The Next Frontier

For 25 years, the Pioneer 10 spacecraft has been exploring the solar system. But now it has come to the end of its useful life. That won’t be the end of the spacecraft, though. In fact, it’s likely it will still be travelling through space long after the earth has disappeared.

If Pioneer 10 is ever seen again, it won't be by human eyes. Since its launch in 1972, the craft has been following a course which will eventually take it into interstellar space, making it one of the first man-made objects ever to leave the Solar System.

But that event lies some way in the future. At present, Pioneer 10 is still safely within the Solar System, despite the fact that it passed the orbit of Pluto — the solar system's outermost planet — over 13 years ago. It is now 10 billion kilometres from the Sun. At this vast distance, its signals, which travel at 300.000km per second — the speed of light — take over nine hours to reach Earth.

For years, astronomers have been waiting for Pioneer 10 to reach interstellar space. At first, it was thought that the Solar System ended just beyond the orbit of Pluto. But, as Pioneer 10 has traveled away from the Sun, it has been sending back messages which reveal that the solar wind — a stream of charged particles that emanate from the Sun — extends way beyond the outermost planet. After each message from the distant craft, astronomers have had to revise their estimates of the size of the solar system, which is proving to be far bigger than anyone imagined. Only when the "solar wind" can no longer be detected will Pioneer 10 have truly reached interstellar space.

Pioneer, as its name implies, heralded the beginning of mankind’s exploration of the solar system. It was the first spaceship to travel beyond the orbit of Mars, and the first to cross the Asteroid Belt – the vast collection of rocks and debris that orbit the Sun between Mars and Jupiter.

Before Pioneer 10, no spacecraft had attempted to go be­yond Mars. One reason was the Asteroid Belt (which was) much feared because, besides the large asteroids, there’s also dust and debris. And with a spacecraft travelling at 30,000 kilometres per hour, a small piece of dust can do a lot of damage.

Once passed the Asteroid Belt, Pioneer's next encounter was with Jupiter, the largest planet in the Solar System. It arrived in November 1973, the first time any man-made craft had ever gone near any of the outer planets.

Jupiter was the last place Pioneer 10 visited. Since then, it has been travelling through interplanetary space, exploring the solar wind. Originally, scientists thought that the solar wind ended close to Jupiter, which is nearly 300 million kilometres from the Sun. But the Sun emits a million tons of material a second into interplanetary space and this becomes the solar wind (which) blows away at a speed of .about 400 kilometres per second. Now, as it expands, it actually takes with it the solar magnetic field, and creates a structure called the heliosphere. And this is what Pioneer has been exploring for the past 20 years.

The heliosphere is believed to ex­tend 18 billion. kilometres into space. This means that Pioneer is only just over halfway to the edge of the solar system. Moreover, be­cause its power will run out within a year, Pioneer will not even be able to tell us when it reaches the edge of the solar system. It's a sad irony that when one of humanity's greatest accomplishments is achieved, it will be by a silent spacecraft, which "died" two decades earlier.

Dead it may be, but Pioneer 10's "corpse" will be around for aeons yet. It may even outlast the Earth, which will be destroyed in about five billion years, when the Sun runs out of hydrogen fuel and expands, enveloping the solar system’s inner planets. In the meantime, Pioneer 10 will continue to drift through interstellar space. But the chances of it ever being detected by another life-form are so remote as to be incalculable. Nevertheless, it does carry some information about its senders. Attached to it is a goldplated plaque, de­signed by the late American astronomer, Carl Sagan. It shows a naked man and woman, atomic hydrogen, and the solar system. Information, Sagan said, that shows "where we are, when we are, and who we are".

After Pioneer. The Pioneer 10 spacecraft may have been put into retire­ment, but two other craft, Voyagers 1 and 2, are still functioning well and should continue to do so well into the next century.

Like Pioneer 10, the Voyagers are now far beyond the orbit of Pluto. They were launched later than Pioneer 10 and its sister craft Pioneer 11, which astronomers lose con­tact with over two years ago.

The Voyagers have been among the most successful craft in the history of solar system exploration. They discovered that Jupiter has a ring around it, which cannot be seen from Earth; they found active volcanoes, ice-oceans on its moon Europa; and they discovered three new Jovian moons— the tiny satellites of Adrastea, Metis and Thebes.

Voyagers 1 and 2 are now ten and eight billion kilometres from the Sun respectively. And astronomers believe they may be about to make their most impressive discovery yet. Evi­dence suggests that, when Voyager 1 reaches the edge of the Solar Sys­tem, it may hit a giant shock wave, which is thought to occur where the solar wind meets interstellar space. This shock wave occurs when par­ticles from the solar wind — trav­elling at up to 800 kilometres per second — collide with the much less dense interstellar medium, where particles travel at speeds of "only" 20 kilometres per second.

But, if it exists, the shock wave lies eight billion kilometres further away than Voyager 1’s current position. Astronomers predict that the Voyagers have enough power to take them just beyond the shock wave. After that they, like the Pio­neers, will fall silent. But the bigger part of their journey will still stretch before them for millions of years.

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