“Sailors on a becalmed sea, we sense the stirring of a breeze.”
— Carl Sagan, “Pale Blue Dot” (1994)
In the last couple of years, there had been considerable uncertainty as to whether NASA’s Voyager 1 spacecraft had crossed the boundary separating the Sun’s magnetic sphere of influence, better known as the heliosphere, from interstellar space. Previous data from Voyager’s onboard instruments provided the first compelling evidence that the spacecraft had crossed that boundary in August 2012, signaling perhaps one of the greatest achievements in the history of our species. New readings of the spacecraft’s surroundings, taken earlier this year, have now come to confirm that the venerable robotic explorer has indeed exited the vast region around the Sun that is dominated by the solar wind, and is now guided by the interstellar winds in the uncharted open sea of interstellar space.
Apart from the flight of Apollo 11, whose 45th anniversary we celebrate next week, and those of the rest of the Apollo missions to the Moon during the 1960s and ’70s, it may be hard to think of another space mission more epic and grandeur than that of the twin Voyager spacecraft. Conceived in the mid-1960s during the same time that humanity was preparing to take its first steps on an alien world, the Voyager missions were NASA’s answer to the planetary science community’s calls for a Grand Tour, albeit unmanned, of the whole outer Solar System. Having been launched in 1977, Voyager 1 and 2 flew by Jupiter and Saturn in the following years, greatly expanding our knowledge of the gas giant planets as well as their dozens of moons, while Voyager 2 continued on to Uranus and Neptune, transforming what had previously been only distant points of light in the sky, into real, dynamic, and ever-changing worlds. Having completed their primary objectives of planetary exploration of the outer Solar System, NASA extended the mission of the twin spacecraft to include the exploration and characterisation of the Sun’s heliosphere and the search for the boundary between it and the interstellar medium, calling it the Voyager Interstellar Mission.
The heliosphere is a vast magnetic bubble that surrounds the Sun, extending well beyond the planets of the Solar System, which 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 of approximately 400 km/second. Traversing interplanetary space along the lines of the Sun’s magnetic field, the solar wind eventually slows down to subsonic speeds at a boundary called the termination shock, until it is finally stopped at the heliopause (the outer limits of the heliosphere) by the pressure of the stellar winds that stream through the interstellar medium.
Even though the mission’s science team had made various predictions through the years concerning the overall structure of the heliosphere and the exact point of the heliopause, they lacked the data needed to accurately map its boundaries, adding further confusion as to when exactly Voyager 1 would enter interstellar space. Theoretical models had shown that the spacecraft’s exit from the heliosphere would be discernible by several distinct markers. For instance, its onboard instruments recorded a sudden drop during the summer of 2012 by more than a factor of 1,000 in the number of solar wind particles originating from inside the Solar System and a subsequent steep increase in the levels of cosmic rays coming from interstellar space. Furthermore, scientists had expected that Voyager’s crossing of the heliopause would result in an abrupt change in the direction of the magnetic field lines, as the spacecraft would stop to be under the influence of the Sun’s magnetic field and those of the interstellar medium would take over. Yet, even though the strength of the magnetic field jumped by 60 percent during the recorded drop off in solar wind particles at the end of the heliosphere, the change in the direction of the magnetic field lines was not greater than 2 degrees, perplexing researchers even more as to Voyager’s exact whereabouts.
The most decisive factor that would finally settle Voyager 1’s exact location would be the plasma density measurements of the interstellar medium, which is rich in ionised gas of much higher densities than those of the rarefied solar wind inside the heliosphere. Unfortunately, the instrument onboard Voyager that could take direct measurements of the plasma environment around the spacecraft, called the Plasma Spectrometer, or PLS, had stopped working following the encounter with Saturn in 1980, adding more difficulty to the already complicated task of mapping the unknown boundaries of the heliosphere. In light of these difficulties, scientists had to rely on indirect, time-consuming measurements. Nevertheless, they were aided in their efforts by the Sun itself.
During its 11-year solar cycle, the Sun goes through periods of increased activity, called the solar maximum, the latest of which peaked in the summer of 2013. 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 about a year after their initial explosion from the Sun. While crossing interplanetary space, CMEs plow through the solar wind, while forming shock waves that 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 cosmic rays and excite oscillations on the plasma in the interstellar medium, similar to the way a pick causes vibrations on a guitar’s strings. “Normally, interstellar space is like a quiet lake,” says Dr. Ed Stone, Voyager Project Scientist at NASA’s Jet Propulsion Laboratory, in Pasadena, Calif. “But when our sun has a burst, it sends a shock wave outward that reaches Voyager about a year later. The wave causes the plasma surrounding the spacecraft to sing.” By measuring the pitch and frequency of these oscillations with Voyager 1’s onboard Plasma Wave System, or PWS, in three separate occasions, scientists were able to confirm that the spacecraft was indeed sailing through the vast expanses between the stars.
