Boldly going in search of Planet X

Four Nasa probes may discover whether something lurks in deepest space, writes David Whitehouse

"It will be a very sad day for all of us", said Manfred Wirth, manager of the Pioneer programme, talking of the decision to cease daily communication with the distant space probe, "but nobody thought that the probe would last even this long."

Pioneer 11, launched into deep space 22 years ago, is being abandoned like a piece of interstellar litter, left to drift towards the stars alone. With Pioneer, we have become a star trekking species. Pioneer 11 is one of only four objects ever to leave our solar system and travel out into the galaxy beyond. Two Pioneer probes and two Voyagers sailed past the largest outer planets in the Seventies and Eighties and then left them behind for the dark of interstellar space.

As the Voyagers and Pioneers race outwards, they fly with their backs to the solar wind, a stream of particles ejected from the Sun at 450km per second. This wind, along with the Sun's magnetic field, forms a bubble around the Sun called the heliosphere. At its edge, the heliopause, which is estimated to be between 50 and 100 times further from the Sun than the Earth, the realm of our Sun ends and galactic space begins. Here the solar wind and the material from between the stars clash in a region of shock waves and turbulence. With special instruments we can look up at this region in the direction of the Sun's travel through space and see a faint glow.

Because of the Sun's motion through space the heliosphere is not spherical but pear-shaped, squashed in the direction of the Sun's motion and elongated behind it. The shortest distance to the heliopause lies upstream, in the general direction taken by three of the four space probes. Voyager 1 and Pioneer 11 are heading towards the region of interaction between the solar wind and the interstellar medium. Voyager 2 is about 75 degrees south of this region and Pioneer 10 is heading in the opposite direction down the tail of the heliosphere. Because of this, Pioneer 10 will be long dead before it crosses into interstellar space.

All four travel between two and four times the Sun-Earth distance (called an AU or astronomical unit) each year. Pioneer 11's energy supply has been petering out for years, falling below the level required to operate its array of scientific instruments. When it reaches the heliopause it will be dead but scientists give Voyager 1 a 75 per cent chance and Voyager 2 a 55 per cent chance of being able to send back data from galactic space. And on their way there they might, if we are lucky, pass by a cometary nursery or even be able to detect the long sought after Planet X.

To many scientists, it's obvious that there is something undiscovered lurking beyond the most distant planets. When Uranus was discovered 200 years ago it seemed to move under the gravitational influence of an even more distant, unseen body. With a great deal of luck, the position of this influencing body was calculated and in 1846 Neptune was found less than a degree from its estimated position. The story was not at an end, however. Neptune, too, was influenced by something even further out and the search began for a more massive planet.

Another one was found, Pluto, in 1930, but Pluto is a lightweight object. Even with its smaller companion, Charon, discovered in 1978, it adds up to less than half of 1 per cent of the Earth's mass and cannot be the cause of the orbital perturbations of Uranus and Neptune. And such is the debate today - some contend that another large object awaits discovery, others that the measurements of the perturbations of the orbits of the outer planets are in error and that there is no fabled Planet X.

Each day, Nasa's Deep Space Tracking Network - radiotelescopes in California, Australia and Spain - turn to pick up the faint signals from the Voyagers and Pioneer. Part of the routine data is used to track the spacecraft. If any of these craft pass near Planet X it should show up as a deviation from their expected trajectories through space. It would be possible to detect a body the size of the Earth from a distance of 350 million miles and a dark body the size of Jupiter from a distance of 2,500 million miles.

In recent years, astronomers have discovered that lying beyond the most distant planets are smaller objects, perhaps 100km in diameter, made of rock and frozen gases. This is the abode of the comets, possibly billions of them, swarming around the distant sun. Perhaps one of the Voyagers and Pioneers will encounter one of them?

After 40 years of operation, barring a catastrophic failure, the signals from the Voyagers will fade and the spacecraft will die. Although we will not be able to track them, we know that their interstellar drift will take them close to some of the nearest stars. Particularly interesting will be a Voyager 2 encounter of Sirius, the brightest star in our sky and the sixth closest star system at 8.6 light years distance. In 358,000 years, Voyager 2 will pass within 0.8 light year of this hot class Al star.

These are the first man-made objects to be cast adrift in the immense vastness between the stars. The first mark of man that could find its way to another star and, at some future age unknown to its makers on planet Earth, perhaps our first contact with an alien race as they capture and study the object we called Pioneer 11.

Three hundred and fifty eight thousand years is a long time. Perhaps by then mankind will have sent space probes across the interstellar void to study Sirius and its dense companion star. I wonder if the records of the Voyagers will survive and mankind to come will remember the enormously productive spacecraft of a much earlier age as it silently passed by in the interstellar dark.

The writer is the science correspondent for the BBC.