Voyager 1 Is in Interstellar Space — the Farthest Humanity Has Ever Reached

In September 1977, NASA launched a spacecraft the size of a small car with three nuclear power sources, a golden record containing sounds and images of Earth, and enough fuel to eventually escape the solar system entirely. Forty-seven years later, Voyager 1 is still traveling outward at approximately 17 kilometers per second, now more than 24 billion kilometers from Earth — more than 160 times the distance from Earth to the Sun — in the interstellar medium, the space between stars.

No human-made object has ever been farther from Earth. Voyager 1's twin, Voyager 2, launched two weeks earlier but on a different trajectory, is currently approximately 20 billion kilometers away, also in interstellar space. Both probes remain in contact with Earth through NASA's Deep Space Network, their signals taking over 22 hours to make the one-way journey at the speed of light.

The Grand Tour and the Gravity Assist

Voyager 1's primary scientific mission was not to travel to interstellar space but to study Jupiter and Saturn. It achieved both objectives with extraordinary success. During its 1979 Jupiter flyby, it discovered active volcanic eruptions on Io — the first confirmed active volcanism observed on any body other than Earth. It imaged Europa's fractured ice surface, hinting at the subsurface ocean now considered a leading candidate for extraterrestrial life. It provided detailed views of Jupiter's Great Red Spot, revealing it as a storm that had been raging for centuries.

The Saturn flyby in 1980 produced equally stunning results: high-resolution imagery of Saturn's ring system revealing far more structure than ground-based observation could detect, discovery of new small moons, and characterization of Titan's thick nitrogen atmosphere. The trajectory required for the Titan encounter placed Voyager 1 on a path out of the ecliptic plane — the flat plane in which most solar system bodies orbit — eliminating the possibility of further planetary encounters but allowing a unique trajectory toward interstellar space.

The propulsion for all of this came largely not from fuel but from gravity. The Grand Tour mission design, conceived by NASA scientist Gary Flandro in 1965, exploited an exceptionally rare alignment of the outer planets that occurs only every 175 years. A spacecraft could use Jupiter's gravity to slingshot toward Saturn, and Saturn's gravity to slingshot toward Uranus, gaining speed with each planetary encounter rather than expending fuel. Voyager 1's trajectory used Jupiter's gravity to reach Saturn; Voyager 2 used Jupiter's gravity to continue to Uranus and Neptune as well.

Crossing into Interstellar Space

The boundary between the solar system and interstellar space is defined by the heliopause: the surface where the solar wind — the stream of charged particles continuously flowing outward from the Sun — finally becomes too weak to push back the interstellar medium. Inside the heliopause, the region of space is called the heliosphere; outside it is the interstellar medium, the space between the stars.

NASA confirmed that Voyager 1 crossed the heliopause in August 2012, entering interstellar space for the first time in history. The crossing was detected through changes in the density and temperature of the charged particles the probe's instruments measured: suddenly higher density and lower energy, consistent with the colder, denser interstellar medium replacing the Sun's outflowing particles. Voyager 2 crossed the heliopause in November 2018, providing the first opportunity to compare heliopause conditions at two different locations simultaneously.

Still Talking

As of 2026, Voyager 1's radioisotope thermoelectric generators — powered by the radioactive decay of plutonium-238 — continue to provide enough electricity to operate a reduced set of instruments. The power output decreases by about 4 watts per year, and mission engineers have progressively shut down non-essential systems to extend operational life. The probe is expected to continue communicating with Earth until approximately 2025–2030, after which power will be insufficient to maintain the transmitter.

When the last signal is received, Voyager 1 will continue traveling silently through the interstellar medium, carrying its golden record — a message from humanity to whoever might ever find it — into the galaxy at 17 kilometers per second. It will not encounter another star system for approximately 40,000 years.