Juno, a four-ton, solar-powered craft, will peer deep below Jupiter’s swirling clouds when it arrives in July 2016, seeking clues to the giant planet’s formation, searching for a hard core and mapping Jupiter’s intense magnetic field and radiation belt.
Juno will even listen for lightning amid Jupiter’s raging storms and gather unprecedented views of the planet’s sparkling auroras.
Planetary scientists say Jupiter was the first planet in the solar system to form, about 4.5 billion years ago. As a great cloud of hydrogen and helium gas coalesced, it spun into a giant ball about 1,300 times larger than Earth. But the ingredient list for this gas giant remains incomplete, its deep structure unknown.
“We don’t know if there’s a core of heavy elements in the middle or if it’s just gas all the way down,” said the mission’s chief scientist, Scott Bolton of the Southwest Research Institute in San Antonio.
To find out, scientists will use Juno — the entire craft — as a gravity probe. As Juno repeatedly zooms past Jupiter, ground stations on Earth will detect the tiny changes in the craft’s velocity. When Juno passes an especially dense region, it will speed up a smidge, tugged by stronger gravity. “If Jupiter is really massive in the middle, it will be clear from this map of the gravity field,” Bolton said.
At the same time, Juno’s microwave detector will peer about 300 miles beneath the top of Jupiter’s colorful surface, mapping the planet’s deep clouds while searching for water and, by proxy, oxygen.
Oxygen is the third-most-abundant element in the universe — after hydrogen and helium — but previous missions have failed to detect much of it inside Jupiter. “It’s the water we’re really after,” Owen said. “That will be very important” for testing theories of how Jupiter — and the other planets — formed.
Owen said that because Jupiter has changed very little since its formation, piecing together its composition will be like looking back in time to the beginning of the solar system.
Other instruments on Juno will map Jupiter’s magnetic field — by far the strongest in the solar system — and probe its stunning auroras, which, like those on Earth, girdle Jupiter’s north and south poles. Jupiter’s auroras are “truly spectacular,” said mission scientist Jack Connerney of NASA’s Goddard Space Flight Center in Greenbelt, but their source is unknown.
Jupiter is the biggest of the planets, and so it’s fitting that Juno enjoys its own list of superlatives. After swinging past Earth for a speed boost in 2013, Juno will be flung outward at 160,000 mph, making it the fastest man-made object in history.
Juno will dive closer to Jupiter — within 3,100 miles of the surface on each of its 30 planned orbits — than any previous spacecraft.