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When news broke recently that communications had finally been reestablished with Voyager 2, I felt a momentary wave of panic. I have literally followed the Voyager missions since the launch of the Twin Space Probes in 1977, and dreaded the inevitable day when the last little piece of plutonium in their radioisotopic thermal generators decays to the point that they are no longer able to talk to us, and they fall silent in the abyss of interstellar space. According to those headlines, Voyager 2 had been off communication for eight months – could it be a little nap before final sleep?
Hopefully not. Turns out the recent blackout at our furthest human-engineering outpost was completely overdue, and completely on the Earth side. Upgrades and maintenance have been performed on the antennas of the Deep Space Network necessary to communicate with Voyager. But that left me with a question: what about the rest of the DSN? Couldn’t they have taken over and kept us in touch with Voyager as it navigates interstellar space? The answer to this is an interesting combination of RF engineering and orbital dynamics.
Under the belt
To understand the failure, you need to know a little more about the Deep Space Network and how it works. I have discussed this in detail in the past, but here is a quick summary. The DSN has three sites: Madrid in Spain, Goldstone in California and the Canberra site in Australia. Each site has a range of satellite dishes ranging from 26 meters in diameter to a massive 70 meter dish. The three sites work together to provide a powerful communications infrastructure that has supported just about every spacecraft launched in the past 50 years.
The interest of DSN sites is their geographical layout. Looking at Earth from the North Pole, the DSN sites are almost exactly 120 ° apart. This means that the sky view from each site overlaps the other for approximately 300,000 km in space, providing 24 hour coverage for each space probe. But space travel isn’t necessarily just two-dimensional, and this is where Earth’s geographic quirks and, oddly enough, the birth of the solar system itself, play into Voyager’s recent blackout.
Almost everything that revolves around the Sun does so in a fairly well-defined plane called the ecliptic. The ecliptic plane is probably a remnant of the first disk of dust and debris that eventually froze in our Sun and the planets. The only major body in the solar system that varies significantly from the ecliptic is Pluto, whose orbit is tilted approximately 17 ° from the ecliptic. Earth is pretty much always in the ecliptic, and therefore anything that leaves Earth will pretty much remain in that plane as well, unless arrangements are made to change its orbit.
And that’s exactly what happened with the Voyager twins. Launched to take advantage of a quirk in the orbital alignment of the outer planets that only occurs once every 175 years, the Voyager probes were able to complete their Grand Tour because each planetary encounter was scheduled to give the probes gravitational assistance. , projecting them to their next destination. The two probes stayed very close to the plane of the ecliptic during the first part of their trip before crossing Jupiter’s orbit and picking up speed for the trip to Saturn.
At Saturn, the twin probes would separate to carry out very different missions. To get a good look at Saturn’s Titan moon, Voyager 1 approached the planet from below the ecliptic, passing below the south pole. The gravitational assistance places it on a trajectory directed above the plane of the ecliptic, in the general direction of the constellation Ophiuchus. Voyager 2, however, continued into the ecliptic, using its gravitational assistance to shoot Uranus first, and ultimately Neptune. There, in the mirror of the trick his twin was using to explore Titan, Voyager 2 flew over Neptune’s North Pole, which set him on a path to fly near his moon Triton and in the general direction of the constellation Sagittarius.
Hello, Canberra is calling
It was this last move that would end up making Voyager 2 completely dependent on Canberra for communications. Canberra is the only DSN site that is below the equator, and although the plane of the equator and the ecliptic are not coplanar – they differ by tilting about 23 ° from the axis from Earth – Voyager would eventually get so far below the plane of the ecliptic. that none of the DSN sites in the northern hemisphere would have a line of sight.
Fortunately, Canberra is well equipped to support Voyager 2. As the probe pulls away from home at 55,000 km / h and its fuel slowly breaks down, Voyager becomes increasingly difficult to speak. Canberra’s giant 70m antenna, nicknamed DSS-43, provides the gain necessary to send a signal strong enough to cross the 17 light hour gap between us and Voyager. Interestingly, Richard Stephenson, a DSN controller in Canberra, reports that although the smaller 34m dishes in the complex can still be used to emit a control signal to Voyager, these contacts are business of “spraying and prayer ”which may or may not be received by the probe and carried out. Only the DSS-43 has the power and gains to still effectively command the spacecraft.
Despite its importance in the pursuit of the Voyager Interstellar (VIM) mission, the DSS-43 was showing its age and needed repair. As we reported in July, the Big Dish went offline in March 2020 and has been the subject of upgrades since. After eight months, repairs progressed to the point where the DSS-43 could try a simple command link to Voyager 2 – just a simple “Are you still here? Ping, which was sent on October 29th.”
Fortunately, despite Voyager having traversed an additional 300 million kilometers of interstellar space in the meantime, the probe returned confirmation of the order 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 2021, but it looks like controllers just couldn’t bear to be out of service. contact with Voyager.
If you’re interested in keeping up with the progress on DSS-43 in particular and the events of DSN Canberra in general, I highly recommend you check out Richard Stephenson’s Twitter feed. He has a ton of great tweets, plenty of photos of the great dishes, and a wealth of inside information. And a big thank you to him for his contribution to this story, and to all the engineers who make the DSN continue to provide important science.
[Featured images sources: CSIRO, JPL/NASA]
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