The Largest Thing in the Solar System

The largest thing in the solar system, Jupiter's magnetosphere (ten times the width of the sun), has for a short time been directly sampled by two spacecraft, Galileo (already on patrol in the Jupiter system) and Cassini-Huygens (on its way toward Saturn).

Just as Cassini was approaching Jupiter in January 2001 the sun obliged scientists by cranking up its already potent wind of particles. The effect of this gale on the Jovian environment could therefore be monitored from two vantage points, not just one.

What the craft saw and measured, supplemented with the observational efforts of earthbound radio telescopes and the Chandra (at x-ray wavelengths) and Hubble (optical) in earth-orbit, were a contraction of the magnetosphere, a brightening of auroras at Jupiter's poles, radio transmissions from Jupiter, synchrotron radiation from electrons with energies as high as 50 MeV, and clear signs of a "planetary wind," a gust of neutral atoms formed from ions spewed by Io's volcanic eruptions and then sent outwards against the incoming solar wind. Such energetic neutral atoms (ENA) were predicted to exist and this is the first evidence in their favor. (Nature, 28 Feb 2002: seven related articles.)