http://www.skyandtelescope.com.au 15Theoretical models give a range for Jupiter’s
combined heavy-element abundance of somewhere
between 3% and 13% by mass. This is a huge
uncertainty, and frankly, to have up to 20 Earth
masses of oxygen unaccounted for is a bit of an
embarrassment. Pinning down the Jovian water
abundance has major compositional implications not
just for Jupiter but also for all planetary systems.
Befuddled by the Galileo probe’s dry descent,
planetary scientists initially clamoured to have NASA
send more probes into Jupiter’s atmosphere to find
out where the water could be hiding. Scott Bolton
and colleagues at the Jet Propulsion Laboratory had a
better idea. Microwaves are absorbed by water (that’s
how a mug of tea gets heated in a microwave oven),
and Jupiter emits microwave radiation from its hot,
deep interior. So, Bolton thought, why not just fly a
spacecraft with a microwave receiver above the cloudsof Jupiter to map the distribution of water? Flying
close enough to Jupiter to do that would also enable
detailed mapping of the planet’s gravitational and
magnetic fields.
But there was a catch: the dense equatorial belt
of energetic charged particles trapped in Jupiter’s
magnetic field. To sound Jupiter’s interior, a spacecraft
would need a polar orbit that ducks under the
radiation belt as it skims above the atmosphere. Thus,
the mission concept that became Juno was born.
In 2005 NASA managers selected Juno as the
second mission in the space agency’s New Frontiers
program. Competing with other missions for NASA
funding meant that the Juno scientists (led by Bolton,
now at the Southwest Research Institute) had to limit
their appetite and focus attention on three main
objectives: Jupiter’s interior, its atmosphere and its
polar magnetosphere.BIG AND
BRAVE Three
giant solar-cell
panels, each 9
metres long,
power NASA’s
Juno spacecraft.
Once it begins
orbiting Jupiter
on July 4, Juno
will have a limited
lifetime before its
electronics absorb
a lethal dose
of high-energy
charged particles.NASA / JPL