Tour Re-planning and Upcoming
Observations
C. J. Hansen
25 July 2005
Tour Tweaks
• Tour changes for icy satellite science
• Tour changes for Titan atmosphere / safe mode avoidance
• Extended mission budget
Tour Tweaks for Icy Satellite Science
“Tethys Tweak”
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NAV / SOST realized that with the expenditure of some delta-v (7 – 8 m/sec)
we could change a ~30,000 km flyby of Tethys into a 1500 km flyby (Rev 15,
September 24)
This also brought the Rev 15 Mimas closest approach down from 118,000 km
to ~70,000 km
Although there was an impact to RADAR Saturn science, the Project Scientist
weighed in and recommended the change
– We also ended up with a different star for our ICYEXO observation
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A substantial amount of re-integration of the sequence took place, including
moving OTM-38
To save delta-v the targeted Hyperion flyby in Rev 15 was also affected:
periapse was lowered from 1000 km down to 510 km
Tour Tweaks for Icy Satellite Science (cont.)
Enceladus “Tweak”
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At the Rev 3 Enceladus flyby MAG detected draping of the magnetic field
lines around Enceladus – this magnetic field signature is consistent with the
presence of an atmosphere
At the Rev 4 Enceladus flyby the MAG team detected an odd distortion of the
field at closest approach
In order to understand the Rev 4 data and to determine whether Enceladus
potentially has an intrinsic or induced field the MAG team requested that the
Rev 11 Enceladus flyby altitude be lowered to 175 km
The NAV team was able to accommodate the MAG request for < 2m/sec and
saved our ICYEXO observation (star changed from alpha Cma to gamma Ori)
The EN011 flyby took place July 14 and the science results were well worth
the changes!
Titan’s Atmosphere vs. Tour
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At the Ta flyby INMS and AACS derived quite different atmospheric densities
UVIS collected stellar occ data on Tb and the Huygens HASI instrument
measured deceleration; both teams came up with different density results
The T5 flyby yielded yet another set of densities from AACS and INMS
– Within a single instrument different flyby geometries have given factors of 2
variation – difficult to sort out temporal and latitudinal and local time differences
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The bottom line is attitude control thruster authority and what is a safe altitude
for Titan flybys –> 950 km seems to be out of the question, but should we go
to 975, 1000, or 1025 km?
Comparison of Derived Densities
T7
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After deliberation at the TAMWG, the project decided to raise the T7 closest
approach flyby altitude
T7 was at 1025 km
– Chance of entering safe mode judged to be 20-30 %
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Altitude will be raised to 1075 km
– Flying by at southerly latitude (67 south)
– RADAR prime at closest approach (worst attitude for control authority)
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Lowers T6 altitude by ~123 km (was 3758 km)
A series of 950 km flybys commences with T16
Impact to Extended Mission
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The project has been using delta-v fairly liberally
It is worth considering what the impact to the extended mission may be
Bi-Propellant Budget
Upcoming Events
• Series of Saturn and ring occultations
– 9 occultations
– Start April 14, end Aug. 20
• Titan Flybys
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T6
T7
T8
T9
T10
August 22
Sept. 7
Oct. 28
Dec. 26
Jan. 15
UVIS HDAC experiment
UVIS Solar Occultation
Occultation Orbits
Upcoming opportunities to observe Saturn’s icy satellites:
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Mimas
Mimas
Tethys
Hyperion
Dione
Enceladus
Rhea
Aug. 2
Sept. 23
Sept. 24
Sept. 26
Oct. 11
Oct. 12
Nov. 26
62,711 km
69,944
1,500
510
500
49,126
500
Aftermarket Process
Aftermarket Process
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Many levels of changes
– Internal to team, e.g. data distribution
– Internal to working groups, e.g. pointing changes
– Science-wide, e.g. tour modifications
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Project “accounting” system hasn’t really been necessary
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Many tour modifications have actually come along after the aftermarket
process ended
TOST Categories of Changes
1.
No project visibility required
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Changes to distribution of bits internal to team’s data volume
TOST “assisted” (facilitation with aftermarket process)
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Prime-rider coordination (mosaic layout, dwell time)
Mutually agreed upon changes to prime instruments that don’t affect the
secondary axis, at times not at closest approach
TOST agreement required
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4.
Changes to overall observation goals
Changes to secondary axis
Mutually agreed upon swaps of prime instrument at closest approach (e.g. T3 T12)
PSG approval required
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Changes to allocation of closest approach time
TOST Adaptability Schedule
Flyby Previews
~2 weeks before
each flyby
Reports on T0, Ta, Tb,T3, T4 and T5 were covered in
flyby previews. Biweekly TOST meetings are
opportunities for minor tweaking of observation
designs.
Workshop #1
Oct 2005
T6 + 6 wks
Identify preliminary requests for changes to T13 to
T19.
Workshop #2
Feb 2006
T10 + 5 wks
Discuss discoveries, instrument performance and
review science priorities based on results from first 10
flybys. Select T20 option.
Workshop #3
June 2006
After T14
Identify requests to T21 to T38. Select T32 option.
Workshop #4
Feb 2007
After T24
Half-way point on prime mission (T22 is Dec. 06).
Detailed discussions on objectives of extended mission
Titan science.
Workshop #5
July 2007
After T34
Identify requests for changes to T39 to T44. Select
flyby type for T44
Negative Data Margin
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We have been dramatically under-utilizing SSR space
Within TOST horrific wailing occurred over 25 Mb, yet we are often >200 Mb under in
actuality
All science planning margin for TOST playback has been released (~70 Mb)
DSN changes for T7 were even greater: we (TOST) decided to allow –54 Mb
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Repercussion is that if SSR fills up we will lose data at the end just before the playback
Project was unwilling to allow negative SSR margin