LISA and LISA Pathfinder

1
LISA and LISA Pathfinder

• Robin StebbinsU.S. Project Scientist
• NASA Goddard Space Flight Center
• LIST Meeting
• 11 December 2004

2
The Seven Questions

• What is the importance of LISA Pathfinder to
  LISA?
• What is the importance of LTP to LISA?
• What is the importance of ST7 to LISA?
• How does ground testing support LISA?
• How might ST7 be descoped and combined with LTP?
• How would the LISA Project respond to loss of
  ST7?
• How would the LISA Project respond to loss of
  LPF/LTP?

3
Importance of LPF to LISA

• LISA technologies being flight-tested
• Complete disturbance reduction system
• Two-axis drag-free operation
• Some aspects of interferometry
• A complete test of a LISA-like disturbance
  reduction system in a flight environment
• How LISA-like are the equipment, the operational
  modes, the environment, etc.
• Fidelity of the test (level of integration)
• Validation of performance models that can be used
  to extrapolate about a factor of 10 to LISA
  performance
• Opportunity for NASA and ESA to work closely
  together on LISA technologies prior to the main
  LISA mission

4
Comparison of GRS Design Features
5
Comparison of GRS Design Features
6
Comparison of GRS Design Features
7
Comparison of Thruster Design Features
8
Comparison of Control Design Features
9
Comparison of Interferometer Design Features
10
Comparison of Requirements
11
Comparison of Requirements
12
Importance of LTP to LISA

• LTP reduces risk to LISA by
• Measuring the performance of a single-axis
  disturbance reduction system capable of meeting
  LISA requirements
• Validating the most LISA-like GRS design
• Measuring the performance of a 12
  degree-of-freedom (DOF) drag-free system that is
  similar, but not identical, to the control modes
  of LISA
• Measuring the performance of liquid metal-ion,
  and possibly cold-gas, thruster systems, suitable
  for use on LISA
• Demonstrating dual-axis drag-free operation using
  one ST7 GRS
• Demonstrating several aspects of LISA
  interferometry in flight.

13
Importance of LTP to LISA

• Flight validation of LISA-like design
• Most LISA-like design of GRS, thruster system,
  drag-free controller, spacecraft and mission
  design features to reduce disturbances
• Could demonstrate residual acceleration within a
  factor of 2 of LISA performance, thereby reducing
  the extrapolation burden
• Dual-axis drag-free operation
• Aspects of LISA interferometry
• Measuring free-falling proof mass displacement,
  tip and tilt
• Optical bench materials, components and assembly
  methods
• Heterodyne interferometrically and precision
  phase measurements
• GRS/interferometer interface
• Etc.
• Programmatic importance
• Prepares ESA for its anticipated responsibilities
  in LISA

14
Importance of ST7 to LISA

• Reducing Pathfinder risk
• Alternate design
• Independently developed
• Flight validation of alternate designs
• Explore design space
• Different error sensitivities
• Dual-axis drag-free operation
• Provides a single GRS
• Programmatic consequences
• Vehicle for NASA to develop technical capability
  and insight into critical LISA technology
• Prepares NASA for its anticipated
  responsibilities in LISA
• Without ST7, NASA will be reduced to a ground
  development program where ESA has a flight
  development program

15
Ground Testing

• Anticipated activities
• Testing a specially adapted GRS on a torsion
  pendulum in two DOFs, using exaggerated noise
  processes. More DOFs may be possible using
  compound torsion pendulums,
• Testing some control properties by servoing the
  housing to follow a proof mass hanging on a
  torsion pendulum,
• Testing some noise processes and validating some
  disturbance models (e.g., volume forces, patch
  fields, unanticipated forces) in special setups,
• Testing some subassemblies (e.g., caging and
  vacuum systems) separately
• Testing thrusters on torsion pendulums and/or
  horizontal/gate pendulums
• Environmental testing of subassemblies expected
  to be sensitive to vibration, thermal or
  radiation effects.
• Bigger extrapolation burden to LISA
• Two decades between ground and LISA
  sensitivities, and a decade in frequencies
  Improvements may be possible
• Test at subsystem level, model the system-level
  behavior
• Limited degrees of freedom
• 1 g limitations, no spacecraft environment, etc.

16
ST7 Descopes and Merger
17
Consequences of ST7 Descope

• Reducing Pathfinder risk
• Merge to mitigate risk
• Options 1 and 2 are the most attractive descopes
• Loss of dual-axis test
• Augmented ground-based development
• Planned technology development needs to adapt
• Design and build GRS suitable for LISA
• NASA ground-testing cannot completely compensate
  for flight-testing
• Programmatic consequences
• Descopes dont directly fulfill the Level 1
  requirements, or at least not with NASA equipment
• NASAs role in LISA would likely be diminished
  because the risk of its contributions would be
  higher.
• NASA could end up paying most of the money for
  LISA, but not participating fully in the central
  technologies, and consequently be disadvantaged
  in the science benefits of the mission.

18
Project Response to ST7 Loss

• Scenarios
• Early cancellation, to be considered
• Loss just before PDR, just accept the increased
  risk of LPF
• Pathfinder strategy
• The NASA side of the Project moves to mitigate
  risk, e.g., thrusters
• Provide other assistance as appropriate (e.g.,
  caging, proof mass)
• Ground-based technology development
• Same as in descopes, except augmented thruster
  development too.
• Higher level of integrated modeling
• Programmatic consequences
• Same as descopes

19
Project Response to LPF/LTP Loss

• Strategy
• Immediate 18-24 month slip of LISA launch date
• Dont relax science requirements, accept greater
  risk
• Anticipate that if performance falls short, it
  wont be worse than LPF and LISA would meet its
  minimum science requirements
• Verification plan based on ground-based
  development is needed.
• Ground-based technology development
• Need to have technology development base in
  place want early investment to avoid later
  marching-army costs
• Design and construct test facilities for GRS,
  thrusters, control system, auxilliary
  measurements torsion pendulum improvements
• Develop and validate integrated models for
  extrapolation
• Programmatic consequences
• Cant fully compensate for the loss
• Greater extrapolation burden

20
Summary

• LISA Pathfinder substantially reduces LISAs
  technical risk
• LPF will validate performance models to be used
  in extrapolation for LISAs verification.
• LTP and LPF have some common benefits (providing
  risk reduction), some unique benefits and some
  joint benefits
• Ground-based development has higher extrapolation
  burden.
• Need contingency plan for ground technology
  development in case of ST7 and/or LTP loss.
• Only one descope for ST7 is practical, and that
  one is unattractive.
• Completing ST-7 is a price that NASA must pay to
  achieve the scientific and technological parity
  with Europe that we have sought from the
  beginning of the NASA/ESA partnership to
  implement the LISA Mission.