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Planetary News: Cassini-Huygens (2005)
Cassini Zooms in on Tethys and Hyperion
By Emily Lakdawalla
22 September 2005
Ithaca Chasma curls around an unnamed multi-ringed impact basin in this Cassini
view from December 15, 2004.
Credit: NASA/JPL/Space Science Institute |
Cassini is prepared for two close encounters in two
days. On Saturday, September 24 at 02:42 UTC, the spacecraft will zoom within
1,500 kilometers (930 miles) of the giant canyon of Ithaca Chasma on Tethys.
Cassini will follow the stunt with an even closer, 514-kilometer (319-mile)
flyby of Hyperion on Monday, September
26 at 02:25 UTC. Both flybys will be the best views that Cassini will get
of the moons during its planned tour of
the Saturn system, so the science plans are packed with every kind of
observation necessary to tease out the geologic histories of these worlds.
During the same days, Cassini will be acquiring a once-in-the-mission radiometry
data set on Saturn. And the spacecraft will pass through not one but two hazardous
dust crossings. And as the encounter begins it will get a close look at one
of the tinier Saturnian moons, Calypso.
It's a busy schedule, but it wasn't originally planned that way, as Cassini
mission planner Amanda Hendrix explains. "Tethys was targeted flyby that
we decided to add just back in March. We were going to do observations of
Tethys anyway, but they were going to be much more distant. One of the navigators,
Brent Buffington, was looking at opportunities and he found that at not a
very high delta-V cost at all, we could go in much closer to Tethys and get
a really good look at Ithaca Chasma. Also a result of the tweak we're also
closer to Hyperion than we were going to be."
Although it packs the schedule, the tweak brings the opportunity for important
science to be performed at Tethys. "We didn't have any targeted flybys
of Tethys planned in the tour at all," Hendrix says. "Now we have
this one. But it's not a normal targeted flyby where we dedicate plus or minus
12 hours to it, because it was added late -- we had observations already planned
of other targets. But we are dedicating a few hours to Tethys, and it will
be really great."
In fact, a condition of the "tweak" was that it not effect RADAR
observations that were planned for several hours surrounding Cassini's closest
approach to Saturn on this orbit, as Hendrix explained. "This is a unique
opportunity to attempt to map the deep ammonia distribution on Saturn," to
a depth in the atmosphere where the pressure reaches 3 times Earth's surface
atmospheric pressure. "The observation measures the ammonia abundance
below the ammonia cloud base, and is relevant to cosmochemistry -- the average
volatile abundances of Saturn -- and meteorology -- ammonia anomalies beneath
the visible features." Because these observations take priority, Cassini
cannot turn to Tethys until it is less than 3 hours away from its closest
approach to the moon.
A movie made from Cassini's views of Hyperion, Saturn's lumpy moon, during a
June 9-11, 2005 flyby shows its spongy, irregular surface. Hyperion is one
of the darker Saturnian moons; the floors of its many craters are filled
with a very dark material. Credit: NASA/JPL/Space Science Institute |
Hyperion is an oddball in the solar system because of its odd rotation state.
Cassini has been studying this aspect of Hyperion closely. "Evidently
the rotation period is chaotic, but not completely chaotic," Hendrix
says. "The rotation axis may change from time to time. So it might not
necessarily be 'tumbling' while we are looking at it, but it might be on a
different spin axis at this point in time compared to when we looked at it
last time." Of course, Cassini's tight schedule only permits 24 hours
of observations of Hyperion during the flyby. Observations over a longer time
period would greatly help understanding of the rotation state. "We are
trying to get a bunch of amateur astronomers to do ground-based observations
of Hyperion," Hendrix said. "Because if people could start looking
at Hyperion now, and look all the way through the flyby, that might really
help to understand its rotation, at least during this period of time."
Following is a look at the detailed science plans for the two flybys.
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Image coverage for Cassini's September 24, 2005 flyby of Tethys (15TE)
This map shows the regions that will be imaged by the cameras during the Tethys flyby on September 24, 2005 (15TE). Some of the best views will be across Ithaca Chasma. Created: 24 September 2005.
Credit: NASA/JPL/Space Science Institute
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| Spacecraft time (UTC) |
Time with
respect to
closest
approach to
Tethys |
Event |
Sep 23
11:15 |
-15h 27m |
Saturn Radiometry - Global map (RADAR prime)
Cassini will be in between Saturn and the Sun
in the ring plane and near its closest approach -- possibly the best
geometry throughout the mission for these observations. |
| 15:30 |
- 11h 12m |
Optical Remote Sensing of Calypso (VIMS prime)
VIMS, ISS, CIRS, and UVIS will all acquire Calypso data for several
hours. |
| 17:16 |
- 09h 26m |
Ascending Ring Plane Crossing
At a distance of 4.46 Saturn radii (269,000 kilometers, or 167,000
miles). |
| 17:28 |
- 09h 14m |
Dust hazard
Cassini will cross the torus of dust in Enceladus' orbit
and must turn to a protective attitude. On either side of this protective
maneuver, CDA will be busy sampling the dust. |
| 18:51 |
- 07h 51m |
Calypso 15CA Flyby Closest Approach
Altitude = 91,096 kilometers (56,600 miles). Unfortunately, due to
the dust hazard and the priority of the Saturn radiometry observations, Cassini cannot
look at Calypso at this time. |
| 18:51 |
- 07h 51m |
Saturn Radiometry - Global map (RADAR prime) |
| 21:36 |
- 05h 06m |
Orbit 15 Saturn periapsis |
Sep 24
00:00 |
- 02h 42m |
Optical Remote Sensing of Tethys (ISS prime)
As Cassini zooms toward Tethys, CIRS and ISS trade priority
and build up global maps while UVIS takes ride-along observations.
