Chapter 7: The Giant Planets and Their Moons

Artist's concept of Voyager in flight

The pair of Voyager spacecraft were the first to explore the outer reaches of our Solar System. Launched in 1977, their missions to the outer planets extended through the 1980s. Voyager 1 flew by Jupiter and Saturn before leaving the Solar System, and Voyager 2 visited all four giant planets. Nearly thirty years later, Voyager 2 remains the only human-made object to reach Uranus and Neptune. The data the Voyagers returned completely revolutionized planetary astronomy.

 

Trajectory of Voyager 1

The mission was designed to take advantage of a rare alignment of the outer planets. Once every 175 years, the gas giants are positioned such that a spacecraft can fly by all of them, slingshotting off each successive planet in a maneuver called a "gravitational assist." The expense of designing such an ambitious mission proved too high and NASA only funded flybys of Jupiter and Saturn. However, the great success of both spacecraft in performing their initial missions inspired a continuation to the outermost two giant planets. Scientists were able to reprogram Voyager 2 remotely with a new trajectory that took it past Uranus and Neptune. Originally designed to last only five years, the mission continued for twelve. In fact, we're still receiving data from the spacecraft as they pass the outermost boundary of the known Solar System and head into interstellar space.

 

The two Voyagers were identical craft — the second one was built simultaneously as a backup. They each had eleven scientific instruments on board, including radio transmitters and detectors to communicate and return data to Earth, imaging cameras, spectrometers working at ultraviolet and infrared wavelengths, and various devices to measure the solar wind, cosmic rays, and magnetic fields. Most of the equipment on the Voyagers had redundant systems built in as backups, which is part of the reason why they were so successful and still survive today. Since the Voyagers would travel too far from the Sun to make efficient use of solar power, the craft were powered by the radioactive decay of plutonium. (It has become a recent crisis in the space program that the U.S. Congress is leery of letting NASA use plutonium as a power source, despite the fact that there has never been an incident or leak, and the radioactive material is being sent millions of miles from Earth.)

This gold aluminium cover was designed to protect the Voyager 1 and 2 "Sounds of Earth" gold-plated records from micrometeorite bombardment, but also serves a double purpose in providing the finder a key to playing the record.

Flying board Voyagers 1 and 2 are identical "golden" records, carrying the story of Earth far into deep space. The 12 inch gold-plated copper discs contain greetings in 60 languages, samples of music from different cultures and eras, and natural and man-made sounds from Earth. They also contain electronic information that an advanced technological civilization could convert into diagrams and photographs.

The Voyagers also included a phonograph record, plated in gold. The "golden record" includes recordings of nature sounds, music from around the world, spoken greetings in many languages, and images of our planet and people. The cover of the disk is marked with symbols explaining where it comes from and how to play and interpret the data on the record. The record is designed to play at half the normal LP speed, or 16 ⅔ rpm, to pack more information on board. It was sent out into space as a "message in a bottle" to any extraterrestrial intelligence who might someday find the record — even though it will be 40,000 years before Voyager 1 passes within a few light-years of another star. The renowned astronomer Carl Sagan headed the committee that chose the material sent out into the universe to represent our race.

 

 

Even though the original scientific mission of the Voyager spacecraft was ambitious, they still far surpassed their initial objectives. Voyager explored the Jovian planets (at first, just Jupiter and Saturn, later expanded to include all four gas giants), their large satellites, magnetic fields, and ring systems. They studied in detail the atmospheres, geology, and internal structure of the planets and satellites. There were surprises at every planet, forever altering our view of the Solar System.

A complete list of Voyagers' scientific findings would be impractical — in fact, scientists are still studying data returned from these missions, more than forty five years later. Highlights of their discoveries include:

• The Galilean satellite Io was found to have active volcanism — making Io the only body other than the Earth to be volcanically active at this time.

• More than twenty new satellites were discovered.

• The magnetic fields of Uranus and Neptune, previously undetected, were both found to be considerably skewed and offset relative to the planets themselves.

• Neptune's satellite Triton was observed to have a thin atmosphere and active geysers.

• Rotation periods for the giant planets were measured precisely for the first time.

• Jupiter's ring system was discovered, structure in Saturn's rings was studied, and new rings were found among Uranus's known rings.

• Neptune's Great Dark Spot and other atmospheric motions in the distant planet were discovered.

• Lightning and aurorae were detected in Jupiter's atmosphere.

• The temperatures and pressures of the giant planets' atmospheres were measured, leading to an understanding of the interior structure and dynamics of their atmospheres.

• Saturn's satellite Titan was found to be covered in a thick, hazy nitrogen atmosphere.

• Compositional studies of the gas giants led to insights into the primordial solar nebula and the origin of planets.

Heliosphere and Interstellar Medium. Shows the current position of Voyager space probes in logarithmatic scale.

After its Saturn encounter, Voyager 1 left the solar system — the path that took it past the large satellite Titan bent its trajectory up out of the plane of the Solar System. Voyager 2 went on to visit Uranus and Neptune in the 1980s, and then turned downward out of the ecliptic. Contact was maintained with the resilient spacecraft as they headed for unexplored space outside our Solar System. The interstellar phase of their extended mission continues to this day.

 

In 2003, Voyager 1 passed through the termination shock, a turbulent zone where the solar wind is slowed by the wind that blows through interstellar space. At that point, Voyager 1 was 86 A.U., or about eight billion miles, away from the Sun. Voyager 2 is traveling slightly behind, at about 70 A.U. They're leaving the Solar System behind at rates of 3 and 4 A.U. per year. Each of these intrepid spacecraft is now more than 12 billion miles from home! As the radioactive decay that powers the ships has slowed over the years, scientists have turned off most of the instruments, one by one. With these power-saving measures, the two explorers have enough fuel to last until at least 2020.