The NASA Voyager 2 probe, which has been travelling in heliosphere since 2007 on its journey towards interstellar space, has recently detected an increase in cosmic rays originating outside the solar system. The Voyager 2 was launched way back in 1977 and is now around 17.7 billion kilometers away from Earth, which is approximately 118 times the distance between Earth and Sun. The probe is travelling in the outermost layer of heliosphere, which is like a vast bubble containing the sun, planets, solar materials and their magnetic fields. Once the probe exits the heliosphere and enters the heliopause, it will become the second man-made object after Voyager 1 to enter interstellar space.
The Cosmic Ray Subsystem instrument on the probe has measured a 5 percent increase in cosmic rays reaching its surface since August and its Low Energy Charge Particle instrument too has detected same type of increase in high-energy cosmic rays. These rays are rapid moving particles originating inside the solar system and some of them are blocked by heliosphere. Mission planners expect that an increase in cosmic rays is an indication that the probe is approaching the boundary of heliosphere and is likely to crossover soon.
The previous probe Voyager 1 also detected an increase in cosmic rays three months before it crossed into interstellar space. But Voyager’s team members state that this is not a definite sign as Voyager 2 is on a different path and location in heliosheath—which is the outer region of heliosphere and so may have a different exit timeline. The heliopause moves during Sun’s activity cycle of 11 years as it experiences solar flares and coronal mass ejections. Voyager’s Project scientist Ed Stone at Pasadena stated that the team is witnessing a change in environment around Voyager 2 and they are still not sure about when it will reach heliopause.