Momentum Exchange Tether to De-­spin a Tumbling Asteroid

1. Momentum Exchange Tether toDe-­spin a Tumbling Asteroid Harold P Gerrish Jr and Les Johnson NASA Marshall Space Flight Center November 22, 2013

2. Outline • Tether Momentum Exchange Physics • Tether Sizing for Momentum Exchange • Asteroid Capture Concept • Asteroid De-Spin Sequence • Ground Demonstration Concept • System Component Technology Readiness Level

3. Concept Overview • Given: A 13m diameter asteroid has a mass of 1000 metric tons and a spin rate of 2 revolutions per minute (RPM). • Goal: De-spin tumbling asteroid to a level that the asteroid redirection vehicle (ARV) can handle. • Concept: Release a small mass from the asteroid using a momentum exchange tether to slow down the spin rate as the tether length is increased. When final spin rate is achieved, release the end mass and tether. 13m Diameter Asteroid Spin Deployer Tether 45kg End Mass • Physics: Law of conservation of angular momentum and changes in moment of inertia caused by tether deployment of a small mass

4. Tether Sizing • The decrease in angular momentum of a tumbling asteroid depends on the length of the rotating tether and the end mass Not as much change with length

5. Tether Tension • Tether tension increases to reasonable levels before dropping due to reduction in asteroid spin rate

6. Asteroid Capture Spacecraft 13m Diameter Asteroid Rendezvous Spacecraft and Tether Deployment System Inflatable StructureCapture Net 2 m

7. Asteroid De-Spin Sequence Asteroid Capture Spacecraft Leaves Mother-ship - Rendezvous with Asteroid - Deploys Capture Device - Captures Asteroid Asteroid Redirection Vehicle Mother-ship Deploys Asteroid Capture Spacecraft Asteroid Capture Spacecraft Closes Capture Net Around Asteroid Small End Mass is Composed of the Asteroid Capture Spacecraft Asteroid Capture Spacecraft Begins Momentum Exchange Tether Deployment on Plane of Rotation Tether Length ~ 3-5 km Momentum Exchange Tether Deployment CompleteAsteroid Spin Rate < 2 Deg./Sec. Asteroid Capture Spacecraft Separates from Asteroid and Mother-ship Retrieves Asteroid

8. Ground Demonstration Concept Self-contained air-bearing spacecraft simulator can be rotated on the floor Overhead view of Flight Robotics Laboratory Flat Floor (44 x 86 ft precision epoxy) Small spheres tested with tethers • The flat floor at MSFC can be used to conduct a sub-scale demonstration of the concept • The test would demonstrate the tether end mass deployment and the effects it has on the spin rate

9. Inflatable Support Structures have been Tested 1996 L’Garde Inflatable Antenna Experiment (IAE) seen from STS-77. 14 meter diameter antenna Inflatable deployment structure for capture net. 2000 Air Force Research Laboratory supported Thiokol and SRS integrated deployment test. Four deployment tests with maximum .5” variation in final position.

10. Tether Technology is Relatively Mature • Small Expendable Deployer System (SEDS) (1993-1994) • SEDS 1: de-orbited a small payload using 20 km tether • SEDS 2: demonstrated controlled deployment of a 20 km tether • Plasma Motor Generator: demonstrated deployment of a 500m conducting wire • Shuttle Tethered Satellite System (TSS) (1992, 1996) • 20 km insulated conducting electrodynamic tether • TSS-1: 200 m deployed, demonstrated stable dynamics • Last-minute S&MA demanded design change resulting in oversized bolt that jammed deployer (configuration control process failure) • TSS-1R: 19.9 km deployed, >5 hours of excellent data validating models of ED tether-ionosphere current flow • Arc caused the tether to fail (tether fabrication/design/handling flaw) • No thrust measurements • TiPS - Survivability & Dynamics investigation (1996-2006) • 4 km nonconducting tether, ~1000 km alt • Survived over 10 years on orbit • T-Rex – Bare Anode Tape Tether deployment (2010) Most tether missions HAVE been successful. Mission failures were due to design process errors, notdue to fundamental physics.

11. Backup Information

12. Tether Technology is Relatively Mature Deployment and Dynamics (Earth Orbit Only) • Ease of Deployment and Control (SEDS-1/2 & TSS-1) • Deployment to 20 km, station keeping for more than 20 hrs, and satellite retrieval have been demonstrated • Dynamic Stability (TSS-1) • Gravity-gradient stabilization achieved at < 300 m. • Recovery from Dynamic Upsets & Slack Tether (TSS-1) • Recover from severer dynamic perturbations, slack tether and satellite pendulous motions. • Retrieval (TSS-1) • Near retrieval (most critical aspect) from 276 m was nominal. Deployer, tether and subsystems continue to mature • 4 Year Technology Maturation by Northrop Grumman/Millennium Space Systems/Tethers Unlimited • Space-survivable conducting tether • Reliable tether deployment mechanism • Attitude control of tethered spacecraft • Method & hardware for enabling precision pointing of tethered spacecraft • Both ‘Bolt-On’ and Dedicated Spacecraft configurations • MSFC worked with NG/MSS/TUI to extend capability to LEO orbit transfer applications.

13. EDT Flight Heritage (1/3)

14. EDT Flight Heritage (2/3)

15. EDT Flight Heritage (3/3)