Active Shielding Technology for Long-Duration Spaceflight

1. Deployed Magnetic Shielding forLong-Duration Spaceflight Darin Knaus, Ph.D. Creare Inc., Hanover, NH dak@creare.com

2. Creare Overview • Founded 1961 by a group of Dartmouth researchers • Owned by a partnership of engineers • ~100 employees • Multiple spin-off companies • Primary business areas • Cryogenics • Biomedical • Fluid Dynamics, Heat Transfer • Advanced Manufacturing • Sensors and Controls • Software and Data Systems

3. Spaceflight Research at Creare • Bioastronautics • Urinary calcium monitoring • ISS hearing assessment • DCS risk for EVA • Cryocoolers • Power generation • Radiation Shielding

4. DISCLAIMER • This is a controversial topic • Human have never traveled beyond LEO for long durations • All shielding concepts are merely concepts • The validity of any approach depends on: • Mission profile • Spacecraft architecture • Operational constraints • Acceptable risk • NASA’s mission is moving target • For a recent discussions see • Eugene Parker, “Shielding Space”, Scientific American, March 2006 • Jay Buckey, “Next Stop Mars”, The Scientist, 19(6):20, March 2005

5. Roadmap • The radiation environment in space presents significant risks that must be addresses in order to enable long duration space travel • Current technology cannot solve the radiation problem for some missions • Active shielding is a radical approach that could potentially solve the radiation problem • Radiation environment on Earth and in space • Radiation risk • Current technology: passive shielding • Active shielding approaches

6. Radiation Dose Units • The SI unit for absorbed dose is the Gray (Gy) • Purely physical unit describing energy deposition to matter • 1 Gy = 100 rad • SI unit for dose equivalent is the Sievert (Sv) • Weighted unit used to estimate tissue damage • 1 Sv = 100 rem • Dose Equivalent (Sv) = Q*N*Absorbed Dose (Gy) • Quality factor: Q • Reflects the relative damage caused by different particles • Electron Q = 1, Alpha particle Q = 20 • Tissue factor: N • Reflects relative sensitivity of different tissues to radiation • Most sensitive tissues reproductive, colon, bone marrow

7. Radiation on Earth • We enjoy shielding from the Earth’s atmosphere and magnetic field • Particles originating in space muons, neutrons and electrons (low Q)

8. Radiation Environment in Space • Radiation Belts • Only a concern in orbit • Solution: mission planning • Solar Particles • High flux, high energy protons • Passive shielding effective • Solution: early warning + shelter • Galactic Cosmic Radiation • Low flux, high energy nuclei • HZE GCR most damaging • Passive shielding ineffective • Solution: ???

9. Radiation Risk for Spaceflight • Dose limits for space travel are based on risk • Primary risk considered is carcinogenesis • 3% increased lifetime cancer mortality typically used • Radiation limits for LEO, Male (35) • 500 mSv/year, 1,000 mSv/career • Most experience is for LEO • Earth’s magnetic field shields GCR • HZE GCR present unique risks • Difficult to establish Q • Non-cancerous risks • Non-repairable cells: CNS

10. Passive Shielding • Passive shield thickness typically expressed in units g/cm2 • Low molecular-weight materials more effective for GCR than high • Typical hull thickness 1 cm Al (2.7 g/cm2) • Significant shielding unsatisfactory for typical Mars mission profiles • Cucinotta et al., “Managing Lunar and Mars Mission Radiation Risks”, NASA/TP-2005-213164 • Mars surface mission • Aluminum shielding • Spallation Radiation Limits GCR Performance • Hydrogen shielding reduces dose by ~50%

11. Passive Shield Analysis: CEV • Current CEV plan is for 5 m diameter capsule, based on Apollo capsule • At 20 g/cm2, shield mass approaches shuttle launch capability (~30,000 kg) • Larger spacecraft will be required for Martian missions • 20 g/cm2 is not adequate • Increasing thickness has limited effect on GCR dose

12. Active Shielding • Deflect particles using electric or magnetic fields • Plasma and electrostatic approaches require large voltages (order kV) that make them generally impractical • Superconducting technology enables magnetic approaches • Two magnetic strategies: • Confined • Deployed • Superconducting technology

13. Confined Magnetic Shield Analysis • Field confined via torus, concentric spheres, cancellation • Equation of motion for particle in magnetic field • F = q (v x B) • Larmor Radius • rL (m) = 3.33 R (GV) / B (T) • Rigidity = Momentum / Charge • Consider HZE GCR particle • 1 GeV/nucleon Fe nucleus • R = 3.65 GV • rL = 1.2 m @ B = 10 T • MRI magnet ~5 T

14. Issues with Confined Approach • Habitability issues close to large magnetic field • Interference with communications • Interference with on-board electronics • Constraints on spacecraft architecture • Quench risk • Energy proportional to B2, order GJ for confined approach • Alpha Magnetic Spectrometer • 0.8 T, LTSC magnet for ISS

16. Overview of Deployed Approach • Spatial dimension is large, so that small bending radii can still miss spacecraft • B is not constant such that Larmor analysis does not apply • Störmer studied the polar auroras • Störmer defined characteristic dimension “protected” by the field of a magnetic dipole • rSt (M/R)1/2 • M = Magnetic Moment of Dipole • For current loop, M = NIA  N I rcoil2

