Having long ago completed its epic journey of exploration and discovery through the outer Solar System, NASA’s Voyager 1 is the spacecraft that keeps on going more than 37 years after it was launched, while having already taken on its new role as humanity’s first robotic emissary to the stars. This historic passage into interstellar space, which occurred in August 2012, was marked by a steep increase in the levels of cosmic rays coming from interstellar space as measured by Voyager 1’s onboard instruments, accompanied by a sudden drop in the number of solar wind particles that originated from the Sun. Yet, despite having exited the Sun’s magnetic sphere of influence, the spacecraft can still feel the effects of its activity. Ongoing measurements taken throughout 2014 show that a “tsunami wave,” which was generated by the Sun in February, is still flying through interstellar space, providing scientists with new insights about the physics of the interstellar medium.
As described in a previous AmericaSpace article, the magnetosphere of the Sun, better known as the heliosphere, is a vast magnetic bubble that extends well beyond the planets of the Solar System. This realm is dominated by the outward flow of the solar wind—the steady stream of charged particles that is released by the Sun’s upper atmosphere at speeds that range between 400 and 800 km/second. Traversing interplanetary space along the magnetic field lines of our home star, the solar wind eventually slows down to subsonic speeds at a boundary called the termination shock, which is located between 84 and 94 Astronomical Units away from the Sun, until it is finally stopped altogether at the heliopause (the outer limits of the heliosphere) by the pressure of the stellar winds that stream through the interstellar medium.
The intensity of the Sun’s activity varies constantly, during a recurring 11-year period of heightened and decreased activity, also known as the “solar cycle.” Throughout this period, our home star can release a great number of solar flares and Coronal Mass Ejections, or CMEs, consisting of electromagnetic radiation and up to several billion tons of plasma in the form of highly energetic protons and electrons, which traverse the entire Solar System before reaching the very edges of the heliosphere several months after their initial explosion from the Sun. While crossing interplanetary space, CMEs plow through the solar wind forming shock waves which produce secondary streams of highly energetic particles that follow along the solar magnetic field lines. After reaching the boundaries of the heliosphere, these CMEs collide with and compress the plasma that is found in the interstellar medium, causing it to oscillate similar to the way the vibrating strings of a guitar create acoustic waves. “The tsunami causes the ionized gas that is out there to resonate – ‘sing’ or vibrate like a bell,” says Dr. Ed Stone, Voyager Project Scientist at NASA’s Jet Propulsion Laboratory, in Pasadena, Calif.
It was such magnetic “tsunamis” from the Sun that Voyager recorded with its onboard Plasma Wave System, or PWS, which helped scientists to ascertain when exactly the spacecraft had made its long-awaited jump to interstellar space in the first place. The first two were recorded in October to November 2012 and April to May 2013 respectively, while Voyager 1 was approximately 122 AU away from Earth, producing high-frequency oscillations which corresponded to a plasma density of over 40 times greater than what the spacecraft had ever measured before. Coupled with a sudden drop of the solar wind’s intensity and a steep rise of cosmic ray particles from interstellar space that were also recorded during that time, the Voyager’s science team was able to determine that the spacecraft had indeed made the big jump out of the heliosphere. The final confirmation came in February of this year, when a third tsunami wave washed over Voyager while it collided with dense interstellar plasma, beyond the boundaries of the heliopause. Yet what has been particularly interesting is that this third tsunami wave, which had been blasted from the Sun approximately a year before it first reached Voyager, seems to be going strong still, propagating outward through interstellar space as measured by the spacecraft’s instruments from February to November of this year, during which time Voyager itself has traveled an additional 400 million kilometers, while currently located at a distance of 130 AU from the Sun.
While these measurements are quite intriguing, it remains uncertain as to why these shock waves can last so long, or how far they can reach into space before dissipating altogether. “The density of the plasma is higher the farther Voyager goes,” says Stone. “Is that because the interstellar medium is denser as Voyager moves away from the heliosphere, or is it from the shock wave itself? We don’t know yet.” Nevertheless, since Voyager 1 is truly into uncharted territory, every single measurement is bound to be a revelation, allowing scientists to directly probe the physical processes that take place in interstellar space for the first time in human history. “This remarkable event raises questions that will stimulate new studies of the nature of shocks in the interstellar medium,” says Leonard Burlaga, astrophysicist emeritus at NASA’s Goddard Spaceflight Center in Greenbelt, Md., who was involved in the analysis of the new Voyager 1 data.
Even though Voyager 1 has already provided us with many revolutionary insights concerning the interstellar medium, its onboard Plasma Spectrometer, which could have taken direct measurements of the plasma environment of interstellar space, had stopped working following the spacecraft’s encounter with Saturn back in 1980. Fortunately, a similar instrument onboard the twin Voyager 2 spacecraft, which is also on an outward trajectory from the Solar System, is still functioning. Voyager 2, which is still located inside the heliosphere at a current distance of 107 AU from Earth, is expected to follow suit and cross the heliopause sometime within the next 4 to 5 years. When that happens, the spacecraft will be able to provide us with much more detailed observations of the environment of interstellar space. “Most people would have thought the interstellar medium would have been smooth and quiet,” said Dr. Donald Gurnett, a physics professor at the University of Iowa and principal investigator of Voyager’s Plasma Wave instrument, during a press conference at the recent American Geophysical Union meeting in San Francisco, earlier this month. “But these shock waves seem to be more common than we thought.”
We cannot be sure for how long exactly the instruments onboard Voyager 1 and 2 will be able to transmit data back to Earth. Nevertheless, both spacecraft are expected to have enough electrical power to do so until at least the mid-2020s, should their instrumentation remain in good condition. What is certain, however, is that as long as these iconic robotic pioneers remain operational, they will keep making history with new results, as they quietly sail the open seas of interstellar space.
Video Credit: NASA/JPL-Caltech