In the 1960s, NASA scientist and engineers began sending robotic probes to explore our planetary neighbors. This greatly expanded our knowledge of theses worlds and gave us entirely new, close-up views of the inner planets that we could only before dream of. This began a new age of exploration, bringing into our reach those distant mysterious discs.
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By the 1970s our sights were set on the outer planets, just as a special opportunity presented itself; the outer planets were soon to be positioned in a way that allowed two spacecraft to efficiently visit the four outer planets, a Grand Tour of the outer solar system. One probe would be sent to Jupiter, and Saturn, with another to all four. Both would utilize the gravity assist maneuver to propel them to their next target.

On August 20, 1977, Voyager 2 was launched on a Titan-Centaur rocket. If the launch had been unsuccessful, the Voyager 1 launch could have been adjusted to have it visit all four outer planets. Thankfully it was successful and on September 5, 1977 Voyager 1 was launched on its accelerated course to Jupiter.

Launch of Voyager 1 and 2 Launch of Voyager 1 and Voyage 2 on their Titan-Centaur vehicles. Credits: NASA/JPL-Caltech/KSC

Thirteen days later after launch, Voyager 1 pointed its camera back home, capturing both Earth and the Moon in a single frame.

The Earth and Moon The Earth and Moon, captured in a single frame by Voyager 1. Credits: NASA/JPL-Caltech

The engineers at NASA's Jet Propulsion Laboratory designed and built both spacecraft. Let's take a closer look at them.

Voyager Spacecraft - Labeled Artist's concept of a Voyager spacecraft in deep space. Labels added. Credits: NASA/JPL-Caltech

Each spacecraft had a main, central body, that housed communication and control systems, including a digital tap recorder for data storage. Attached to the top is a 3.66 meter diameter parabolic antenna for communications with Earth.

Voyager Antenna Dish Engineer working on a parabolic Voyager high-gain antenna. Credits: NASA/JPL-Caltech

On one side is the instrument boom, which holds the majority of the probe's scientific instruments and cameras. Opposite the instrument boom are two antennas that make up the radio astronomy sensors. On the same side, extending up is the magnetometer boom, with the sensitive magnetometer at the end, far away from any magnetic interference from the probe's electronic systems.

Low-Energy Charged Particle Instrument Low-Energy Charged Particle Instrument Assembly. Credits: NASA/JPL-Caltech/JHU-APL

The spacecraft's power source is an array of three radioisotope thermoelectric generators (RTGs). Each RTG contains samples of Plutonium-238 (PuO2). As the plutonium decays, it creates heat that is then converted into electricity.

A Voyager RTG One of the RTGs used by the Voyager probes. Credits: NASA/JPL-Caltech

Voyager 1 was the first to arrive at Jupiter, arriving on March 5, 1979. Voyager 2 arrived on July 9, 1979. During their flybys, three moons were discovered and several other observations lead to surprising discoveries about Jupiter and its moons.

Approach of Voyager 1 to Jupiter Animation of Voyager 1's approach to Jupiter. Credits: NASA/JPL-Caltech

Flying by the moon Jupiter, Voyager 1 made the first observation of active volcanic activity other than on Earth. Together Voyager 1 and 2 observed nine separate eruptions with evidence of several others in the four months between.

Io with Volcanic Plume Io with Volcanic Plume by Voyager 1; brightened plume. Credits: NASA/JPL-Caltech

Europa was found to have many long rifts, evidence of tectonic activity, but with Voyager 2's flyby, its surface features were found to be very flat. It is believed that Europa has a thin icy crust on top of an ocean of liquid water.

Europa Europa During Voyager 2 Closest Approach. Credits: NASA/JPL-Caltech

With their close up views, the Voyager probes also discovered a faint ring system around Jupiter that had never been observed before.

Jupiter's Faint Ring System False Color of Jupiter's Faint Ring System by Voyager 2. Credits: NASA/JPL-Caltech

Voyager 1 arrived at Saturn on November 12, 1980, with Voyager 2 arriving on August 25, 1981. One fascinating discovery was of Saturn's active atmosphere. When in visible color and observed from Earth, Saturn's atmosphere appears very plain, but with a closer look in ultraviolet, bands of clouds with storms were found.

Approach of Voyager 2 to Saturn Approach of Voyager 2 to Saturn in false color. Credits: NASA/JPL-Caltech

With scientist's interest in Titan and its atmosphere, Voyager 1's trajectory was optimized for a flyby, the closest flyby of any body during the Voyager Missions. Information about the atmosphere's density and composition were made, but no observations of the surface could be done because of the thick clouds.

Titan's Atmosphere False color of Titan's atmospheric haze by Voyager 1. Credits: NASA/JPL-Caltech

This flyby of Titan put Voyager 1 on a course above the ecliptic of the Solar System, flying out of the Solar System.

Saturn and its rings Saturn and its rings as Voyager 1 leaves. Credits: NASA/JPL-Caltech

Still fascinated by Jupiter and Saturn more missions were sent by NASA to study the outer Solar System. The Galileo and Juno orbiters were sent to Jupiter, Cassini with its Huygens lander probe visited Saturn and Titan.

