Scientists will begin to use the Voyager I spacecraft to make in-situ measurements of interstellar space, having applied a “gift from the Sun” to confirm that the venerable probe has traveled into the region characterized by plasma originating in other stars. Analyzing plasma-pressure data that have trickled back across the 11.7 billion mi. to Earth at a rate of about 160 bps, the science team has concluded that Voyager I is outside the relatively low-pressure plasma at the outer edge of the heliosphere, where the solar wind from Earth's star slows. The spacecraft also has entered a region where the density is 40 times greater because it is generated by material from other stars and stellar explosions. Voyager I actually made the transition to interstellar space in August 2012, but because its plasma sensor failed years ago, the calculations had to be based on data from the Plasma Wave Subsystem (PWS), which measures perturbations in the surrounding plasma. That plasma was so thin near the heliopause that it did not register on the PWS until the arrival of fallout from a coronal mass ejection in March 2012, according to Don Gurnett, head of the plasma wave team at the University of Iowa. Scientists expect to continue receiving data from the twin Voyager spacecraft, launched in 1977 to conduct fly-by science at Jupiter, Saturn, Uranus and Neptune, for seven more years. Voyager II is on a slightly different trajectory than its counterpart, but it, too, should reach interstellar space before its radioisotope thermoelectric generator stops producing enough power to drive the instruments.
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