The US space agency's (Nasa) venerable Voyager mission is celebrating its 30th anniversary.
The probes will go on for a few more years yet
Its two probes were launched within weeks of each other in 1977 to make a detailed study of the outer planets.
The probes were then sent on trajectories that will eventually take them out of the Solar System and into interstellar space.
Three decades on, they continue to return data from distances more than three times farther away than Pluto.
Currently, Voyager 1 is farthest away. Launched on 5 September 1977, it is about 15.5 billion km (9.7 billion miles) from the Sun.
Voyager 2, which was lofted on 20 August 1977, is about 12.5 billion km (7.8 billion miles) away from the Sun.
They are travelling in the general direction of the centre of our galaxy. Mission managers believe the probes' power packs should maintain their working systems until at least 2020.
Although there are no planets in their vicinity to investigate, they are helping scientists understand the extent of the heliosphere, the huge "bubble" within which the Sun dominates its region of space.
It contains electrically charged particles that have been blown off the Sun at high speed and which are now pressing up against matter from other stars.
'Much to learn'
At some point in the next few years, the Voyagers will cross the "official" edge of the Solar System - the heliopause. Past here, the probes would be in interstellar space and, by that stage, probably some 40 years on from their launches.
"The Voyager mission has opened up our Solar System in a way not possible before the Space Age," said Edward Stone, the Voyager project scientist at the California Institute of Technology, Pasadena, California.
"It revealed our neighbours in the outer Solar System and showed us how much there is to learn and how diverse the bodies are that share the Solar System with our own planet Earth."
Both Voyagers carry gold-plated copper discs containing sounds and images selected to portray the diversity of life and culture on Earth. Music on the discs includes Bach and Chuck Berry.
Glossary of terms that describe features of the heliosphere:
Solar wind: Stream of charged particles blown off the Sun and travelling at "supersonic" speeds
Termination shock: Area where particles from the Sun begin to slow and clash with matter from deep space
Heliosheath: A vast, turbulent expanse where the solar wind piles up as it presses outward against interstellar matter
Heliopause: The boundary between the solar wind and the interstellar wind, where the pressure of both are in balance
Interstellar wind: The gas and dust between the stars
Bow shock: The shock wave caused by the edge our Solar System travelling through deep space