Have you heard of a light-year? It is the distance light travels in one year. Since light travels at about 300,000 kilometres per second, a light-year equals roughly 9.46 trillion kilometres.
Now consider another fascinating fact. Voyager 1, launched by NASA on September 5, 1977, remains the farthest human-made object ever sent into space. Yet, despite travelling for nearly five decades at more than 17 kilometres per second, the spacecraft has still not reached a distance of one light-day from Earth, highlighting the staggering scale of the universe.
A light-day is the distance light travels in 24 hours, or about 25.9 billion kilometres. According to current estimates, Voyager 1 is expected to cross that milestone in November 2026. From then on, a radio signal sent from Earth will take a full 24 hours to reach the spacecraft, while a reply will require another 24 hours to return.
Voyager 1 was originally designed to study Jupiter, Saturn and their moons. After successfully completing its primary mission, it continued its journey into deep space. In 2012, it became the first human-made object to cross the heliosphere—the outer boundary of the Sun's influence—and enter interstellar space. It continues to transmit scientific data back to Earth.
The spacecraft is now about 25 billion kilometres from Earth, with its signals taking more than 22 hours to arrive. Despite ageing instruments and dwindling electrical power, NASA engineers have kept Voyager 1 operational through careful power management.
Although this distance is extraordinary by human standards, it remains tiny on a cosmic scale. Proxima Centauri, the nearest star to Earth after the Sun, is about 4.24 light-years away. At its current speed, Voyager 1 would need tens of thousands of years to reach it.