Reconstructing Cassini’s Plunge into Saturn

As NASA’s
Cassini spacecraft made its
fateful
dive
into the upper
atmosphere of Saturn on Sept.
15, the spacecraft was live-streaming data from
eight of its
science instruments, along with readings from a variety of
engineering systems. While analysis of science data from the
final plunge will
take some time, Cassini engineers already
have a pretty clear understanding of
how the spacecraft itself
behaved as it went in. The data are useful for
evaluating
models of Saturn’s atmosphere the team used to predict the
spacecraft’s behavior at mission’s end, and they help provide a
baseline for
planning future missions to Saturn.

Chief among
these engineering data, or telemetry, are
measurements indicating the
performance of the spacecraft’s
small attitude-control thrusters. Each
thruster was capable of
producing a force of half a newton, which is roughly
equivalent
to the weight of a tennis ball on Earth.

During the final moments of its plunge, Cassini was
traveling
through Saturn’s atmosphere, which was about the same density
as the
tenuous gas where the International Space Station
orbits above Earth. In other
words, there’s barely any air
there at all. Despite the fact that this air
pressure is close
to being a vacuum, Cassini was traveling about 4.5 times
faster
than the space station. The higher velocity greatly
multiplied the force, or
dynamic pressure, that the thin
atmosphere exerted on Cassini. It’s like the
difference
between holding your hand outside the window of a car moving at
15
mph versus one moving at 65 mph.

Data show
that as Cassini began its final approach, in the
hour before atmospheric entry
it was subtly rocking back and
forth by fractions of a degree, gently pulsing
its thrusters
every few minutes to keep its antenna pointed at Earth. The
only
perturbing force at that time was a slight tug from
Saturn’s gravity that tried
to rotate the spacecraft.

“To
keep the antenna pointed at Earth, we used what’s called
‘bang-bang
control,'” said Julie Webster, Cassini’s spacecraft
operations chief at
NASA’s Jet Propulsion Laboratory,
Pasadena, California. “We give the
spacecraft a narrow range
over which it can rotate, and when it bangs up
against that
limit in one direction, it fires a thruster to tip back the
other
way.” (This range was indeed small: just two
milliradians, which equals
0.1 degree. The reconstructed data
show Cassini was subtly correcting its
orientation in this way
until about three minutes before loss of signal.)

At this
point, about 1,200 miles (1,900 kilometers) above the
cloud tops, the
spacecraft began to encounter Saturn’s
atmosphere. Cassini approached Saturn
with its 36-foot-long
(11-meter) magnetometer boom pointing out from the
spacecraft’s side. The tenuous gas began to push against the
boom like a lever,
forcing it to rotate slightly toward the
aft (or backward) direction. In
response, the thrusters fired
corrective gas jets to stop the boom from
rotating any
farther. Over the next couple of minutes, as engineers had
predicted, the thrusters began firing longer, more frequent
pulses. The battle
with Saturn had begun.

With its
thrusters firing almost continuously, the spacecraft
held its own for 91
seconds against Saturn’s atmosphere — the
thrusters reaching 100 percent of
their capacity during the
last 20 seconds or so before the signal was lost. The
final
eight seconds of data show that Cassini started to slowly tip
over backward.
As this happened, the antenna’s narrowly
focused radio signal began to point
away from Earth, and 83
minutes later (the travel time for a signal from
Saturn),
Cassini’s voice disappeared from monitors in JPL mission
control.
First, the actual telemetry data disappeared, leaving
only a radio carrier
signal. Then, 24 seconds after the loss
of telemetry, silence.

These data explain
why those watching the signal — appearing
as a tall green spike on a squiggly
plot of Cassini’s radio
frequency — in mission control and live on NASA TV — saw
what appeared to be a short reprieve, almost as though the
spacecraft was
making a brief comeback. The spike of the
signal first began to diminish over a
few seconds, but then
rose briefly again before disappearing with finality.

“No,
it wasn’t a comeback. Just a side lobe of the radio
antenna beam pattern,”
Webster said. Essentially, the reprieve
was an unfocused part of the otherwise
narrow radio signal
that rotated into view as the spacecraft began to slowly
tip
over.

“Given that Cassini wasn’t designed to fly into a
planetary
atmosphere, it’s remarkable that the spacecraft held on as long
as it
did, allowing its science instruments to send back data
to the last second,”
said Earl Maize, Cassini project manager
at JPL. “It was a solidly built craft,
and it did everything
we asked of it.”


This animation shows the last 30 seconds of Cassini's X- and S-band radio signals as they disappeared from mission control on Sept. 15, 2017.

This animation shows the last 30 seconds of Cassini’s X- and
S-band radio signals as they disappeared from mission control
on Sept. 15, 2017. The video has been sped up by a factor of
two. Credit: NASA/JPL-Caltech
Click to play movie

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.dyches@jpl.nasa.gov

2017-266

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