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When news broke recently that communications had finally been re-established with Voyager 2, I felt a momentary wave of panic. I have literally followed the Voyager missions since the twin space probes were launched in 1977, and dreaded the inevitable day when the last bit of plutonium in their radioisotope heat generators decays to the point where they are no longer able to talk to. us, and are silent in the abyss of interstellar space. According to these headlines, Voyager 2 had stopped communicating for eight months – could it have been a short nap before final sleep?
Fortunately no. It turns out that the recent blackout at our farthest outpost of human engineering was fully anticipated and completely from the side of the Earth. Upgrades and maintenance were performed on the Deep Space Network antennas needed to communicate with Voyager. But that left me with a question: what about the rest of the DSN? Could they have missed the problem and keep us in touch with Voyager as it navigates through interstellar space? The answer to this question is an interesting combination of RF engineering and orbital dynamics.
Below the belt
To understand the outage, you need to know a little about the Deep Space Network and how it works. I’ve discussed this in detail in the past, but here’s a quick rundown. The DSN consists of three sites: Madrid in Spain, Goldstone in California and the site in Canberra, Australia. Each site has a series of satellite dishes ranging from 26 meters in diameter to a huge 70 meter dish. The three sites work together to provide a powerful communications infrastructure that has supported nearly every spacecraft launched in the last 50 years or so.
The interesting thing about DSN sites is their geographic arrangement. Looking down on Earth from the north pole, the DSN sites are spaced almost exactly 120 ° from each other. This means that each site’s view of the sky overlaps the other for approximately 300,000km into space, thus providing 24-hour coverage for each space probe. But space travel isn’t necessarily just 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 on a fairly well-defined plane called the ecliptic. The ecliptic plane is likely a remnant of the first disk of dust and debris that eventually froze in our Sun and planets. The only major body in the solar system that varies appreciably from the ecliptic is Pluto, whose orbit is tilted approximately 17 ° to the ecliptic. The Earth is practically always in the ecliptic, and therefore anything that leaves the Earth will practically remain on that plane as well, unless steps are taken to alter its orbit.
And that’s exactly what happened with the Voyager twins. Launched to exploit an oddity in the 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 provide the probes with gravitational aid. hurling them on their next destination. Both probes stayed very close to the ecliptic plane for the first part of their journey before intersecting Jupiter’s orbit and picking up speed for the journey to Saturn.
At Saturn, the twin probes would have separated to carry out 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 assistance places it on a trajectory pointed above the ecliptic plane, in the general direction of the constellation Ophiuchus. Voyager 2, however, continued into the ecliptic, using its gravitational assist to shoot first at Uranus and finally at Neptune. There, in the makeup mirror used by her twin to explore Titan, Voyager 2 flew over Neptune’s north pole, which set her on a path to fly near her moon Triton and in the general direction of the constellation Sagittarius.
Hi, Canberra Calling
It was the latter move that would ultimately make Voyager 2 completely dependent on Canberra for communications. Canberra is the only DSN site that lies below the equator, and although the equatorial plane and the ecliptic are not coplanar – they differ in the approximately 23 ° inclination of the Earth’s axis – ultimately Voyager it would come so far below the ecliptic plane that none of the Northern Hemisphere’s DSN sites would have a line of sight on it.
Fortunately, Canberra is well equipped to support Voyager 2. As the probe moves away from home at a speed of 55,000 km / h, with its slowly deteriorating fuel, it becomes increasingly difficult to talk to Voyager. The giant 70-meter dish in Canberra, dubbed the DSS-43, provides the gain needed to launch a signal powerful enough to cross the 17 light-hour gap between us and Voyager. Interestingly, Richard Stephenson, a DSN controller in Canberra, reports that although the complex’s smaller 34m dishes can still be used to radiate a control signal to Voyager, such contacts are “spray and pray” affairs that can either they cannot be received by the probe and acted upon. Only the DSS-43 has the power and gains to still effectively command the spacecraft.
Despite its importance in continuing the Voyager Interstellar Mission (VIM), the DSS-43 was showing its age and needed to be scheduled for repairs. As we reported back in July, the big pot went offline in March 2020 and has received updates since. After eight months the repairs progressed to the point where DSS-43 was able to test a simple command connection to Voyager 2 – just a simple “Are you still there?” ping, sent on October 29.
Fortunately, despite Voyager having crossed another 300 million kilometers of interstellar space in the meantime, the spacecraft 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 returns to full service in January 2021, but it seems the controllers can’t stand being out of touch with Voyager anymore.
If you want to keep abreast of progress on DSS-43 in particular and what’s going on at DSN Canberra in general, I highly recommend you check out Richard Stephenson’s Twitter feed. It has a lot of great tweets, a lot of pictures of the great dishes and a great deal of inside information. And a heartfelt thanks to him for putting himself out there on this story, and to all the engineers who make the DSN continue to provide important scientific data.
[Featured images sources: CSIRO, JPL/NASA]
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