Page 21 - Astronomy - October 2017 USA

ntil the mid-1990s, when
planets were at last being
discovered orbiting distant
stars, Jupiter was the larg-
est world known. Eleven
times Earth’s diameter
and 318 times our planet’s
mass, Jupiter possesses
more than twice the com-
bined matter of all the
other planets, moons, asteroids, and com-
ets in the solar system. Made mostly of
hydrogen (89.8 percent by volume) and
helium (10.2 percent), Jupiter is essentially
a bottomless atmosphere that, beneath the
clouds, transitions into an exotic fluid. Its
largest moons, so big they rival Mercury in
size, appear to hold substantial amounts of
liquid water and beckon scientists as poten-
tial abodes for life beyond Earth.
For centuries, humanity could view this
giant world only through ground-based
telescopes. But in 1973 and 1974, respec-
Voyager’s views of Jupiter’s colorful cloud tops reminded many people of an impressionist painter’s
tively, the Pioneer 10 and 11 spacecraft canvas. Fine details in and around the Great Red Spot changed significantly in the four months
raced past the planet, providing the first between the Voyager 1 flyby and Voyager 2 (pictured). NASA/JPL
close-up images of its stormy atmosphere,
probing its internal structure, and charting
its intense radiation belts and magnetic cruise away from our planetary system and By early February, the resolution and
field. The Pioneer probes blazed a trail for journey into the unending night of inter- image quality were comparable to the best
further exploration of the outer solar sys- stellar space. Voyagers 1 and 2 are now so pictures returned by the Pioneers. From
tem. Even as scientists reveled in the data far away that their radio signals take more that point on, Jupiter would be seen as
the probes returned, NASA already was than 15 hours to reach us. The more dis- never before.
working on a far more ambitious encore. tant probe, Voyager 1, has passed beyond When viewed from Earth through a
The mission, originally called Mariner the heliopause, a boundary separating the small telescope, the planet’s atmosphere
Jupiter-Saturn 1977, received a name interstellar environment from charged par- shows alternating bright white zones and
change before launch to the one we know ticles and magnetic fields flowing from the darker brown belts. These are the visible
today: Voyager, the longest-running space Sun, known as the solar wind. Voyager 2 is manifestations of east-west jet streams that
mission in history. The twin Voyagers car- cruising through the heliosheath, a zone alternate direction from the equator to the
ried better cameras, more advanced instru- where pressure from interstellar gas begins poles and carry oval-shaped weather sys-
ments, and more computing power than slowing the solar wind. tems of all sizes.
the Pioneers. Both spacecraft launched in “It’s been a remarkable journey,” says The Voyager imaging team tracked a
1977 and are still returning data as they Voyager project scientist Ed Stone at the pair of similarly sized brown ovals, watch-
California Institute of Technology in ing them merge, tumble, and eject a dark
Pasadena. “We keep discovering things streamer. Atmospheric models had not
nobody knew we were going to discover.” predicted such strange behavior. Eddies,
He says both spacecraft may be able to waves, and turbulent clouds churned
return measurements from a single science everywhere. At a February 28 press brief-
instrument until 2030 if their power ing, imaging team leader Bradford Smith
sources — called radioisotope thermoelec- described his team as “happily bewildered”
tric generators — hold out as expected. by what they were seeing. “Jupiter is far
more complex in its atmospheric motions
Encounter with Jove than we had ever imagined,” he said.
On January 6, 1979, Voyager 1 was 36 mil- The planet’s largest feature, a vast
lion miles (58 million kilometers) from southern storm called the Great Red Spot,
Jupiter and two months from its closest had been observed continuously from
approach. Views of the planet’s cloudy, Earth for 150 years. But now, for the first
banded disk already exceeded the best time, scientists could study its rotation and
Voyager 1 captured this view of Jupiter on images from Earth. Among other assign- watch it interact with neighboring features.
January 9, 1979. Even though the spacecraft
was 34 million miles (54 million km) away, ments, the probe began accumulating a Large enough to hold a pair of Earth-sized
its camera captured details impossible time-lapse movie by taking images every planets, the Great Red Spot rolls between
to see from Earth. NASA/JPL 10 hours, one for each Jupiter rotation. two jet streams and completes a rotation in
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