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Task Objective

To look at the fundamental issues and challenges presented by formations of spinning tethered spacecraft as an alternative to monolithic and formation flying space-based interferometers in the 10m to ~1km aperture range (optical to sub-millimeter to Infrared bands).

Task Description

Spacecraft in orbit with a tetherTethers provide a unique capability to deploy, maintain, reconfigure, and retrieve any number of collaborative vehicles in orbit around any planet. Control techniques for tethered formation reconfiguration must allow the tethered spacecraft to act as a single unit, while the tether length can change depending on the mission profile.
Tethers also offer a high survivability low fuel alternative to scenarios in which multiple vehicles and light collectors must remain in close proximity for long periods of time. In this way, distributed tethered observatories with kilometer class apertures can be built that enable the resolution needed in the optical and microwave bands. To achieve this goal, tether length control must ensure controllability and noiseless operation in the presence of environmental and orbital disturbances.
Our work builds on previous research done on tethered spacecraft for low Earth orbit applications. Previous relevant work includes the TSS-1 flights, SEDS flight, and TiPS flight in the ‘90s.

Goals and Challenges

Two spacecraft tethered together (artist's concept)Our main goal has been to develop dynamics and control algorithms for spinning tethered formations enabling a new class of system architecture and missions for planet finding, and space-based interferometry. To address these challenges, we have been developing an innovative framework of integrated dynamics, control, guidance and estimation algorithms enabling feasibility studies of LEO and deep space tethered configurations, including: the design of precision tether deployment and retrieval algorithms for baseline stabilization of the interferometer; the design of formation retargeting and accurate pointing control algorithms for planet imaging and planet finding; and the conceptual development
of tether dynamics vibration isolation and slack mitigation schemes.

Our studies have pointed out that several key technologies need further development before autonomous and reliable precision applications of tethered spacecraft can be made:

- controlled tethered system retargeting strategies to different sky sources
- precision stationkeeping.
- very smooth reeling in and out of tether suitable for precision baseline control
- disturbance rejection of tether lateral modes caused by transient maneuvers
- smooth thrust control during retargeting
- tether long term survivability to micrometeoroids
- accommodation of tether material degradation
- compensation of tether backscatter during laser metrology
- robust algorithms to manage tether slackness during retargeting
- decoupling of spacecraft motions from tether dynamic noise
- autonomous tether tension and length control logic

Artist's concept of a spacecraft near MarsUltimately, the development of these technologies will provide a unique capability to precisely deploy, maintain, reconfigure, and retrieve many tether-connected collaborative vehicles in orbit to satisfy the needs of the NASA science community in the areas of space interferometry, and planetary exploration.




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