Nearly 49 years after it left Cape Canaveral, Voyager 1 is closing in on a distance no human-made object has ever reached. At 12:16:07 a.m. CST on Nov. 18, NASA calculates the spacecraft will be exactly one light-day from Earth — far enough that a radio signal traveling at the speed of light takes a full 24 hours to make the trip.
That distance works out to about 16.1 billion miles, or 173 astronomical units — 173 times the distance between Earth and the sun — according to figures published by EarthSky based on NASA tracking data. Voyager 1 launched Sept. 5, 1977, just over a week from now marking its 49th anniversary, and has spent decades outrunning its own twin, Voyager 2, which trails well behind in a different direction after both probes toured the outer planets in the 1980s.
What a one-day delay actually means
Communicating with Voyager 1 has always required patience — a round trip already runs close to two days — but crossing the one-light-day mark turns that delay into something closer to a metaphor for the mission itself. Engineers who send a command on a Monday effectively wait until Wednesday for a reply, if the aging spacecraft is able to send one at all. Voyager 1 exited the heliosphere, the bubble of charged particles the sun projects around the solar system, back in 2012, and now flies through interstellar space, still transmitting data from a handful of working instruments.
NASA has been managing that aging hardware carefully. Engineers at the Jet Propulsion Laboratory have shut down onboard instruments one by one over the past several years to conserve power, including a low-energy charged-particle detector switched off earlier this year, and mission planners have said Voyager 1's three plutonium power generators are expected to run too low to operate any instruments sometime in the 2030s. Until then, the probe that once beamed back the first close-up portraits of Jupiter and Saturn keeps adding to a distance record it has held, uncontested, since it overtook Pioneer 10 in 1998.
Reaching a full light-day out does not change how Voyager 1 operates day to day, engineers say. It is simply a milestone in the arithmetic of a mission that was never expected to still be running six presidential terms after launch.
