Interstellar probe

The interstellar probe became reality on September 5, 1977, when NASA launched Voyager 1 from Cape Canaveral—a spacecraft that would become the first human-made object to leave our solar system and enter interstellar space. Voyager 1 crossed the heliopause on August 25, 2012, at about 122 astronomical units from the Sun, where influences from outside our solar system become stronger than those from the Sun itself.

The adjacent possible for interstellar probes opened through a rare celestial alignment. In the late 1960s, aerospace engineer Gary Flandro at JPL discovered that Jupiter, Saturn, Uranus, and Neptune would align in the late 1970s in a configuration that occurs only once every 176 years. This 'Grand Tour' alignment would allow a spacecraft to use gravitational assists—slingshotting off each planet's gravity to reach the next—visiting all four outer planets in a single mission. Miss this window, and humanity would wait nearly two centuries for another chance.

NASA's original Grand Tour proposal was too expensive and was cancelled in 1972. But engineers at the Jet Propulsion Laboratory in Pasadena proposed a scaled-down mission targeting just Jupiter and Saturn: the Voyager program. Two identical spacecraft would be built, with Voyager 2 launching first (August 20, 1977) despite its higher number. Voyager 1, launched 16 days later, followed a faster trajectory that would reach both planets sooner.

The engineering challenges were formidable. The spacecraft had to operate for years without maintenance, survive the radiation belts of Jupiter, and communicate with Earth across billions of miles. Power came from radioisotope thermoelectric generators using plutonium-238 decay heat—solar panels would be useless in the outer solar system. The computer systems used radiation-hardened memories with just 69.63 kilobytes of storage. Every component had to be designed for extreme reliability.

Voyager 1's scientific discoveries transformed our understanding of the outer solar system. It discovered active volcanoes on Jupiter's moon Io—the first evidence of volcanic activity beyond Earth. It found five new moons of Saturn and discovered the G-ring. Its trajectory took it close enough to Titan to study that moon's atmosphere, at the cost of not being able to continue to Uranus and Neptune (Voyager 2 would complete the Grand Tour instead).

On February 14, 1990, from 3.7 billion miles away, Voyager 1 turned back toward Earth and captured the famous 'Pale Blue Dot' image—Earth as a tiny speck suspended in a ray of sunlight. Carl Sagan used that image to eloquent effect in arguing for humanity's cosmic perspective.

Each Voyager carries a Golden Record: a 12-inch gold-plated copper disc containing greetings in 55 languages, 35 sounds from life on Earth (including whale songs and laughter), and 90 minutes of music. Instructions for playing the record use symbolic diagrams referencing pulsar locations to indicate Earth's position in the galaxy.

By 2024, Voyager 1 was still transmitting data from over 15 billion miles away, traveling at about 17 kilometers per second. No spacecraft has gone farther. The probe that was designed for a four-year mission to Jupiter and Saturn had become humanity's first ambassador to interstellar space.