Humanity’s farthest spacecraft Voyager 1 nears one light-day from Earth: Its 26 billion km journey equals 650,000 trips around Earth

Imagine sending a message to a spacecraft and having to wait an entire day just for it to arrive. Then imagine waiting another day for its reply. That is the extraordinary distance NASA's Voyager 1 is approaching, nearly half a century after it left Earth. The spacecraft is expected to cross the 16-billion-mile (26-billion-km) mark on Wednesday at 5:16 am ET (10:16 GMT), according to NASA.

Launched in September 1977, Voyager 1 is now on the verge of reaching a distance of one light-day from Earth, a milestone that puts into perspective just how far humanity's most distant spacecraft has travelled.

To understand the scale, 26 billion kilometres is roughly equivalent to travelling around Earth's equator more than 650,000 times. It is also enough distance to circle the Earth nearly 650,000 times, compared with the roughly 40,000 km needed for a single trip around the planet.


At its current speed of approximately 38,000 miles per hour (61,000 km per hour), Voyager 1 continues to move through interstellar space, far beyond the region dominated by the Sun's solar wind.

The milestone is particularly striking because a light-day is not simply a measure of how long Voyager has been travelling. It describes how far light itself can travel in 24 hours. Since light moves at nearly 300,000 km per second, it covers approximately 26 billion km in a day.

In other words, if NASA sends a radio command to Voyager 1 once it reaches this distance, even a signal travelling at the speed of light will need around 24 hours to get there.

Why Voyager 1 reaching one light-day is such a stunning achievement

The distance is difficult to comprehend because everyday measurements become almost meaningless at this scale. A journey that would take a commercial aircraft travelling at 900 km per hour more than 3,000 years to cover is being completed by Voyager 1 over several decades.

Even at its extraordinary speed, the spacecraft has taken nearly 49 years to approach the one-light-day mark. And unlike a car or aircraft, Voyager 1 cannot stop for maintenance, refuel at a station or return to Earth for repairs.

Its continued operation is therefore as remarkable as the distance it has covered.

Astrophysicist Alan Cummings, a senior research scientist at Caltech's Space Radiation Laboratory and a Voyager programme scientist since 1973, described the milestone as an engineering achievement.

"I view it as an engineering milestone, marking the amazing longevity of Voyager 1 — and also an engineering achievement to be able to communicate with Voyager 1 at its distance," Cummings said.

The communication challenge is particularly significant. Radio signals travel at the speed of light, meaning the time taken to exchange information increases as Voyager 1 moves farther away. At one light-day, a message from Earth will take 24 hours to reach the spacecraft, while its response will take another 24 hours to return.

That means NASA would need to wait roughly two days to send a command and receive a response, even if the spacecraft responded immediately.

Voyager 1 has already left the Sun's protective bubble

Voyager 1's journey is not just about setting distance records. The spacecraft has travelled into a region of space that no human-made probe had directly explored before.

In 2012, it crossed the outer boundary of the heliosphere, a vast protective bubble created by the Sun's charged particles and magnetic field, and entered interstellar space.

This transition allowed scientists to study conditions beyond the Sun's immediate influence, including galactic cosmic rays — high-energy particles that originate outside our solar system.

Cummings said Voyager 1 provided the first measurements of the energy spectra of low-energy galactic cosmic rays in interstellar space.

These particles are largely prevented from entering the heliosphere by the outward pressure of the solar wind, making measurements beyond that boundary particularly valuable.

The spacecraft also made important discoveries during its earlier planetary encounters. It identified a thin ring around Jupiter and two previously unknown moons, Thebe and Metis.

During its encounter with Saturn, it discovered five new moons and identified a ring known as the G-ring.

These findings helped reshape scientists' understanding of the giant planets and their surrounding environments.