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Humanity's farthest spacecraft went quiet. For two days in October, NASA could not hear Voyager 1 at all — no signal, no telemetry, nothing from a probe 16 billion miles away. Then, without any command from Earth, the signal came back. Engineers are still investigating what happened, and the timing could hardly be more dramatic: on November 18, Voyager 1 is set to become the first human-made object ever to sit a full light-day from Earth.
The silence fell on October 18 and 19, when NASA's Deep Space Network — the global array of giant radio antennas that talks to interplanetary spacecraft — failed to detect Voyager 1's signal during two scheduled contacts 24 hours apart. On October 20, the signal unexpectedly returned, before engineers could send any troubleshooting commands. Communication has been maintained since, NASA said in an update on October 29.
What went wrong on board
Engineers found that one group of Voyager 1's thrusters had gone offline, which automatically switched on the backup thrusters. That is the spacecraft's fault-protection system doing exactly what it was designed to do: when onboard computers detect a possible problem with the active thrusters, the probe swaps to backups, settles into a stable, safe configuration and pauses normal operations so engineers have time to diagnose the fault.
But one important caveat remains. NASA has not confirmed that the thruster problem directly caused the communication blackout, and the exact trigger is still under investigation. "This is the first time we've seen the spacecraft do something like this," mission manager Kareem Badaruddin of NASA's Jet Propulsion Laboratory said, "but we are also at a point in the mission where we have to expect the unexpected." Voyager 1 launched on September 5, 1977, with 10 science instruments, and was never designed to operate into the 21st century, let alone for nearly half a century.
The power-saving gamble that came first
The outage followed a major energy-saving operation in early September that engineers nicknamed the "Big Bang." Three devices on board were switched off and three lower-power alternatives switched on at the same moment — all three had to be swapped together to keep the spacecraft warm and its heat evenly spread. Both Voyagers run on plutonium-powered generators that lose about 4 watts of power a year, and NASA says that without the Big Bang, another science instrument would probably have had to be switched off as early as summer 2027.
That slow power decline is the mission's real clock. Every watt saved buys more science from a spacecraft now travelling through interstellar space, which it entered in August 2012 after crossing the heliopause, the boundary where the Sun's influence gives way to the interstellar medium.
What one light-day actually means
On November 18, Voyager 1 will be about 16.09 billion miles (25.89 billion kilometres) from Earth — the distance light travels in one day. A radio message sent to the probe will take 24 hours to arrive, and a reply will take another 24 hours to come back. Every command now effectively takes two days to answer, at a cruising speed of roughly 38,000 miles per hour. Nothing physically changes for the spacecraft at that distance; the milestone is a measure of how far human engineering has reached, and how lonely its operation has become.
The October silence was a reminder that this record-breaking run is fragile. A 49-year-old spacecraft, running on fading power, in a place no human instrument has ever been, can still surprise its own engineers — and still, so far, come back. Related reading: NASA CHAPEA: 378 Days in a Simulated Mars Habitat and Hubble: Supernova Winds and the Early Universe. Sources: Gizmodo and NDTV.