On 26 April, 2017, at 2 am Pacific Daylight
Time, the Grand Finale began. The spacecraft Cassini started its first dive
into the space between Saturn and its magnificent rings – the first of 22 such
dives it will perform before finally crashing into that remote planet. For 13
years, Cassini has been sending back its remarkable readings and photographs of
Saturn and its moons – the latest just two weeks ago, with data that indicated
that Enceladus, the sixth-largest moon of Saturn, contains an ocean of liquid
water underneath its icy surface, which may be capable of supporting life.
Cassini started on its journey almost 20 years
ago, launched on 15 October 1997 from Cape Canaveral Space Launch Complex 40 in
a Titan IV/Centaur rocket, as the Cassini-Huygens Mission. A joint project of
NASA, the European Space Agency and the Italian Space Agency, it cost about US$
3.2 billion. Cassini itself was designed and constructed by NASA, with the ASI
providing its high-gain radio antenna for communication with Earth, as well as
its radar and its radio science subsystem. The ESA constructed the Huygens
probe and lander, destined for Titan, Saturn’s largest moon (which is in fact
larger than the planet Mercury).
Fitting for a mission to Saturn, these two
were named after those who had made discoveries about Saturn first. Giovanni
Domenico Cassini had discovered the divisions between Saturn’s rings, and 4 of
its moons, while Christiaan Huygens had discovered its moon Titan.
Cassini-Huygens, though, had to travel a bit further than they did to get a
closer look, and utilized some quirks of physics to do so.
First it dropped into the Sun’s gravity
well, and approached Venus. On 26 April 1998 it did a close fly-by of Venus to
get a gravitational-assist, or gravity slingshot, to place it into orbit around
the sun. This was not enough, however, and it made a second fly-by of Venus to
slingshot it out to the asteroid belt on 24 June 1999. With the sun’s gravity
pulling it back once more, it was time for a final slingshot manoeuvre around
Earth on 18 August 1999 before it finally had enough momentum to propel it into
the outer solar system.
By 30 December 2000, Cassini-Huygens made
its closest approach to the planet Jupiter, sending back over 26 000 images,
which contributed to major findings about Jupiter’s atmospheric circulation. On
its way to Saturn, it further participated in a test of the General Theory of
Relativity on 10 October 2003, when its radio waves passed close to the sun,
and its frequency shift could be measured as predicted by Einstein.
On 11 June 2004, Cassini-Huygens flew by
Saturn’s moon Phoebe, returning the first images of that moon. Finally, on 1
July 2004, Cassini-Huygens became the first spacecraft to ever orbit Saturn,
after a seven-year journey. Now its work would really begin. On 24 December
2004, Huygens was released, and began its decent to Titan. On 14 Jaruary 2005,
it entered Titan’s atmosphere and after a two and a half hour descent, landed
on solid ground, the first landing ever in the outer solar system.
Cassini, now on its own, continued its
mission of exploration. During the next 4 years of its prime mission, it
encountered Titan 45 times, explored 10 icy satellites and orbited Saturn 76
times. On 15 April 2008, its mission was extended for another 27 months, called
the Equinox mission. This involved 64 more orbits, 28 Titan encounters, 8
Enceladus encounters, 3 with other smaller icy satellites and an equinox
crossing in August 2009. Still, its job was not done, for in February 2010 it
received another missions extension, called the Solstice Mission. With its
mission extended by seven years, it involved another 155 orbits, 54 Titan
encounters, 11 Enceladus encounters, 5 other icy moon encounters, and the
opportunity to observe spring–early summer in northern hemisphere of Saturn.
The wealth of data Cassini sent back was
enormous. The spacecraft had sent back more than 500 gigabytes of data, that
enabled the publication of more than 3000 scientific papers, and in its time
there, had discovered seven more moons of Saturn – Methone, Pallene,
Polydeuces, Daphnis, Anthe, Aegaeon and the as yet unnamed S/2009 S 1.
Besides the discovery of water via plumes
on Enceladus as mentioned, it also uncovered that Titan has rain, rivers, seas
and lakes and is covered in a nitrogen-rich atmosphere. It discovered that
there are vertical structures in the rings of Saturn, piled in bumps of almost
3km high. It captured the first complete view of the odd hexagon at Saturn’s
North Pole, and discovered giant hurricanes at both poles. These are but the
tip of the iceberg of its remarkable discoveries.
But all good things come to an end. By
2016, Cassini had started to run out of fuel. Due to the discoveries made by
Cassini about the potential existence of life on Saturn’s moons, it could not
be allowed to drift indefinitely and potentially crashing on them – it was
imperative to protect them from any potential biological contamination.
Thus, it now embarks on its final mission,
and its most daring one. Climbing high above Saturn’s poles before diving
between the rings to plunge close to the planet, before climbing up again and
repeating it 22 times. As it does this, it will collect rich and valuable
information about the planet’s magnetic and gravitational fields, revealing how
it is composed on the inside, and possibly solving the mystery of just how fast
the interior is rotating. It will improve our knowledge of the materials in the
rings, and help us to understand its origins. All this, while taking
ultra-close images of the planets rings and clouds, before finally, on 15
September 2017, crashing into the planet in a blaze of glory. To quote NASA:
“It’s inspiring, adventurous and romantic — a fitting end to this thrilling
story of discovery.”
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