Voyager 1 and 2: The Probes That Left the Solar System

Voyager 1 and Voyager 2 are twin robotic spacecraft that NASA launched in 1977 to study the giant planets of the outer solar system. Their tour was made possible by a rare planetary alignment that comes around roughly once every 175 years. Both probes kept sending data for decades and became the first human-made objects to reach interstellar space.

Why the Voyager Probes Were Built

The Voyager program grew out of an opportunity in the late 1970s. Jupiter, Saturn, Uranus, and Neptune were positioned so that one spacecraft could visit several of them in sequence. Each planet's gravity could bend the probe's path and add speed, so the trip took far less fuel than a direct flight would have needed.

Engineers at NASA's Jet Propulsion Laboratory built two nearly identical probes. If one failed, the other could still complete the mission. Each spacecraft had a mass of roughly 800 kilograms at launch and carried about ten science instruments, including cameras, magnetometers, and plasma and radio detectors.

The designers faced one major constraint. Sunlight at Jupiter is about 1/27 as intense as sunlight at Earth, so large solar panels would have been impractical for the outer planets. Instead, each probe used radioisotope thermoelectric generators (RTGs) fueled by plutonium-238, which turn heat from radioactive decay into electricity.

How Gravity Assists Work

A gravity assist lets a spacecraft borrow a small amount of orbital energy from a planet it passes. The probe swings around the planet and leaves on a new path with higher speed, while the planet's own orbit barely changes. Mission planners used this technique at Jupiter and Saturn to reach the outer planets without the very large rockets a direct trip would have required.

Launch and the Jupiter Encounters

Voyager 2 launched first, on August 20, 1977. Voyager 1 followed on September 5, 1977. Both rode Titan IIIE rockets with Centaur upper stages from Cape Canaveral in Florida. Voyager 1 took the faster path and reached Jupiter first, passing the planet in March 1979. Voyager 2 passed Jupiter in July 1979.

At Jupiter, the probes changed what scientists knew about the planet's moons. Voyager 1 found active volcanoes on Io, the first active volcanism confirmed beyond Earth. The images also revealed a faint ring system around Jupiter and detailed storm patterns in its atmosphere. The smooth, cracked surface of Europa suggested a possible subsurface ocean, an idea that later missions helped support.

Saturn and Titan

Voyager 1 reached Saturn in November 1980 and flew close to Titan, the largest Saturnian moon, which has a thick nitrogen atmosphere. The close Titan flyby pulled the probe out of the plane of the planets, which ended its planetary tour. Voyager 2 reached Saturn in August 1981 and studied the rings and moons in more detail. Its images showed Enceladus with a bright, young-looking icy surface.

Uranus and Neptune: The Second Half of the Tour

Voyager 2 is the only spacecraft that has visited Uranus and Neptune. It reached Uranus on January 24, 1986, passing about 81,500 kilometers above the cloud tops. The flyby found new moons and rings, and the magnetometer showed that Uranus's magnetic field is tilted sharply and offset from the planet's center.

Voyager 2 reached Neptune on August 25, 1989, passing about 4,950 kilometers above the cloud tops. Images showed the Great Dark Spot, a large storm, and winds among the fastest recorded in the solar system. Triton, Neptune's largest moon, showed geysers of nitrogen gas, which surprised scientists for such a cold world.

Planet	Voyager 1	Voyager 2
Jupiter	March 1979	July 1979
Saturn	November 1980	August 1981
Uranus	not visited	January 1986
Neptune	not visited	August 1989

The Golden Record

Each probe carries a gold-plated copper phonograph record, called the Golden Record, as a message for any finder who might intercept it. The record holds more than 100 encoded images, natural sounds such as thunder and surf, greetings in 55 languages, and about 90 minutes of music. The selections span many cultures and include works by Bach and Beethoven along with Chuck Berry's "Johnny B. Goode." A committee led by astronomer Carl Sagan chose the contents. Each record includes a cartridge, a stylus, and instructions for playing it.

Into Interstellar Space

The heliosphere is the bubble of solar wind that surrounds the Sun and the planets. Its outer boundary is called the heliopause. Voyager 1 crossed the termination shock, where the solar wind slows sharply, in December 2004 at about 94 astronomical units (AU). It crossed the heliopause in August 2012 at about 121 AU, the first spacecraft to enter the interstellar medium.

Voyager 2 crossed the termination shock in 2007 and the heliopause in November 2018, at about 119 AU. Its plasma instrument recorded a sharp rise in plasma density, which confirmed that the probe had entered the denser material between the stars.

What the Crossings Showed

The interstellar crossings gave researchers direct measurements of conditions beyond the Sun's influence. Magnetic field readings changed direction, and charged particles from outside the solar system were detected. Before the Voyagers, this environment could only be inferred from distant observations.

Power, Distance, and Time Delay

The probes depend on their RTGs, whose electrical output drops about 4 watts each year as plutonium-238 decays. Each spacecraft started with about 470 watts. Engineers have shut down instruments one by one to save power. The data rate fell by orders of magnitude as the probes moved away, and the 3.7-meter high-gain antenna sends signals to NASA's Deep Space Network on Earth.

Parameter	Value
Launch electrical power	about 470 W
Power loss per year	about 4 W
High-gain antenna	3.7 m dish
One-way delay beyond 100 AU	about 14 hours

The long light delay means that every command takes many hours to arrive, and any reply takes as long to return. Mission teams must plan commands well ahead of time and accept that a problem may take a day or more to diagnose.

Key takeaways

Voyager 1 and Voyager 2 used a rare planetary alignment and repeated gravity assists to reach four giant planets. Voyager 2 remains the only spacecraft to have visited Uranus and Neptune, so its flybys are still the only close-range data for those worlds.

The Golden Record remains a message in a bottle. It shows how a 1977 team chose to represent humanity to any future finder.

The interstellar crossings opened direct measurement of the space between stars. That data changed how scientists model the boundary between the Sun's influence and the wider galaxy.