NASA's deep-space probe Voyager 1 will approach a historic milestone in November 2026. Launched in 1977, the spacecraft will at that point reach a distance of one light-day from Earth, becoming the first human-made object ever to do so.
A "light-day" refers to the distance light travels in 24 hours—about 16 billion miles, or 26 billion kilometers. Suzy Dodd, project manager for Voyager at NASA's Jet Propulsion Laboratory, notes that at such distances, a signal sent from Earth takes 24 hours to reach the spacecraft, and its response takes another 24 hours to return.
"If I send a command at 8:00 a.m. on Monday saying 'Good morning Voyager 1,' I'll receive the answer around 8:00 a.m. on Wednesday," Dodd said.
Voyager 1 and its twin, Voyager 2, are the only spacecraft to have exited the "heliosphere"—the bubble of magnetic fields and particles extending beyond Pluto's orbit that surrounds the Sun. Voyager 1 is currently about 15.8 billion miles from Earth, continuing its journey through interstellar space.
After decades of operation, both probes have shut down some scientific instruments to conserve power. The instruments still functioning collect critical data about particle density, magnetic fields, and plasma waves in this region, where the Sun's influence fades. These measurements will guide the design of future missions into interstellar space.
Voyager 1 has been traveling in the same direction and at the same speed since its flyby of Saturn in 1980. Moving at about 38,000 miles per hour, it heads outward from the Solar System. After Saturn, its trajectory lifted above the plane of the planets, while Voyager 2 moved below it following its 1989 encounter with Neptune. Neither spacecraft has had a trajectory correction since.
At such distances, communication is extremely difficult. The spacecraft can transmit only 160 bits of data per second—a speed compared to early "dial-up" internet. "At this distance, the signal is so weak that we need to use multiple large antennas simultaneously to collect it," Dodd explained.
This low data rate means scientific measurements and health updates from the spacecraft arrive slowly and in small amounts. When an issue arises, diagnosing and correcting it can take days.
Voyager probes were designed with autonomy in mind, anticipating long communication delays. When a serious fault is detected, they can enter "safe mode," point their antennas toward Earth, and await instructions.
To prolong their lives, engineers have had to shut down systems and instruments to reduce power usage. Keeping the spacecraft warm is crucial; if the thruster lines freeze and the antennas drift away from Earth, communication could be lost forever.
Scientists especially hope to better understand how the Sun's magnetic field interacts with interstellar space at the heliopause—the boundary of the heliosphere. Dodd compares it to an ocean shoreline: the movement of waves and currents gradually becomes calmer beyond a certain point. Voyager's instruments measure the "ripples" created as solar wind mixes with the interstellar medium.
Voyager 1 and Voyager 2 will mark 50 years in space in 2027. But until then, additional systems and instruments will likely be shut down. The team hopes to keep cosmic-ray detectors, magnetometers, and plasma-wave instruments running as long as possible, allowing the probes to function like "weather satellites" for interstellar space.
Dodd remains hopeful at least one spacecraft will operate for several more years. But each year, managing power, heat, and communication becomes more complex. Meanwhile, the Voyager team has become a multigenerational effort, including NASA retirees in their 80s and young engineers who weren't born when the probes launched.
"This cross-generational effort is truly special," Dodd said. "I love these spacecraft. They're ambassadors wandering through interstellar space on behalf of all of us on Earth."
