Before embarking on his Ph.D., Ralph McNutt had never been east of the Mississippi River. But soon after the young Texan arrived at the Massachusetts Institute of Technology (MIT) in the fall of 1975, he found himself on a voyage to the edge of the Solar System—and beyond. Casting around for a research assistantship, he ended up in the office of plasma physicist Herbert Bridge, a towering figure in space science who had overseen the cloak-and-dagger effort to dismantle and ship Harvard University’s cyclotron to New Mexico for the Manhattan Project during World War II. Bridge evidently saw a familiar spark in McNutt and invited him to work on a plasma detector for Voyager, the epic mission to the outer planets that began in 1977. “I said, ‘Where do I sign up before you change your mind?’”
Now, this veteran of Voyager, one of NASA’s greatest scientific triumphs, wants to wheel his own passion project onto the launchpad. McNutt and colleagues at the Johns Hopkins University Applied Physics Laboratory (APL) have laid out a concept for Interstellar Probe (IP), a $3.1 billion mission to pick up a scientific gauntlet that the two Voyager probes threw down a decade ago after leaving the heliosphere, the Sun’s zone of influence. Few expected the spacecraft to survive that long, yet their beguiling observations, still trickling in, have upended many beliefs about the Solar System’s outer limits. “A lot of our preconceived notions didn’t work out too well,” McNutt says.
The Voyager data are so mystifying that some prominent researchers assert the probes haven’t made it to interstellar space yet, perhaps because the bounds of the heliosphere stretch farther than generally thought. Gazing out from Earth’s perch won’t settle the matter. “The only way to see what our fishbowl looks like is to be outside looking in,” McNutt says. “We need to get modern instruments out there,” adds Lennard Fisk, a space physicist at the University of Michigan (UM), Ann Arbor. “In that sense, Interstellar Probe would be revolutionary.”
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