Nearly 50 years after leaving Earth, NASA's Voyager 1 is approaching an extraordinary milestone that will highlight just how far humanity's most distant spacecraft has traveled. But keeping the aging explorer operational has become an increasingly difficult challenge as its power supply slowly fades.
Both Voyager spacecraft generate electricity using radioisotope thermoelectric generators. These systems produce electrical power from the heat released as plutonium undergoes radioactive decay. As that process continues, the spacecraft gradually lose generating capacity, with each probe experiencing a decline of approximately 4 watts annually.
Both Voyager spacecraft have crossed into interstellar space, making them the only spacecraft ever to directly examine the environment outside the Sun's heliosphere, the enormous region of particles and magnetic fields produced by the Sun.
NASA temporarily lost contact with the spacecraft on October 18 and 19, when the agency's Deep Space Network failed to receive its radio signal during two scheduled communication attempts separated by 24 hours.
After two unsuccessful attempts to detect the spacecraft, its signal unexpectedly returned on October 20, roughly 48 hours after the interruption began. Communication has remained established since then, and engineers subsequently sent commands to investigate the problem.
NASA has not yet determined exactly what caused the incident. However, engineers discovered that one set of Voyager 1's thrusters had stopped operating and that the spacecraft had automatically activated its backup thrusters.
This behavior is part of a built-in safety system known as fault protection. When the onboard computers recognize a potential problem with active equipment, they can switch to backup hardware, place the spacecraft into a stable configuration, and suspend normal activities. This gives controllers on Earth an opportunity to investigate and determine the appropriate response.
Traveling through interstellar space at approximately 38,000 mph (61,155 kph), the spacecraft is expected to reach a distance of 16.09 billion miles (25.89 billion kilometers) from Earth on November 18. That distance equals one light-day, meaning it takes light an entire 24 hours to travel between Earth and the probe.
No other human-made object has ever reached such a distance.
Radio waves travel at the speed of light, so commands transmitted from Earth will require a full day to reach Voyager 1. Any response sent back will take another 24 hours, creating a total communication delay of 48 hours.
Crossing the one-light-day threshold will not produce any physical change in Voyager 1 or its surroundings. The spacecraft will continue traveling through the same interstellar environment, unaware of this milestone of special significance to people on Earth.