Voyager 2 Finds the Edge of the Heliosphere

Voyager 2 probe sends data from interstellar space back to Earth

In December a year ago, NASA announced the spacecraft had left the solar system, saying data returned suggested there had been a major change in the environment, with a marked increase in galactic cosmic rays and decrease in heliospheric particles from the Sun. Scientists can use the new numbers from Voyager 2 to more accurately model the shape and thickness of the heliosphere's hydrogen wall, where the solar winds run out of momentum and are pushed back by the interstellar medium. The findings were published Monday in a series of five papers in Nature Astronomy. These include magnetic field sensor, two instruments to detect energetic particles in different energy ranges and two instruments for studying plasma, which is a gas composed of charged particles. Additionally, interstellar space contains cosmic rays, particles accelerated by exploding stars.

Determining when an object leaves the solar system is not easy. The plasma in interstellar space is colder and more dense than the heliosphere. The intensity of cosmic rays there was just 90% of their intensity farther out.

The Voyager probes launched in 1977, each equipped with an identical suite of instruments for exploring the outer solar system. From the first Voyager-1 and now Voyager-2 we come to know that there remains a strong boundary.

Scientists hope to continue learning new details about interstellar space as the Voyager probes return new measurements.

Now Voyager 2 has sent back the most detailed look yet at the edge of our solar system - despite Nasa scientists having no idea at the outset that it would survive to see this landmark.

Voyager 2 also made some observations that don't square up with a sharp boundary-at least not what we'd expect. The edge of the heliosphere is called the heliopause.

Even though they made it out of the sun's bubble, astronomers maintain that the Voyagers are still in the solar system because it stretches to the outer edge of the Oort cloud - comprising water ice, ammonia and methane - which surrounds the sun.

It is also similar to the plasma density jump experienced by Voyager 1 when it crossed into interstellar space, they said.

"The Voyager probes are showing us how our Sun interacts with the stuff that fills most of the space between stars in the Milky Way galaxy", said Ed Stone, project scientist for Voyager and a professor of physics at Caltech.

Both Voyager 1 and Voyager 2 were launched in 1977, designed for five-year missions.

"The two Voyagers will outlast Earth", said Dr. Bill Kurth, a research scientist at the University of Iowa. The marked increase in plasma density is evidence of Voyager 2 journeying from the hot, lower-density plasma characteristic of the solar wind to the cool, higher-density plasma of interstellar space.

Voyager 1 passed through the heliosphere in 2012 but its crucial plasma instrument had been damaged during a fly-by with Saturn.

The heliosphere is somewhat leaky, revealed Voyager's particle instruments. One of the Voyager's particle instruments picked up on particles trickling out of our heliosphere and into interstellar space. Voyager 1 exited close to the very "front" of the heliosphere, relative to the bubble's movement through space. To understand what's happening at and near this boundary, they study the information the Voyager probes collected about magnetic fields and charged particles on either side of the heliopause.

With Voyager 1, scientists had only one sample of these magnetic fields and couldn't say for sure whether the apparent alignment was characteristic of the entire exterior region or just a coincidence. Some had thought Voyager 2 would make that crossing first, based on models of the heliosphere. Their interstellar mission began in 1989.

Currently, Voyager 1 is located more than 22 billion kilometers (13.6 billion miles) from the Sun, and Voyager 2 is 18.2 billion kilometers (11.3 billion miles) from it.

The Voyager probes became world famous thanks to the gold plates they carry, full of music, sounds and images of the earth, meant as a kind of time capsule that tells the story of our world to any aliens they might encounter along the way.



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