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THEMIS A,B EFI Deploy Plan: An Overview of Risk Posture

THEMIS A,B EFI Deploy Plan: An Overview of Risk Posture. Science Requirements, and EFI need Placement and Deploy Plan Statement of Risk and Cases Risk Likelihood and Consequences The Path to Optimal Science. Science Objectives Note: Primary Science is sole Mission Design Driver.

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THEMIS A,B EFI Deploy Plan: An Overview of Risk Posture

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  1. THEMIS A,B • EFI Deploy Plan: • An Overview of • Risk Posture • Science Requirements, and EFI need • Placement and Deploy Plan • Statement of Risk and Cases • Risk Likelihood and Consequences • The Path to Optimal Science

  2. Science ObjectivesNote: Primary Science is sole Mission Design Driver • THEMIS HAS FOCUSED MINIMUM (TO BASELINE) OBJECTIVES: • Time History of Events… • Auroral breakup (on the ground) • Current Disruption [CD] (2 probes at ~10RE) • Reconnection [Rx] (2 probes at ~20-30RE) • … and Macroscale Interactions during >5 (>10) Substorms (Primary): • Current Disruption and Reconnection coupling • Outward motion (1600km/s) of rarefaction wave • Inward motion of flows (1000km/s) and Poynting flux. • Ionospheric coupling • Cross-tail current reduction (P5u/P4) vs flows • Field aligned current generation by flow vorticity, pressure gradients (dP/dz, dP/dx). • Cross-scale coupling to local modes • Field line resonances (10RE, 5 min) • Ballooning modes, KH waves (1RE, 1min) • Weibel instability, cross-field current instability, kinetic Alfven waves (0.1RE, 60Hz) • Production of storm time MeV electrons (Secondary) • Control of solar wind-magnetosphere coupling by the bow-shock, magnetosheath and magnetopause (Tertiary) EFI EFI

  3. EFI Science Top-Level RequirementsNote: Primary Science is sole Mission Design Driver • S-3. Rx Onset Time: The EFI instrument shall measure the DC electric field at the reconnection site at <10s resolution A pair of EFI Spin Plane Booms required on each: P1 and P2 • S-6 Flow vorticity: The EFI instrument shall deduce flows and flow vorticity at the current disruption region at <10s resolution using the ExB approximation, and • S-8 non-MHD: The EFI instrument shall be used to determine the MHD portion of the flow to compare against ion flow measurements at <10s resolution A pair of EFI Spin Plane Booms required on each P3, P4 and P5 • S-10 waves: The EFI instrument shall be used to determine the 3D nature and propagation of the cross-field current instabilities (1-60Hz) and high frequency waves (<600Hz) at the current disruption region Full EFI deployment on two out of three of P3, P4 and P5

  4. EFIs EFIa SCM ESA SST FGM Tspin=3s Instruments required: Redundancy and Overlap 1 FGM: Low freq. B-field (0-64Hz) 1 ESA: Thermal plasma2 SSTh (heads): Super-thermal plasma1 SCM: High freq. B-field (1Hz-4kHz) 4 EFIs (spin plane) & 2 EFIa (axials): Low&High freq. E-field

  5. Probe placementand EFI deployment plan • Currently probes are in string of pearls configuration • Probe assignment made on accounts of RF performance (TH-B = P1; TH-A=P5) • EFIs have been deployed on TH-C,D,E Decision made as best compromise between: Early science and placement operations • Placement Sept 7 – Dec 21: P1,2 to higher apogee, P3,4,5 to lower apogee P2,3,4 take less deltaV to place; side-thrusting not too onerous P1 and P5 (outer-most and inner-most) take dV ~ 250m/s to place With EFI booms UNDEPLOYED: 5 axial thrusts: a ~2 week maneuver sequence With EFI booms DEPLOYED: 30 sidethrusts: a ~2 month maneuver sequence 3% loss of margin (out of 21% total) • EFI deployment Dec 21 – Jan 15, a period of 2 weeks before tail season start • Why deploy EFI late on P1, P5? • Late EFI deploy reduces operational complexity during placement • Late EFI deploy conserves fuel margin (P1,5 have least fuel margin, ~21%) • Healthy P1,5 fuel margin reduces operations risk on both probes

  6. Risk and Consequences • Statement of Risk of EFI non-deploy: • EFI antennas have yet to be deployed on P1, P5 probes.Action: Quantify risk to the nominal mission science • Consequences to Mission Objectives: • If no more EFI SPBs deploy: • Minimum mission fully achievable • P3 or P4 becomes replacement to P1 • Side-thrusting up, ~2 months, before tail season • P5 (EFI-less probe) continues nominal mission plan • P1 stays at 30Re • Baseline mission partially compromised • Risk posture and mission profile same as per CSR proposal • Risk posture unaffected by transition to Phase-E • If only one pair of EFI SPBs deploys per probe: • Baseline mission is achieved 100% • No replacement necessary

  7. Risk Likelihood • Facts: • Six EFI SPB pairs deployed flawlessly 3-4 months after launch: • One motor out of 12 “out of family” (slower) but operated nominally. • Final lengths well within specifications on all (better than 1%) • First probe deployment (THC; P2) required two weeks of operations. • Second and third probes (THD, E; P3,4) required two weeks of operations total (parallel deploy operations). • Lab testing of long term motor performance reduces risk further: • No change in motor function after 18 months @ 60 C (starting or running current). • Likelihood of 2 out of 2 failing, after 6/6 deploys is very low: • 6 out of 6 wire boom pairs deployed on THEMIS successfully. • Over 100 wire booms of this type deployed on-orbit over past 30 years. • Design and workmanship of THEMIS EFI wire boom systems are by now proven.

  8. The path to optimal science • Nominal EFI deployment on P1,5 after probe placement allows: • Optimal science both now (dayside) and later (tail science in early 2008): • Late EFI deploy allows for a short placement (Sept-Dec), still in time for tail season: • A September start of placement allows dayside magnetopause science in an unprecedented five-probe string-of-pearls configuration • Mid-January start of tail season captures full night-side science • Optimal fuel utilization optimizes long term science: • Late EFI deploy conserves fuel margin (P1,5 have least fuel margin, ~21%) • Leftover fuel on P1 can be used to negate lunar torques for better 2nd year science Leftover fuel on P5 can be used for a more capable and longer extended phase

  9. Summary P3,4,5 EFI deployed nominally and provide excellent EFI dayside science now P1,5 deploy plan is both operationally and scientifically optimal The proposed 3 now - 2 later plan is the best compromise between optimal science and overall risk reduction for the mission Risk and risk mitigation strategy are same whether Phase E starts now or later Phase E start now enables best utilization of NASA’s assets in space

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