Cosmic ray detection could mean Voyager 2 is close to interstellar space

Laurel Kornfeld

Now almost 11 billion miles (17.7 billion kilometers) from Earth, NASA’s Voyager 2 spacecraft has detected an increasing amount of cosmic rays that originate beyond our Solar System, leading some scientists to believe the probe is on the verge of entering interstellar space.

Launched on Aug. 20, 1977, Voyager 2 completed a “Grand Tour” of the Solar System, flying by Jupiter, Saturn, Uranus and Neptune. Its twin, Voyager 1, launched on Sept. 5 of the same year and flew by Jupiter, Saturn and Saturn’s largest moon, Titan. Both spacecraft are on trajectories taking them out of the Solar System.

Voyager 2 entered the outer region of the bubble surrounding the Sun and planets, known as the heliosphere, in 2007. This “bubble” encompasses the magnetic fields of the Sun and planets. Its outer boundary, the heliopause, marks the transition from the Sun’s sphere of influence to interstellar space.

Beginning in August, the probe has detected a five percent increase in cosmic rays coming from beyond the Solar System as measured by its Cosmic Ray Subsystem instrument, a detection Voyager 1 made in May 2012. Cosmic rays are made up of particles that move rapidly but are blocked upon hitting the heliosphere.

Voyager 1 was declared by mission scientists to have entered interstellar space in August 2012 based on its measurements of a coronal mass ejection (CME) that had occurred earlier that year. A shock wave generated by the CME caused increased vibration of the particles around the spacecraft, making it possible for scientists to measure the density of the spacecraft’s environment.

When they found the density in Voyager 1’s environment to be significantly higher than that in the outer heliosphere, mission scientists determined it had entered interstellar space.

However, exactly where interstellar space begins remains blurry. Voyager 1 has not exited the Oort Cloud, a sphere of comets orbiting the Sun extending as far as 50,000 to 200,000 astronomical units (one astronomical unit, or AU, is equal to the average Earth-Sun distance—about 93 million miles) or 0.8 to 3.2 light years. The spacecraft will leave the Oort Cloud sometime between 14,000 and 28,000 years from now.

Furthermore, in 2014 two Voyager 1 scientists said they believe the spacecraft was still in the heliosphere, the region dominated by the solar wind. George Gloeckler of the University of Michigan led a study published in the journal Geophysical Research Letters that claimed the spacecraft will remain in the Solar System until it detects a reversal of the Sun’s magnetic field.

The writers pointed out that although Voyager 1 was detecting an increased level of cosmic rays from beyond the Solar System, no change had been detected in the level of magnetism around the probe, indicating it was still within the Sun’s influence.

However, in the same study, Gloeckler also said if Voyager 1 does not experience a magnetic field reversal within a year or two, scientists could conclude it had already entered interstellar space.

Not static, the heliosphere itself could be regularly contracting and expanding, resulting in the spacecraft exiting and re-entering it several times.

Voyager 2 took a completely different path than its twin, and could find the heliopause either closer or farther than it is on Voyager 1’s journey. The increased level of cosmic rays detected by the spacecraft may therefore not mean it is approaching the heliopause.

“We’re seeing a change in the environment around Voyager 2, there’s no doubt about that,” said Voyager project scientist Ed Stone of Caltech. “We’re going to learn a lot in the coming months, but we still don’t know when we’ll reach the heliopause. We’re not there yet—that’s one thing I can say with confidence.”