The NASA Voyager 2 probe, currently on a trip to interstellar space, has detected a increase in cosmic rays that originate outside of our Solar System.
Launched in 1977, Voyager 2 is about 17.7 billion kilometers from Earth, or more than 118 times the distance from Earth to the Sun.
Since 2007, the probe has been traveling through the outermost layer of the heliosphere: the vast bubble that surrounds the Sun and the planets dominated by solar material and magnetic fields. Voyager scientists have been watching the spacecraft to reach the outer limit of the heliosphere, known as the heliopause. Once Voyager 2 leaves the heliosphere, it will become the second object created by man, after Voyager 1, to enter interstellar space.
As reported by the Jet Propulsion Laboratory (JPL), since the end of August, the instrument of the Cosmic Ray Subsystem in Voyager 2 has measured around 5% increase in the rate of cosmic rays that impact the spacecraft compared to early August. The low energy charged particle instrument of the probe has detected a similar increase in cosmic rays of higher energy.
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 Voyager 2 to measure an increase in the cosmic ray rate as it approaches and crosses the boundary of the heliosphere.
In May 2012, Voyager 1 experienced an increase in the cosmic ray rate similar to that which Voyager 2 is detecting. That was approximately three months before Voyager 1 crossed the heliopause and entered interstellar space.
However, members of the Voyager team note that the increase in cosmic rays is not a definitive signal that the probe is about to cross the heliopause. Voyager 2 is located in a different location in the outer region of the heliosphere than Voyager 1, and the possible differences in these locations mean that Voyager 2 can experience a Exit timeline different to that of Voyager 1.
The fact that Voyager 2 can approach the heliopause six years after Voyager 1 is also relevant, since the heliopause moves in and out during the 11-year solar activity cycle. Solar activity refers to the Sun's emissions, including solar flares and eruptions of material called coronal mass ejections. During the solar cycle of 11 years, the Sun reaches a maximum and minimum activity level.
"We're seeing a change in the environment around Voyager 2There is no doubt about that, "said Voyager project scientist Ed Stone, of the California Institute of Technology, who is participating in the mission." We are going to learn a lot in the coming months, but we do not yet know when we will reach the heliopause We have not reached that yet, that's something I can say with confidence, "added the researcher.