The first such measurements were recorded in October to November 2012, when a plasma wave from a CME washed over Voyager 1. At the time, these oscillations weren’t recognised as such because of their relative low frequency. It took a second wave in April to May 2013 by another CME from the Sun for Voyager to clearly register its oscillations. These data showed that the material from the Sun’s outburst had collided with the plasma of the interstellar medium producing high-frequency oscillations, which corresponded to an interstellar plasma density of over 40 times greater than that of the solar wind inside the heliosphere. Where plasma wave measurements in previous years while Voyager 1 was still inside the heliosphere had recorded oscillations with frequencies around 300 Hz, the pair of plasma waves recorded in 2012-2013 had frequencies between 2 and 3 Khz, signaling the spacecraft’s transition into a new region of space.
Video Credit: NASA/JPL-Caltech/University of Iowa
This high-pitched interstellar “singing” was music to everyone’s ears in Voyager’s mission team. “We literally jumped out of our seats when we saw these oscillations in our data – they showed us the spacecraft was in an entirely new region, comparable to what was expected in interstellar space, and totally different than in the solar bubble,” says Dr. Donald Gurnett, a space physicist at the University of Iowa and Principal Investigator of Voyager’s PWS instrument. “Clearly we had passed through the heliopause, which is the long-hypothesized boundary between the solar plasma and the interstellar plasma.” It was after reviewing these data from the PWS that the mission’s science team recognised the previous plasma wave measurements from 2012 as well, leading it to acknowledge in September 2013, after many months of speculations and introspection, that Voyager had made the long-awaited crossing to interstellar space in 25 August 2012, at a distance of 122 Astronomical Units, or 11 billion miles from the Sun. “We have been cautious because we’re dealing with one of the most important milestones in the history of exploration,” said Stone, following the official announcement by the space agency. “Only now do we have the data — and the analysis — we needed. Now that we have new, key data, we believe this is mankind’s historic leap into interstellar space. The Voyager team needed time to analyze those observations and make sense of them. But we can now answer the question we’ve all been asking — ‘Are we there yet?’ Yes, we are.”
Now, a new series of measurements recorded in February of this year of a third “tsunami” plasma wave that overtook Voyager 1 comes to further confirm the iconic spacecraft’s exit from the Sun’s magnetic sphere of influence. “All is not quiet around Voyager,” says Gurnett. “We’re excited to analyze these new data. So far, we can say that it confirms we are in interstellar space.” In addition to the measurements taken with the PWS instrument, Voyager’s onboard Cosmic Ray System, or CRS, also recorded the plasma wave resulting from the collision of material of yet another solar outburst into the interstellar medium. “The tsunami wave rings the plasma like a bell,” explains Stone. “While the plasma wave instrument lets us measure the frequency of this ringing, the cosmic ray instrument reveals what struck the bell – the shock wave from the Sun.”
As monumental as the ongoing mission of Voyager 1 is, the emblematic robotic spacecraft hasn’t escaped the Solar System and the complete grasp of the Sun yet. Although now outside of the realm dominated by our home star’s magnetic field, Voyager 1 will still spend its next 30,000 years in the Sun’s gravitational grip, slowly crossing the vast reservoir of trillions of comets that envelopes the Solar System, known as the Oort cloud, at a speed of 17 km/s, or 3.5 Astronomical Units per year. By then, the spacecraft will be long silent, unable to record and transmit any scientific observations of any possible wonders it might fly by. Currently operating on a feeble 23 watts of electrical power down from their full capacity of 470 watts at the time of launch, Voyager’s radioisotope thermoelectric generators will have depleted their plutonium-238 power sources by the end of the next decade, leading to the spacecraft’s inevitable end of operations, after having long completed their roles as both space explorers and sources of inspiration for society and art. Even then, Voyager 1 and its twin Voyager 2, far from being worthless heaps of junk, will continue on circling the Mikly Way galaxy far from their place of origin, while silently carrying their onboard Golden Records, loaded with humanity’s sights and sounds of a bygone era, finally taking on their ultimate role as interstellar robotic ambassadors of a species called Homo sapiens, in the Universe’s infinity of space and time.
Video Credit: Callum C. J. Sutherland