The Grand Tour: The Historic Voyager Missions

Lorne Trottier
1 February 2013

Curiosity Update

In my last post I mentioned that the Curiosity Rover had stopped at a spot called Rocknest to do an analysis of a soil sample using its highly sophisticated mineral and chemical analysis instruments. The press conference in early December, at which the results of this analysis were announced, generated a media frenzy when John Grotzinger, the project scientist, said that this announcement would be “one for the history books”. Speculation was rife that scientists had found evidence for life on Mars. As it turned out, Grotzinger was referring to the excellent quality of the data that had been received, which proved that the sophisticated instruments were in perfect working order. This means that Curiosity has the capability to find organic compounds if any are present.

Curiosity will have to be lucky if it is to find such compounds. Radiation from space actively destroys all but the simplest organic compounds up to a depth of about a meter. Curiosity’s drill can only penetrate to a depth of about 5 cm. Much better drills are planned for future missions. Curiosity is preparing to use its drill at a site named John Klein containing a diverse set of sedimentary rocks that were laid down by water. Perhaps it will get lucky and strike “gold”, but understanding the composition of these unusual rocks will be interesting. Curiosity’s best chance of finding organics will come if it finds a fresh small crater, which has excavated a hole in the right type of rock.

Even though they will be hard to find, it would be surprising if there were no organic compounds on Mars, since they are formed by natural processes and are found in meteorites and comets throughout the solar system. Such objects have bombarded the surfaces of all the planets and moons of the solar system including Mars, seeding them with organics. Titan, the moon of Saturn covered in my last article, is saturated with organic molecules called tholins. But, the presence of organic compounds is merely one of the necessary preconditions for life to emerge. If organics are found on Mars, the ultimate question will be whether or not they are of natural origin or if they came from life.

The Grand Tour

Of all the planetary missions in the history of space exploration, the Voyager missions stand out as among the greatest. These missions revolutionized and forever changed our understanding of the solar system. Both spacecraft are still functioning and are exploring the outer boundary of the solar system on their way to interstellar space. An interesting item carried on both spacecraft is a golden record the size of an old fashioned LP disc that was prepared by the late planetary scientist and science popularizer Carl Sagan and his wife Ann Druyan. The record contains both images and sounds from earth. It is a kind of cosmic message in a bottle which includes instructions that would enable extraterrestrials figure out where it came from.

The gas giantsThe Voyageur missions were conceived in the late 1960’s when NASA was sending the first probes to Venus and Mars. When scientists started thinking about the outer planets, Jupiter, Saturn, Uranus, and Neptune an engineer named Gary Flandro realized that in the following decade, these planets would be aligned in such way that a single spacecraft could fly by each of them in succession using a technique called gravity assist. This was like getting four missions for the price of one! But such a favorable alignment, which was dubbed the Grand Tour, only happens once every 177 years! These four planets are known collectively as the gas giants. Unlike the so-called rocky planets such as Earth and Mars, they are much larger and consist mostly of gas. Jupiter is by far the largest, being big enough to swallow 1000 earths.

Like Curiosity, the Voyager 1 & 2 spacecraft were built for NASA by the Jet Propulsion Laboratory and were launched in 1977. Voyager 2 was propelled on the Grand Tour trajectory to all four planets. But Voyageur 1 was sent on a faster path instead of the Grand Tour. This enabled it to fly by Jupiter and to observe the south pole of Saturn as well as to make a close flyby of Titan. The planetary encounters phase of the Voyager missions lasted from Jan 1979 when Voyager 1 began its encounter with Jupiter, through to Dec. 1989 when Voyager 2 completed its encounter with Neptune

Before the Voyager missions it was thought that the gas giant planets would all be about the same. It was also thought that the numerous moons of these planets, with the exception of Titan, would be like our own moon: heavily cratered and geologically dead. This turned out to be completely wrong!

Stranger than science Fiction

Jupiter's moonsOnce the pictures and data from Voyager 1’s encounter with Jupiter and its moons started coming in, scientists quickly realized that Jupiter and especially its four Galilean moons were full of amazing surprises. These moons were discovered in 1610 by Galileo Galilie, the famous Italian who many consider to be the founder of modern science.

The first surprise was that Io, the innermost moon of Jupiter has hundreds of active volcanoes spewing hot lava up to 500 km into space. Europa, the second moon of Jupiter, has a smooth crystalline surface of ice that is covered with thousands of fractures. Ganymede, the third moon, has a surface that is crisscrossed with strange grooved terrain. Only the fourth moon, Callisto has a heavily cratered moon like appearance.

