Page 36 - BBC Sky at Night - September 2017 UK

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2010 2011

" Cassini witnessed a churning white storm in Saturn’s northern hemisphere, which appeared to grow and leave a tail in its wake
< Lightning was seen on
Saturn (left) and the same
spot after the flash had
dissipated (right)

> Saturn’s north polar
hexagon, a jetstream that
NASA/JPL/SPACE SCIENCE INSTUTE X 6, NASA/JET PROPULSION LABORATORY-CALTECH/ERICK
evolves with the seasons
STURM NASA/JPL-CALTECH/SOUTHWEST RESEARCH INSTITUTE, NASA/JPL-CALTECH
> within 1,700 km of Saturn’s one-bar level, the first time and resolved features as large as the
roughly equivalent to sea level pressure here on tallest buildings on Earth.
Earth. “This planned conclusion for Cassini’s
journey was far and away the preferred choice The final countdown
for the mission’s scientists,” says Spilker. “Cassini The ring grazing phase ended on 22 April, when
will make some of its most extraordinary Cassini gained a gravitational tweak from Titan
observations at the end of its long life.” to establish the proper conditions for the Grand
" Saturn’s Death Star-like
Early in 2016, the spacecraft completed five moon Mimas, dominated Finale. Four days later, moving at 34.4km/s, it
manoeuvres to drastically reshape its orbit from by crater Herschel made its initial dive through a gap between
an equatorial one into a higher inclination. Last Saturn’s cloud-tops and the inner edge of the
November, a series of 20 ‘ring-grazing’ orbits innermost D ring. This positioned Cassini closer
commenced, during which Cassini hurtled within to the planet and its rings than ever before.
! Cassini has made 22
a thousand kilometres of the F ring – the outermost As a precautionary measure, its high-gain
dives between Saturn
boundary of Saturn’s dazzling necklace – every and its rings; the orange antenna was used to shield the delicate scientific
week for almost five months. At this distance, it track is the final loop instruments against particle impacts, but the
could directly sample particles from the rings for before its final descent region proved surprisingly dust-free. It was even
nicknamed “the big empty” by Cassini project
manager Earl Maize. Over the following weeks,
the spacecraft dived through the rings every
6.5 days, imaging their entire radial extent at
breathtaking resolution, measuring Saturn’s
gravitational and magnetic fields and extensively
mapping its northern and southern hemispheres.
Rhythmically looping out to 1.3 million km
from the planet, then sweeping inbound for
another dive, the spacecraft’s last five orbits from
mid-August 2017 promised great drama, great risk
and even greater reward. At this stage, Cassini was
so close to Saturn that it would physically pass
through its tenuous upper atmosphere, sampling
various species of molecular hydrogen and helium,
examining small-scale structures and observing its
powerful polar aurorae and fast-moving vortices.
As it completes its 292nd and final full orbit on 12
September, Cassini will perform a distant flyby of
Titan. The moon’s gravity will offer a ‘goodbye kiss’,

skyatnightmagazine.com 2017