This document proposes a laser tether method for ultra-precise formation flight using photon thrusters and tethers. It would use the counterbalancing forces of tether tension and photon thrust produced via intracavity laser arrangements to maintain inter-spacecraft distances to the nanometer level over tens of kilometers. The system aims to enable propellant-free and contamination-free long-term precise formation flying for applications such as space telescopes and interferometry missions. It analyzes the technology readiness of components and outlines a development roadmap.

1. Laser Tether
A Contamination-Free Ultrahigh Precision
Formation Flight Method
Based on Intracavity Photon Thrusters and Tethers
2006 NIAC Fellow Meeting Presentation
Young K. Bae, Ph.D.
Bae Institute
Tustin, California, USA
www.baeinstitute.com
Collaborators: C. W. Larson, Ph.D., AFRLT. Presilla, Ph.D., Northrop GrummanC. Phipps, Ph.D., Photonic Associates
J. Carroll, Tether Applications, Inc.

2. Laser Tether
Precision Formation Flying
TPF
SI Mission
MAXIM
LISA
SPECS

3. Laser Tether
Prior Propellant-Free Formation Flying Concepts
Tether Concepts
•Spin-Stabilization
•Propulsive Conducting Tether
Electrodynamics Concepts
•Microwave Scattering Concept --M. R. LaPointe(NIAC)
•Coulomb Force Concept --L. B. King et al. (NIAC)
•Magnetic Dipole Interaction Concept --D. W. Miller (NIAC)
Present Concepts
Tether + Electrodynamics →Ultrahigh Precision (nano-m accuracy) Baseline Distance Maintenance

4. Laser Tether
Proposed Formation Flying (FF) Method
zForce Structure: Counter Balance of Two Forces:
Contracting Force: Tether Tension
Extending Force: Photon Thrust
-Intracavity Arrangement
-Thrust Multiplied by Tens of Thousand Times by
Bouncing of Photons between Spacecraft
zGeometrical Structure: Crystalline Structure
zInterspacecraftDistance Accuracy: better than nm
zMaximum Operation Range: Tens of km (Limited by Mirror Size)
zCan be Used for both Static and Dynamic Applications

5. Laser Tether
Advantages of the Proposed FF Method
zPropellantless
--System Mass Savings
--Contamination Free
--Long Operation Lifetime
zInherent Capability of Efficient Damping of Tether Vibration by Modulating Laser Thrust
zDual Usage of Photon Thruster Laser for
Interferometric Ranging System
--Simplified System Architecture and Control
--Low System Weight
zReadily Downscalableto Nano-and Pico-Satellites Usage

6. Laser Tether

7. Laser Tether
Satellite ISatellite IIPrecision Laser Power MeterHR MirrorHR MirrorLaser Gain MediaUltrahigh Precision CW Photon ThrustTetherTensionPiezo-TranslatorStepper MotorIntracavity Laser BeamTetherReelClampLensDiodePumpLaserPumpLaser BeamHR MirrorHR MirrorPartal MirroriPhotodetectorPartal MirroriPartal MirroriTether SystemPhoton ThrusterSystemInterferometricRanging SystemNano-Precision Formation Flying System Architecture

8. Laser Tether
Photon Thruster System: TRL 3Satellite ISatellite IIPrecision Laser Power MeterHR MirrorHR MirrorLaser Gain MediaUltrahigh Precision CW Photon ThrustIntracavity Laser BeamLensDiodePumpLaserPumpLaser Beam
zLaser System
--Diode Pumped Intracavity Laser
--Lifetime of Diodes
1 Year for Continuous Operation
zPump Diode Carousel Design –Tens of Years

9. Laser Tether
101001000100001000000.111010010001000010 W System Intracavity Photon Thrust as a Function of the Mirror Reflectance (R) Photon Thrust (μN) 11 - ROff-the-ShelfSuper MirrorPredicted Capability of the Proposed System

10. Laser Tether
Specific thrusts as functions of Ispof various conventional and photon thrusters. Isp (sec) 102103104105106107108 Specific Thrust (mN/W) 10-710-610-510-410-310-210-1100X 1,000X 10,000X 100X 20,000Electric ThrustersPhoton ThrustersIntracavity Multiplication Factors

11. Laser Tether
Intersatellite Distance (km) 0.010.1110100 Mirror Diamter (cm) 110Photon Thruster System: Mirror Diameter vs. Operation Distance

12. Laser Tether
Interferometric Ranging System: TRL 5Satellite ISatellite IIPrecision Laser Power MeterHR MirrorHR MirrorLaser Gain MediaUltrahigh Precision CW Photon ThrustIntracavity Laser BeamLensDiodePumpLaserPumpLaser BeamHR MirrorHR MirrorPartal MirroriPhotodetectorPartal MirroriPartal Mirrori
zDual Usage of Photon Thruster Laser for Interferometric Ranging System Source Laser
--System Architecture Simplification
--System Mass Reduction

13. Laser Tether
Heterodyne Interferometric Ranging System Integrated with PhotonThruster SystemSatellite ISatellite IIPrecision Laser Power MeterHR MirrorHR MirrorLaser Gain MediaUltrahigh Precision CW Photon ThrustIntracavity Laser BeamLensDiodePumpLaserPumpLaser BeamPartal MirroriBeam SplitterMeasurementDetectorReferenceDetectorAOMAOMRetroreflectorMirrorODL

14. Laser Tether
Tether System: TRL 5Satellite ISatellite IITetherClampTetherReelInchwormPiezo-TranslatorElectromechnical Damper
zCoarse Control: Reel System--mm Accuracy
zFine Control: Inchworm or Stepper Motor--μm Accuracy
zUltrafineControl: Piezo-Translator (off-the-shelf) --0.1 nm Accuracy

