If spacecraft have feelings, little Voyager 2 must have been feeling pretty lonely and dejected out there in the bleakness of space after Earth went quiet in March. The spacecraft faithfully sent regular signals — readings from the few instruments still powered by its ebbing batteries — but received nothing in return. Its loyalty was finally requited on Oct. 29, when it promptly acknowledged a test signal transmitted by NASA. "Promptly" in this case was 32 hours, the time it takes radio waves traveling at the speed of light to get there and back.
The seven-month hiatus was due to engineers carrying out repairs and upgrades to the Deep Space Station 43 (DSS43) dish outside Canberra, Australia, the only radio antenna capable of sending command signals to Voyager 2. The spacecraft's 1989 flyby of Triton, Neptune's largest moon, sent it on a trajectory well south of the ecliptic, Earth's imaginary plane around the sun, so that two of NASA's three Deep Space Network antennae — Goldstone, California, and Madrid, Spain — could no longer "see" the craft. Leaving DSS43 as the sole dish capable of transmitting signals to Voyager 2. (Voyager 1, north of the ecliptic, regularly communicates with Goldstone and Madrid.)
Why was DSS43 taken down while we're still in touch with Voyager 2? According to Philip Baldwin, operations manager for NASA's Space Communications and Navigation Program, "For an antenna that is almost 50 years old, it's better to be proactive than reactive with critical maintenance." So from March to October, this 70-meter (230 feet), old warhorse of a radio dish was upgraded throughout, from its foundation pedestal to the feed cones at the focus of the antenna (see photo).
The fact that we can still "talk" to the two Voyager spacecraft is barely short of miraculous, given that they were designed in the early 1970s, before the age of personal computers. And talk about faint signals! Both Voyagers are now well over 100 times the distance of Earth from the sun (AU), and they transmit with about the same power as the light bulb in your fridge, a little more than 20 watts. At the Canberra station, three smaller dishes — "only" 34 meters (112 feet) in diameter — detect Voyager 2's whisper of a signal, passing it through exquisitely sensitive amplifiers and receivers (cooled by liquid nitrogen to help separate out noise). The refurbished DSS43 antenna handles the uplink.
How long will we be able to stay in touch with these two distant craft as they head out into space, sending back daily information on the density and temperature of the interstellar plasma? I discussed the future of Voyager 1 in my Jan. 4, 2018 column, in particular how engineers finagled a cunning workaround after its failing attitude control thrusters threatened communication with Earth. Voyager 2 hasn't had that problem and will, with some luck, stay alive for several more years — hopefully into 2027, 50 years after its August of 1977 launch. At some point, power from the three radioisotope thermoelectric generators ("nuclear batteries") will decay to the point where its remaining instruments can no longer function. However, NASA is hopeful that a faint engineering signal may still be detectable into the 2030s. May we all endure that long.