According to NASA mission data, Voyager 1 crossed the boundary of the heliosphere on August 25, 2012, becoming the first human-made object to enter interstellar space. The historic crossing occurred without a direct sensor reading, as the spacecraft’s primary plasma instrument had failed decades earlier, forcing scientists to rely on secondary data captured by the Plasma Wave Subsystem.
How Voyager 1 Lost Its Primary Instrument for Measuring Interstellar Space
When NASA launched Voyager 1 and Voyager 2 in 1977, the mission’s primary objective was to explore the outer solar system and gather observations directly at the source, from outer planets we had only seen with remote studies. According to NASA mission history, Voyager 1 flew past Jupiter in March 1979 and Saturn in November 1980, operating as intended during both encounters. However, the instrument built to feel it broke first. NASA’s own mission history states plainly that the instrument “stopped working in 1980 and was turned off in 2007 to save power.”
John Richardson, the MIT physicist who inherited the role of principal investigator for that instrument, later put its odd, permanent half-relevance this way: “Although not designed to measure the LISM, PLS constantly measured the interstellar plasma currents beyond the heliosphere,” referring to the local interstellar medium, the very region the mission would eventually reach with its best tool for reading it no longer working. Because this sensor degraded years before the spacecraft actually reached the boundary, the mission lost its direct tool for identifying the threshold.
Did you know? Although Voyager 1 and Voyager 2 were originally designed to measure the properties of the giant planets, they have continued operating, pushing past the sun’s magnetic bubble into deep space.
The Backup Instrument That Recorded the Interstellar Crossing
With the primary sensor disabled, confirmation of the historic boundary crossing relied on an instrument that was never intended to measure density at all. The Plasma Wave Science instrument, built and still operated by the University of Iowa, was designed for something narrower: catching plasma waves and low frequency radio phenomena during the Jupiter and Saturn flybys specifically, the kind of short lived, planet specific bursts a spacecraft only sees up close, utilizing two thin antennas trailing about ten meters behind the spacecraft.
The instrument stayed on anyway, mostly because turning working hardware off saves so little power that nobody bothered. In April of 2013, the instrument recorded electron plasma oscillations triggered by a solar disturbance that had spent months crossing the gap between the sun and the spacecraft, and the reading corresponded to an electron density roughly forty times greater than anything measured inside the heliosphere. Don Gurnett, the University of Iowa physicist who had led the instrument’s design since Voyager launched, called it plainly: “This is the first solid evidence that Voyager 1 has crossed the heliopause.”
Reading a Boundary Crossing More Than a Year Later
Because the confirming signal depended entirely on the sun happening to throw a strong enough burst toward Voyager, and because that burst then had to travel outward for months before reaching the spacecraft, the actual crossing could only be dated once the delayed proof showed up. Two bursts, one in late 2012 and one in the spring of 2013, let the Iowa team place the real crossing on the 25th of August, 2012, more than a year after Voyager had actually left.
Data from other onboard systems corroborated this timeline. According to analysis of the Magnetometer (MAG) instrument—which undergoes a calibration maneuver that allows Voyager to differentiate between the spacecraft’s own magnetic field and the magnetic fields of the space it’s traveling through—the high variability in magnetic strength typical of the heliosphere vanished once the spacecraft crossed into interstellar space. Bill Kurth, another University of Iowa physicist on the same instrument team, said that once the readings from beyond the boundary started coming in, “Now that we’re on the outside, we are learning that interstellar space isn’t a bland region.” Ocker’s new study, published on Monday in Nature Astronomy, reports what may be the first continuous measurement of the density of material in interstellar space, co-authored by space physicist Jim Cordes.
Pro Tip: When analyzing deep space telemetry, researchers combine data from multiple legacy instruments—such as magnetic field strength and plasma wave oscillations—to verify events that single sensors can no longer capture directly.
Frequently Asked Questions
When did Voyager 1 enter interstellar space?
According to NASA, Voyager 1 changed that on Aug. 25, 2012, as it crossed the heliopause.
How do scientists measure plasma density if the primary instrument failed?
Researchers use the Plasma Wave Science instrument, which records plasma waves and low frequency radio phenomena, as well as data from the onboard Magnetometer.
Are the Voyager spacecraft still sending data back to Earth?
Yes, now in interstellar space, they are pushing the limits of spacecraft and exploration, journeying through the cosmic neighborhood, and continuing on, exploring and measuring the interstellar medium.
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