Why Nasa’s Voyager 1 spacecraft reaching one light-day distance from Earth is a milestone moment

Imagine sending a message to someone and having to wait two days for a reply. You send a greeting, put down your phone and go about your daily routine. Only after 48 hours will you know whether the person at the other end has got your message and responded. This is not an unusual communication problem in the age of interstellar exploration. It is about to become the reality of communicating with Voyager 1, already the most-distant human-made object ever built.

It is a milestone without any dramatic event on board the spacecraft. Nothing will happen when Voyager 1 crosses this imaginary boundary. Its instruments will continue operating and it will carry on along its course through interstellar space. Yet the occasion offers a striking measure of how far human ingenuity has taken us, and that we can operate a spacecraft that is almost 50 years old.

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Why do astronomers measure this distance in terms of the time light takes to travel? The idea is not very different from describing a distance in terms of how long sound takes to travel. We say that someone is within shouting distance when they are neither very close nor too far away, but close enough to hear us call. Similarly, astronomers use the time light takes to travel to express the enormous distances across space.

Light, however, travels enormously faster than sound. In the vacuum of space, it moves at approximately 300,000 kilometres every second. A light-day is simply the distance that light travels in 24 hours: about 25.9 billion kilometres.

The measure helps us appreciate the scale of distances in the universe. The Moon is, on average, about 384,400 kilometres from Earth. Light takes just 1.3 seconds to travel that distance, although a spacecraft carrying astronauts takes several days to make the journey. The Sun is about 150 million kilometres away, and its light takes approximately eight minutes and 20 seconds to reach us. We therefore say that the Sun is about 8.3 light-minutes away.

The distances become much larger as we move towards the outer planets. Neptune, the most distant planet in our solar system, is on average about 4.5 billion kilometres from Earth. Light takes a little over four hours to cover that distance. Voyager 2, travelling at tens of thousands of kilometres per hour, took about 12 years to reach Neptune.

Astronomers commonly use an even larger unit, the light-year. It is the distance light travels in one year, approximately 9.46 trillion kilometres. The nearest star system to our Sun, Alpha Centauri, is about 4.3 light-years away. Its faintest member, Proxima Centauri, is the closest individual star to the Sun. Even travelling at the speed of light, a journey there would take more than four years.

These units measure distance, not time. They help convey the immensity of space in a way that kilometres, however large the numbers, often fail to do.

Voyager 1 began its journey on September 5, 1977, from Cape Canaveral, Florida. Its twin, Voyager 2, had been launched a few weeks earlier, on August 20. The two spacecraft were designed to explore the outer planets of the solar system. Their launch coincided with a rare alignment of the giant planets, making it possible to use their gravitational fields to propel the spacecraft from one planet to another.

Voyager 1 was assigned to study Jupiter and Saturn. Voyager 2 followed a longer route, visiting Jupiter, Saturn, Uranus and Neptune. Both missions were extraordinarily successful, transforming our knowledge of the giant planets and their moons.

When Voyager 1 flew past Jupiter in 1979, a great surprise was awaiting us back on Earth. Its cameras took pictures of massive volcanic eruptions on Io (one of the Jovian moons). Up to that point, active volcanoes had only been known on Earth. Voyager 1 demonstrated that Io was geologically active, with plumes of volcanic material rising hundreds of kilometres into the atmosphere.

The spacecraft also detected a faint ring around Jupiter, showing that Saturn was not the only planet in the solar system with rings. Its close-up observations of Jupiter's Great Red Spot revealed a vast, turbulent atmospheric storm, larger than Earth.

In November 1980, Voyager 1 flew past Saturn. Its observations showed the remarkable complexity of the planet's rings, which are made up of numerous narrow ringlets rather than a few smooth, continuous bands. The spacecraft also discovered several small moons associated with the rings, including Atlas, Prometheus and Pandora.

Saturn's largest moon Titan also threw a surprise. Voyager 1 found that Titan possesses a dense atmosphere abundant in nitrogen and containing hydrocarbons. The thick atmosphere prevented its cameras from seeing the surface clearly. The discovery made Titan an especially interesting target for later exploration. More than two decades later, the European Huygens probe, carried by the Cassini spacecraft, descended through Titan's atmosphere and revealed a world with rivers and lakes of liquid methane and ethane.

Voyager 1 had completed its primary planetary mission. But its journey was far from over.

The Sun does not simply illuminate the planets. It continuously releases a stream of charged particles called the solar wind. This wind, together with the Sun's magnetic field, creates a vast protective region around the solar system known as the heliosphere. Its outer boundary, the heliopause, marks the region where the solar wind gives way to the interstellar environment.

On August 25, 2012, Voyager 1 crossed the heliopause and entered interstellar space. It became the first human-made object to do so. Its twin, Voyager 2, crossed the same boundary in November 2018.

This was not merely a record-breaking journey. The spacecraft were now exploring a region of space that had never before been directly sampled by human instruments. Their measurements showed that interstellar space is not an empty void. It contains charged particles, magnetic fields, and plasma (a gas of electrically charged particles, like lightning).

Voyager 1's plasma wave instrument has picked up oscillations in the plasma. In 2021, scientists reported a very weak, persistent signal which they described as a kind of ‘hum’ in the interstellar medium. The spacecraft has also detected shock waves and other disturbances propagating through the plasma, giving us information about the environment beyond the Sun.

