Key figures: Edward C. Stone (Voyager Project Scientist, California Institute of Technology); Don Gurnett (Principal Investigator, Plasma Wave Science instrument, University of Iowa); Stamatios “Tom” Krimigis (Principal Investigator, Low-Energy Charged Particle instrument, Johns Hopkins Applied Physics Laboratory); the Voyager Mission operations team at NASA’s Jet Propulsion Laboratory, Pasadena, California.
Background: The Voyager Program
NASA’s twin Voyager spacecraft were launched during the summer of 1977 to exploit a rare alignment of the outer planets — a configuration that occurs once every 176 years — allowing a single trajectory to fly past Jupiter, Saturn, Uranus, and Neptune without additional propulsion. Voyager 2 lifted off first, on August 20, 1977, on a slower, wider trajectory; Voyager 1 launched on September 5, 1977, on a faster course that allowed it to reach Jupiter and Saturn ahead of its sibling.
Voyager 1 made its closest approach to Jupiter on March 5, 1979, discovering active volcanism on the moon Io, and to Saturn on November 12, 1980, producing the first detailed images of the rings and revealing that Titan’s dense atmosphere prevented direct surface imaging. At Saturn, mission controllers used the planet’s gravity to slingshot the spacecraft sharply northward and out of the ecliptic plane — the flat disk in which the planets orbit — placing it on a trajectory that would carry it faster and farther from the Sun than any other human-made object.
Each Voyager spacecraft carries a golden record: a 12-inch gold-plated copper disk containing 115 images, greetings in 55 languages, music samples, and natural sounds from Earth, intended as a message to any extraterrestrial civilization that might retrieve it. The project was led by a committee chaired by Carl Sagan.
By August 2012, Voyager 1 had been operational for nearly 35 years. Its nuclear power source — three Radioisotope Thermoelectric Generators (RTGs) fueled by plutonium-238 — had declined from an initial ~470 watts at launch to approximately 270 watts, a rate of roughly 4 watts per year. To conserve power, the mission team had progressively shut down instruments that were no longer scientifically necessary. The Plasma Science (PLS) instrument, which directly measured solar-wind plasma density and velocity, had been powered off in 1980 after Saturn — a gap that would complicate the initial interpretation of the heliosphere crossing.
The Heliosphere and the Heliopause
The heliosphere is the vast bubble-shaped region of space dominated by the Sun’s magnetic field and the solar wind — a continuous stream of charged particles (mostly protons and electrons) flowing outward from the Sun at roughly 400–800 km/s. This expanding plasma pushes back the interstellar medium, creating a protective cocoon that extends far beyond the orbits of the planets. The boundary where the solar wind first slows to subsonic speeds is called the termination shock; beyond that lies the heliosheath, a turbulent transition zone where slowed solar-wind plasma mingles with interstellar particles; and the outermost boundary — the heliopause — is where the solar wind’s outward pressure is exactly balanced by the pressure of the local interstellar medium.
Voyager 1 crossed the termination shock in December 2004, at approximately 94 AU from the Sun. Over the following eight years it traversed the heliosheath, instruments registering the increasingly chaotic and compressed plasma environment of this boundary region.
The precise location and nature of the heliopause had been predicted theoretically for decades but never directly observed. Scientists knew it existed because of the behavior of galactic cosmic rays — high-energy particles from supernovae and other violent events beyond the solar system — which are partially deflected and modulated by the heliosphere’s magnetic field. They expected that once Voyager 1 crossed the heliopause, the flux of galactic cosmic rays would rise sharply while the flux of lower-energy particles originating within the heliosphere would fall.
Detection: Particle Data and the August 25 Crossing
On or around August 25, 2012, instruments aboard Voyager 1 recorded an abrupt change in the particle environment. The Cosmic Ray Subsystem (CRS) detected a rapid and sustained increase in galactic cosmic ray intensity (particles above approximately 70 MeV), while the Low-Energy Charged Particle (LECP) instrument registered a simultaneous and equally sharp drop in the count of lower-energy heliospheric particles — the kind accelerated by the termination shock and the heliosheath. The change was not gradual; the transition occurred within days, suggesting the spacecraft crossed a relatively thin, well-defined boundary.
Specifically, the galactic cosmic-ray rate increased by roughly a factor of three over about a week, while counts of anomalous cosmic rays — particles trapped and accelerated within the heliosheath — plummeted to near zero. Scientists monitoring the data at JPL recognized the signature as consistent with exit from the heliosphere, but the absence of plasma measurements (the PLS instrument being off) created initial uncertainty about whether the magnetic-field environment had also changed in the way expected at the heliopause.
