Onward to Interstellar Space!

For the first time, a probe is nearing the edge of the solar system.
NASA launched Voyager 1 and its sister spacecraft Voyager 2 in 1977 to explore the giant planets Jupiter and Saturn. After both had sent back dramatic photos and measurements of the main targets and their moons and rings, Voyager 2’s trajectory was adjusted so it also would encounter Uranus and Neptune, the farthest planet.

A third of a century after launch, both craft are soaring outward, continuing to measure the hot ionized particles that blow from the sun at supersonic speed and extend throughout the solar system.
Now Voyager 1, the more distant at 10.8 billion miles from the sun, has reached a region where the charged particles no longer flow outward, the Jet Propulsion Laboratory, Pasadena, Calif., announced earlier this month. This stream of particles, called the solar wind, “has turned the corner,” said Ed Stone, Voyager project scientist based at the California Institute of Technology, Pasadena.

[An artist’s concept of the Voyagers nearing interstellar space. Image credit: NASA/JPL]
According to the JPL press release that quoted Stone, the solar wind forms a bubble around our solar system called the heliosphere. The particles stream from the sun “at supersonic speed until it crosses a shockwave called the termination shock,” the release says.
“At this point, the solar wind dramatically slows down and heats up in the heliosheath.” The heliosheath is the thick edge of the heliosphere, with interstellar space beyond. The shockwave forms because material from other stars, described by JPL as the interstellar wind between stars, impacts the solar wind.
In June, when Voyager 1 was 10.6 billion miles from the sun, it reported that the solar wind had slowed dramatically, not heading outward anymore. Scientists checked the readings for four months before they decided the wind really had died down. The solar wind’s velocity in August 2007 was 130,000 miles per hour, but since then it had slowed by about 45,000 mph each year.
The release quotes Rob Decker, senior staff scientist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. — a member of the Voyager Team — as saying he was amazed when the readings of the outward solar wind came back as “solid zeros ….
“Here was Voyager, a spacecraft that has been a workhorse for 33 years, showing us something completely new again.”
Nightly News asked Jia-Rui C. Cook, a NASA public affairs expert at JPL, whether the solar wind had disappeared. “There is solar wind where Voyager 1 is,” she replied by email.
“It is just not moving outward radially from the sun. The scientists believe the wind has turned the corner and may be moving up or down or to the side.”
The shock wave far ahead is bending back the solar wind.
Voyager 1 has entered the heliosheath but has yet to cross through it into interstellar space. When it does, its instruments should detect “a sudden drop in the denisity of hot particles and an increase in the density of cold particles,” the JPL statement adds. That may happen in about four years. Voyager 2, at 8.8 billion miles from the sun in a different direction, is expected to reach the heliosheath in a few years.
When the probe finally leaves the solar system behind, what conditions will it discover?

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