Introduction to special issue

Grand Tour

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Science  12 Oct 2007:
Vol. 318, Issue 5848, pp. 215
DOI: 10.1126/science.318.5848.215

Traveling out to the farthest reaches of the solar system, to Pluto and the Kuiper belt where it will arrive in 2015, the New Horizons probe has to endure a long and mostly uneventful journey. But luckily there are some spectacular sights along the way. On 28 February 2007, New Horizons flew past Jupiter, where it used the gas giant's gravity to slingshot it to even greater speeds and also test its instruments in flight. New Horizons' transit took it to unvisited areas of the planet's spacescape. The papers in this special issue record how the probe witnessed lightning and aurorae in Jupiter's atmosphere, volcanic eruptions on the moon Io, and the pulsing of Jupiter's magnetosphere, a cocoon of charged particles that swathes the entire system.

On Earth, although seen planetwide, the most powerful thunderstorms concentrate near the equator and in the tropics. Not so on Jupiter. Lightning flashed near both poles as well as elsewhere, suggesting that convective electrical storms bubble up everywhere in Jupiter's atmosphere because of global heat imbalances. Nighttime auroral glows, on the other hand, were not as widespread as expected.

Skirting the giant planet, New Horizons also flew by Jupiter's rings and attendant moons, big and small. Surprisingly, no moonlets smaller than a kilometer in size were seen in Jupiter's faint rings, a puzzle if they are built from the debris of shattered moons. Rubble also clumps together in locations favored by gravity resonances with larger moons.

An eruption of the Tvashtar volcano on the satellite Io was caught in the act, allowing the mechanics of the sulfurous plume and the lava temperature to be measured. Pollution from Io's volcanoes has even reached the shores of Europa, an icy moon that may harbor oceans beneath its frozen surface. Io's volcanic emissions feed extra sulfur and oxygen ions into a vast particle cloud that circles the entire Jupiter system, held in place by the planet's strong magnetic field. Behind the planet, it is pulled into a magnetic shadow billions of kilometers long, streaming away from the Sun as the solar wind deflects around Jupiter. Acting like a giant pipe, this magnetic tail drains half a metric ton of charged particles out of the jovian system each second. New Horizons' route took it down the magnetotail, to regions unexplored by earlier Galileo or Voyager missions (see the Perspective on p. 216). Pulses of energetic particles flow along the tail in synchrony with Jupiter's 10-hour rotation rate and also every few days as plasma blobs are fed down the tube.

With Pluto still in its sights, New Horizons' snapshots show that Jupiter inhabits an active landscape, experiencing storms, the pumping of the magnetosphere, and volcanic ash falls. A pity then that it is the last time we will visit Jupiter until the Juno mission in 2016. So sit back, enjoy these views, and think of New Horizons as it races along the solar system's back roads to an even stranger destination.

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