Where's the Edge of the Solar System? It's Complicated...


If you thought finding a definition for Pluto was contentious, try defining the edge of the solar system.

A press release from the American Geophysical Union (AGU) last week announced that on August 25, 2012, NASA’s Voyager 1, officially entered interstellar space. This milestone comes after speeding across the solar system for 35 years following its landmark flybys of the Jovian and Saturnian system. The AGU release title read: “Voyager 1 Has Left The Solar System, Sudden Changes In Cosmic Rays Indicate.”

The threshold is described by the authors of the paper, published in Geophysical Journal Letters, as Voyager’s measurement of a substantial increase in the level of galactic cosmic rays slamming into the 1,700 pound spacecraft. This is seen as evidence that Voyager may have crossed a cliff called the heliopause, the edge of tenuous immense bubble of plasma and charged particles blown into space by the solar wind.

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The paper abstract reports: “ has crossed a well-defined boundary for energetic particles at this time possibly related to the heliopause.”

But hold on.

Within hours of the AGU release NASA’s Jet Propulsion Laboratory posted a terse press statement that the Voyager team insists the spacecraft has not yet reached interstellar space. The team has a different criterion: a change in the direction of the magnetic field between the of the sun and the interstellar medium. They said the this transition has not yet been measured as Voyager hurtles toward the stars at 38,000 miles per hour.

The same day the AGU news office back peddled and changed the press release title to “Voyager 1 Has Entered A New Region Of Space, Sudden Changes In Cosmic Rays Indicate.” (yawn)

Stories of Voyager crossing something new on its departure from the solar system have been perennial.

A year ago it was reported that Voyager passed the so-called termination shock. This is where the solar wind of charges particles abruptly slows, indicating Voyager’s entry into an outer region called the heliosheath.

Only last August the Voyager team reported that its robot explorer had entered an unknown “magnetic highway” where magnetic fields inside and outside the heliosphere connect up. (Don’t worry; there won’t be a quiz after all this.)

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If you are befuddled by all this, so am I, especially because I can’t see any of it like I can see auroras on Jupiter or the delicate gravitational ripples in the rings of Saturn.

What’s humbling is that Voyager 1 has only traveled 0.02 light-year (11 billion miles) since its launch in 1977. Star Trek’s Captain Kirk could cover that distance in less time than it would take him to finish his morning latte.

Does it sound a little presumptive to say that Voyager is on the cusp of entering an interstellar mission phase?

More fundamentally, who gets to unequivocally define the official edge of the solar system?

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If it’s the heliopause, then it is compounded by the fact that the heliosphere is teardrop- shaped due to the sun plowing through the interstellar medium like a ship on the ocean. The “edge” is at a significantly different distance from the sun depending on a spacecraft’s outbound trajectory.

I’d say that the entire debate is terribly esoteric for the public even though it involves specific milepost signs that are important to astrophysicists for mapping the sun’s relationship to the cold and vacuous interstellar medium.

This “exo-solar system” is an unexplored phantom frontier as intriguing to scientists as exploring the Louisiana Purchase was to Lewis and Clark.

Why not simply use a dynamical argument for defining the solar system’s perimeter that is not as subject to data interpretation? This could be when Voyager 1 reaches the hypothetical Oort cloud of comets, 18,000 years from now. That’s where the sun’s gravitational field keeps a weak hold on primeval objects — at a distance of 6 trillion miles, or one light-year.

Beyond the Oort cloud the sun’s grip eventfully grows so weak that the pull of a nearby star could capture a slow moving object into it gravitational sphere of influence.

Whatever you call solar system’s edge, there doesn’t seem to be a consensus among scientists. Voyager’s bold entry into interstellar space may happen in the near future, or perhaps not until the year 20,000 A.D.  – depending on your definition.

Image Credit: NASA

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