Voyager 1: One Light-Day from Home
AIn September 1977, a small spacecraft called Voyager 1 was launched from Florida on a mission that was originally designed to last only a few years. Almost half a century later, it is still working, and it has travelled farther from Earth than any other object ever built by humans. Its twin, Voyager 2, had been launched just over two weeks earlier and followed a different route through the solar system. After flying past Jupiter and Saturn, Voyager 1 continued outwards and eventually left the region dominated by the Sun's influence, entering interstellar space in 2012. Today it is moving at roughly 38,000 miles per hour, and every day carries it deeper into the darkness between the stars.
BIn November 2026, Voyager 1 is set to pass an unusual milestone. On 18 November, it is expected to be one light-day away from Earth, which means that a radio signal travelling at the speed of light will need a full day to reach it. No object made by humans has ever been so far away that light needs a whole day to reach it. That distance is approximately 25.89 billion kilometres, or a little over 16 billion miles. Because every message must travel out and back again, engineers who send a command have to wait about 48 hours to learn whether it has been received. The project manager, Suzanne Dodd, has pointed out that after sending any message, the team must wait two days for a response. The milestone itself, however, is purely symbolic: crossing this invisible line will cause no physical change to the spacecraft.
CA far more practical concern for the mission team is power. Voyager 1 has no solar panels, because sunlight is far too weak at such enormous distances to be useful. Instead, it relies on generators that turn the heat released by decaying plutonium into electricity. As the plutonium slowly decays, the amount of power available to the spacecraft falls by about four watts every year. When it was launched, Voyager 1 carried ten scientific instruments, and two of them have been switched off in recent years to save energy. Every decision about what to keep running is therefore a careful balance between the value of the science and the limited electricity that remains.
DIn early September, engineers carried out a delicate operation that they nicknamed the "Big Bang". They switched off three devices on board and turned to alternatives that use less power. These three devices belonged to a group of six that help to keep the spacecraft warm, and they all had to be changed at the same moment so that heat would remain evenly spread across the craft. The same procedure had been performed on Voyager 2, Voyager 1's twin, about six weeks earlier. If it works as hoped, the change should allow the remaining instruments to continue operating for longer. Without it, another instrument might have had to be shut down as early as the summer of 2027.
EOnly a few weeks after the operation, the team faced an unexpected scare. On 18 and 19 October, the large antennas of NASA's Deep Space Network could not detect any signal from Voyager 1. For two days, the engineers had no way of knowing whether the spacecraft was still working. Contact was finally restored on 20 October. When engineers examined the data, they discovered that one set of the small thrusters that help to control which way the spacecraft faces had stopped working. The craft's built-in fault protection system had responded automatically by switching to a backup set. Exactly why the first set failed is still being investigated.
FThe mission manager, Kareem Badaruddin, said that this was the first time the spacecraft had behaved in this way. He added that, at such a late stage in the mission, the team has to expect the unexpected. For now, the engineers are relieved to be in contact again, and they want to understand what happened. Each year carries Voyager 1 further from home and leaves it with a little less energy. Nobody knows exactly how many more years it will be able to keep in touch with Earth. Yet the ageing spacecraft continues to send back information from farther away than any other machine has ever reached.