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AGRAND TOUR OF THE GIANT PLANETS

On August 20, 1977, the Voyager 2 spacecraft was launched from Cape Canaveral in Florida. Two weeks later, its sister craft Voyager 1 was launched. Thus began the most ambitious exploration of the solar system ever. The launch was the culmination of more than a decade’s work. The core mission would run for 12 years, but an interstellar mission continues.

A Titan 3E rocket lifts off carrying the Voyager 1 spacecraft as its payload. The Titan 3E was the most powerful launch vehicle of its time.

Going interplanetary

By the early 1960s, both the Soviet and US space agencies were sending missions to other planets. There were more failures than successes, but over the decade robotic spacecraft began sending back close-up images of Venus and Mars. The NASA craft were part of the Mariner program, run largely from the Jet Propulsion Laboratory (JPL) in California. The mathematicians at JPL perfected the art of the “flyby”—sending a spacecraft on a trajectory that had it fly past a planet close enough to photograph and observe it, albeit too quickly to enter its orbit. In 1965, a graduate student named Gary Flandro, who was working at JPL for the summer, was given the task of figuring out routes to the outer planets and discovered that, in 1978, all the outer planets would be on the same side of the sun. His calculations revealed that this had not happened since 1801, and would not occur again until 2153. Flandro saw the opportunity for a Grand Tour of the outer solar system, but the distances involved were far beyond the capabilities of the spacecraft of the day. In 1965, Mars’s alignment made it the closest planet to Earth at that time at 35 million miles (56 million km), but Neptune was 2.5 billion miles (4 billion km) away, and a journey to it would take several years.

Planetary slingshots

A Grand Tour spacecraft would have to change course several times in order to fly past all its planets. Flandro’s plan had to use gravity assists to fling the craft from planet to planet. Also known as a gravitational slingshot or swingby, a gravity assist had first been used by the Soviet Luna 3, which had swung around the far side of the moon in 1959, photographing as it went. It had never been used to guide spacecraft as far from Earth as the outer planets. The planned slingshot required the craft to approach the planet head-on, traveling in the opposite direction from the planet’s orbital motion. The planet’s gravity would speed up the craft as it made a loop around the planet. It would then slow down again as it headed off into space, having done an about turn. If the motion of the planet were ignored, the craft’s escape speed would be more or less equal to its approach speed. However, taking the motion of the planet into account, the craft would leave the planet having added approximately twice the speed of the planet to its own velocity. A slingshot would not only redirect the craft, but also accelerate it on to its next target.

“It was a chance that came around once in every 176 years and we prepared for it. Out of that emerged the greatest mission of planetary exploration to this day.” Charles Kohlhase

An artist’s impression shows Voyager 1 in space. This craft and its twin, Voyager 2, communicate with Earth via radio waves transmitted and received by a 12-ft (3.7-m) dish.

Taking a Grand Tour

In 1968, NASA set up the Outer Planets Working Group. It proposed the Planetary Grand Tour mission, which would send one spacecraft to visit Jupiter, Saturn, and Pluto, and a second one toward Uranus and Neptune. The plan required a new long-range spacecraft and costs grew steadily. Then in 1971, NASA canceled the Grand Tour as it needed cash to fund the Space Shuttle program.

The exploration of the outer planets was handed back to the Mariner program. The mission was named Mariner Jupiter–Saturn, or MJS77 for short —77 referred to the launch year. To reduce costs, Pluto was removed from the tour itinerary. Instead, one craft was to visit Jupiter, Saturn, and finally Saturn’s huge moon Titan. Titan was considered more intriguing than distant Pluto. It was larger than Mercury, and thought at the time to be the largest moon in the solar system. It was also the only moon known to have its own atmosphere. This change meant that the mission would be budgeted as an exploration of the two gas giants, not a Grand Tour. However, the spacecraft, code-named JST, was to have a back-up, JSX. Its mission would also include Jupiter and Saturn if JST failed. The X represented an unknown quantity. If required, JSX would go to Titan, but if JST achieved its mission, then JSX would be sent to Uranus and Neptune.

