Titan Flyby Altitude – Tour Updates Upcoming Observations in 2006 C. J. Hansen 6 January 2005
Background: Safe Titan Flyby Altitude The atmospheric density experienced by the s/c on the Ta flyby was higher than expected, leading to concerns about low altitude flybys possibly putting the s/c in safe mode The thrusters fire to overcome torque on the s/c from the atmosphere If the density is too high the thrusters cannot compensate and the s/c goes off point If the thrusters fire continuously for >7 sec the s/c enters safe mode The altitude of the T3, T5, and T7 flybys was raised by lowering the altitudes of the T4, T6, and T8 flybys Starting with T16 we won’t have that option because we have long series’ of 950 km flybys – the tour must be re-designed to fly at higher altitudes The AACS team has been working to define the density at which the s/c will lose control This density is dependent on the orientation of the s/c at closest approach, also on the type of activity (slewing) taking place TAMWG has been meeting to develop an atmospheric model that predicts density as a function of altitude, given the differing values measured to-date
Titan Flybys in 2006-2008 2006 2007 21 low altitude Titan flybys 2008
Safe Titan Flyby Density Low-altitude Titan flyby are all performed with thrusters. Thrusters are used: To overcome Titan atmospheric torque To perform target motion compensation (TMC) To slew the S/C about X, Y, Z-axis (or multi-axis) for science observation To maintain a set of commanded per-axis attitude controller dead-band The safe Titan flyby density is highly scenario dependent. It is a function of many variables: Thruster magnitude Spacecraft’s attitude time history Spacecraft’s projected area and c.p. location near TCA Spacecraft’s inertia properties (including c.m.) RCS controller dead-band Per-axis slew rates near TCA Titan-relative S/C flyby velocity AACS team used a test bed named Flight Software Development System (FSDS) to determine the tumbling densities of all low-altitude flybys
Reference Trajectory Modifications The project started development of the new tour in December Changes won’t be huge – petal rotation, icy satellite flybys, occultations will still happen, but times may move, sometimes considerably The AACS team has used the current TOST plan to define a safe density for each flyby based on the s/c attitude If the tour is designed at precisely those altitudes / attitudes we lose some amount of flexibility – any attitude which is not safe at that altitude will not be possible Discussed at length at TOST November workshop TOST made recommendations to Project Altitude decisions made at Project meeting November 30 Used INMS density multiplied by AACS-driven fudge factor, with cosine latitude dependence General policy was to be conservative on early inbound Titan flybys, take more risk on later outbound flybys (but thruster torque dropping so these still turn out to be relatively high
TOST Input to Process The correlation of s/c attitude with safe flyby altitude is a concern to TOST planning A workshop was held 18 Nov. to discuss the impact on TOST plans and flexibility of fixing an altitude and thereby fixing the s/c attitude One particular outlier was T20 option a Attitude of the spacecraft at closest approach is predominantly either –Z to Titan, -X to ram (RADAR + INMS) or –Y to Titan, -X to ram (ORS + INMS) Comparison of T25 and T26 results (see Frautnick presentation) shows that –Y to Titan, -X to ram differs from –Z to Titan, -X to ram by only 5 km T25 and T26 best for comparison because at ~ same latitude, timeframe
TOST “Flexibility” Recommendation Preserve the capability to switch between ORS + INMS and RADAR + INMS 5 km difference in altitude TOST recommends that this be subsumed in overall project margin, not added on top Preserve the capability to fly either the a or b options of T20 and T32 Choose the higher altitude, or Subsume difference in project margin For some flybys science gain is worth additional risk MAG, INMS would like to go low to get data deeper in atmosphere (INMS) and search for an internal field (MAG) On many flybys however outbound science is too important to risk
Flyby recommendation summary
Implementation: Tour Modifications Tour redesign is underway New tour plus limited case studies are out for review Team review due January 5 Comments to TWT and OST’s, TWT and OST feedback due January 11 UVIS is more impacted than any other team because so much of our science comes from occultations Timing shifts necessitate re-integration of science sequences already on the shelf Actual occultation geometry may change Initial response from UVIS was to spend delta-v to maintain timing of old tour Based on initial NAV analysis of a cost of 35 m/sec That turned out to be incorrect – cost is > 100 m/sec New strategy is to emphasize importance of occ’s to the project, try to get commitment to do re-integration
Implementation schedule Feedback on the new reference trajectory will be reviewed at the PSG New (final) tour developed February 3 – 13 Modifications begin with T17 (S23), September ’06 Sequence re-integration will be handled in aftermarket process
Upcoming Observations in 2006
Petal Rotation
Rev 20 – 25 Summary Titan Flybys T10 15 January UVIS solar occultation T11 27 February RSS gravity science T12 18 March RSS earth occultation T13 30 April RADAR T14 20 May RSS earth occultation T15 2 July MAPS – unique local time T16 22 July RADAR, UVIS stellar occultation Tour phase: petal rotation to magnetotail In equatorial plane until July Only one relatively close icy satellite flyby (no targeted flybys) Rhea 21 March 82,000 km
Backup Charts
Safe Density (Phase A) [But without the 5% DKDK margin] Safe = 0.932Tumble
Safe Density (Phase B) [1] [But without the 5% DKDK margin] Safe = 0.932Tumble
Safe Density (Phase B) [2] [But without the 5% DKDK margin] Safe = 0.932Tumble