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Planetary News: Cassini-Huygens (2008)
Cassini to Shoot Past Enceladus for Its Fifth Close Encounter
By Emily Lakdawalla
August 8, 2008
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Fountains of Enceladus
On November 27, 2005, Cassini captured a series of images of Enceladus from its night side. The back-lit view lights up fine particles streaming from Enceladus' south-polar geysers.
Credit: NASA / JPL / Space Science Institute
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Cassini is barreling toward Saturn's active geyser moon Enceladus for the
fifth time. The spacecraft's course will take it close over the south
pole, the source of those geysers, deeper into the plume than it has ever
traveled before. Unlike the previous
close encounter, which was designed
to study the composition of the plumes, the main science goal of the August
11, 2008 flyby will be to acquire the highest-resolution-ever images of the
south polar vents, along with detailed maps of the composition and temperature
of the geologically active region.
Even before Cassini arrived at Saturn, Enceladus had
been singled out from Saturn's other mid-sized moons for special attention because
of Earth-based and Voyager data indicating
it had a relatively youthful surface, and because of its location at the densest
part of the E ring. Data
returned by Cassini from four close encounters in the primary mission has
proven Enceladus to be one of the few bodies in the solar system at which humans
have observed active geology (the others being Io, Triton,
and, of course, Earth).
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Saturn's Moons
How tiny Enceladus (bottom row, left) can be so active when most of Saturn's
moons, which are much larger, apparently ceased geologic activity long ago,
is one of the many mysteries being investigated by the Cassini mission.
These
photos of Saturn's eight largest moons were taken at different times
by Cassini. The moons are: top row: Dione, Mimas, Titan;
middle row: Tethys, Rhea; bottom row: Enceladus, Hyperion, Iapetus. Credit:
NASA / JPL / SSI
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We now know Enceladus
is active, with a youthful surface including four "tiger
stripes" (formally known as "sulci") crossing its south pole,
which are thermal
hot spots spewing active
plumes of water vapor, water particles, and other gases. But we
don't know why or how such a relatively tiny moon can presently have such
active geology. We don't know what
is driving the geysers, we don't know if there
is near-surface liquid water or not,
and we aren't sure how the composition of the plumes tells us about the
composition and history of the interior of Enceladus. So when
the Cassini team laid out the two-year
Equinox Mission, they planned seven
more close Enceladus flybys. This is the first.
Skeet Shooting Enceladus
Performing the science observations will be a challenge for Cassini. At
the moment of closest approach, Enceladus will flash by Cassini at a relative
speed of nearly 18 kilometers per second (40,000 miles per hour), too fast
to point its main cameras, the Imaging Science System (ISS), at specific locations
on the ground. The Cassini imaging team described the imaging efforts
in an online
article this way: "The challenge is equivalent to trying to capture
a sharp, unsmeared picture of a roadside billboard about a mile [about 1.5
kilometers] away with a 2,000-millimeter telephoto lens held out the window
of a car moving at 50 miles [80 kilometers] per hour."
Instead of pointing and shooting, Cassini has been commanded to perform an
unusual and difficult set of observations, spinning at its maximum angular
rate to partially compensate for Enceladus' fast motion while shuttering its
camera at specific times designed to catch seven points of interest near Enceladus'
south pole as the moon rushes through the camera field of view. Exposure
times for the each of the planned images have been carefully set to balance
the requirements of imaging at low light levels and attempting to avoid motion
smear.
If successful, the maneuver -- which is being referred to as "skeet
shooting" by the Cassini science team -- will result in images of the
south polar terrain with resolutions as high as eight meters per pixel. Many
of the images are designed to look straight into the canyons of Enceladus'
tiger stripes (formally known as sulci). Others are targeted specifically
at unusually hot spots measured by Cassini's Composite Infrared Spectrometer
(CIRS) during the previous flyby in March.
Skeet shooting Enceladus
On August 11, 2008, Cassini flies close past Enceladus' south pole at very high speed. The speed is too high for Cassini to track features on the surface. Instead, the spacecraft will rotate at its maximum rate, shuttering the camera at seven carefully selected times to capture high-resolution images of Enceladus' south polar sulci, or tiger stripes, including over some spots seen by the CIRS instrument to be "hot spots" and the sources of vents.
