Nearly 50 years after it left Earth, a small spacecraft launched to tour the outer planets is about to cross a distance marker no human-made object has ever reached. The milestone is not really about time, even though it is usually described using a unit built from it. Instead, it is a way of grasping just how far a probe the size of a small car has traveled since 1977, carrying instruments that are, one by one, being switched off to conserve the little power that remains.
One Light-Day, Explained
According to NASA’s own explainer, Voyager 1 is set to become the first spacecraft to reach a distance of one light-day from Earth, at 2:16:07 a.m. PST on November 18, 2026. A light-day is the distance light travels in 24 hours, moving at about 186,000 miles every second, which works out to 16,094,799,096 miles, or roughly 25.9 billion kilometers. Because radio signals travel at the same speed as light, that distance also means a command sent from Earth will take nearly a full day to reach the spacecraft, with a reply taking just as long to come back.
NASA frames the scale of that distance by comparison with closer objects. Sunlight takes about eight minutes and 20 seconds to cross the roughly 93 million miles between the sun and Earth, and a radio signal to Mars can take anywhere from three to 22 minutes depending on the planets’ relative positions. Neptune, the most distant planet in the solar system, sits about four light-hours and ten light-minutes from Earth, a gap that took Voyager 2 twelve years to cross after launch.
Even the moon, humanity’s most familiar destination beyond Earth, illustrates how quickly the comparisons break down at interplanetary scale. NASA notes that the moon averages about 238,855 miles from Earth, a trip the Artemis II crew is expected to make in roughly three days, while light covers that same distance in only 1.3 seconds. Voyager 1’s light-day distance is tens of thousands of times farther than that lunar gap, a scale NASA describes as difficult to convey in miles or kilometers at all, which is why the agency reaches for light-based units instead.
A Journey That Began in 1977
Voyager 1 launched in 1977 alongside its twin, Voyager 2, which took a different route through the solar system in order to visit Uranus and Neptune. Both spacecraft have since become the only human-made objects to operate outside the heliosphere, the bubble of particles and magnetic fields generated by the sun that encloses the solar system, according to NASA’s mission-status page. Voyager 1 is currently traveling at nearly 38,000 miles per hour, or about 61,000 kilometers per hour, a pace that made the light-day milestone a matter of decades rather than years.
Voyager 2 is following the same trajectory toward that marker on a slower timeline, and NASA projects it will reach its own one-light-day distance from Earth in November 2035, nine years after its twin. The gap between the two milestones reflects the different paths the spacecraft took during their planetary flybys in the late 1970s and 1980s, which set each probe on a distinct course out of the solar system.
Why Engineers Keep Switching Instruments Off
Reaching this distance has come at a cost to the spacecraft’s capabilities. Voyager 1’s power source, a radioisotope generator, produces steadily less electricity each year, so mission engineers have spent decades selectively shutting down instruments to keep the spacecraft’s core systems and remaining science instruments running. NASA’s own instrument-status table, last updated in April 2026, shows Voyager 1’s cosmic ray subsystem was switched off in February 2025 and its low-energy charged particle instrument followed in April 2026, joining cameras and other sensors that were powered down years or decades earlier.
Not every instrument has gone dark. Voyager 1’s magnetometer and plasma wave subsystem remain active, continuing to gather data about the interstellar environment the spacecraft now travels through, more than a decade after it first crossed into that region of space. That trickle of data is still transmitted back to Earth through NASA’s Deep Space Network, even though each exchange now takes nearly two full days to complete a round trip.
Where Voyager 1 Is Headed Next
Voyager 1 is not aimed at any particular destination in the human sense, but its trajectory will eventually carry it toward a star. NASA has identified that star as AC +79 3888, also known as Gliese 445, an obscure star in the constellation Ursa Minor more than 17 light-years from Earth. The spacecraft is expected to pass within about 1.7 light-years of that star in roughly 40,000 years, a distance that sounds close in astronomical terms but is still vastly larger than the gap between the sun and its nearest neighbors.
For comparison, the nearest star system to the sun, Proxima Centauri and its planets, sits about 4.24 light-years away, or roughly 24.95 trillion miles. Voyager 1 will not come anywhere near that system, but its slow drift toward Gliese 445 underscores how much emptier interstellar space is than the solar system it left behind, and how long the spacecraft’s silent journey is likely to continue after its instruments and radio finally go quiet.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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