Enrico Lorenzini

Overview of future NASA tether applications

Abstract The groundwork has been laid for tether applications in space. NASA has developed tether technology for space applications since the 1960s. Important recent milestones include retrieval of a tether in space (TSS-1, 1992), successful deployment of a 20-km-long tether in space (SEDS-1, 1993), closed loop control of tether deployment (SEDS-2, 1994), and operation of an electrodynamic tether with tether current driven in both directions—power and thrust modes (PMG, 1993). Various types of tethers and systems can be used for space transportation. Short electrodynamic tethers can use solar power to “push” against a planetary magnetic field to achieve propulsion without the expenditure of propellant. The planned Propulsive Small Expendable Deployer System (ProSEDS) experiment will demonstrate electrodynamic tether thrust during its flight in early 2000. Utilizing completely different physical principles, long nonconducting tethers can exchange momentum between two masses in orbit to place one body into a higher orbit or a transfer orbit for lunar and planetary missions. Recently completed system studies of this concept indicate that it would be a relatively low-cost, in-space asset with long-term, multimission capability. Tethers can also be used to support space science by providing a mechanism for precision formation flying and for reaching regions of the upper atmosphere that were previously inaccessible.

Electrodynamic Tethers for Spacecraft Propulsion

Relatively short electrodynamic tethers can use solar power to push against a planetary magnetic field to achieve propulsion without the expenditure of propellant. The groundwork has been laid for this type of propulsion. NASA began developing tether technology for space applications in the 1960s. Important recent milestones include retrieval of a tether in space (TSS-1, 1992), successful deployment of a 20-km-long tether in space (SEDS-1, 1993), and operation of an electrodynamic tether with tether current driven in both directions-power and thrust modes (PMG, 1993). The planned Propulsive Small Expendable Deployer System (ProSEDS) experiment will demonstrate electrodynamic tether thrust during its flight in early 2000. ProSEDS will use the flight-proven Small Expendable Deployer System (SEDS) to deploy a 5 km bare copper tether from a Delta II upper stage to achieve approximately 0.4 N drag thrust, thus deorbiting the stage. The experiment will use a predominantly bare tether for current collection in lieu of the endmass collector and insulated tether approach used on previous missions. Theory and ground-based plasma chamber testing indicate that the bare tether is a highly-efficient current collector. The flight experiment is a precursor to utilization of the technology on the International Space Station for reboost application and the more ambitious electrodynamic tether upper stage demonstration mission which will be capable of orbit raising, lowering and inclination changes - all using electrodynamic thrust. In addition, the use of this type of propulsion may be attractive for future missions at Jupiter and any other planetary body with a magnetosphere.

Analysis of Passive System to Damp the Libration of Electrodynamic Tethers for Deorbiting

In the last decade, the continuous and alarming growth of space debris prompted many space agencies all over the world to adopt debris mitigation strategies. Present guidelines indicate the need to deorbit new satellites launched into low Earth orbit (LEO) within 25 years from their end of life. At present, a space-proven technology suitable to carry out a complete deorbit utilizes classical chemical propulsion. However, a deorbit maneuver by means of chemical rocket strongly affects the satellite propulsion budget, thus limiting the operational life of the satellite. These issues bring the need to develop innovative deorbiting technologies. One of these consists in using electrodynamic tethers that, through its interaction with the Earth ionosphere and magnetic field, can take advantage of Lorentz forces for deorbiting. Previous studies have shown the effectiveness of such a technology to deorbit LEO satellites from different altitudes and inclinations in a relatively short time. However, the continuous injection of small amount of energy produced by Lorentz forces into the tether system can cause dynamic instabilities. This paper addresses this issue through the analysis of the benefits provided by a damping device installed at the attachment point of the tether to the spacecraft. The damped tether system is modeled with a two-bar model to represent the dynamics of the tether and damping device. A key issue is how to maximize the energy transfer from the electrodynamic tether to the damper and its dissipation. The analysis carried out by means of linearization of dynamics equations and numerical simulations show that a well-tuned damper can effciently damp out the tether kinetic energy thus greatly increasing the system stability.

A Review of Electrodynamic Tethers for Space Applications

Space applications of electrodynamic tethers, and basic issues and constraints on their operation are reviewed. The status of the bare-tether solution to the problem of effective electron collection from a rarefied magnetized plasma is revisited. Basic modes of tether operation are analyzed; design parameters and parametric domains where a bare electrodynamic tether is most efficient in deorbiting, rebooking, or power generation, are determined. Use of bare tethers for Radiation Belt Remediation and generation of electron beams for ionospheric research is considered. Teiher heating, arcing, and bowing or breaking, as well deployment strategies are discussed.

Electrodynamic Tether for Scientific Mission at Low Jovian Orbit

An electrodynamic bare tether is shown to allow carrying out scientific observations very close to Jupiter, for exploration of its surface and subsurface, and ionospheric and atmospheric in-situ measurements. Starting at a circular equatorial orbit of radius about 1.3/1.4 times the Jovian radius, continuous propellantless Lorentz drag on a thin-tape tether in the 1-5 km length range would make a spacecraft many times as heavy as the tape slowly spiral in, over a period of many months, while generating power at a load plugged in the tether circuit for powering instruments in science data acquisition and transmission. Lying under the Jovian radiation belts, the tape would avoid the most severe problem facing tethers in Jupiter, which are capable of producing both power and propulsion but, operating slowly, could otherwise accumulate too high a radiation dose . The tether would be made to spin in its orbit to keep taut; how to balance the Lorentz torque is discussed. Constraints on heating and bowing are also discussed, comparing conditions for prograde versus retrograde orbits. The system adapts well to the moderate changes in plasma density and motional electric field through the limited radial range in their steep gradients near Jupiter.