15. Laser Tether
Method of Tether Vibration Suppression•Longitudinal Tether Wave Damping•Tether Material Friction•Modulation of Photon Thruster Power•Major Tether Vibrations will Result from Reorientation of the Whole Formation Structure, and other Sudden Environmental Perturbations, such as Meteoroid Impacts.
•Transverse Tether Wave Damping
•Electromechanical Damper with Impedance MatchingDamping Applied
Electromechanical Damping
Simulation by Lorenziniet al.
For 1 km Baseline System

16. Laser Tether
Example of Formation Flying at L2TetherLaser1 km
Altitude: 1.5 x 106km
Satellite Mass: 100 kg
Cross-sectional Area per Spacecraft: 1m2
Base Line Distance: 1 km
Tether Material: Kevlar
Tether Diameter: 4 mm
(99.9 % survival at L2
for 5 years) Not to Scale

17. Laser Tether
Exemplary System Design
zMajor Perturbation Forces
Solar Pressure Force Per Pair: < 20 μN
Other Perturbations Including Gravitational Perturbations Per Pair: < 30 μN
Total Differential Force Per Pair : < 50 μN
zThe Tethers are extended with ~ 100 μN with Photon Thrust Per Pair
--0.16 μm Extension
zThe Change in Tether Length due to the Perturbation:
Countered with Length Adjustment with Piezo-Translator (sub nm Accuracy)
zLaser Requirements with Off-the-Shelf Components:
Power Requirement ~ 1 W with 0.99995 Mirrors
With 20 % Wall-Plug Efficiency: The Total Laser System Power ~ 5 W
Stability Requirement: ~10-3(Lab Laser Stability ~ 10-5)
Mirror Diameter: > 7 cm

18. Laser Tether
Application ExampleRequirements for New World Imager Freeway MissionBy Prof. W. Cash –2005 NIAC Fellow Meeting--Searching for Advanced Civilization in Exo-Planets•300 m resolution at 10 parsecs = 0.02 nano-arcseconds•500,000 km based line distance between Collectors•Huge collecting area –one square kilometer“Right now this is impossibly expensive, but not necessarily tomorrow,”by Prof. Cash 2005

19. Laser Tether
One-Year Later…“Km-Diameter Membrane Space Telescope
Based on Photon Thrusters and Tethers” James WebbSpace TelescopeMembrane Mirror (NIAC)Image Processing With Real-TimeHolographic AberrationCorrection (NIAC)

20. Laser Tether
Roadmap
zOptimized Photon Thruster Design and Development
zOverall System Integration including the Interferometric RangingSystem and Tether System
zOverall System Stability and Control including Tether
Vibration Related Issues
zDevelopment of Methods for Reorientation and Alignment of the Whole Formation Structure
zMission Specific Studies

21. Laser Tether
Technology Readiness Assessment Summary
zPhoton Thrusters: TRL 3
zInterferometric Ranging System: TRL 5
zTether System: TRL 5
zSystem Integration and Control: TRL 2
zR&D3:II -III (moderate -high) (Degree of Difficulty)
Requires to optimize photon thrust design based on the current laboratory system and system integration, and to develop control system.

22. Laser Tether
Phase I Study Accomplishment Summary
zTheoretically proved that the proposed FF method is capable of maintaining the interspacecraftdistance with accuracy of nm at the maximum baseline distance of tens of kms.
zSuccessfully developed the engineering architecture of unification of photon thruster system with interferometricranging system for simplified architecture control and system weight reduction.
zDeveloped the method of controlling tether vibrations using electromechanical dampers and photon thruster power modulation.
zOrbit specific mission applications have been identified and investigated.
zIdentified Phase II program topics and designed the Phase II experimental system.

23. Laser Tether
Phase II Proposed Work
zProof-of-Concept Demonstration of Photon Thruster
zConstruction of a Thrust Stand with nNAccuracy
zOverall System Stability and Control
zTether Vibration Dynamics
zEnvironment Perturbation
z3-D Simulation
zDesign of Prototype Interferometric Ranging System
zDesign of Prototype Tether System
zDetailed Study of Specific Applications
zIn-Depth Revisits of Existing Concepts --SPECS and MAXIM
zUltralargeMembrane Space Telescopes
zUltralargeSparse Aperture Space Telescopes
zOthers

24. Laser Tether
Phase II Proposed Work
Photon Thruster Development with Nano-NewtonAccuracy Test StandLaser Power MeterConcave HR MirrorHR MirrorLaser MediaIntracavity Laser BeamTorsionFiberCounterWeightVacuum ChamberInterference PatternLow Power LaserCorner CubeWindowsOptical FiberPhoto Detectorfor Fringe CountingPump Laser Diode

25. Laser Tether
Conclusions
zThe proposed system needs thorough study.
zIf successful, the proposed system will open new innovative (revolutionary) ways to implementing new and existing mission concepts.
zMission Specific Applications
zSimplifies the Architecture and Reduces the Weight in Distributed InterferometeryMissions --TPF, DARWIN, MAXIM, SPECS etc.
zUltralargeMembrane Space Telescopes --For New World Imager (300 m Resolution –Freeway Mission with km Mirror) and Earth Imaging/Monitoring/Surveillance (10 cm Resolution Monitoring at GEO with 200 m Mirror)
zUltralargeSparse Aperture Space Telescopes

26. Laser Tether
“I believe in intuitions and inspirations. I sometimes feel that I am right. I do not know that I am.” by Albert EinsteinThe Support by NIACand NASAfor this project is greatly appreciated.