17. Störmer Analysis • Consider 1 GeV/nucleon Fe nucleus (R = 3.65 GV) • rSt = 5 m • Assumes 120 A HTS wire (commercially available) • B value is for loop axis • Mass calculation based on HTS wire only

18. Advantages of Deployed Approach • Benefits • No spallation radiation • Rigidity cutoff can theoretically be set anywhere • Minimal energy requirement • Reasonable mass requirement for large shield • Challenges • Storage/deployment/retraction of coil • HTS cooling • Redundancy • Structural/control issues for maneuvers, external disturbances (spacecraft only)

19. Overcoming Challenges • Cooling • HTS temperatures currently ~70 K • Selective emitter coatings have been proposed • Orientation dependent • Active cooling strategies • Inflatable structures • Deployment • NASA Tethered Satellite System • 250 m, then jammed • Shield stored on spool for launch • Deployment in LEO • B induces hoop stress to aid deployment

20. Solar Magnetic Sails • Zubrin primary champion • Interaction of solar wind and magnetic field imparts acceleration on spacecraft • Acceleration is very small (order mm/s2) but is continuous • 10 mm/s2 over 6 days results in final velocity of 18,000 km/hr (5g rocket over 100 sec) • Cosmos 1 (non-magnetic) • Private launch 2005 • Russian submarine! • Launch failure

21. Lunar Surface Applications • GCR dose for lunar surface 50% of deep space • In a habitat, astronauts can be well-protected by creating large-thickness regolith barriers • During EVA/surface exploration, astronauts will be exposed • Applications include large-area shielding and shielding rover vehicles

22. Selected References Buckey, J.C., “Radiation Hazards: Establishing a Safe Level,” Space Physiology, Oxford University Press, New York, in press 2005. Cocks, F.H., “A Deployable High Temperature Superconducting Coil (DHTSC): A Novel Concept for Producing Magnetic Shields Against Both Solar Flare and Galactic Radiation During Manned Interplanetary Missions,” J. British Interplanetary Soc., Vol. 44, 1991, pp. 99–102. Cocks, J.C., Watkins, S.A., Cocks, F.H., Sussingham, C., “Applications for Deployed High Temperature Superconducting Coils in Spacecraft Engineering: A Review and Analysis,” J. British Interplanetary Soc., Vol. 50, 1997, pp. 479–484. Curtis, S.B., Vazquez, M.E., Wilson, J.W., Atwell, W., Kim, M., Capala, J., “Cosmic Ray Hit Frequencies in Critical Sites in the Central Nervous System,” Advanced Space Research, Vol. 22 No. 2, 1998, pp. 197–207. Hilinski, E.J., Cocks, F.H., “Deployed High-Temperature Superconducting Coil Magnetic Shield,” J. Spacecraft, Vol. 31, No. 2, 1993, pp. 342–344. Landis, G.A., “Magnetic Radiation Shielding: An Idea Whose Time Has Returned?” Space Manufacturing 8: Energy and Materials from Space, AIAA, 1991, pp. 383–386. Letaw, J.R., Silberberg, R., Tsao, C.H., “Radiation Hazards on Space Mission Outside the Magntosphere,” Adv. Space Res., Vol. 9, No. 10, 1989, pp. 285–291. Levy, R.H., “Radiation Shielding of Space Vehicles by Means of Superconducting Coils,” ARS Journal, Nov 1961, pp. 1568–1570. Masur, L.J., Kellers, J., Li, F., Fleshler, S., Podtburg, E.R., “Industrial High Temperature Superconductors: Perspectives and Milestones,” IEEE Trans. Applied Superconductivity, Vol. 12, No. 1, Mar 2002, pp. 1145–1150. Simonsen, L.C., Nealy, J.E., “Radiation Protection for Human Missions to the Moon and Mars,” NASA Technical Paper 3079, Feb 1991. Simonsen, L.C., Nealy, J.E., Townsend, L.W., Wilson, J.W., “Space Radiation Shielding for a Martian Habitat,” SAE Technical Paper 901346, 20th Intersociety Conference on Environmental Systems, Williamsburg, VA, July 9–12, 1990. Sussingham, J.C., Watkins, S.A., Cocks, F.H., “Forty Years of Development of Active Systems for Radiation Protection of Spacecraft,” J. Astronautical Sciences, Vol. 47, No. 3–4, July–Dec 1999, pp. 165–175. Townsend, L.W., “HZE Particle Shielding Using Confined Magnetic Fields,” J. Spacecraft, Vol. 20, No. 6, 1983, pp. 629–630. Townsend, L.W., Nealy, J.E., Wilson, J.W., “Large Solar Flare Radiation Shielding Requirements for Manned Interplanetary Missions,” J. Spacecraft, Vol. 26, No. 2, 1989, pp. 126–128. Townsend, L.W., Wilson, J.W., Shinn, J.L., Nealy, J.E., Simonsen, L.C., “Radiation Protection Effectiveness of a Proposed Magnetic Shielding Concept for Manned Mars Missions,” SAE Technical Paper Series 901343, 20th Intersociety Conference on Environmental Systems, Williamsburg, VA, July 9–12, 1990. Townsend, L.W., “Overview of Active Methods for Shielding Spacecraft from Energetic Space Radiation,” Physica Medica, Vol. 17, Sup. 1, 2001, pp. 84–85. Townsed, L.W., Fry, R.J.M., “Radiation Protection Guidance for Activities in Low-Earth Orbit,” Advanced Space Research, Vol. 30, No. 4, 2002, pp. 957–963.