Don Gurnett, on the left, during his talk at the Dartmouth physics colloquium. Photo: Merritt Losert '17

Don Gurnett, on the left, during his talk at the Dartmouth physics colloquium. Photo: Merritt Losert ’17

On Friday, April 4, Don Gurnett, a professor of physics and astronomy at the University of Iowa, gave a talk about the history and science of the Voyager missions. The Voyager probes have been two of the most important ever launched. They explored the far reaches of the solar system, taking beautiful images of the outer planets and their moons and performing interesting science along the way. These probes have travelled farther from earth than any other man-made object in history—so far, in fact, that Voyager 1 has left the region dominated by the sun’s magnetic field and entered interstellar space.

Scientists conceived the Voyager missions in the late 1960s, when they realized that the outer planets in 1977 would align in a way that allowed a probe to fly closely by each one. This alignment only occurs once every 177 years, so scientists made sure not to miss this opportunity. The missions were proposed in 1969 and launched in 1977.

The Voyager missions have contributed immensely to our understanding of the solar system. They revealed, for example, volcanoes on Io and vast swaths of ice on Europa, both moons of Jupiter. Gurnett himself, an expert in plasma physics, designed a plasma wave instrument on Voyager 1 that has revealed a shock wave between solar radiation and Jupiter’s magnetic field as well as lightning storms on Jupiter.

The Voyager missions are so far from earth that they are leaving the heliosphere, the region of space dominated by the sun’s magnetic field. In fact, Voyager 1 has crossed the boundary between the heliosphere and the interstellar space, the so-called heliopause.

Determining that Voyager 1 has crossed the heliopause, though, was no trivial task. Estimates of the distance from the sun to the heliopause ranged wildly. Gurnett, though, noticed two abnormal influxes of radio waves, each after a period of low cosmic ray detection. He hypothesized that shock waves propagating from the sun blocked cosmic rays. These shock waves then interacted with the heliopause, creating radio waves that were then detected. Using the time between the beginning of the cosmic ray lows and the radio events, Gurnett predicted the distance to the heliopause: 11 to 16 billion miles from earth.1

In August 2012, scientists saw an increase in cosmic ray detection from Voyager 1, evidence that it had crossed the heliopause. This evidence alone, though, could not convince everyone. But in the spring of 2013, scientists detected an increase in the space plasma density around the probe, again evidence that it had crossed the heliopause. Calculating backward, Gurnett showed that Voyager 1 crossed the heliopause around August 2012, at a distance right within the range Gurnett predicted. The probe officially became the first man made object to ever leave our solar neighborhood.1

Voyager 1 will collect data for about 10 more years before its power depletes. In about 40,000 years the probe will pass near the star AC+79 3888.2 With nothing to stop it, the probe will drift indefinitely through interstellar space as the farthest flung object mankind has ever created.



1. Rebecca Jacobson, Voyager 1 Has Entered Interstellar Space (September 12, 2013).  Available at (April 6, 2014).

2. Interstellar Mission.  Available at (April 6, 2014).