Credit: NASA / JPL / SSI
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Science Plans
Cassini's science
instruments are of three main types: optical remote sensing
(including cameras and spectrometers, which study electromagnetic radiation
reflected or emitted from distant objects); fields and particles (which examine
magnetic fields, molecules, ions, and particles in situ, in the space
through which Cassini flies) and radio. All four of the optical remote sensing
instruments share a common boresight, so when one instrument controls the pointing
of the spacecraft, the other three usually also take data. The instrument that
is controlling the pointing of Cassini at any given time is "prime," while
the others "ride along."
Enceladus' south polar vents are hot
Heat radiating from the entire length of 150 kilometer (95 mile)-long
fractures is seen in this March 12, 2008 heat map of the active south
polar region of Saturn's ice moon Enceladus. The warmest parts of the
fractures tend to lie on locations of the plume jets identified in
earlier images, shown with yellow stars. The measurements were obtained
by the Cassini spacecraft's Composite Infrared Spectrometer. Remarkably
high temperatures, at least 180 Kelvin (minus 135 degrees Fahrenheit)
were registered along the brightest fracture, named Damascus sulcus,
in the lower left portion of the image. For comparison, surface temperatures
elsewhere in the south polar region of Enceladus are below 72 Kelvin
(minus 330 degrees Fahrenheit). Credit: NASA / JPL / GSFC / SwRI /
ISS |
In practice, the main cameras (the ISS instrument) are not often "prime" because
their exposure times are much shorter than for the other instruments, VIMS,
UVIS, and CIRS. Observations designed only for ISS would not point long enough
at a target for VIMS, UVIS, and CIRS to take good-quality data. So those instruments
are more often given "prime" status, but work cooperatively with
the ISS team to make sure that ISS gets the brief opportunities it needs to
take important images. However, for this flyby, Cassini is moving so fast at
closest approach that only ISS can take data quickly enough, so, for once,
ISS gets prime status during the most active part of the flyby.
Shortly after the "skeet shoot" has ended, Enceladus
will pass into Saturn's shadow, removing the main source of light on its surface. This
is a perfect opportunity for CIRS to study the temperatures of those south
polar sulci, generating high-resolution maps of the locations of hot spots
that can be compared to the March
data to see if the hot spots have shifted
with time.
Cassini will begin downlinking data from this flyby on Tuesday, August 12
at 02:06 UTC. Following is a detailed timeline of science plans for
the encounter. It is a summary of information presented in the article "Skeet
Shooting Enceladus" on the Cassini imaging team's website, written by
Anne Verbiscer.
Date/
Time
(UTC) |
Event |
Aug 11
06:40:19 |
Inbound low-resolution plume observations
If visible, the plumes should appear off the
nightside limb of Enceladus. |
| 12:06:19 |
Turn to point high-gain antenna at Earth for gravity studies
The turn will take 24 minutes. For 3.5 hours, Cassini
will broadcast a radio signal that will be tracked by radio antennas
at Earth. Subtle shifts in the frequency of the signal caused by changes
in Enceladus' gravity field will hint at the presence of "mass
anomalies" --
places of unusually high or low density -- below Enceladus' surface.
