Voyager 2 On Verge Of Interstellar Space

This graphic shows the position of the Voyager 1 and Voyager 2 probes in relation to the protective bubble created by the sun that extends well past the orbit of Pluto. Voyager 1 crossed the heliopause, or the edge of the heliosphere, in 2012. Voyager 2 is still in the heliosheath, or the outermost part of the heliosphere. This diagram shows the heliosphere as teardrop-shaped but another NASA study shows it “closed” on both ends, ie. much more rounded. See hereNASA/JPL-Caltech

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.

This illustration shows the paths of the Voyager 1 and 2 spacecraft on their journey through the outer solar system and into interstellar space. Each is traveling in a different direction with Voyager 1 headed north of the solar system plane and Voyager 2 south. About 40,000 years from now, both spacecraft will pass near stars: Voyager 1 comes within 1.6 light-years the red dwarf GJ 445, and Voyager 2 will pass about 1.7 light-years from Ross 248, another red dwarf. NASA, ESA, and Z. Levay (STScI)

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.

This curve plots the number of cosmic rays detected by Voyager 2 from July through early October. It sharply increased starting in late August.  NASA

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!

This artist’s concept puts huge solar system distances in perspective. The scale bar is measured in astronomical units (AU), with each set distance beyond 1 AU representing 10 times the previous distance. Each AU is equal to the distance from the sun to the Earth. It took from 1977 to 2013 for Voyager 1 to reach the edge of interstellar space. NASA/JPL-Caltech

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.

8 Responses

  1. kevan hubbard

    Wouldn’t pioneer 11,and possibly 10 although that went a different way out of the syestem than 11,the 2 Voyagers and new horizons,also be beyond the terminator shock?as we no longer hear from the pioneer 10 or 11 we have to guess where they are based on their speed at the last contact,this would seem to put pioneer 11 slightly ahead of Voyager 1?

    1. astrobob

      Hi Kevan,

      Last contact with Pioneer 10 was Jan. 23, 2003. Last contact with Pioneer 11 was Nov. 24, 1995, so we really can’t say if they’ve reached interstellar space or not because there’s no signal. Voyager 1 exceeded Pioneer 10’s distance in Feb. 1998 and is currently well past both Pioneer 10 and 11 in distance from the sun. Voyager 2 is 8 AUs past Pioneer 11 and 4 AUs shy of Pioneer 10.

  2. Unless I am missing something, it seems to me there is something wrong with the first image that shows the position of the Voyager 1 and Voyager 2 probes in relation to the protective bubble created by the sun that extends well past the orbit of Pluto. The Voyager probes travel along the plane of the solar system disc, but the solar system travels at right angles to this, so the edge of the heliosphere should be roughly to the north of out planet, the same direction in which the solar system as a whole is travelling.

    1. astrobob

      Hi Ben,
      The Voyagers are on different trajectories on their way out of the solar system. Voyager 1 is headed north of the plane of the solar system and Voyager 2 south of the plane.

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