Launched in 1977, NASA Voyager 2 is a bit less than 17.7 billion miles from Earth, or more than 118 times the distance from Earth to the Sun.
NASA Voyager 2 probe has now been on a journey to interstellar space NASA Voyager 2 will enter interstellar space
The Voyager Spacecrafts, 1 and 2, were built by NASA's Jet Propulsion Laboratory in Pasadena, California.
Since 2007, the Voyager 2 probe has been traveling through the outermost layer of the heliosphere – the vast bubble around the Sun and the planets dominated by solar material and magnetic fields.
Voyager scientists have been watching for the spacecraft to reach the outer boundary of the heliosphere, known as the heliopause.
Posters NASA made for Voyager's 40th anniversary. (Image: NASA)
Once Voyager 2 exits the heliosphere, it will become the second human-made object, after Voyager 1, to enter interstellar space.
The increase in cosmic rays detected by NASA Voyager 2
Cosmic rays are fast moving particles that originate outside the solar system.
Since late August, the Cosmic Ray Subsystem Instrument on Voyager 2 has measured about a 5 percent increase in the rate of cosmic rays hitting the spacecraft compared to early August
The probe's Low-Energy Charged Particle instrument has detected a similar increase in higher-energy cosmic rays.
A book on the chronicles of NASA's attempt to send spacecrafts 'beyond Earth'. (Image: NASA)
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.
When Voyager 1 faced a similar increase
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.
We do not know when NASA Voyager 2 will reach heliopause
However, Voyager team members note that the increase in cosmic rays is not a definitive sign that the probe is about to cross the heliopause.
Voyager 2 has a different location in the heliosphere – the outer region of the heliosphere – than Voyager 1 had been, and possible differences in these locations means Voyager 2 may experience a different exit timeline than Voyager 1.
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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.
Voyager 2 in Launch Vehicle. (Image: NASA)
During the 11-year solar cycle, the Sun reaches both a maximum and a minimum level of activity.
" We're seeing a change in the environment around Voyager 2, there's no doubt about that "said Voyager Project Scientist Ed Stone, based at Caltech in Pasadena.
" We are going to learn a lot in the coming months, but we still do not know when we'll reach the heliopause. We're not there yet – that's one thing I can say with confidence "he added.
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