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| Credit: NASA/JPL-Caltech |
Voyager 2 is currently on a journey toward interstellar space. According NASA the probe has detected an increase in cosmic rays which travel from outside our solar system. Voyager was launched in 1977 and is about 17.7 billion kilometers from Earth, this is more than 118 times the distance from Earth to the Sun.
Since August, the Cosmic Ray Subsystem instrument on Voyager 2 has measured a 5% increase in the rate of cosmic rays hitting the spacecraft. The probe's Low-Energy Charged Particle instrument has detected a similar increase in higher-energy cosmic rays.
Cosmic rays are fast-moving particles that originate outside the solar system. Some of these cosmic rays are blocked by the heliosphere, so mission planners expect that Voyager 2 will measure an increase in the rate of cosmic rays as it approaches and crosses the boundary of the heliosphere.
In May 2012, Voyager 1 experienced an increase in the rate of cosmic rays similar to what Voyager 2 is now detecting. That was about three months before Voyager 1 crossed the heliopause and entered interstellar space.
The fact that Voyager 2 may be approaching the heliopause six years after Voyager 1 is also relevant, because the heliopause moves inward and outward during the Sun's 11-year activity cycle. Solar activity refers to emissions from the Sun, including solar flares and eruptions of material called coronal mass ejections. During the 11-year solar cycle, the Sun reaches both a maximum and a minimum level of activity.
