University of Iowa space physicist Don Gurnett says that NASA’s Voyager 1 spacecraft — the most distant manmade object at some 94 astronomical units (AU) or more than 8.7 billion miles from the sun — has crossed a boundary called the “termination shock,” one of the last milestones it will encounter before leaving the solar system and entering interstellar space.
Gurnett, professor of physics in the UI College of Liberal Arts and Sciences and principal investigator for the plasma wave instrument on Voyager 1, will present his findings at a May 24 news conference during the spring 2005 meeting of the American Geophysical Union in New Orleans. Evidence for the crossing, which occurred December 16, 2004, included steadiness in the strength of energetic particle beams, a near reversal of the direction of these beams and a jump in the magnetic field strength. An electron beam generated by the termination shock created the electron plasma oscillations observed by Gurnett’s team.
“We saw a burst of plasma oscillations before we went through the termination shock,” he says. “There’s a consensus among Voyager scientists that we crossed the termination shock at 94 AU. This is something that Bill Kurth, (UI research scientist and Voyager co-investigator) and I predicted several years ago in a published paper.”
Kurth compares the termination shock to what happens when water is allowed to run from a kitchen faucet onto the center of a dinner plate. The water — representing the solar wind, a stream of electrically charged particles flowing outward from the sun — strikes the center of the plate and smoothly flows outward in all directions. Somewhere near the edge of the plate, the smooth stream becomes rippled as it runs into slower moving water. This rippled band of turbulence represents the termination shock and the region where it occurs, the heliosheath. Similarly, the solar wind slows from supersonic to subsonic speed as it approaches the gas generated by stars beyond our sun.
“The solar wind creates a bubble (the heliosphere) around the sun, and near the edges of the bubble is a place where the solar wind piles up as it encounters the interstellar wind,” says Ed Stone, Voyager project chief scientist and professor of physics at the California Institute of Technology. “We think the sun is currently in a phase where the heliosphere is shrinking. If so, Voyager would continue to be in this thicker and hotter region until it reaches the heliopause, the outer edge of the bubble. This is a wonderful opportunity to reach interstellar space, and we hope we can keep the spacecraft operating through the year 2020.”
Nobody knows precisely when Voyager will reach the heliopause, but Gurnett notes that estimates of the distance of the heliopause from the sun have varied widely, ranging from one scientist’s 1956 estimate of 5 AU to Gurnett’s estimate of 126-168 AU made in 1993. Today, he says that the termination shock just encountered is probably about three-quarters of the way there, placing interstellar space about 25 to 35 AU beyond Voyager’s current position. With the spacecraft moving at about 3.5 AU per year, Voyager 1 may reach the heliopause in another 10 years or so — a long journey for a mission that began with a Sept. 5, 1977 launch and successful fly-bys of both Jupiter and Saturn. A sister spacecraft, Voyager 2 was launched Aug. 20, 1977, on a flight path that took it to encounters with Jupiter, Saturn, Uranus and Neptune. At present Voyager 2 is about 76 AU from the sun and traveling at about 3.3 AU per year.
The sounds of Voyager’s encounter with the termination shock and other sounds of space can be heard by visiting Gurnett’s Web site at: http://www-pw.physics.uiowa.ed u/space-audio/
NASA’s Jet Propulsion Laboratory (JPL), Pasadena, Calif., a division of Caltech, manages the Voyager mission for NASA’s Office of Space Science, Washington, D.C.
