NASA’s Voyager 1 spacecraft continues its journey through interstellar space powered by assembly language and hardware computing capacity smaller than a single smartphone image, relying on a plasma-wave instrument initially built to monitor Jupiter and Saturn flybys.
Assembly Code and Apple II Computing Power in Deep Space
Launched in 1977, NASA’s twin Voyager spacecraft operate with onboard computers that run assembly language written for custom General Electric processors. Each probe carries three separate computer systems with a total memory of roughly 64 to 70 kilobytes across all three systems stored on custom hardware. That capacity amounts to less storage than a single small image file on a modern smartphone.
Mission personnel have noted the extreme technological gap. Suzy Dodd has compared operating Voyager to flying an Apple II, illustrating how primitive the computing resources appear by modern standards. While popular shorthand often attributes Voyager’s flight software to Fortran, that description conflates separate systems. Fortran has been associated with ground systems and older mission tooling rather than the onboard flight software itself, which relies entirely on low-level assembly programming for specialized early-1970s hardware.
Around the start of the interstellar mission following Voyager 2’s flyby of Neptune in August 1989, the flight software received updates designed to give each spacecraft greater autonomy. That updated version, augmented by command sequences uploaded every few months, serves as the operational baseline for both probes today. When NASA sought a replacement engineer in 2015, the job posting required both assembly language skills and a comprehensive understanding of the spacecraft’s unique hardware architecture.
Fading Institutional Memory and Aging Engineering Teams
Continuous operations have created knowledge gaps that present greater challenges than the legacy programming language itself. Much of the original paper documentation has been lost or fragmented over time, while the original engineers are disappearing faster than the spacecraft.
Larry Zottarelli served as the final original Voyager engineer actively working on the project until his retirement in 2016 at the age of 80. All other original engineers are either deceased or over 90 years old, including Dr. Gary Flandro, an aerospace/trajectory engineer who now lives in retirement. Dodd reported to Live Science that the people who built the spacecraft are no longer alive, leaving a shrinking team to maintain code that no one fully understands anymore.
Communication lags compound these operational hurdles. The Voyager signal now takes more than 23 hours to reach Earth. By the time NASA receives a status check, the spacecraft will already be 1.5 million kilometers further into interstellar space. The mission continues, but institutional memory is fading faster than the plutonium power sources keeping the probes alive.
The Interstellar Crossing Measured by an Afterthought
Voyager 1 crossed the heliopause—the outer skin of the bubble the sun blows around our entire solar system—blind and without its originally intended sensor working. Inside the heliopause, a steady outflow of charged particles called the solar wind pushes against the thin gas drifting between stars. Crossing that threshold places a spacecraft into the interstellar medium, where interstellar gas is measurably thicker than the solar wind.
The instrument built specifically to take that measurement failed years before the crossing. Designed and built at MIT in the 1970s under physicist Herbert Bridge, the Plasma Science instrument (PLS) used four detector cups to measure density, speed, and temperature. It flew past Jupiter in March 1979 and Saturn in November 1980, operating as intended during both encounters before its utility slipped away.
John Richardson, the MIT physicist who inherited the role of principal investigator for the PLS instrument, highlighted its indirect utility: Although not designed to measure the LISM, PLS constantly measured the interstellar plasma currents beyond the heliosphere
, describing the local interstellar medium, the region the mission eventually reached after its primary sensor had already shut down.
How a Flyby Tool Became an Interstellar Detector
Because the primary sensor broke down, NASA relied on an instrument built for an entirely different purpose: the Plasma Wave Science instrument, constructed and still operated by the University of Iowa. Designed to capture plasma waves and low-frequency radio phenomena during the Jupiter and Saturn flybys via two thin antennas trailing about ten meters behind the spacecraft, the instrument stayed active after the flybys simply because turning off working hardware saves minimal power.
That oversight provided the means to detect the interstellar crossing. When an electron cloud is disturbed, it rings at a specific note called the plasma frequency, which rises and falls depending on how tightly packed the electrons are. By listening to those plasma oscillations, scientists detected the density shifts that confirmed Voyager had stepped beyond the sun’s reach.
