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This graph shows the position of the Voyager 1 and Voyager 2 probes in relation to the heliosphere, a protective bubble created by the Sun that extends far beyond Pluto's orbit. The Voyager 1 passed through the heliopause, or the edge of the heliosphere, in 2012. The Voyager 2 is still in the heliosheath, or the outermost part of the heliosphere. Credit: NASA / JPL-Caltech
NASA's Voyager 2 spacecraft, currently en route to interstellar space, has detected an increase in cosmic rays from outside our solar system. Launched in 1977, Voyager 2 is just under 11 billion miles from Earth, more than 118 times the distance from Earth to the Sun.
Since 2007, the probe travels the outermost layer of the heliosphere – the vast bubble around the Sun and the planets dominated by solar material and magnetic fields. Voyager scientists are monitoring the passage of the probe into the outer limit of the heliosphere, the heliopause. Once Voyager 2 has left the heliosphere, it will become the second human-made object, after Voyager 1, to penetrate into interstellar space.
Since the end of August, the instrument of the Cosmic Ray subsystem on Voyager 2 has measured an increase of about 5% in the rate of cosmic rays reaching the spacecraft compared to early August. The low energy charged particle instrument of the probe has detected a similar increase in high energy cosmic rays.
Cosmic rays are fast-moving particles that come from the solar system. Some of these cosmic rays are blocked by the heliosphere. Mission planners therefore expect Voyager 2 to measure the increase in cosmic rays at the approach and beyond the limits of the heliosphere.
In May 2012, Voyager 1 experienced an increase in the cosmic ray rate similar to that detected by Voyager 2. It was about three months before Voyager 1 passed through the heliopause and entered the space interstellar.
However, members of Voyager's team note that increasing cosmic rays is not a definite sign that the probe is about to pass through the heliopause. Voyager 2 is at a different location than Voyager 1 in the heliosheath, and the possible differences at these locations mean that Voyager 2 might experience a different Voyager 1 release schedule.
The fact that Voyager 2 can approach heliopause six years after Voyager 1 is also relevant, as the heliopause moves inwards and outwards during the cycle. 11 years of activity of the Sun. Solar activity refers to Sun's emissions, including solar flares and material eruptions called coronal mass ejections. During the eleven-year solar cycle, the sun reaches a maximum and minimum level of activity.
"We are observing a change in the environment around Voyager 2, no doubt," said Ed Stone, Caltech-based project researcher based in Pasadena. "We will learn a lot in the months to come, but we still do not know when we will hit the heliopause – we are not there yet – that is one thing I can say with confidence. "
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John Richardson and John Belcher on Interstellar Crossing and Voyager Exploration 1
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