One of the longstanding dreams of humanity has been to explore our Solar System by space travel. In his 1865 novel De la terre à la lune, Jules Verne told the story of members of a “Baltimore Gun Club” building a huge cannon to shoot a projectile with three of the Club’s members on board, for a trip to and landing on the Moon. About one century later, the USA–Soviet space race of the 1960s turned this dream into reality by developing the necessary technology and providing the resources for such ambitious endeavors. After several failures, the US Mariner 2 mission succeeded in 1962 in the first flyby of Venus, followed by the 1964 Mariner 4 flyby of Mars. Between 1966 and 1967 the Soviet missions Luna 9–13 and Venera 4 pioneered the first lunar landing and investigation of Venus’s atmosphere, followed by the 1969 culmination of the Apollo 11 manned lunar landing.

In 1972/1973 the Pioneer spacecraft 10 & 11 flew by Jupiter and Saturn. The next major NASA program, Voyager, consisted of two spacecraft launched in August/September 1977 to undertake a “grand tour” of the outer solar system. Both spacecraft flew past Jupiter and Saturn in the period 1979–1981. Voyager 2 visited Uranus in 1986, and Neptune in 1989. The Voyager data were far superior to those obtained by Pioneer. The two Voyagers’ high-resolution images of the outer Solar system planets and their moons fascinated all of humanity, experts and laypersons alike. These unique data are also particularly important for the study of extra-solar planets, since Uranus and Neptune now appear to be more representative of the bulk of the exoplanet population than the other solar-system planets.

The Voyager probes sent back to Earth unique information of fascinating and often strange worlds, including

• the discovery that Jupiter’s satellite Io has many volcanoes, powered by tidal heating of its interior;
• the first images of the rings of Jupiter, Uranus, and Neptune, and the discovery of complex structure in Saturn’s rings including gaps, narrow ringlets, waves and transient “spokes”;
• the discovery that Uranus and Neptune have magnetic fields, but surprisingly with a magnetic symmetry axis that is both strongly tilted and offset relative to the planetary spin axis. Voyager also provided the first measurements of the magnetospheres of these planets, including size, density, composition, and plasma waves;
• the first detailed measurements of the atmospheres of Saturn’s enigmatic satellite Titan and Neptune’s satellite Triton;
• the discovery that Neptune radiates about 2.5 times as much energy as it receives from the Sun. The nature of this energy source is not yet understood;
• measurements of the composition, winds, temperature and pressure profiles of the planetary atmospheres. Neptune’s atmosphere has winds of up to 2,000 km/h and a vast storm system called the Great Dark Spot;
• dramatic improvements of our knowledge of the masses, sizes, shapes, and gravitational fields of all the giant planets and many of their satellites.

Voyager 1 is now 145 times as far from us as the Sun, and has become the most distant human artifact. Many of the instruments continue to send back valuable data, more than forty years after the launch date. The Shaw Prize Astronomy Committee is convinced that now is the time to recognize these achievements. The most natural definition of the ‘boundary’ of the solar system is the heliopause. It marks the outer boundary of the solar system, where the interstellar gas halts the solar wind. Inside the heliopause, space is filled by low-density material from the Sun, while outside it contains material from other stars. The Voyager spacecraft crossed the heliopause in 2012 and 2018, and returned data on the physical properties of the ambient plasma as the spacecraft crossed into interstellar space. This was the final milestone of the Voyager mission.

One of the explicit aims of the Shaw Prize is to recognize advances that have “enriched humanity’s spiritual civilization”. Arguably more than most other scientific endeavors, Voyager has achieved this goal through its spectacular images of unfamiliar worlds. Each spacecraft carried a “golden record” containing sounds and images selected to portray the diversity of life and culture on Earth and intended for a possible encounter with an advanced civilization. In 1990 Voyager 1 looked back to take a famous “family portrait” of the solar system planets including the image of Earth known as the “pale blue dot”, which became a symbol for how small we are in the larger Universe. The mission is unparalleled in its duration and the unique science it has returned. The story of the researchers who devoted their entire careers to Voyager resonates strongly with a wide audience.

The dominant figure in the Voyager mission is Edward C Stone, Professor of Physics at the California Institute of Technology. He has served as Project Scientist from 1972 to the present¾over 45 years¾and is Principal Investigator on one of the spacecraft’s 11 instruments. During the planetary encounters he became internationally known as the public spokesperson for Voyager and explained Voyager’s scientific discoveries to the public with lucidity and scientific authority. His far-ranging contributions to and leadership of this epochal space mission make Professor Edward C Stone an excellent recipient of the 2019 Shaw Prize in Astronomy.