COLLEGE PARK, Md. – University of Maryland researchers played a key role in a historic moment in interstellar exploration, with the announcement that Voyager 1 has travelled beyond the outer limit of our solar system.
Carrying Earthly greetings on a gold plated phonograph record and still-operational scientific instruments – including the Low Energy Charged Particle detector designed, built and overseen, in part, by UMD's Space Physics Group – NASA's Voyager 1 has traveled farther from Earth than any other human-made object. Thirty-six years after it was launched on a mission to the outer planets and beyond, the space craft is “setting sail on the cosmic seas between the stars,” said the mission’s chief scientist, Ed Stone, in a Sept. 12 press conference.
The announcement ends a period of intense scientific debate about whether Voyager 1 had traveled beyond our solar system or was still in a fuzzily-defined transition zone between the Sun's sphere of influence and the rest of the galaxy. In quick succession, two scientific research teams independently concluded that the spacecraft had, in fact, entered a region of space beyond the influence of the Sun in the late summer of 2012.
A UMD team led by research scientist Marc Swisdak was the first to present evidence supporting that claim in a research paper published online in The Astrophysical Journal Letters on August 14. "It's a somewhat controversial view, but we think Voyager has finally left the Solar System, and is truly beginning its travels through the Milky Way," Swisdak, a plasma physicist and the paper’s lead author, said at the time.
On Sept. 12 a separate team of Voyager scientists led by the University of Iowa’s Don Gurnett published its own findings in the journal Science, seconding the conclusion that Voyager has left the solar system. The two teams differ on the precise date of the historic event – Swisdak’s team places it on July 27, 2012, while Gurnett’s team singled out August 25, 2012 – but their research complements one another, Swisdak said today.
At issue is what the boundary-crossing should look like to Earth-bound observers 11 billion miles (18 billion kilometers) away. The Sun's envelope, known as the heliosphere, is relatively well-understood as the region of space dominated by the magnetic field and charged particles emanating from our star. The heliopause transition zone is both of unknown structure and location. According to conventional wisdom, we'll know we've passed through this mysterious boundary when we stop seeing solar particles and start seeing galactic particles, and we also detect a change in the prevailing direction of the local magnetic field.
NASA scientists recently reported that last summer, after eight years of travel through the outermost layer of the heliosphere, Voyager 1 recorded "multiple crossings of a boundary unlike anything previously observed." Successive dips in, and subsequent recovery of, solar particle counts caught researchers' attention. The dips in solar particle counts corresponded with abrupt increases in galactic electrons and protons. Within a month, solar particle counts disappeared, and only galactic particle counts remained. Yet Voyager 1 observed no change in the direction of the magnetic field.
To explain this unexpected observation, many scientists theorized that Voyager 1 has entered a "heliosheath depletion region," but that the probe is still within the confines of the heliosphere. Swisdak and colleagues James Drake, also of UMD, and Merav Opher of Boston University, who are not part of the Voyager 1 mission science teams, said there was another explanation.
In previous work, Swisdak and Drake have focused on magnetic reconnection, or the breaking and reconfiguring of close and oppositely-directed magnetic field lines. It's the phenomenon suspected to lurk at the heart of solar flares, coronal mass ejections and many of the sun's other dramatic, high-energy events. The UMD researchers argued that magnetic reconnection is also key to understanding NASA's surprising data.
Though often depicted as a bubble encasing the heliosphere and its contents, the heliopause is not a surface neatly separating "outside" and "inside." In fact, Swisdak, Drake and Opher assert that the heliopause is both porous to certain particles and layered with complex magnetic structure. Here, magnetic reconnection produces a complex set of nested magnetic "islands," self-contained loops which spontaneously arise in a magnetic field due to a fundamental instability. Interstellar plasma can penetrate into the heliosphere along reconnected field lines, and galactic cosmic rays and solar particles mix vigorously.
Most interestingly, drops in solar particle counts and surges in galactic particle counts can occur across "slopes" in the magnetic field, which emanate from reconnection sites, while the magnetic field direction itself remains unchanged. This model explains observed phenomena from last summer, Swisdak and his colleagues said.
In the 36th year after their 1977 launches, the twin Voyager 1 and 2 spacecraft continue exploring where nothing from Earth has flown before. Their primary mission was the exploration of Jupiter and Saturn. After making a string of discoveries there — such as active volcanoes on Jupiter's moon Io and intricacies of Saturn's rings — the mission was extended. Voyager 2 went on to explore Uranus and Neptune, and is still the only spacecraft to have visited those outer planets. The current mission for both spacecraft, the Voyager Interstellar Mission, is to explore the outermost edge of the Sun's domain and beyond. Both Voyagers are capable of returning scientific data from a full range of instruments, with adequate electrical power and attitude control propellant to keep operating until 2020. Voyager 2 is expected to enter interstellar space a few years after its twin. The Voyager spacecraft were built and continue to be operated by NASA's Jet Propulsion Laboratory, in Pasadena, Calif.
University of Maryland scientists lead the Deep Impact spacecraft science team and are part of the science teams of many of the other spacecraft exploring our Solar System, including both Voyagers and Cassini.
This work by Swisdak, Drake and Opher was supported by National Science Foundation (NSF) grant AGS-1202330 to the University of Maryland, and NSF grant ATM-0747654 and NASA grant NNX07AH20G to Boston University.
Written by Barbara Brawn-Cinani and edited by Lee Tune.