On Friday, 15 September 2017, at 11:55:46
UTC, the Grand Finale ended. The spacecraft Cassini entered and crashed into
the atmosphere of the gas giant Saturn, and signal was lost. This was the final
of 22 orbits, its ‘Grand Finale,’ as it entered the atmosphere about 10 degrees
north of Saturn’s equator at a speed of over 120 000 km/h. For 13 years,
Cassini has been sending back its remarkable readings and photographs of Saturn
and its moons, and now its mission was finally over.
So what has happened over the last 4 months
during Cassini’s Grand Finale? In the first orbit, the Visible and Infrared Mapping
Spectrometer (VIMS) as well as the Composite Infrared Spectrometer (CIRS)
produced a movie of Saturn’s north polar region, including the planet’s hexagon
jet stream for a full rotation of the planet. The second orbit provided the
opportunity for the magnetometer to make high-intensity magnetic field
observations of the planet for the first time ever. The Imaging Science
Subsystem (ISS) simultaneously observed its rings while the sun was hidden,
allowing the observations of faint ringlets.
The third orbit provided the opportunity to
use the Radio Science Subsystem (RSS) to perform gravity field measurements to
study the interior of Saturn. The fourth has the ISS observe the most prominent
ring propellers, while the RSS ran a gravity experiment to study the planet’s
gravitational field. The Cosmic Dust Analyser (CDA) scooped up ring particles
to determine their age. The fifth orbit continued the RSS and CDA experiments,
while the ISS and CIRS observed and mapped the planet’s atmosphere.
The sixth obit had the ISS takes pictures
of Saturn’s rings, while the RADAR mapped the rings down to 100 meters per
pixel. During the seventh orbit, the CIRS made high-resolution thermal scans of
the rings while the RADAR continued its experiment. During the eight orbit,
Saturn’s southern hemisphere was mapped. In the ninth orbit, the Ultraviolet
Imaging Spectrograph (UVIS) observes Saturn’s northern aurora, while the Ion
and Neutral Mass Spectrometer (INMS) measures the density and composition of
particles in the ring plane.
During the tenth orbit, the CIRS observes
Saturn’s A ring to compare its ice with those of its moons. During the eleventh
orbit, the UVIS studies the edge of the atmosphere – airglow, auroras and
hydrocarbons – while the CIRS maps the temperature of the north polar vortex
and the VIMS captures a movie of the southern polar vortex. In the twelfth
orbit, the UVIS studies small scale structures in the rings, and in the
thirteenth orbit, the CIRS observes the A ring to determine its composition and
structure, while the ISS makes a high-resolution scan of the B and C rings. By
now, Cassini is within 50 000 km of the atmosphere, and the INMS samples
the planet’s exosphere and ionosphere.
On the fourteenth orbit, the RSS performs
its last observations as Cassini skims the atmosphere, while feeling the
gravitational pull of the rings, determining their mass very accurately, while
the VIMS observes Saturn’s southern hemisphere at night. In the fifteenth
orbit, the UVIS observes the auroral zones on both poles, while the Radio and
Plasma Wave Science (RPWS) captures ‘whistlers’ produced by the planet’s
lightning. During the sixteenth orbits, both the UVIS and VIMS focus to produce
high-resolution auroral images.
The seventeenth orbit sees the ISS
targeting Saturn’s ring propeller features, and marks the start of the end for
Cassini. Only five orbits remain – the final five that are low enough to start
passing through the atmosphere. During the eighteenth orbit, the CIRS observes
the atmosphere’s edge to determine temperatures at different altitudes. The ISS
observes the mysterious streaks in the C ring, while the INMS performs the
first ever direct sampling of Saturn’s atmosphere.
Less than a month is now left for Cassini.
In the nineteenth orbit, CIRS observes temperatures in Saturn’s southern pole,
while the VIMS stares at the southern auroras to produce a mosaic. During the
twentieth orbit, the CIRS maps the northern hemisphere to study temperatures in
the troposphere, while the VIMS maps the equatorial regions. The RADAR studies
the atmosphere in active mode, while the INMS performs a second direct sampling
of the upper atmosphere. In the twenty-first orbit, both the VIMS and the CIRS
work together to study Saturn’s atmosphere, while the INMS made its third direct
sampling as well.
During the final, twenty-second orbit, the
CIRS and VIMS together focused to determine the amount of helium in the
atmosphere, while the INMS performs its fourth direct sampling from the
atmosphere. The RADAR was focused on studying the ammonia in the atmosphere,
while the ISS captured an iconic image of the rings as seen looking outward
from Saturn. And so, the Cassini-Huygens missions that started on 15 October
1997 came to a close.
On 12 September, 2017, at 5h27 UTC,
Cassini’s final plunge began. During its final days, data from the CIRS and
UVIS were transmitted to Earth in real-time, seconds after each observation. By
15 September, 2017, at 10h31, thrusters were activated to maintain altitude
control for its final minute. Thereafter, Cassini started to tumble into
Saturn’s atmosphere, unable to maintain radio contact with Earth. Its final
signals were received, after the 84 minute lightspeed delay of radio signals,
at 11h55:46 by NASA’s Deep Space Network.
The total cost of the missions was a mere
US$ 3.9 billion over the 20 years, costing about US$11 per American, US$1 per
European and US$ 3 per Italian, the mission’s three main sponsors. A small
price to pay for the wealth of information brought back. Cassini programme
manager Earl Maize described the probe as a ‘superb machine’ that ‘to the very
end did everything we asked.’ He continued, ‘Cassini has ended its mission high
over the clouds of Saturn. Thanks and farewell, faithful explorer.’