Only VIMS is not in use. |
| 01:10 |
- 01h 32m |
Optical Remote Sensing of Tethys (CIRS prime) |
| 01:40 |
- 01h 02m |
Optical Remote Sensing of Tethys (ISS prime) |
| 02:10 |
- 00h 32m |
Radio and Plasma Wave observations |
| 02:42 |
+ 00h 00m |
Cosmic Dust Analyzer observations |
| 02:42 |
+ 00h 00m |
Tethys 15TE Flyby Closest Approach
Altitude = 1,500 kilometers (930 miles), speed = 9.0 kilometers/second
(20,000 miles/hour). The closest approach occurs over Ithaca Chasma. |
| 03:30 |
+ 00h 47m |
Optical Remote Sensing of Tethys (CIRS prime) |
| 05:05 |
+ 02h 12m |
Dust hazard
Cassini will be crossing the torus of dust in Dione's orbit. |
| 06:00 |
+ 03h 17m |
Tethys Scatterometry (RADAR prime) |
| 07:31 |
+ 04h 48m |
Saturn Radiometry (RADAR prime) |
| 07:42 |
+ 04h 59m |
Descending Ring Plane Crossing
At a distance of 7.58 Saturn radii (457,000 kilometers, or 284,000
miles). |
Data playback from the Tethys encounter should begin Saturday, September
24 at 11:45 UTC (04:45 PDT), and end no later than Sunday, September
25 at 14:44 UTC (07:41 PDT). As soon as that playback has ended, Cassini will
get on with its flyby of Hyperion.
| Spacecraft time (UTC) |
Time with
respect to
closest
approach to
Hyperion |
Event |
Sep 25
14:25 |
- 11h 59m |
Optical Remote Sensing of Hyperion (ISS prime)
As Cassini zooms toward Hyperion, VIMS and ISS trade priority
and build up global maps while UVIS and CIRS take ride-along observations. |
| 15:11 |
- 11h 14m |
Optical Remote Sensing of Hyperion (VIMS prime) |
| 17:20 |
- 09h 05m |
Optical Remote Sensing of Hyperion (ISS prime) |
| 17:22 |
- 09h 03m |
Optical Remote Sensing of Hyperion (ISS prime) |
| 17:31 |
- 08h 54m |
Optical Remote Sensing of Hyperion (VIMS prime) |
| 18:00 |
- 08h 25m |
Radio Science
As Cassini approaches, it will broadcast a radio signal to
Earth. From careful analysis of the Doppler shift of the signal, radio
scientists will be able to determine Hyperion's mass. These analyses
can take months or even years. |
| 20:20 |
- 06h 05m |
Optical Remote Sensing of Hyperion (ISS prime) |
| 20:31 |
- 05h 54m |
Optical Remote Sensing of Hyperion (VIMS prime) |
| 21:21 |
- 05h 04m |
Optical Remote Sensing of Hyperion (ISS prime) |
| 21:23 |
- 05h 02m |
Optical Remote Sensing of Hyperion (ISS prime) |
| 21:32 |
- 04h 53m |
Optical Remote Sensing of Hyperion (VIMS prime) |
| 21:55 |
- 04h 30m |
Optical Remote Sensing of Hyperion (ISS prime) |
| 22:21 |
- 04h 04m |
Optical Remote Sensing of Hyperion (CIRS prime)
Now quite close to Hyperion, CIRS can acquire a detailed map. VIMS
now rides along as CIRS, ISS, and UVIS trade priority. |
| 23:21 |
- 03h 04m |
Optical Remote Sensing of Hyperion (ISS prime) |
| 23:41 |
- 02h 44m |
Optical Remote Sensing of Hyperion (UVIS prime) |
Sep 26
00:40 |
- 02h 04m |
Optical Remote Sensing of Hyperion (ISS prime) |
| 01:10 |
- 01h 15m |
Optical Remote Sensing of Hyperion (CIRS prime) |
| 01:30 |
- 00h 55m |
Optical Remote Sensing of Hyperion (ISS prime) |
| 01:55 |
- 00h 30m |
Radio and Plasma Wave observations of Hyperion through closest approach |
| 02:18 |
- 00h 06m |
Optical Remote Sensing of Hyperion (CIRS prime) |
| 02:25 |
+ 00h 00m |
Hyperion-1 (15HE) Flyby Closest Approach
Altitude = 514 kilometers (319 miles), speed = 5.6 kilometers/second
(12,500 miles/hour). |
| 02:55 |
+ 00h 29m |
Radio and Plasma Wave observations of Hyperion |
| 03:18 |
+ 00h 53m |
Optical Remote Sensing of Hyperion (UVIS prime) |
| 04:10 |
+ 01h 45m |
Radio Science
The Doppler tracking will resume on the outbound
leg of the flyby to help determine Hyperion's mass. |
| 05:55 |
+ 03h 30m |
Hyperion Scatterometry (RADAR prime) |
| 08:57 |
+ 06h 32m |
Optical Remote Sensing of Hyperion (VIMS prime) |
Playback from the Hyperion flyby will begin on Monday, September 26, at
12:10 UTC (05:10 PDT) and will finish no later than Tuesday, September
27, at 19:05 UTC (12:05 PDT).
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