By Dr David Whitehouse BBC News Online science editor

Twenty-five years ago the two Voyager spacecraft embarked on an unprecedented mission of exploration into the depths of the outer Solar System.

Back in 1977, we had no way to know they would last so long. We were initially just on a four-year journey to Jupiter and Saturn

Edward Stone, Voyager chief scientist

After the most spectacular discoveries scientists say the mission is not yet over.

Both spacecraft flew past the giant planets Jupiter and Saturn, and one went on to the mysterious Uranus and Neptune.

Now they are reaching even further out into space, and will soon be the first to escape to wander the galaxy.

In a way the Voyager spacecraft came to define a golden age of planetary exploration. Voyager 2 was launched first, on 20 August, followed by Voyager 1 some 16 days later.

During the first 12 years after launch in 1977, they produced a wealth of discoveries about the four gas-giant worlds and their 48 moons.

They found fast winds on Neptune, kinks and spokes in Saturn's rings and spectacular volcanoes on Jupiter's moon Io, the most volcanically active world known.

Jupiter composite

Jupiter's atmosphere has dozens of huge storms. The hazy atmosphere of Saturn's moon Titan may hold the secrets of the origin of life. Miranda, a small moon of Uranus, has a jumble of old and new surfaces. Neptune's moon Triton has active geysers.

Most distant object

Voyager 1 is now the most distant man-made object, about 85 times as far from the Sun as Earth is. Voyager 2 is now about 68 times the Sun-Earth distance.

"After 25 years, the spacecraft are still going strong," says Edward Stone, Voyager project scientist since 1972.

"Back in 1977, we had no way to know they would last so long. We were initially just on a four-year journey to Jupiter and Saturn."

The Voyagers are still in almost daily contact as they travel through the distant ripples of the wind from the Sun.

Their eventual goal is to become the first spacecraft to escape the Sun's influence. Both spacecraft are heading towards the heliopause, the boundary between the Sun's influence and interstellar space.

Neptune and its moon Triton from Voyager

Voyager 1 is rushing toward the heliopause at about 1.6 million kilometres a day (one million miles a day). Whether it gets there before about 2020, while it still has adequate electrical power, depends on how far away the heliopause is.

Recent estimates are that, depending on that distance, it would take Voyager 1 between seven and 21 years to reach the Solar System's edge.

Voyager 1 has already discovered that the outbound solar wind around it is slowing from effects of inbound interstellar particles leaking through the boundary.

A much better prediction of the boundary's location will come when the spacecraft encounters the termination shock, the zone where the solar wind begins piling up against the heliopause. That encounter may come within the next three years.

But their final task may last longer than humanity's lifetime.

Last mark

Long after they fall silent, the Voyager twins will keep speeding away from our solar system, each carrying a disk of recorded sounds and images from Earth.

The message from Earth

Included are greetings from many Earth languages, images of life on our planet and Man's achievements.

Long after our Sun has swelled to become a Red Giant, probably destroying the Earth in the process, the Voyager craft will still be moving among the stars.

Perhaps long after mankind itself has disappeared from the Cosmos they will still be wandering.

If they are ever found by another intelligence I wonder what they would make of the images of the creatures who made it so long ago and so far away.

The Voyagers could be our final mark on the Universe, what mankind is judged by.