ARLA/CLUSTER: Sonda espacial Cassini vai amanhã despenhar-se controladamente em Saturno

[João Costa > CT1FBF]

Cassini spacecraft makes its final approach to Saturn

NASA's *Cassini* spacecraft is on final approach to Saturn, following
confirmation by mission navigators that it is on course to dive into the
planet’s atmosphere on Friday, Sept. 15.

Cassini is ending its 13-year tour of the Saturn system with an intentional
plunge into the planet to ensure Saturn's moons – in particular Enceladus,
with its subsurface ocean and signs of hydrothermal activity – remain
pristine for future exploration. The spacecraft's fateful dive is the final
beat in the mission's Grand Finale, 22 weekly dives, which began in late
April, through the gap between Saturn and its rings. No spacecraft has ever
ventured so close to the planet before.

The mission’s final calculations predict loss of contact with the Cassini
spacecraft will take place on Sept. 15 at 7:55 a.m. EDT (4:55 a.m. PDT).
Cassini will enter Saturn's atmosphere approximately one minute earlier, at
an altitude of about 1,190 miles (1,915 kilometers) above the planet's
estimated cloud tops (the altitude where the air pressure is 1-bar,
equivalent to sea level on Earth). During its dive into the atmosphere, the
spacecraft's speed will be approximately 70,000 miles (113,000 kilometers)
per hour. The final plunge will take place on the day side of Saturn, near
local noon, with the spacecraft entering the atmosphere around 10 degrees
north latitude.


This illustration shows NASA's Cassini spacecraft heading for the gap
between Saturn and its rings during one of 22 such dives of the mission's
finale. The spacecraft will make a final plunge into the planet's
atmosphere on Sept. 15, 2017.
Credits: NASA/JPL-Caltech
When Cassini first begins to encounter Saturn's atmosphere, the
spacecraft's attitude control thrusters will begin firing in short bursts
to work against the thin gas and keep Cassini's saucer-shaped high-gain
antenna pointed at Earth to relay the mission's precious final data. As the
atmosphere thickens, the thrusters will be forced to ramp up their
activity, going from 10 percent of their capacity to 100 percent in the
span of about a minute. Once they are firing at full capacity, the
thrusters can do no more to keep Cassini stably pointed, and the spacecraft
will begin to tumble.

When the antenna points just a few fractions of a degree away from Earth,
communications will be severed permanently. The predicted altitude for loss
of signal is approximately 930 miles (1,500 kilometers) above Saturn's
cloud tops. From that point, the spacecraft will begin to burn up like a
meteor. Within about 30 seconds following loss of signal, the spacecraft
will begin to come apart; within a couple of minutes, all remnants of the
spacecraft are expected to be completely consumed in the atmosphere of
Saturn.

Due to the travel time for radio signals from Saturn, which changes as both
Earth and the ringed planet travel around the Sun, events currently take
place there 86 minutes before they are observed on Earth. This means that,
although the spacecraft will begin to tumble and go out of communication at
6:31 a.m. EDT (3:31 a.m. PDT) at Saturn, the signal from that event will
not be received at Earth until 86 minutes later.

"The spacecraft's final signal will be like an echo. It will radiate across
the solar system for nearly an hour and a half after Cassini itself has
gone," said Earl Maize, Cassini project manager at NASA's Jet Propulsion
Laboratory (JPL) in Pasadena, California. "Even though we'll know that, at
Saturn, Cassini has already met its fate, its mission isn't truly over for
us on Earth as long as we're still receiving its signal."

Cassini's last transmissions will be received by antennas at NASA's Deep
Space Network complex in Canberra, Australia.

Cassini is set to make groundbreaking scientific observations of Saturn,
using eight of its 12 science instruments. All of the mission's
magnetosphere and plasma science instruments, plus the spacecraft’s radio
science system, and its infrared and ultraviolet spectrometers will collect
data during the final plunge.

Chief among the observations being made as Cassini dives into Saturn are
those of the Ion and Neutral Mass Spectrometer (INMS). The instrument will
directly sample the composition and structure of the atmosphere, which
cannot be done from orbit. The spacecraft will be oriented so that INMS is
pointed in the direction of motion, to allow it the best possible access to
oncoming atmospheric gases.

For the next couple of days, as Saturn looms ever larger, Cassini expects
to take a last look around the Saturn system, snapping a few final images
of the planet, features in its rings, and the moons Enceladus and Titan.
The final set of views from Cassini's imaging cameras is scheduled to be
taken and transmitted to Earth on Thursday, Sept. 14. If all goes as
planned, images will be posted to the Cassini mission website beginning
around 11 p.m. EDT (8 p.m. PDT). The unprocessed images will be available
at: https://saturn.jpl.nasa.gov/galleries/raw-images

Live mission commentary and video from JPL Mission Control will air on NASA
Television from 7 to 8:30 a.m. EDT (4 a.m. to 5:30 a.m. PDT) on Sept. 15. A
post-mission news briefing from JPL is currently scheduled for 9:30 a.m.
EDT (6:30 a.m. PDT), also on NASA TV.

NASA TV is available online at: https://www.nasa.gov/live

A new NASA e-book, The Saturn System Through the Eyes of Cassini,
showcasing compelling images and key science discoveries from the mission,
is available for free download in multiple formats at:
https://www.nasa.gov/ebooks

An online toolkit of information and resources about Cassini's Grand Finale
and final plunge into Saturn is available at:
https://saturn.jpl.nasa.gov/grandfinale

Follow the Cassini spacecraft’s plunge on social media using #GrandFinale,
or visit:
https://twitter.com/CassiniSaturn
https://www.facebook.com/NASACassini

The Cassini-Huygens mission is a cooperative project of NASA, ESA (European
Space Agency) and the Italian Space Agency. JPL, 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
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