These measurements are extremely valuable because no other spacecraft has travelled far enough to study this region directly. Voyager 1 and Voyager 2 remain our only instruments working beyond the heliosphere, and for years to come will provide valuable insights into interstellar space.

Voyager 1's scientific contribution is only one part of its heritage. It has also given humanity one of the most memorable images of its own existence.

In February 14, 1990, as Voyager 1 moved away from the planets, astronomer Carl Sagan persuaded NASA to turn the spacecraft's camera back toward Earth. From a distance of about six billion kilometres, the camera captured Earth in the vast empty space. Earth appeared as a virtually imperceptible speck suspended in a ray of sunlight. It also took a family portrait of the Solar System, images of Neptune, Uranus, Saturn, Jupiter, and Venus in addition to Earth in one shot.

The photograph became known as the Pale Blue Dot. Sagan later used it as the title of his book, Pale Blue Dot: A Vision of the Human Future in Space. Reflecting on the image, he wrote about the significance of that tiny point of light: it contained everyone we know, every human life, every civilisation and every conflict in our history.

The photograph offers a perspective that is difficult to achieve while living on Earth. The divisions that occupy so much of human attention — national boundaries and religious differences — disappear when our planet is seen from such a distance. All of humanity shares that tiny world. It reminds us that we are just a speck in the universe, teaching us humility while also showing that the Earth is one.

The golden disc also includes sounds of nature, human activities intended for any intelligent civilisation that might one day encounter the spacecraft. The record was conceived as a time capsule of our planet, a representation of human life and culture sent into the cosmic expanse.

Whether anyone will ever find it is another matter. The chances are extraordinarily small. But the record expresses something important about the mission: even as we travel through the universe, we carry with us a desire to communicate who we are.

Perhaps the most remarkable aspect of Voyager 1 is that it continues to function after nearly five decades in space. It has no solar panels. At its distance from the Sun, sunlight is far too weak to provide useful electrical power. Instead, the spacecraft relies on three radioisotope thermoelectric generators (RTGs).

Unlike a conventional nuclear reactor, an RTG does not produce electricity through a chain reaction. It uses the natural decay of radioactive material as a long-lasting heat source. The radioactive decay of plutonium-238 releases heat. The heat is converted into electrical energy by the thermocouples.

The system has served Voyager remarkably well. But its output steadily declines as the plutonium decays and the thermocouples degrade. At launch, the generators produced about 470 watts of electrical power. The spacecraft now has only a fraction of that available.

Every watt matters. The radio transmitter, scientific instruments, heaters and onboard electronics all compete for a diminishing supply of electricity. To keep the mission alive, engineers have had to switch off instruments and other systems one by one, following a carefully planned sequence.

Voyager 1 carried ten science instruments. Only two are still working today: the magnetometer (which measures the magnetic field) and the plasma wave subsystem (which detects oscillations in the plasma around the spacecraft). The low-energy charged particle instrument (which measured particles such as ions, electrons and cosmic rays originating from the solar system and galaxy, providing crucial data about the structure of the interstellar medium among other things) was turned off in April this year to save power. In September, engineers completed a significant power-reduction effort on the rest of the spacecraft systems by replacing three devices with lower-power ones. With this, they hope to prolong its life.

This is an extraordinary exercise in engineering ingenuity. The spacecraft was designed in the 1970s, using technology and components that are no longer manufactured. Its engineers must solve problems remotely, often with limited information, and send commands that take nearly a day to reach the spacecraft. Even a simple troubleshooting exercise can become a multi-day operation.

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NASA's Voyager team continues to find ways to extend the mission. But the available power will eventually become insufficient to operate the remaining instruments and maintain communication with Earth. The spacecraft will then continue its silent journey, even after it has stopped sending data.

Voyager 2, although launched before Voyager 1, is not as far away. It travelled on a different trajectory, visiting all four giant planets, and is now moving through interstellar space in a different direction. It is about 3.2 billion kilometres closer to Earth than its twin. NASA estimates that Voyager 2 will reach the one-light-day distance in November 2035.

The two spacecraft are heading towards different regions of the sky. Voyager 1 is moving in the general direction of the constellation Ophiuchus, while Voyager 2 is travelling towards the constellations Sagittarius and Pavo. In about 40,000 years, Voyager 1 is expected to pass within roughly 1.7 light-years of the star Gliese 445. Voyager 2 will make a similar close approach to another star, Ross 248, over a similar timescale.

Neither spacecraft is likely to reach another star system in any meaningful sense. They will pass through the immense distances between stars, carrying their instruments, their records and the traces of the civilisation that built them.

When Voyager 1 crosses the one-light-day mark this month, it will not encounter a new boundary in space. The importance lies in what the distance means to us. For the first time, a human-made object will be so far away that a message travelling at the fastest speed permitted by nature will need an entire day to reach it.

One day, perhaps around 2035, its radio will fall silent. But Voyager 1 will continue travelling, long after the engineers who built it and the people who followed its journey are gone. It will remain a small artefact of Earth, moving through the immense darkness of interstellar space, a sign that, from our tiny planet, humanity once reached out to explore the stars.