The most distant man-made object 鈥� the Voyager 1 spacecraft 鈥� is finally leaving the Solar System. Astronomers think the probe has reached a boundary where the Sun鈥檚 influence starts to wane.
The spacecraft has just entered a region no one has ever explored before, according to Voyager project scientist Edward Stone, at the California Institute of Technology in Pasadena. 鈥淭his is a very exciting time,鈥� he told a NASA news conference in Washington DC. 鈥淰oyager is beginning to explore the final frontier of the Solar System.鈥�
Voyager 1 and its companion Voyager 2 were launched on a journey to the outer planets in 1977. Voyager 1 is now about 90 astronomical units from the Sun (one AU is the distance between the Earth and the Sun). It is the most distant spacecraft in the Solar System, having overtaken the Jupiter probe Pioneer 10 in 1998. Voyager 2 lags behind, at about 73 AU (see graphic).
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For years, scientists thought Voyager 1 must be getting close to the Solar System鈥檚 鈥渢ermination shock鈥�. This is the region where supersonic particles streaming out from the Sun plough into interstellar particles and slow down to subsonic speeds. This region, often considered to mark the Solar System鈥檚 edge, should energise lots of particles and have a strong magnetic field.
Particle peak
When the Voyagers were launched, astronomers predicted that the termination shock would lie 40 to 50 AU away from the Sun. But Voyager 1 clocked up 60, 70 and 80 AU, and still had not reached the shock. Now, at last, scientists have evidence that Voyager 1 is close to the shock, or has even passed through it.
In the summer of 2002, when Voyager 1 was about 85 AU from the Sun, it measured a sharp increase in the number of energetic particles over a period of six months. Stamatios Krimigis of Johns Hopkins University in Laurel, Maryland, and his colleagues say the data suggests Voyager 1 crossed the termination shock, which then washed back over the spacecraft. The shock moves in and out depending on the Sun鈥檚 activity.
鈥淭his is a really exciting milestone,鈥� says Krimigis. 鈥淚t鈥檚 the first time a machine has gone outside the cocoon of the solar atmosphere.鈥�
However, a team led by Frank McDonald at the University of Maryland in College Park does not think Voyager 1 has passed the termination shock yet. McDonald points out that the spacecraft has not registered the expected large increases in the local magnetic field in the shock region.
鈥淲e鈥檙e in the neighbourhood of the termination shock, but we haven鈥檛 crossed it,鈥� he told the NASA conference.
Around 2020, Voyager 1 is expected to reach the heliopause at roughly 135 AU. This is where the Sun鈥檚 influence fades away entirely and interstellar space begins. Astronomers will then get their first chance to measure the magnetic fields and energetic particles of interstellar space.
Journal reference: Nature (vol 426, p 45, 48)