probes would part to carry on very different missions. To get a good look at Saturn’s moon Titan, Voyager 1 approached the planet from below the ecliptic, coming under the south pole. The gravitational assist put it on a trajectory aimed above the ecliptic plane, in the general direction of the constellation Ophiuchus. Voyager 2, however, continued on in the ecliptic, using its gravitational assist to shoot first to Uranus, and eventually to Neptune. There, in the mirror of the trick its twin used to explore Titan, Voyager 2 flew over the north pole of Neptune, which put it on a path to fly close to its moon Triton and on in the general direction of the constellation Sagittarius.
Hello, Canberra Calling
It was this last move that would eventually make Voyager 2 completely dependent on Canberra for communications. Canberra is the only DSN site that lies below the equator, and even though the plane of the equator and the ecliptic are not coplanar — they differ by the roughly 23° tilt of the Earth’s axis — eventually Voyager would get so far below the ecliptic plane that none of the northern hemisphere DSN sites would have line-of-sight to it.
Luckily, Canberra is well-equipped to support Voyager 2. As the probe streaks away from home at 55,000 km/h, with its fuel slowly decaying, Voyager is getting harder and harder to talk to. The giant 70-m dish at Canberra, dubbed DSS-43, provides the gain needed to blast a signal powerful enough to cross the 17 light-hour gap between us and Voyager. Interestingly, Richard Stephenson, a DSN controller at Canberra, reports that although the smaller 34-m dishes at the complex can still be used to radiate a control signal to Voyager, such contacts are “spray and pray” affairs that may or may not be received by the probe and acted upon. Only DSS-43 has the power and gains to still effectively command the spacecraft.
Despite its importance in continuing the Voyager Interstellar Mission (VIM), DSS-43 was showing its age and had to be scheduled for repairs. As we reported back in July, the big dish was taken offline in March of 2020 and has been getting upgrades ever since. After eight months the repairs have progressed to the point where DSS-43 could try out a simple command link to Voyager 2 — just a basic “Are you still there?” ping, which was sent on October 29.
Happily, despite the fact that Voyager had crossed an additional 300 million kilometers of interstellar space in the meantime, the probe returned confirmation of the command almost a day and a half later. There are still a number of DSS-34 upgrade tasks to complete before the antenna is returned to full service in January of 2021, but it seems like the controllers just couldn’t bear to be out of touch with Voyager any longer.
If you want to keep up on progress on DSS-43 specifically and the goings-on at DSN Canberra in general, I highly recommend checking out Richard Stephenson’s Twitter feed. He’s got a ton of great tweets, plenty of pictures of the big dishes, and a wealth of insider information. And a hearty thanks to him for pitching in on this story, and to all the engineers making the DSN continue to deliver important science.