Saturn Plunge Nears for Cassini Spacecraft

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Saturn Plunge Nears for Cassini Spacecraft
Jet Propulsion Laboratory
August 29, 2017

NASA's Cassini spacecraft is 18 days from its mission-ending dive into
the atmosphere of Saturn. Its fateful plunge on Sept. 15 is a foregone
conclusion -- an April 22 gravitational kick from Saturn's moon Titan
placed the two-and-a-half ton vehicle on its path for impending destruction.
Yet several mission milestones have to occur over the coming two-plus
weeks to prepare the vehicle for one last burst of trailblazing science.

"The Cassini mission has been packed full of scientific firsts, and our
unique planetary revelations will continue to the very end of the mission
as Cassini becomes Saturn's first planetary probe, sampling Saturn's atmosphere
up until the last second," said Linda Spilker, Cassini project scientist
from NASA's Jet Propulsion Laboratory in Pasadena, California. "We'll
be sending data in near real time as we rush headlong into the atmosphere
-- it's truly a first-of-its-kind event at Saturn."

The spacecraft is expected to lose radio contact with Earth within about
one to two minutes after beginning its descent into Saturn's upper atmosphere.
But on the way down, before contact is lost, eight of Cassini's 12 science
instruments will be operating. In particular, the spacecraft's ion and
neutral mass spectrometer (INMS), which will be directly sampling the
atmosphere's composition, potentially returning insights into the giant
planet's formation and evolution. On the day before the plunge, other
Cassini instruments will make detailed, high-resolution observations of
Saturn's auroras, temperature, and the vortices at the planet's poles.
Cassini's imaging camera will be off during this final descent, having
taken a last look at the Saturn system the previous day (Sept. 14).

In its final week, Cassini will pass several milestones en route to its
science-rich Saturn plunge. (Times below are predicted and may change
slightly; see https://go.nasa.gov/2wbaCBT for updated times.)

Sept. 9 Cassini will make the last of 22 passes between Saturn itself
and its rings -- closest approach is 1,044 miles (1,680 kilometers) above
the clouds tops.
Sept. 11 -- Cassini will make a distant flyby of Saturn's largest
moon, Titan. Even though the spacecraft will be at 73,974 miles (119,049
kilometers) away, the gravitational influence of the moon will slow down
the spacecraft slightly as it speeds past. A few days later, instead of
passing through the outermost fringes of Saturn's atmosphere, Cassini
will dive in too deep to survive the friction and heating.
Sept. 14 -- Cassini's imaging cameras take their last look around
the Saturn system, sending back pictures of moons Titan and Enceladus,
the hexagon-shaped jet stream around the planet's north pole, and features
in the rings.
Sept. 14 (5:45 p.m. EDT / 2:45 p.m. PDT) -- Cassini turns its antenna
to point at Earth, begins a communications link that will continue until
end of mission, and sends back its final images and other data collected
along the way.

Sept. 15 (4:37 a.m. EDT / 1:37 a.m. PDT) -- The "final plunge" begins.
The spacecraft starts a 5-minute roll to position INMS for optimal sampling
of the atmosphere, transmitting data in near real time from now to end
of mission.
Sept. 15 (7:53 a.m. EDT / 4:53 a.m. PDT) -- Cassini enters Saturn's
atmosphere. Its thrusters fire at 10 percent of their capacity to maintain
directional stability, enabling the spacecraft's high-gain antenna to
remain pointed at Earth and allowing continued transmission of data.
Sept. 15 (7:54 a.m. EDT / 4:54 a.m. PDT) -- Cassini's thrusters are
at 100 percent of capacity. Atmospheric forces overwhelm the thrusters'
capacity to maintain control of the spacecraft's orientation, and the
high-gain antenna loses its lock on Earth. At this moment, expected to
occur about 940 miles (1,510 kilometers) above Saturn's cloud tops, communication
from the spacecraft will cease, and Cassini's mission of exploration will
have concluded. The spacecraft will break up like a meteor moments later.

As Cassini completes its 13-year tour of Saturn, its Grand Finale -- which
began in April -- and final plunge are just the last beat. Following a
four-year primary mission and a two-year extension, NASA approved an ambitious
plan to extend Cassini's service by an additional seven years. Called
the Cassini Solstice Mission, the extension saw Cassini perform dozens
more flybys of Saturn's moons as the spacecraft observed seasonal changes
in the atmospheres of Saturn and Titan. From the outset, the planned endgame
for the Solstice Mission was to expend all of Cassini's maneuvering propellant
exploring, then eventually arriving in the ultra-close Grand Finale orbits,
ending with safe disposal of the spacecraft in Saturn's atmosphere.

"The end of Cassini's mission will be a poignant moment, but a fitting
and very necessary completion of an astonishing journey," said Earl Maize,
Cassini project manager at NASA's Jet Propulsion Laboratory in Pasadena,
California. "The Grand Finale represents the culmination of a seven-year
plan to use the spacecraft's remaining resources in the most scientifically
productive way possible. By safely disposing of the spacecraft in Saturn's
atmosphere, we avoid any possibility Cassini could impact one of Saturn's
moons somewhere down the road, keeping them pristine for future exploration."

Since its launch in 1997, the findings of the Cassini mission have revolutionized
our understanding of Saturn, its complex rings, the amazing assortment
of moons and the planet's dynamic magnetic environment. The most distant
planetary orbiter ever launched, Cassini started making astonishing discoveries
immediately upon arrival and continues today. Icy jets shoot from the
tiny moon Enceladus, providing samples of an underground ocean with evidence
of hydrothermal activity. Titan's hydrocarbon lakes and seas are dominated
by liquid ethane and methane, and complex pre-biotic chemicals form in
the atmosphere and rain to the surface. Three-dimensional structures tower
above Saturn's rings, and a giant Saturn storm circled the entire planet
for most of a year. Cassini's findings at Saturn have also buttressed
scientists' understanding of processes involved in the formation of planets.

The Cassini-Huygens mission is a cooperative project of NASA, ESA (European
Space Agency) and the Italian Space Agency. NASA's Jet Propulsion Laboratory,
a division of Caltech in Pasadena, manages the mission for NASA's Science
Mission Directorate, Washington. JPL designed, developed and assembled
the Cassini orbiter.

More information about Cassini:

https://www.nasa.gov/cassini

https://saturn.jpl.nasa.gov

News Media Contact
Preston Dyches
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-7013
preston...@jpl.nasa.gov

Dwayne Brown / Laurie Cantillo
NASA Headquarters, Washington
202-358-1726 / 202-358-1077
dwayne....@nasa.gov / laura.l....@nasa.gov

2017-231


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