This illustration shows the position of NASA’s Voyager 1 and Voyager 2 probes. Credit: NASA/JPL-Caltech.

NASA's Voyager 2 probe has reached interstellar space at about 11 billion miles from Earth, making it only the second human-made object to do so, according to the space agency.

Why it matters: Voyager 2, which joins its twin Voyager 1 in crossing the heliopause (the boundary between the hot solar wind and cold, interstellar space), will provide scientists with crucial observations about the kinds of particles and forces the spacecraft encounters, such as cosmic rays. This is huge, given that a key instrument aboard Voyager 1 stopped working before that spacecraft crossed the boundary.

  • The information collected by Voyager 2 will also be useful to planners of future human spaceflight missions, since journeys that expose astronauts to cosmic rays can pose health risks.

The details: NASA said data from instruments aboard Voyager 2 show that on Nov. 5 the spacecraft exited the heliosphere (the protective shield of particles and powerful magnetic fields caused by the sun).

  • Scientists detailed their findings at a press conference at the American Geophysical Union's annual meeting in Washington, D.C., on Monday, including new data from the spacecraft's Plasma Science Experiment, or PLS.
  • The PLS is used to detect the speed, density, temperature, pressure and flux of the solar wind, and on Nov. 5 it showed a steep decline in solar wind particles. The PLS has not shown any solar wind flow since, which confirms scientists' judgment that it has left the heliosphere.
  • Other instruments aboard the spacecraft also showed an uptick in powerful cosmic rays, which scientists expected to see once the spacecraft entered interstellar space.

The backdrop: Both Voyager 1 and 2 launched in 1977, and scientists said they hope they will last 50 years.

What they're saying: “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,” said John Richardson, a principal research scientist at MIT, in a press release.

  • “One of the great things about Voyager is it keeps surprising us,” said Ed Stone, Voyager project scientist based at Caltech in Pasadena, California, at the press conference.
  • “I like to say that Voyager 1 and 2 are pretty healthy if you consider them senior citizens," said Suzanna Dodd, JPL's director for the Interplanetary Network Directorate.

Yes, but: While the probes have entered interstellar space, they have not left the solar system. It would take Voyager 2 as many as 30,000 more years to reach the boundary of the solar system, NASA said.

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