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NEXT week, NASA will call out one last time to Earth鈥檚 most remote explorer. After 25 years in space, Pioneer 10 is now 10 billion kilometres from the Sun, a distance so great that data from the spacecraft take a little over 9 hours to travel home at the speed of light.

Sadly, NASA can no longer afford to listen to Pioneer鈥檚 swan song. Fred Wirth, who as project manager has devoted most of his career to the Pioneer mission, says, 鈥淭he volume and scientific importance of the data was not judged to warrant NASA spending $500 000 a year to staff the control centre and operate the spacecraft.鈥� In any case, Pioneer 10鈥檚 power levels are dropping. By the end of next year it will have died, just like its sister spacecraft, Pioneer 11, which fell silent two years ago.

Bon voyage

Nevertheless, the exploration of space beyond the Solar System is just beginning. And two far more advanced spacecraft are hot on the heels of the Pioneers. Voyagers 1 and 2 have travelled well beyond the orbit of Pluto, they are better equipped than their predecessors, and they have the power to keep operating well into the 21st century.

The Voyager missions are already among the most successful in the history of spaceflight. The spacecraft are still returning data that are changing the way scientists understand the Sun, the Solar System and its place in the Milky Way.

To cap it all, Voyager 1 may be on the brink of its most spectacular discovery yet. Astronomers believe that the spacecraft is about to hit a giant shock wave at the edge of the Solar System created by the motion of the Sun through the interstellar medium. This huge bow wave has never been seen before but its discovery would be hugely significant. The bow wave separates the Solar System from the rest of the Galaxy. Beyond it, the Voyagers may be able to sample primordial matter left over from the big bang for the first time. The wave may even form a huge shield that protects the Solar System from the ravages of powerful cosmic rays, in the same way that the Earth鈥檚 magnetic field protects the surface from radiation. For the moment, nobody knows.

Spectacular discoveries are nothing new for the Voyager and Pioneer spacecraft nor for the teams of scientists now in their sixties who built them. During the course of their distinguished careers the spacecraft have also had some close shaves. Pioneer 10 was launched in 1972. At that time, no spacecraft had ever ventured beyond the orbit of Mars and the prospect of traversing the Asteroid Belt appeared daunting.

Astronomers had calculated the total mass of the belt but did not know the distribution of this mass. Should the belt be packed with tiny particles rather than large chunks, the spacecraft would stand a high chance of being hit. And any collision with a microscopic particle would be fatal since the debris would be travelling at 33 kilometres per second relative to the spacecraft. 鈥淲e were extremely nervous, and there was a great scientific debate that we would get hit,鈥� says Wirth, who has now retired. 鈥淎s it turns out, we had no impacts.鈥�

Because Pioneer 10 lived to tell the tale, astronomers concluded that the Asteroid Belt was sparsely populated with relatively large chunks-a discovery that cleared the way for all future outer planetary missions. NASA promptly launched Pioneer 11-originally a backup for Pioneer 10.

Shortly before the launch an idea occurred to a group of mathematicians. If Pioneer 11 approached Jupiter with the right velocity and distance, the tremendous mass of the planet would sling the spacecraft out in a different direction. 鈥淚f you got this game of celestial billiards just right, you鈥檇 get a freebie Saturn mission out of it. So that鈥檚 what we did,鈥� explains Wirth.

In the meantime, Pioneer 10 became the first spacecraft to reach Jupiter. During the flyby, it analysed the vast Jovian magnetic field. Most planets have their own magnetic fields, and because these interact with the solar wind they form comet-shaped cavities around the planets. These cavities, or magnetospheres, capture charged particles and accelerate them to huge speeds, forming rings of radiation around a planet.

Radiation belts are a serious problem for spacecraft. The ones around Earth can fry the electronics on board spacecraft that pass through. The Jovian belts are 1000 times more intense than Earth鈥檚 and Pioneer 10 was heading straight into them. 鈥淲e were literally biting our nails,鈥� says Wirth.

Irradiated chaos

Although Pioneer 10 was designed to survive high levels of radiation, nothing had prepared the team for what was to come. During the encounter, the two onboard radiation detectors rapidly became saturated with radiation levels off the scale. The onboard computer began to receive spurious commands and one of the spacecraft鈥檚 cameras malfunctioned and was unable to take important images of the Jovian moon Io.

In December 1973, Pioneer 10 skimmed the Jovian cloud tops at a speed of 37 kilometres per second. 鈥淭he worst part was that the Pioneer disappeared behind Jupiter-for four hours during closest approach. That was the most nerve-wracking,鈥� says Wirth. But the spacecraft鈥檚 huge speed meant it spent only a few hours in Jupiter鈥檚 intense radiation belts. Pioneer survived and its charts of the Jovian radiation belts paved the way for Voyager.