Confirmation: The 2013 Plasma Wave Evidence
The definitive confirmation that Voyager 1 had entered interstellar space came not from the August 2012 data alone, but from subsequent plasma density measurements enabled by a solar event. In April 2013, a powerful coronal mass ejection (CME) launched from the Sun reached the region around Voyager 1 roughly 13 months later — solar phenomena travel through space more slowly than light. The CME triggered oscillations in the surrounding plasma that the Plasma Wave Science (PWS) instrument could detect as electron plasma oscillations at a frequency of approximately 2.6 kHz. Using the relationship between oscillation frequency and electron density, Don Gurnett and colleagues at the University of Iowa calculated a plasma density of approximately 0.08 electrons per cubic centimeter — consistent with the density of the local interstellar medium and more than 40 times denser than the heliosheath plasma Voyager 1 had measured before August 2012.
The plasma wave observations were published in the journal Science on September 12, 2013, in a paper by Gurnett et al. that formally announced Voyager 1’s interstellar crossing and identified August 25, 2012 — the date of the particle-data transition — as the best estimate of when the spacecraft actually crossed the heliopause. Edward Stone, Voyager’s project scientist since 1972, described the discovery as “a historic moment in the history of science.”
At the time of crossing, Voyager 1 was approximately 121 AU (18.1 billion km, or about 11.3 billion miles) from the Sun. At that distance, a radio signal traveling at the speed of light took approximately 16 hours and 38 minutes to reach Earth. The spacecraft was moving at roughly 17.0 km/s (about 38,000 mph, or 3.6 AU per year) — heading northward out of the ecliptic plane at an angle of about 35 degrees.
Significance
Voyager 1’s passage into interstellar space on August 25, 2012 — confirmed publicly in September 2013 — stands as one of the most consequential milestones in the history of exploration. No other human artifact had ever operated beyond the Sun’s protective influence; for the first time, a craft of human construction was sampling the interstellar medium directly rather than inferring its properties from remote observations.
The measurements from interstellar space immediately challenged and refined theoretical models. The direction of the local interstellar magnetic field, inferred from Voyager 1’s magnetometer readings, differed from predictions based on observations of the “ribbon” of energetic neutral atoms detected by the IBEX satellite in 2009. Scientists concluded that the boundary region between the solar system and interstellar space is more complex and dynamic than pre-mission models had anticipated.
The achievement also underscored the extraordinary engineering longevity of the Voyager spacecraft. Designed to operate for at most five years to complete the Grand Tour of the outer planets, Voyager 1 had by August 2012 survived 35 years in the space environment — surviving radiation, temperature extremes, and the gradual decline of its nuclear power supply — while still returning data that scientists could not obtain any other way. The spacecraft’s continued operation, supported by a dedicated team at JPL, demonstrated the enduring value of long-horizon investment in robotic exploration.
Voyager 2 crossed the heliopause on November 5, 2018, at approximately 119 AU from the Sun, providing a second data point — at a different latitude and longitude relative to the solar system’s bow shock — that confirmed the heliopause’s existence as a consistent boundary while also revealing unexpected asymmetries in its shape.
Sources
- Voyager 1 – Wikipedia
- Voyager Mission – NASA Science
- Gurnett et al. (2013), “In Situ Observations of Interstellar Plasma with Voyager 1,” Science 341, 1489–1492
- Stone et al. (2013), “Voyager 1 Observes Low-Energy Galactic Cosmic Rays in a Region Depleted of Heliospheric Ions,” Science 341, 150–153
See Also
- Curiosity Rover Mars Landing — the other landmark space milestone of summer 2012, landing on Mars just 19 days before Voyager 1’s heliosphere crossing
- Higgs Boson Discovery — the other transformative physics discovery of 2012, announced on July 4 from CERN
- Venus Transit of June 2012 — a 2012 planetary-science event, like Voyager 1’s crossing, with relevance for understanding distant atmospheres and exoplanet detection
- 2012 Arctic Sea Ice Minimum — a record-setting Earth-science measurement of the same year, illustrating how 2012 was marked by milestones at both the cutting edge of science and in climate observation
- Felix Baumgartner Red Bull Stratos (October 2012) — the October 2012 supersonic stratospheric jump, a human-achievement milestone complementing Voyager 1’s robotic reach into interstellar space