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Tethers
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
 Tethers
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
 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
 Ultimately,
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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