Both Pioneer missions were a resounding scientific success. But in 1995, 6.6 billion kilometres from the Sun, communications with Pioneer 11 ceased when the Earth moved out of the antenna鈥檚 field of view. The spacecraft鈥檚 power supplies were close to exhaustion and an earlier attempt to swap a failed receiver system to a backup had failed. Today, this powerless ghost ship is travelling towards interstellar space at the rate of 375 million kilometres a year.

Although Pioneer 10 is slightly older, its power supply has lasted longer. The spacecraft鈥檚 power is supplied by heat from radioactive pellets which generates electricity. But by the end of the year, several months after NASA has bid farewell, the radioactivity will have decayed to unusable levels and the spacecraft will fall silent.

Today, it has only enough power for essential systems-its antenna and onboard computer-with a little left over for one other instrument. This power is shared between an ultraviolet photometer and a cosmic ray detector.

These instruments are still sending back valuable data. Hydrogen molecules in interstellar space reflect ultraviolet light, obscuring astronomers鈥� view of the Universe at these frequencies. Just how much hydrogen is out there is hotly debated and Pioneer鈥檚 first-hand measurements have provided a unique viewpoint. This will be compared with measurements from the Voyager spacecraft which are heading in the opposite direction. The cosmic ray measurements at this distance are also important since the Sun鈥檚 magnetic field may shield Earth from some of this bombardment. Pioneer鈥檚 final measurements will help to show just how important this shielding effect is.

Rare line-up

After next week鈥檚 final contact, only the Voyagers will be left. These missions were designed to take advantage of a rare geometric arrangement of the outer planets. Occasionally, the layout of Jupiter, Saturn, Neptune and Uranus allows a spacecraft to swing from one planet to the next without the need for large engines. This gravity-assist technique reduces the flight time to Neptune from 30 years to 12. 鈥淭he planetary line-up occurs every 176 years. It is indeed very rare,鈥� says Edward Stone, who headed the Voyager project in the 1970s and is now director of NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California.

The Voyagers have a distinguished body of discoveries behind them. Their cameras have returned stunning images of storms in the Jovian atmosphere. They discovered Jupiter鈥檚 ring, a feature that is impossible to see from Earth and one that was missed by the Pioneers. In 1979, the spacecraft provided the first photos of the major Jovian moons-pictures that are only now being bettered by the Galileo probe. Voyager 1 found evidence of active volcanoes on Io that produced volcanic plumes 30 times higher than Everest and showered lava over an area the size of France. These plumes are so high that many particles of sulphur and oxygen escape into orbit around Jupiter and are now thought to be the primary source of charged particles in Jupiter鈥檚 radiation belts.

Planetary astronomers have the Voyagers to thank for the discovery of permanent ice-oceans on Europa. Some scientists now believe that if liquid water oceans exist beneath this layer, Europa may be one of the most likely candidates in the Solar System for extraterrestrial life. The spacecraft also found the curious mixture of old and new crust on Ganymede, returned images of Callisto where craters stand shoulder to shoulder on its ancient surface, and spotted three new moons, Adrastea, Metis, and Thebe none of which is more than 80 kilometres in diameter.

Voyagers 1 and 2 are now almost 10 billion and 8 billion kilometres from the Sun respectively. They continue to perform well and their radioactive power generators are working even better than scientists had dared hope.

In the absence of any nearby objects to photograph, the spacecraft鈥檚 cameras have long been switched off, but every day Voyager鈥檚 science teams receive valuable data. The spacecraft are still measuring the strength and direction of the Sun鈥檚 magnetic field. This field is inescapably entangled with the solar wind, the stream of charged particles, or plasma, that blows away from the Sun鈥檚 surface. The plasma flows under the control of this magnetic field but at the same time supports and maintains the field.

This interdependence causes huge complexity. The field is anchored by the Sun which rotates once every 27 days. But the solar wind does not rotate at this speed and consequently the magnetic field at the edge of the Solar System becomes twisted and coiled like a spring. Voyager鈥檚 measurements are providing a valuable insight into the complex structure of the field at this distance.

But the Voyagers鈥� most spectacular discovery may still be to come. Astronomers believe that Voyager 1 is close to hitting the giant shock wave that the Solar System creates as it travels through interstellar space. The Sun and its family of planets are wanderers in our Galaxy and the journey is through a sea of primordial matter left over from the birth of the Universe, from material ejected by supernovas and from other star鈥檚 solar winds. This rarefied mixture of gas and plasma is called the interstellar medium (ISM). The Sun鈥檚 motion through this ocean compresses the solar wind to form a bow-wave in the direction of motion and a comet-like tail in the other. This region of the Sun鈥檚 influence is called the heliosphere and nothing has travelled beyond it.

