Voyager 1 has just ticked off another milestone: on Tuesday it reached 138 astronomical units from Earth, or about 20,600,000,000km from the planet on which you're (presumably!) reading this story.
It's not an achievement that will be widely noticed or celebrated, because every kilometre it travels sets a new record for the most-travelled artefact humans have ever created. Passing an entirely notional milestone makes little difference, but we noticed the “Miles Since Left Planet” counter tick over and so here we are!
It now takes 38 hours and 15minutes for a radio signal to make it to Voyager and back to Earth, a period of time that is both remarkable and terrifying: we think that the Universe is about 27 billion light years across, but we're just 19 light hours into it. Both Voyagers stopped sending pictures decades back and in 2015 NASA stopped publishing even weekly mission updates because so few people bothered to read them and it was decided to save the mission team the time it took to compile them.
NASA has, however, recently updated the Spacecraft Lifetime page for both Voyagers, confirming that both craft's Radioisotope Thermoelectric Generators continue to produce less power, but are also achieving “better performance than the pre-launch predictions”. But before long NASA will have to decide which instruments it has to shut down to allow the probes to operate with reduced power budgets.
One tool already scheduled to shut down in 2018 is Voyager 1's data tape recorder, as NASA says it's “Limited by ability to capture 1.4 kbps data using a 70m/34m antenna array.” We think that means the tape spools out data at a faster rate, but that Voyager 1 is now so distant that even a big antenna can't make a sufficiently fast connection to receive its output.
It's hoped that four instruments – the detector of Low-Energy Charged Particles, Cosmic Ray Subsystem, Magnetometer and Plasma Wave Subsystem – will operate on both craft after 2020.
But the space agency's advice is that “no earlier than 2025”, neither Voyager will produce enough power to allow operation of a single on-board instrument. Once Voyager 1 runs out of power, it'll head towards planet AC +79 3888, aka Gliese 445, 17.6 light years from home. ®
Bootnote: Your correspondent visited the Canberra Deep Space Communication Complex late last year, just in time to see a connection to Voyager 2. Bucket list item, ticked!
Visited Canberra deep space tracking centre. Here's today's schedule, Inc Voyager 2 and New Horizons downloads pic.twitter.com/vw762m3Zd3— Simon Sharwood (@ssharwood) December 30, 2016