Titan's Surface Titan's Surface after the landing of the Huygens probe. Credits: NASA/JPL/ESA/University of Arizona

As its hyperbolic trajectory brought Voyager 2 further from the Sun, its speed decreased, and the distance between planets increased, so Voyager 2's journey between the planets grew longer. With it being four year after Saturn, it finally arrived, this time alone, at Uranus on January 24, 1986.

Uranus Voyager 2 approaches Uranus. Credits: NASA/JPL-Caltech

During its flyby, Voyager 2 visited several known moons and discovered 10 previously unknown moons. It also took the first quality pictures of Uranus as previously ground based telescopes could only get a small, mostly uniformly colored circle.

The ice-rock conglomerate moon, Miranda was found to be particularly interesting as its surface was a mix of old and new, with large features and canyons ten times deeper than the Grand Canyon on Earth.

Miranda The icy moon Miranda by Voyager 2. Credits: NASA/JPL-Caltech

Also interesting was that the previously known ring system was found to be very different from Jupiter and Saturns' rings and are relatively young.

The moon Ariel was found to have a young surface showing evidence of recent geological activity. With the distance from the Sun and cold temperatures, flows of not volcanic rock, but rather icy material were found on its fractured surface.

Ariel The icy moon Ariel by Voyager 2. Credits: NASA/JPL-Caltech

Probably most anticipated was the question of if Uranus had a magnetic field and what its characteristics were. They were surprised to find a very odd magnetic field, its center being offset from the center and tilted 60 degrees from the planet's already sideways axis of rotation. This lead to its magnetotail being twisted behind it.

After another four years of travel and twelve years after launch, Voyager 2 reached Neptune on August 25, 1989. Again scientist were in for quite the surprise. Even after the launch of the Hubble Space Telescope, Voyager 2's pictures remain the best yet of the planet, but Hubble did provide excellent follow up data from some of the discoveries made.

Proteus (1989N1) Proteus (1989N1): a moon discovered by Voyager 2. Credits: NASA/JPL-Caltech

One of the largest discoveries of the planet made was the presence of a large dark storm, similar to The Great Red Spot, and was appropriately dubbed The Great Dark Spot. About the size of the Earth, speeds were measured to be up to 1,200 miles per hour (2,000 km/h), the fastest observed in our Solar System. Hubble has since shown that this storm has disappeared.

Neptune Voyager 2's approach of Neptune. Credits: NASA/JPL-Caltech

Of particular interest to scientists was Neptune's largest moon, Triton. With no further targets after Neptune engineers optimized Voyager 2's flyby trajectory to accommodate Triton's odd orbit with its high inclination. Also odd was that they discovered that Triton had a relatively high density. That with its retrograde orbit, it is believed that it is a captured satellite, not originally from the Neptune system. Also they found geysers of nitrogen, contributing to a thin atmosphere, with occasional clouds of nitrogen ice.

Triton Voyager 2's image of Uranus' largest moon, Triton. Credits: NASA/JPL-Caltech

Especially after the surprise of Uranus' magnetic field, scientist were anxious to learn about Neptune's magnetic field, and they were not disappointed. While not as extreme as Uranus', they found its magnetic field was off center and was tilted from its axis of rotation.

Neptune's Rings Image of Neptune and its outermost rings by Voyager 2. Credits: NASA/JPL-Caltech

To this day the Voyager spacecraft continue to sent back valuable scientific data as they have continued out of the solar system. As time has gone on, some of the Plutonium in the RTGs has decayed, reducing their power output. To have enough energy to continue operations, instruments and other supporting systems have been shutdown.

Before shutting down the cameras on Voyager 1, scientists and engineers decided to take one more set of images. The subject: our Solar System. On February 14, 1990 A mosaic of 60 images was taken capturing Venus, Earth, Jupiter, Saturn, Uranus, and Neptune. Pluto was too small and dim, Mars was not visible because of sunlight scattering inside the optics, and Mercury is too close to the Sun.

Solar System Family Portrait Solar System Family Portrait by Voyager 1 in 1990. Credits: NASA/JPL-Caltech

With the 60 wide angle frames, narrow angle images were taken of the planets as well, including the famous Pale Blue Dot. These pictures are the furthest pictures ever taken of our Sun and Planet.

Earth: Pale Blue Dot Earth, the "Pale Blue Dot", from the Solar System Family Portrait. Credits: NASA/JPL-Caltech

The next milestone for the Voyager spacecrafts was to pass the termination shock, or where because of interaction with the interstellar medium, the speed of the solar wind slows down to subsonic speeds relative to the sun. After about nine more years of travel, both probes eventually crossed the heliopause and entered the interstellar medium, and exiting the Sun's protective magnetosphere. The probes still continue to send data from the cosmic ray system, low-energy charged particle instrument, plasma sensors, magnetometers, and radio astronomy antennas.

Engineers hope to continue the mission for as long as the spacecraft continue to send data. Current plans power management strategies will keep the probes operational for a few more years. As they continue on, silent, they will pass by other stars in the galaxy, and possibly be discovered by intelligent life, on which they will find a golden record, containing a message in a bottle. A message from the human race saying "Hi, this is who we are."

The Golden Record Cover The Golden Record Cover installed on one of the Voyager probes. Credits: NASA/JPL-Caltech