Sphere of Influence

It’s an exciting time right now, when humans have finally made it to interstellar space. If you haven’t heard, Voyager 1 has exited the heliosphere. And this time we mean it!


Seriously, though, the news has been all over the internet lauding this most recent confirmation that Voyager 1 has made it out (see JPL Press Release). The real question, however, is out of what? Put simply, the discovery being presented is that the plasma densities that Voyager 1 is measuring have increased significantly, suggesting it has moved out of the sphere of influence created by the constant outflow from the Sun we call the Solar Wind. This outflow creates a cozy little bubble called the heliosphere, but this is only one way to measure the extent of the Sun’s influence.


Artist’s conception of the location of Voyager 1 and 2 from December 2012

When the Solar wind slows down because of the increased pressure of the interstellar medium, it sets up a termination shock as it decelerates past the speed of sound (think of this as a sonic boom, backwards). There is then a region of the outer heliosphere called the heliosheath, and the idea right now is that Voyager 1 has made it out of this region.

“But wait!” you exclaim. “That’s just a boundary based on pressure and density! What about gravity?” That’s a very astute observation. To put everything into perspective, I’ll call on the graphic design skills of NASA/JPL-Caltech professionals…


A schematic of the key components of the solar system. Please note: the scale on this graphic is logarithmic, not linear

You’re likely familiar with the eight (not nine!) planets of the Solar System and the fact that they orbit the Sun because they are gravitationally bound (if this second point is troublesome, perhaps review this wikipedia article) What you may be less aware of are the other main sources of material in our little neighborhood.

  • The asteroid belt, containing dwarf planet Ceres, lies between Mars and Jupiter.
  • The Kuiper belt, containing among other things dwarf planets Pluto and Eris, lies just beyond the orbit of Neptune.
  • The Oort Cloud best represents the extent of the Sun’s gravitational influence, as it is a spherical cloud extending from a few thousand AU from the Sun to a few hundred thousand AU.

The Oort Cloud is the source of most of the long-period comets we see coming through the inner solar system. Voyager 1 and 2 will be long dead before their trajectories bring them anywhere close to this “edge of the Solar System,” so it isn’t surprising that as a species we’d prefer to take the smaller victory of making it out of the heliosphere.

Finally, if we consider the Sun’s furthest possible influence, we would have to travel five billion light-years away, where the first rays of light emitted by our newborn star are currently streaming outward at the speed of light. Five billion light-years is equivalent to 2.9×10^22 miles i.e. 29,000,000,000,000,000,000,000 miles (for the curious, this would be 29 sextillion miles in America or 47 trilliard kilometers in France). This certainly puts our accomplishments with the Voyagers into perspective. To track these satellites real-time, check out http://voyager.jpl.nasa.gov/where/.

…and knowing what it had to tell of the night passing swiftly on
toward further darknesses, but moving also toward a new sun.
Fahrenheit 451, Ray Bradbury


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