Jupiter’s moon Europa was photographed by Voyager 2. It is covered in a thick crust of ice, which has been fractured and filled in with material from the moon’s interior.

Mission profile

In 1974, mission design manager Charles Kohlhase began to make a master plan for the MJS77 mission. He had to consider every aspect, from the spacecrafts’ design, size, and launch system to the many variables they would
encounter along their routes—the radiation levels, light conditions, and contingencies for altering the missions. It took Kohlhase and his team eight months to eventually settle on two trajectories that met all the criteria and would take the spacecraft as close to as many points of interest as possible. Neither Kohlhase nor anyone else working on MSJ77 liked the name. As the launch date approached, a competition for a new name was organized. Nomad and Pilgrim made the shortlist, but by the time the two identical spacecraft were ready, they were known as Voyager 1 and Voyager 2. At 1,590 lb (720 kg), the two Voyager spacecrafts were nearly 50 percent heavier than any previous flyby craft. About 220 lb (100 kg) of that was scientific equipment, comprising two cameras, magnetic field sensors, spectrometers that would analyze light and other radiation to show which chemicals were present in atmospheres, and particle detectors for investigating cosmic rays. In addition, the radio system could be used for a variety of experiments, such as probing atmospheres and Saturn’s rings. The spacecrafts’ trajectories were to be controlled by 16 hydrazine thrusters. However, it would be too dark beyond the asteroid belt for solar panels to generate enough electricity for the spacecraft, and batteries would not last nearly long enough. The answer was nuclear power in the form of radioisotope thermoelectric generators (RTG), held out on a boom to isolate them from sensitive equipment. Each RTG contained 24 balls of plutonium that gave out heat, which was converted into an electric current by thermocouples. The power supply was built to last for nearly 50 years.

Voyager 1 captured a 100-mile- (150-km-) high eruption on Jupiter’s moon Io. Strongly affected by Jupiter’s gravity, Io is the most volcanically active place in the solar system.

Jupiter and its moons

By December 1977, Voyager 1 had overtaken Voyager 2, which was taking a more circular trajectory. It reached the Jupiter system in January 1978. Most of Voyager 1’s important discoveries were made in a frenetic 48-hour period around March 5, when it made its closest approach, coming within 217,000 miles (349,000 km) of the planet’s cloud tops. In addition to sending back images, Voyager 1 analyzed the compositions of the clouds and measured the planet’s immense magnetic field. It also showed that Jupiter had a faint ring system. Its most memorable discoveries came from the flybys of the Galilean moons. These were not sparse, cratered balls but active worlds. Photographs of Io showed the largest volcanic eruptions ever seen, spurting ash clouds into orbit. Fresh measurements of Ganymede revealed that it superseded even Titan in size, while images of Europa’s eerily smooth yellowish disk had astronomers puzzled.

Voyager 2 arrived at Jupiter more than a year later, and did not approach as close as Voyager 1, but it took some of the mission’s most iconic images of Io transiting Jupiter. Voyager 2 also got a closer look at Europa, showing that it was covered in a crust of water ice riven by cracks. Later analysis revealed that these cracks were caused by upwellings in a liquid ocean under the crust, an ocean that is estimated to hold at least twice as much water as Earth and which is thought by scientists to be a prime candidate for the presence of alien life.

The latter half of the next decade abounds in interesting multiple planet opportunities. Of particular interest is the 1978 “grand tour” which would make possible close-up observation of all planets of the outer solar system.” Gary Flandro

Voyager 2’s images of Saturn’s rings revealed a complex structure made of small satellites, none of which were larger than 3–6 miles (5–9 km) across.