Voyager also discovered that Jupiter has a set of faint rings, just like Saturn. In fact Voyager discovered rings around all four of the gas giants. Seen up close, Saturn’s strange rings were far more complex and beautiful than anyone had imagined. As I explained in my previous article, Titan the only moon in the solar system with a dense atmosphere, hid its secrets due to the thick haze in its atmosphere. Mimas has a giant crater that nearly destroyed it. It looks like the death star in Star Wars. Mysterious Iapetus, the two faced moon, is snow white on one side blacker than coal on other.

As Voyageur 2 continued on its grand tour to Uranus and Neptune, scientists expected things to get duller due to the extremely cold temperatures farther from the sun.

Neptune & it's spotAgain they were surprised. Neptune has clouds, and a giant dark spot that is similar to Jupiter’s famous red spot. But the last big surprise came from Triton, the giant moon of Neptune. Triton is unusual because it orbits Neptune in the opposite direction from the other moons of the solar system. This indicates that Triton must have once been a separate body that was gravitationally captured by Neptune. Scientists now believe that Triton is part of the same class of dwarf planets as Pluto. Scientists have since found dozens of these dwarf planets and believe that they number in the thousands.

Scientists expected the surface of Triton to be completely dead and frozen at a surface temperature of -237 °C. They were delighted to discover that there are active geysers on Triton, which operate at these frigid temperatures. Frozen nitrogen is heated by the sun to the point where it turns into a gas which explodes from below the surface to form geysers that reach a height of 8 km.

The brief descriptions I’ve given of some of Voyagers’ discoveries literally only “scratch the surface” of the new scientific knowledge gained about these alien worlds.

The Pale Blue Dot

After completing their grand tour, the Voyager spacecraft have continued on their journey out of the solar system. Carl Sagan had an idea for a final picture to be taken by the Voyager camera. Voyager turned its camera on the whole solar system and captured all of the major planets, including the earth. In these images, the earth appears as a pale blue dot. Carl wrote a rather poetic piece that provides food for thought about our existence on a tiny planet in the vastness of space.

the 'family' portrait

Pale Blue Dot“From this distant vantage point, the Earth might not seem of any particular interest. But for us, it's different. Consider again that dot. That's here. That's home. That's us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives. The aggregate of our joy and suffering, thousands of confident religions, ideologies, and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilization, every king and peasant, every young couple in love, every mother and father, hopeful child, inventor and explorer, every teacher of morals, every corrupt politician, every "superstar," every "supreme leader," every saint and sinner in the history of our species lived there – on a mote of dust suspended in a sunbeam.”

The final frontier

The Voyager spacecraft have continued to function and make new scientific discoveries since their launch more than 34 years ago. They are now exploring the outer limits of the solar system. Voyager 1 has reached a boundary called the heliopause, where particles from the sun’s solar wind, collide with the tenuous gas between the stars known as the interstellar medium. Within the next few years, it is expected that it will cross completely through this boundary into interstellar space. Voyager 1 is now the most distant object made by man. It is at a distance of 18.5 billion km. At the speed of light it takes a radio signal 34 hours and 17 minutes for a round trip. Yet the nearest stars are so distant, that it would take Voyager another 40,000 years to reach one – if it were going in the right direction.


The profound lesson that scientists learned about the solar system from the Voyager missions is that the universe is full of surprises. The planets and moons of our solar system are wildly different. Scientists now realize that alien worlds exist with a vast variety of possible geological, chemical, and temperature regimes. They are far more diverse that anyone can even imagine. Scientists have only begun to explore this unexpected cornucopia of diversity. The reality of science is far stranger and more interesting than any fantasy, even science fiction.

View other Lorne's Lens on Space articles

Links to Images & Other Resources

1. Curiosity preparing to drill:

2. Plunging Into the Unknown: Landing on Titan:

3. The Voyager disc: the Golden Record:

4. The gas giants Jupiter, Saturn, Uranus and Neptune:

5. Jupiter’s Galilean moons Io, Europa, Ganymede, and Callisto:

6. Volcanic eruption on Io:

7. The cracked surface of Europa:

8. Grooves on Ganymede:

9. Saturn’s rings:

10. Titan’s haze:

11. Saturn’s moon Mimas « the death star »:

12. Iapetus: the two faced moon:

13. Neptune and the Great Dark Spot:

14. Geysers on Triton:

15. Solar system family portrait:

16. The pale dot:

17. Where is Voyager?

Lorne Trottier

Lorne shares with us his love for space exploration in our Lorne's Lens on Space series!

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