Like Mars, Jupiter has received a lot of atten-
tion during the era of space exploration.
That began with Pioneer 10, which f lew bythe gas giant in 1973. It was rapidly followed by Pioneer 11
(1974), Voyager 1 (1979), and Voyager 2 (1979). Each of these
missions had their own scientific objectives, but collec-
tively, they revealed a world of confounding com-
plexity. The Pioneer probes took the first close-up
shots of the behemoth’s storm-strewn atmo-
sphere, as well as studied its super-charged
radiation belts and powerful magnetic field
(the strongest of any planet in our solar sys-
tem). The Voyagers provided improved views
of the king of planets, tracking Jupiter’s alter-
nating bands of bright white zones and darker
brown belts. This new perspective revealed strange
atmospheric behavior that models did not predict,
including eddies churning in the clouds and a pair of colliding
oval storms that ejected streamers upon merging. Such unex-
pected sights left the imaging team “happily bewildered.”
In the decades since, NASA has continued slinging space-
craft at Jupiter. The Galileo mission included an orbiter and
the first-ever probe to dive into the atmosphere of one of the
outer planets. As the Galileo probe plunged into Jupiter’s
swirling bands of multicolored clouds, it transmitted back
data for nearly 58 minutes before finally succumbing to face-
melting temperatures of almost 25,000 degrees Fahrenheit
(14,000 degrees Celsius). Still, Jupiter’s beautifully chaotic
atmosphere — including its famous Great Red Spot, a storm
the size of Earth that’s been brewing for centuries — isn’t the
only intriguing aspect of the jovian system.
Jove’s natural satellites total nearly four score and run the
gamut in size, ranging from tens to thousands of miles wide.
But of these moons, four truly stand above the rest. These are
Jupiter’s Galilean moons: Io, Europa, Ganymede, and Callisto.
The more we learn about these surprisingly complicated
JUPITER
worlds, first recognized as moons by Galileo Galilei him-
self in March 1610, the more intriguing they get. Io is
the most volcanically active world in the solar
system, thanks to Jupiter’s immense gravity cre-
ating tides in Io’s solid surface that reach some
300 feet (100 m) tall. Europa’s surface, on the
other hand, is mostly water ice and might even
hide an underground global ocean of slushy
water. Ganymede, which is wider than Mercury
and the largest moon in the solar system, gener-
ates its own internal magnetic field. Callisto, mean-
while, shows little evidence of recent resurfacing, which
means its heavily cratered face likely preserves a record of
stray detritus streaming through the early solar system.
These intriguing features are just some of the reasons the
European Space Agency (with NASA as a partner) plans to
launch the JUICE (JUpiter ICy moons Explorer) mission in
2022, which will see an orbiter reach the jovian system in- Once there, JUICE will spend several years closely scru-
tinizing Ganymede,
Callisto, and
Europa — while
also evaluating the
potential of these
watery worlds
to harbor
life.
STATS
Mass: 318 Earth masses
Equatorial diameter: 88,850 miles
(143,000 km)
Average temperature: –162 F (–108 C)
Rotation period (day):
9 hours 56 minutes
Orbital period (year): 11.9 Earth years
Moons: At least 79 moonsNASA’s Europa Clipper spacecraft, seen here in this artist’s concept, is expected
to launch to Jupiter aboard a SpaceX Falcon Heavy rocket in October 2024.
Once in orbit, it will conduct a series of flybys of Europa, using a suite of science
instruments to study the moon’s surface and composition, measure the
thickness of its icy crust, and even investigate whether the world’s subsurface
ocean sports conditions that are suitable for life. NASA/JPL-CALTECH
Jupiter’s swirling bands are created by differences in the thickness, height, and
composition of its icy clouds. The colorful bands flow in opposite directions at
various latitudes; the lighter bands hold thicker clouds and stretch higher than
the darker bands. This image of the gas giant was captured by Hubble in June- NASA/ESA/A. SIMON (GSFC)/M.H. WONG (UNIVERSITY OF CALIFORNIA, BERKELEY