Consequently, little is known about the ISM. From Earth, astronomers can see that it absorbs light from other stars and these measurements give an average figure of its density of about 0.05 particles per cubic centimetre. By comparison, the solar wind has about 10 particles per cubic centimetre near the Earth, and the air we breathe has around 27 脳 10¹⁸ particles in the same volume. Astronomers think these particles move at subsonic speeds of around 20 to 26 kilometres per second. This is much slower than the solar wind which travels at supersonic speeds of up to 800 kilometres per second.

The structure of the boundary will be complex. Researchers expect to find a shock wave where the solar wind meets the ISM and suddenly slows to subsonic speeds-the termination shock. Beyond that, they expect to find a point where the incoming pressure of particles from the ISM equals the pressure of the solar wind outgoing from the Solar System. This is the heliopause, where the gusting solar wind is blown back by the interstellar winds.

The distances of these boundaries depend on a number of factors. First of all, there is the density and velocity of the ISM, then the strength and direction of the interstellar magnetic field. While these factors are unknown today, they can be quantified as soon as Voyager discovers the location of the boundary. Knowing the density of the local ISM also tells astronomers how the Sun鈥檚 immediate neighbourhood compares with the interstellar average.

When Voyager crosses the boundary the spacecraft will even be able to use its mass spectrometer to analyse the stuff from which the Solar System formed. Voyager鈥檚 plasma spectrometers will work out the mass of the individual ions it meets to determine the densities of the elements of the periodic table in interstellar space. Knowing exactly what interstellar space is made of will tell astronomers about the chemical evolution of the galaxy and what conditions were like before the birth of the Solar System.

So when will Voyager hit the bow wave? Nobody is really sure, says Wirth, but Voyager has already found tantalising clues to its existence. In May 1993, the spacecraft picked up mysterious radio signals that seemed to be coming from beyond the Solar System. Researchers believe these signals are produced when solar bursts strike the boundary, causing it to resonate, rather like striking a bell. 鈥淭hese radio emissions are the most powerful radio source in the Solar System,鈥� says Don Gurnett, the physicist in charge of Voyager鈥檚 plasma wave detector which picked up the signals. 鈥淲e鈥檝e estimated that the total power radiated is more than 10 trillion watts. However, the signals are at such low frequencies that they cannot be detected from Earth.鈥�

By measuring the time that the solar bursts take to reach the boundary, astronomers can work out how far away it is. 鈥淚t turns out that two of the largest solar events lead the onset of the two major radio emission events seen at Voyager by about 400 days,鈥� explains William Kurth, a physicist at the University of Iowa who has analysed the data. 鈥淵ou can then basically do a time of flight calculation with the solar wind speed and calculate how far it goes in the 400 days.鈥�

The results are surprising. Astronomers had believed that these kinds of radio signals would emanate from the termination shock, when the Solar Wind decelerates from supersonic to subsonic speeds. Since Voyager is well beyond this distance, something must be wrong.

False prediction

The calculations showed that emissions were being generated much further away, at up to 18 billion kilometres. So astronomers have had to change their ideas. These signals, they say, can only be coming from the heliopause. 鈥淭here is no other known structure out there that could be causing these signals,鈥� says Gurnett. Using these arguments, the termination shock should be about 13 billion kilometres away, says Stone. If he is right, Voyager 1 will probably cross this boundary in 2001.

Just how long Voyager can last beyond that is anybody鈥檚 guess. The vehicles have power to keep going until they are at least 20 billion kilometres away. But by then NASA may not have the money or resources to continue the mission. The Voyagers are tracked for up to 14 hours every day by a team of 19 researchers. If anything goes wrong outside of office hours, a computer pages one of the team members, who must be on call 24 hours a day. This is an expensive business. 鈥淚鈥檓 hoping we will find some way to keep it going,鈥� says Stone.

Whenever the heliopause is found, the powerless Pioneers and the Voyagers still have one last service to perform for humankind-as messengers to the stars. Researchers needed the termination shock to beat 7 billion kilometres to explain the high frequency signals they were seeing.

The spacecraft, without fuel and without any significant gravitational forces acting on them, will continue to drift deeper into space. Eventually, in 33 000 years, Pioneer 10 will reach the star Ross 248, in the constellation of Andromeda, and 325 000 years later, a battered Voyager 2 will pass within 0.7 light-years from the bright star Sirius. Pioneer 11 will slowly drift towards the centre of the Galaxy, but is destined to be destroyed long before it gets there by pulverising collisions with interstellar dust.

鈥淚t will be sad when we lose them, but hopefully they won鈥檛 fail before we discover one or two more important things,鈥� says Stone. Either way, he admits that the Pioneer and Voyager missions have exceeded all but their wildest dreams. 鈥淚t has been a wonderful journey.鈥