After a voyage lasting more than 34 years, a spacecraft that has travelled further than any man-made object is on the verge of leaving the Solar System and entering the mysterious region of interstellar space, where nothing terrestrial has gone before.
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Scientists at the US National Aeronautics and Space
Administration (Nasa) said that the Voyager 1 space probe, which
has travelled about 11 billion miles since its launch in 1977, has
entered the cosmic equivalent of the doldrums, where the high-speed
solar winds die down at the very edge of the Solar System.
Voyager 1, launched within weeks of its twin probe,Voyager 2,
was originally designed to explore Jupiter and Saturn. After making
a string of important observations, such as active volcanoes on
Jupiter's moon Io and the intricacies of Saturn's rings, the
mission was extended. Voyager 2 went on to explore the faraway
planets of Uranus and Neptune.
However, long after the official planetary missions ended, both
spacecraft continued to plough through the farthest regions of the
Solar System, while maintaining radio contact with mission control
through its Deep Space Network.
Nasa expects that within the next few months – or possibly years
if margins of error are taken into account – Voyager 1 will finally
leave the Solar System for good and begin its journey through the
vast void of interstellar space that comprises most of the Milky
Way galaxy. Voyager 2 – travelling not far behind – will follow
suit.
Scientists at Nasa said that over the past year, Voyager 1 had
entered a kind of "cosmic purgatory" where the wind of electrically
charged particles streaming from the Sun has calmed.
Both spacecraft are now in a region known as the "heliosheath",
the outermost layer of the Solar System, where the solar wind,
which can travel 16 miles per second, is being slowed down by the
rising pressure of interstellar gas. Nasa scientists believe this
indicates the imminent entry of Voyager 1 into the interstellar
region, which is dominated by another kind of magnetic wind coming
from a different direction of deep space.
"Voyager tells us now that we're in a stagnation region in the
outermost layer of the 'bubble' around our Solar System. Voyager is
showing that what is outside is pushing back. We shouldn't have
long to wait to find out what the space between the stars is really
like," said Ed Stone, Voyager project scientist at the California
Institute of Technology in Pasadena.
Nasa changed the orientation of Voyager 1 four times this year
to see whether the solar wind and magnetic field lines had switched
direction. Data released at the American Geophysical Union meeting
in San Francisco shows the magnetic field lines have not changed,
indicating that Voyager 1 is still just within the "heliosphere",
the magnetic bubble of charged particles created by the Sun.
"We have seen the same east-west direction of the magnetic field
since we launched. That's the solar magnetic field. Once we leave
the heliosphere we will enter the magnetic field of the galaxy and
all the data to date suggest that this field is orientated more
north-south," Dr Stone told the meeting.
The final frontier: Between the stars
* The English philosopher Francis Bacon appears to have been the
first person to attempt to describe the space between stars when he
wrote in 1626 about the "interstellar skie".
* In the 19th century scientists postulated an invisible
luminiferous ether between the stars that allowed the transport of
light. It is now known that electromagnetic waves, whether cosmic
rays or light in visible wavelengths, can travel through
interstellar space without the need for a physical "ether".
* Astronomers today talk about an interstellar medium that fills
the void between stars. Rather than a complete vacuum, consists of
about 99 per cent dust and 1 per cent charged particles or ions,
but in incredibly low densities. Cosmic rays from deep space fill
the void.
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