The Shaw Prize in Astronomy 2019 is awarded to Edward C Stone, David Morrisroe Professor of Physics and Vice Provost for Special Projects, California Institute of Technology, USA for his leadership in the Voyager project, which has, over the past four decades, transformed our understanding of the four giant planets and the outer solar system, and has now begun to explore interstellar space.

The Voyager project consists of two spacecraft launched by NASA in August and September 1977 to explore the outer solar system. Both spacecraft flew past Jupiter and Saturn in the period 1979–1981 and Voyager 2 also visited Uranus in 1986 and Neptune in 1989. For the foreseeable future Voyager 2 will remain the only spacecraft to visit Uranus and Neptune. These unique data are particularly important for the study of exoplanets, since Uranus and Neptune now appear to be more representative of the bulk of the exoplanet population than the other solar-system planets.

The highlights of the encounters and from Voyager’s journey include:

• the discovery that Jupiter’s satellite Io has many volcanos, powered by tidal heating of Io’s 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, density and bending waves, and transient “spokes”;
• the first high-resolution images of the four giant planets in the solar system as well as their larger satellites, and the discovery of almost two dozen new satellites;
• the discovery that Uranus and Neptune have magnetic fields, and these fields differ from those of other solar-system planets in that the magnetic pole is strongly tilted and offset relative to the north pole or spin axis. Voyager also provided our first data on the magnetospheres — extended atmospheres of ionized gas — surrounding these planets, including size, density, composition, and plasma waves. It also showed that Jupiter, Saturn and Neptune all have aurorae in their upper atmospheres;
• the first detailed measurements of the atmospheres of Saturn’s 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; in particular Voyager showed that Neptune’s atmosphere has winds of up to 2,000 km/h and a vast storm system called the Great Dark Spot;
• Voyager dramatically improved our knowledge of the masses, sizes, shapes, and gravitational fields of all the giant planets and many of their satellites.
• Each spacecraft carried a ‘golden record’ containing sounds and images selected to portray the diversity of life and culture on Earth and intended to be played only if Voyager encountered an advanced spacefaring civilization.
• Voyager 1 looked back to take a famous ‘family portrait’ of the planets including the image of Earth known as the ‘pale blue dot’, which became an icon for how small we are in the larger universe.

Voyager 1 is now 145 times as far from us as the Sun, and has become the most distant human artifact. Voyager 2 is not quite as far away. Many of their instruments continue to send back valuable data, more than forty years after the launch date. After this long voyage, the spacecraft have finally reached the outer boundary of the solar system.

There are several possible ways to define the ‘boundary’ of the solar system, but the most natural one is the heliopause. The heliopause marks the end of the heliosphere, where the wind of ionized gas emitted by the Sun is finally halted by the pressure from interstellar gas. Inside the heliopause, space is filled by low-density material from the Sun, while outside the heliopause it contains material from other stars. Voyager 1 and 2 crossed the heliopause in 2012 and late 2018, respectively, and returned data on the velocity, density, temperature and other properties of the ambient plasma as the spacecraft crossed into interstellar space. This was the final major milestone of the Voyager mission.

Although many scientists and engineers have devoted much or most of their careers to Voyager, the dominant figure in the mission is Edward Stone, who has served as Project Scientist from 1972 to the present — over 45 years — and is also in charge of one of the spacecraft’s 11 instruments. During the planetary flybys, he became internationally known as the public spokesman for Voyager and explained Voyager’s scientific discoveries to the public with remarkable lucidity and scientific authority.

Astronomy Selection Committee
The Shaw Prize

21 May 2019  Hong Kong