At the same time, Cassini will empty its solid-state recorders, getting
ready to fill them with Enceladus data. |
| 16:06:19 |
Optical remote sensing observations (VIMS prime; others ride along)
Starting at a distance of 280,000 kilometers above
the northern hemisphere, VIMS will study the spectral properties of
Enceladus' surface. Dwell times are long becuase they are focusing
on a part of the spectrum from 2 to 5 microns where water is incredibly
dark, in the hopes of seeing subtle features due to other molecules
besides water. At the end of the observation, Enceladus will almost
fill the ISS narrow-angle camera filed of view, so the camera will
acquire 18 images through various color and polarizing filters. |
| 19:36:19 |
Optical remote sensing observation (UVIS prime; others ride along)
For 50 minutes, UVIS will scan across Enceladus,
including in the sky above its sunlit limb, searching for the signature
of oxygen (and, therefore, the presence of an atmosphere). When the
UVIS field of view crosses Enceladus, ISS will capture six images in
color filters at about 325 meters per pixel. |
| 20:07:00 |
Cosmic Dust Analyzer (CDA) begins riding along with optical observations
Although the pointing of the spacecraft will not
be ideal for CDA, it will gather data throughout the period of closest
approach, scooping up particles in the space around Enceladus, particularly
above the south pole and its vents. |
| 20:26:19 |
Optical remote sensing observations (ISS prime)
ISS will shoot two 1-by-2 color mosaics of Enceladus'
northern cratered plains, with resolution as high as 250 meters per
pixel. |
| 20:34:19 |
Turn to set up "skeet shoot"
Cassini will turn to point at a spot ahead of
Enceladus in order to prepare for the imaging sequence that follows
closest approach. The turn will take 27 minutes to complete. |
| 20:36:25 |
Cassini Plasma Spectrometer (CAPS) begins riding along
CAPS will study the interaction between Enceladus
and Saturn's magnetosphere. |
| 21:06:19 |
Closest Approach |
| 21:07:19 |
Start "skeet shoot" imaging
The relative speed of Cassini and Enceladus is
too high for Cassini to be able to track points on the surface. Instead,
Cassini begins by pointing ahead of Enceladus in its orbit. Then
the spacecraft begins rotating at its highest speed in the same direction
that Enceladus is moving. Enceladus' motion is faster than Cassini's
highest rotation speed, so the moon overtakes Cassini and hoves into
the camera field of view. Enceladus will track across the camera
field of view in such a way that three of the sulci will sweep by.
ISS will shutter at times when the sulci, in particular hot spots
on the sulci, are predicted to be passing across the camera field
of view.
Before the "skeet shoot" starts, both
the wide-angle and narrow-angle cameras will shutter simultaneously
on the night side of the terminator. Then a
wide-angle camera image will capture the first sunlit pixels at
a resolution of 48 meters per pixel. The following narrow-angle
camera image will be the highest-resolution one to date, at 8 meters
per pixel, adjacent to Cairo sulcus. Resolutions of subsequent
images, all from the narrow-angle camera, get lower, as Cassini
is past closest approach and fast receding form Enceladus. The
next two images will be of Cairo sulcus and a hot spot. The next
image will be between Cairo and Baghdad sulci; the next, atop Baghdad
and another known hot spot. The next lies in between Baghdad and
Damascus sulci. The next will be atop Damascus and more hot spots.
For the last image, the spacecraft makes a very slight turn off-track
to bring the CIRS instrument in line to measure the temperature
on the floor of Damascus sulcus, as ISS snaps an image at 28 meters
per pixel. All of this action unfolds over a period of less than
four minutes.
Skeet shooting Enceladus
On August 11, 2008, Cassini flies close past Enceladus' south pole at very high speed. The speed is too high for Cassini to track features on the surface. Instead, the spacecraft will rotate at its maximum rate, shuttering the camera at seven carefully selected times to capture high-resolution images of Enceladus' south polar sulci, or tiger stripes, including over some spots seen by the CIRS instrument to be "hot spots" and the sources of vents.
Credit: NASA / JPL / SSI
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| 21:10:09 |
End "skeet shoot," resume tracking Enceladus (ISS prime; UVIS, VIMS
ride along)
ISS will capture an eight-tile color mosaic of
the south polar region. Dwell times are long in order that VIMS can
also gather compositional data, searching for ammonia, carbon dioxide,
and hydrogen compounds, and information about the grain size of the
ice. VIMS will also be searching for hot spots (although VIMS is only
sensitive to spots at temperatures above 150 Kelvin).
Cassini ISS plans for images after August 11, 2008 Enceladus close approach
Following closest approach on the August 11, 2008 flyby of Enceladus, Cassini will capture a high-resolution, color mosaic of the south pole at 8 positions.