Titan and Saturn

By November 12, 1980, Voyager 1 was skimming 77,000 miles (124,000 km) above the atmosphere of Saturn. On the approach, and despite some instrument failures, it revealed details of the rings, which were made of billions of chunks of water ice and were as thin as 30 ft (10 m) in places. Kohlhase had sent Voyager 1 to visit Titan before approaching Saturn to prevent any damage caused by Saturn’s atmosphere and rings from endangering this crucial phase. The spacecraft swung behind Titan so the sun’s light shone through the atmosphere, allowing measurements of its thickness and composition. The Titan trajectory then sent the craft over Saturn’s pole and away to the edge of the solar system. Voyager 2 arrived at Saturn in August 1981, and was able to study the planet’s rings and atmosphere in more detail, but its camera failed during much of the flyby. Fortunately, it was restored, and the order was given to continue to the ice giants.

Voyager 2 sent back this image of Neptune’s ice moon Triton. During the craft’s flyby, only the southern ice cap was visible. Highly reflective, it is made of frozen nitrogen and methane.

Uranus and Neptune

Voyager 2 is the only craft to have visited the ice giants Uranus and Neptune. It took 4.5 years to travel from Saturn to Uranus, where the craft passed 50,500 miles (81,500 km) above the pale blue atmosphere. It looked at the planet’s thin rings and discovered 11 new moons, all of which are now named after Shakespearian characters, as is the rule for Uranus. The most curious thing to be examined on this otherwise relatively quiet planet is the tilt of its axis, which is roughly 90°. As a result, Uranus does not spin as it orbits, but “rolls” around the sun.

The final port of call was Neptune, reached in August 1989. This deep-blue planet was found to have the strongest winds in the solar system, up to 1,500 mph (2,400 km/h)—nine times stronger than anything experienced on Earth. The Voyagers’ mission controllers were able to abandon caution as the planetary mission drew to an end. Without regard for the safety of its final trajectory, Voyager 2 was redirected to fly past Neptune’s moon Triton. The images of the huge ice moon showed geysers blasting fountains of slush from the surface.

The golden records carried by the Voyager spacecraft included a selection of music, greetings in 55 different languages, and images of humans, animals, and plants.

Continuing mission

The Voyager program continues and the two craft are still in touch with NASA. As of 2016, Voyager 1 was 12.5 billion miles (20 billion km) and Voyager 2 was 10 billion miles (16 billion km) away. Six times a year, the craft spin around to measure the cosmic rays around them. This data shows that the craft are approaching the edge of the heliosphere, the region of space that is influenced by the sun. Soon they will enter interstellar space and measure the cosmic wind from ancient stellar explosions.

In 2025, the two spacecraft will power down and go quiet forever, but their mission may still not yet be complete. A committee chaired by Carl Sagan selected content for a gold-plated phonograph record (its analog groove would be easier to read than a digital format). They included greetings from the world, the sounds and sights of Earth, and even human brain waves. The record is a calling card from humankind to an alien civilization. The Voyagers are not heading for any star systems; the closest they will get is when Voyager 1 passes 1.6 light-years from a star in 40,000 years’ time. In all likelihood, they will never be found by intelligent life, but the golden records are a symbol of the hope with which the two interplanetary spacecraft were sent on their way.

“The spacecraft will be encountered and the record played only if there are advanced spacefaring civilizations in interstellar space. But the launching of this “bottle” into the cosmic “ocean” says something very hopeful about life on this planet.” Carl Sagan

By 2005, the Voyagers had reached the termination shock, where the solar wind slows and becomes turbulent as it mixes with the interstellar medium (matter in the space between star systems), entering the heliosheath region. By 2016, they were nearing the heliopause, where the solar wind is stopped by the interstellar medium.

CHARLES KOHLHASE

Charles “Charley” Kohlhase was born in Knoxville, Tennessee, and graduated with a degree in physics. He briefly served in the US Navy before joining JPL in 1960, where he turned his life-long fascination with exploration into work on the Mariner and Viking projects, before joining the Voyager team. In 1997, Kohlhase left Voyager to design the Cassini– Huygens mission to Saturn, which succeeded in dropping a lander onto the surface of Titan in 2005. In the late 1970s, he worked with computer artists to create accurate animations of space missions for advancing the public understanding of NASA’s work. Now retired, Kohlhase remains involved in several projects that blend art and space science, aiming to educate and inspire the next generation of rocket scientists and interplanetary explorers.

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