NASA is trying to solve a mystery emanating from its Voyager 1 spacecraft, which is currently 23.3 billion kilometres away from Earth.
The 45-year-old spacecraft appears to be operating normally and is still gathering and sending back scientific data as well as receiving and executing commands, the space agency said.
But the attitude articulation and control system (AACS), which controls Voyager 1's orientation, has started sending random data that doesn't make any sense.
"A mystery like this is sort of par for the course at this stage of the Voyager mission," said Suzanne Dodd, project manager for Voyager 1.
That's partly due to the age of the spacecraft, with four and a half decades of flying well beyond what the project planners had anticipated.
"We're also in interstellar space - a high-radiation environment that no spacecraft have flown in before," Dodd said.
"So there are some big challenges for the engineering team. But I think if there's a way to solve this issue with the AACS, our team will find it."
The AACS system keeps Voyager 1's antenna pointed towards home, allowing data to be transferred. Commands still being executed is an indication the antenna is still pointing the right way, but the telemetry data readouts are invalid.
Troubleshooting is difficult with the spacecraft so far away from Earth. It currently takes light 20 hours and 33 minutes to travel the distance, meaning it takes nearly two days to send a message to Voyager 1 and then get a response.
Voyager 1's signal hasn't weakened, NASA said, and the onboard fault protection systems haven't been triggered which is giving engineers the ability to try and diagnose the problem.
However until the issue is resolved, the potential impact on how long it can collect and transmit scientific data remains unknown.
It is not the first time Voyager 1 has had problems. Five years ago degradation of its primary thrusters forced engineers to switch to another set, originally used in the early days of the mission.
Voyager 1's 'twin' Voyager 2 is currently 19.5 billion kilometres from Earth and continues to operate normally.
"The information they provide from this region has helped drive a deeper understanding of the heliosphere, the diffuse barrier the Sun creates around the planets in our solar system," NASA said.