Credit: NASA / JPL / SSI
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| 21:34:49 |
Optical remote sensing observations (CIRS prime; others ride along)
Seven minutes after the start of this 3.5-hour
observation, Enceladus will pass into Saturn's shadow. The lack of
sunlight will reduce the quality of observations by ISS, UVIS, and
VIMS (though there is enough light reflected around the Saturn system
by rings and, this time, Titan, to faintly illuminate the surface,
even in eclipse). But CIRS sees infrared radiation emitted by all surfaces
above 60 Kelvin regardless of whether or not they are sunlit. CIRS
will obtain full-disk maps using its medium and longest-wavelength
subinstruments, mapping surface temperatures and searching for hot
spots. As Enceladus comes out of eclipse, CIRS will stare to see how
its surface temperature rebounds, as ISS, UVIS, and VIMS ride along. |
| 21:41:26 |
Enceladus passes into eclipse |
21:48
21:56 |
Cassini passes through "Voyager-class" encounter distances of Mimas
and Daphnis without observing
During the Enceladus flyby, neighboring moons Mimas and Daphnis will also be in the area, as close as about 81,000 kilometers
from Mimas and 110,000 kilometers from Daphnis. Cassini sometimes takes
advantage of such "nontargeted" encounters to perform opportunistic
observations. But it will not interrupt these high-priority Enceladus
science plans for distant images of other moons. This is the kind of
tradeoff Cassini frequently has to make in the crowded Saturn system. |
Aug 12
00:07:34 |
Eclipse ends |
| 01:06:19 |
Optical remote sensing observations (VIMS prime; others
ride along)
More compositional mapping and full-disk imaging. |
| 02:06:19 |
Turn to Earth to broadcast data and perform gravity observations
Cassini's high-gain antenna will perform double
duty, beginning to relay the data from the flyby at the same time as
radio telescopes perform Doppler trackin og its carrier signal for
the purpose of mapping Enceladus' gravity field. |
| 12:42:19 |
Turn back to Enceladus |
| 13:06:19 |
Optical remote sensing observations (UVIS prime; others ride along)
More compositional mapping and full-disk imaging
ensue. |
Future Flybys
Enceladus in eclipse
This view of the south pole of Enceladus was captured by Cassini
as it sped away from its March 2008 encounter with the moon. At
the time, Enceladus was in Saturn's shadow, so it received no direct
illumination from the Sun. All the illumination here is by light
reflected from various other objects. The main source of sunlight
is Saturn and its rings (including light reflected from the rings
onto Saturn and then onto Enceladus). More faint light sources
are Rhea, faintly illuminating the left side, and Tethys and Dione,
faintly illuminating the right side. Because of the long exposure
necessary to take this image, many stars are visible in the background.
Credit: NASA / JPL / SSI |
There will be two more close flybys this year, on October 9 and October
31. The next will not be until after the equinox, on November 2,
2009. Once the Saturn system passes through its equinox, the south
poles of Saturn and Enceladus and all
the other bodies that orbit in Saturn's ring plane will pass into winter
darkness for almost 15 years, which is, in all likelihood, well after the Cassini
mission will have ended. So
the three flybys of 2008 represent Cassini's last opportunity for sunlit
images of the vents.
However, all those other moons and especially
the rings bounce a great deal of sunlight around the Saturn system, illuminating
nighttime surfaces. Cassini's cameras are more than sensitive enough
to take photographs of such a bright, snowy moon as Enceladus by the reflected
light of the rings. In fact, this encounter will include camera images
taken while Enceladus is in eclipse, with its surface lit by sunlight bounced
off of the rings and Titan.
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Cassini's close flybys of Enceladus
During the prime and extended missions, Cassini will have a total of eight very close flybys of Enceladus. This graph compares their geometry. The Rev 4, 120, 130, and 131 encounters happen when Cassini is on a near-equatorial orbit about Saturn; the other encounters happen with Cassini on more inclined orbits.
Rev 3: March 9, 2005, 1,264 km (not pictured)
Rev 4: March 9, 2005, 500 km
Rev 11: July 14, 2005, 168 km
Rev 61: March 12, 2008, 52 km
Rev 80: August 11, 2008, 54 km
Rev 88: October 9, 2008, 25 km
Rev 120: November 2, 2009, 103 km
Rev 121: November 21, 2009, 1,607 km (not pictured)
Rev 130: April 28, 2010, 103 km
Rev. 131: May 18, 2010, 201 km
Credit: NASA / JPL / SSI / John Spencer
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