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Voyager probes' positions
This illustration shows the positions of NASA’s Voyager 1 and Voyager 2 probes, outside of the heliosphere, a protective bubble created by the Sun that extends well past the orbit of Pluto. Sizes and distances are not shown to scale. Click on the image for a larger version. (NASA / JPL-Caltech Illustration)

NASA says its Voyager 2 probe has become the second human-made object to fly into interstellar space — six years after its twin, Voyager 1, became the first.

Based on readings from its onboard instruments, the mission’s scientists have determined that Voyager 2 has left the solar system’s heliosphere, a protective bubble of particles and magnetic fields created by the sun. The spacecraft is now journeying in a region where the cold, dense interstellar medium takes the place of the tenuous, hot solar wind — more than 11 billion miles from Earth.

The milestone came more than 41 years after Voyager 2’s launch in 1977 on what was then a grand interplanetary mission, and is now a grand interstellar mission. During the 1970s and 1980s, Voyager 2 took on a “Grand Tour” with close flybys of Jupiter, Saturn, Uranus and Neptune, while Voyager 1 took a different course that featured a close-up of the Saturnian moon Titan.

Scientists discussed the mission’s status today in conjunction with this week’s American Geophysical Union meeting in Washington, D.C.

The most compelling evidence of the probe’s passage across the heliopause — that is, the boundary between the heliosphere and interstellar space — came in the form of data from its Plasma Science Experiment, or PLS, an instrument that stopped working on Voyager 1 in 1980.

The detector monitors the electrical current in plasma flowing out from the sun to measure the speed, density, temperature, pressure and flux of the solar wind. On Nov. 5, Voyager 2’s PLS detected a sharp decline in the speed of solar wind particles, and since then, no solar wind has been detected in the environment surrounding the probe.

“Working on Voyager makes me feel like an explorer, because everything we’re seeing is new,” MIT’s John Richardson, principal investigator for the PLS instrument, said in a news release from NASA’s Jet Propulsion Laboratory. “Even though Voyager 1 crossed the heliopause in 2012, it did so at a different place and a different time, and without the PLS data. So we’re still seeing things that no one has seen before.”

Data from three other instruments on Voyager 2 — the cosmic ray subsystem, the low-energy charged particle instrument and the magnetometer — were consistent with the plasma readings. Mission scientists will continue to analyze the data to get a better fix on the environment through which Voyager 2 is traveling at more than 35,000 mph.

“There is still a lot to learn about the region of interstellar space immediately beyond the heliopause,” said Voyager project scientist Ed Stone, a JPL veteran who’s now based at Caltech.

Both Voyager probes are powered by radioisotope thermal generators, which convert heat from the radioactive decay of plutonium into electricity. The spacecraft thus don’t rely on solar power to keep themselves alive, but power usage must be carefully managed to extend the probes’ operating life.

Even though the Voyagers are traveling through interstellar space, they are still well within the solar system’s gravitational sphere of influence.

The easiest way to express their distance is in terms of astronomical units, or AU. A single AU is equal to the distance from Earth to the sun, or 93 million miles. By that measure, Voyager 1 is almost 145 AU from Earth, and the distance to Voyager 2 is nearly 120 AU. (In comparison, the next-farthest-out operational spacecraft, New Horizons, is less than 44 AU away.)

Both Voyagers are heading toward the Oort Cloud, a huge haze of comets swirling at a distance of between 1,000 and 100,000 AU from the sun. It’s expected to take about 300 years for Voyager 2 to reach the inner edge of the Oort Cloud, and perhaps 30,000 years to fly beyond it.

The nearest star beyond the sun, Proxima Centauri, is about 4.2 light-years or 268,770 AU away.

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