I’m happy many of my friends from the 1970s are still around. I feel the same way about the dual space probes Voyagers 1 and 2, launched by NASA in 1977 and still kicking to this day. Both went on to explore Jupiter and Saturn with Voyager 2 continuing to Uranus and Neptune. On August 25, 2012 Voyager 1 became the first spacecraft to cross the heliopause, the boundary where the pressure of the solar wind is in balance with the winds blowing from the stars. The heliopause is considered the transition zone between solar and interstellar space. Now, Voyager 2 appears poised to enter interstellar space too.
Voyager 2 is a little less than 11 billion miles (~17.7 billion km) from Earth at the moment, or more than 118 times the distance from Earth to the Sun. Since 2007 it’s been traversing the outermost layer of the heliosphere, a giant bubble surrounding the sun and planets dominated by streams of subatomic particles and magnetic fields from the sun called the solar wind. Within the bubble, we’re somewhat shielded from cosmic rays — high-speed protons bounding about the galaxy that can damage satellite electronics or life outside the protection of the atmosphere and Earth’s magnetic field. But outside of it, their number ticks up and space travel becomes more hazardous.
Since late August, the Cosmic Ray Subsystem instrument on Voyager 2 has measured about a 5% increase in the rate of cosmic rays hitting the spacecraft compared to early August. Another instrument has detected a similar increase in higher-energy cosmic rays. Both are signs we’re approaching the edge of the sun’s magnetic influence.
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. That doesn’t necessarily mean Voyager 2 will be fancy free come December — the spacecraft’s in a different location and moving along a different path.
We also know that the sun’s domain expands and contracts in sync with the 11-year sunspot cycle. At cycle maximum, the solar wind blows faster and the heliosphere expands; at minimum it contracts. Voyager 1 is making the crossing very close to solar minimum in a shrinking heliosphere, so you’d think it would already be in interstellar space by now. My hunch is that the sun’s house is far from an idealized, perfect bubble!
Whatever the timeline, astronomers expect to know soon. Interestingly, just because interstellar space begins doesn’t mean the solar system ends! A vast roughly spherical cloud of comets called the Oort Cloud may be influenced by interstellar winds and even other stars, but it’s still dominated by the sun’s gravity and a proper part of the solar system. Incredibly far away, its inner edge is estimated to be 186 billion miles (300 billion km) away — 50 times farther than Pluto. It’s estimated that the Voyager won’t breach the Cloud for another 300 years.
That’s so remote that passing stars and big clouds of gas and dust can wiggle one or more comets loose and send them into the inner solar system. Every year, astronomers trace a subset of new comet discoveries back to the Oort Cloud.