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Voyager Mission reveals unexpected pressure at the edge of the solar system

NASA astronomers have used data from the Voyager probes to measure the motion of particles as ripples in the very edge of our solar system, and found that the pressure in the far boundary regions of our star is higher than expected.

The results suggest "there are some other parts to the pressure that are not being considered right now that can contribute," says Princeton University astrophysicist Jamie Rankin.

Maybe there are whole populations of particles out there. Not yet taken into account. Or maybe it's just a little warmer than anyone expected. The researchers have a number of possible explanations to explore in future research.

Although the discovery itself is interesting enough, it is as if they found it that makes a truly fascinating science.

As plasma in the form of solar wind comes from our sun, it forms a "bubble" which we call the heliosphere. Fourteen billion kilometers away from the star, the wind is running out of steam, as charged particles slowly sink to subsonic speeds.

The edge of this bubble, called heliosheath, is a zone where the density of the charged particles drops off and the magnetic field grows weakly.

Beyond this dirty boundary there is a thin shell called the heliopause, where the plastic breeze blown out by the sun is making away, nudged by the subtle influence of our galactic neighbors as our star moves through space.

During this "break", the pressure from the local interstellar space that protrudes and the helio protector that protrudes must balance. However, knowing exactly what this looks like is no easy task. We can make models to estimate, but nothing beats hard evidence.

Fortunately, we have two probes that pass through that part of the solar system. Take a look at NASA's practical chart below to see how it all fits together.

 NASA chart of a heliosheath (NASA's Goddard Space Flight Center / Mary Pat Hrybyk-Keith)

Voyager 1 is approximately 20 billion kilometers away , effectively out in the wild emptiness that we regard as an interstellar space. Its partner, Voyager 2, is not far behind, right at the tip of making an exit.

Neither has a direct way of telling much about the space pressure in that area, but a new inflation in solar activity called a Global Merged Interaction Region (GMIR) provided an excellent opportunity to accomplish it.

"It was a truly unique time for this event because we saw it right after Voyager 1 passed into the local interstellar space," Rankin says.

"And while this is the first event that Voyager saw, there is more data that we can continue to look at to see how things in heliosheath and interstellar space change over time."

The solar activity was effectively a cry into space, sending a pulse of particles rushing into the distance. This cry rippled into the 2012 heliosheath, where Voyager 2 saw and listened. About three months later, Voyager 1 also felt its effects.

From each set of observations, the researchers calculated the pressure at the boundary to about 267 femtopascals, which is an absolutely minuscule fraction of the type of atmospheric pressure we experience here on earth.

It may be a relatively small squeeze, but the researchers were surprised.

"When we add the pieces that are known from previous studies, we found that our new value is still greater than what has been measured so far." says Rankin.

The team was also able to calculate the speed of sound waves passing through this medium – a fast 314 kilometers per second. Or a thousand times faster than sounds traveling through our own atmosphere.

Another surprise came. The wave's passenger line up with an apparent decrease in the intensity of high-particle particles called cosmic rays. The fact that each of the probes experienced the same thing in two different ways gives astrophysicists yet another mystery to solve.

"Trying to understand why the change in the cosmic rays is different inside and outside the heliosheath is an open question," Rankin says.

The Voyager probes may be a little old, but given how busy it looks on the edge of the solar system, we're glad they haven't gone out quite yet.

This research was published in The Astrophysical Journal .

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