Search

The use of four Lagrange points of the Sun/Earth system for fundamental physics experiments in space is presented. L1, L2, L4 and L5 rotating rigidly together with the Earth form a natural reference frame at the scale of the inner solar system. The idea which is discussed in the paper considers the possibility of locating four spacecraft in the four cited Lagrange points and exchanging electromagnetic pulses among them. Including stations on earth, various closed paths for the pulses are possible. Time of flight measurements would be performed. The time of flight difference between right- and left-handed circuits is proportional to the angular momentum of the Sun and the detection of the effect would reach accuracies better than 1% depending on the accuracy of the clock. The four points could also be used as "artificial pulsars" for a relativistic positioning system at the scale of the solar system. Additional interesting possibilities include detection of a galactic gravito-magnetic field and also, using a global configuration as a zero area Sagnac contour, detection of gravitational waves. More opportunities are also mentioned.

This paper presents a novel configuration of a space system which addresses the issue of rendezvous and capture of a spacecraft in a lower (or higher) energy orbit for later injection into a higher (or lower) energy orbit. We first demonstrate that a slender long tether loop in space can maintain its shape while spinning and that the width of its opening depends on the boundary conditions of the loop arms near the center of rotation. Subsequently, we show how the tether loop could be utilized to allow a rendezvous and capture of a spacecraft approaching the loop from a lower (or higher) energy orbit while the natural target area of the loop enables an extended time opportunity for spacecraft capture which is tolerant of errors. We also discuss at the conceptual level a possible device/technique for making a soft contact between the loop and the incoming spacecraft. These results are directly applicable to spinning tether systems for space transportation from low Earth orbit to geotransfer orbit and to escape trajectory or for reentering payloads into the Earth atmosphere (from higher energy orbits) with high Δ V and a predictable reentry trajectory.

A long tether connects a Martian orbiter to a small probe orbiting in the dense atmosphere of the planet. The minimum altitude reachable by the probe is estimated based upon considerations of dynamic stability, tether temperature, resistance of the tether material, and propellant consumption required to maintain a constant orbital altitude. The results obtained by means of a static-force model are then verified by dynamics simulation carried out by using a lumped-mass computer code. The scientific applications of this tethered system include probing of the Martian upper atmosphere and gravity mapping of the planet carried out by collecting gradiometric data from the probe. Another application involves the release of Martian rovers and penetrators from the probe at low altitude.

The Propulsive Small Expendable Deployer System (ProSEDS) space experiment was ready to fly as a secondary payload on a Delta–II expendable launch vehicle in late March 2003. Concerns raised by the International Space Station (ISS) after the February 2003 Columbia shuttle accident resulted in the delay of the launch of ProSEDS. Issues associated with both the delayed launch date and a change in starting altitude resulted in the ultimate cancellation of the mission. ProSEDS was intended to deploy a tether (5-km bare wire plus 10-km non-conducting Dyneema) from a Delta–II second stage to achieve adequate electrodynamic drag thrust that would lower the orbit of the system over days—as opposed to months due to atmospheric drag. The experiment was also designed to utilize the tether-generated current to provide limited spacecraft power. Considerable effort and testing went into developing the ProSEDS system by a dedicated team. Throughout this effort, important technological issues were identified and addressed and this paper discusses some of the important technical issues and hurdles that had to be addressed to successfully prepare for flight. It is intended that this information will be of use for future tether mission and experiment designers.

Relatively short electrodynamic tethers can extract orbital energy to 'push' against a planetary magnetic field to achieve propulsion without the expenditure of propellant. The Propulsive Small Expendable Deployer System experiment 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 lowering the altitude of 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. The flight experiment is a precursor to a more ambitious electrodynamic tether upper stage demonstration mission which will be capable of orbit raising, lowering and inclination changes - all using electrodynamic thrust. The expected performance of the tether propulsion system during the experiment is described.

Electrodynamic tether thrusters can use the power provided by solar panels to drive a current in the tether and then the Lorentz force to push against the Earth's magnetic field, thereby achieving propulsion without the expenditure of onboard energy sources or propellant. Practical tether propulsion depends critically on being able to extract multiamp electron currents from the ionosphere with relatively short tethers (10 km or less) and reasonably low power. We describe a new anodic design that uses an uninsulated portion of the metallic tether itself to collect electrons. Because of the efficient collection of this type of anode, electrodynamic thrusters for reboost of the International Space Station and for an upper stage capable of orbit raising, lowering, and inclination changes appear to be feasible. Specifically, a 10-km-long bare tether, utilizing 10 kW of the space station power could save most of the propellant required for the station reboost over its 10-year lifetime. The propulsive small expendable deployer system experiment is planned to test the bare-tether design in space in the year 2000 by deploying a 5-km bare aluminum tether from a Delta II upper stage to achieve up to 0.5-N drag thrust, thus deorbiting the stage.

The groundwork has been laid for tether applications in space. NASA has developed tether technology for space applications since the 1960's. 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). 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 non-conducting 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 multi-mission 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.

The Propulsive Small Expendable Deployer System (ProSEDS) space experiment will demonstrate the use of an electrodynamic tether propulsion system. The flight experiment is a precursor to the more ambitious electrodynamic tether upper stage demonstration mission which will be capable of orbit raising, lowering and inclination changes-all using electrodynamic thrust. ProSEDS which is planned to fly in 2000, will use the flight proven Small Expendable Deployer System (SEDS) to deploy a tether (5km bare wire plus 15 km spectra) from a Delta II upper stage to achieve approximately 0.4N drag thrust, thus demonstrating deorbit thrust. The experiment will use a predominantly 'bare' tether for current collection in lieu of endmass collector and insulated tether approach used on previous missions. ProSEDS will utilize tether-generated current to provide limited spacecraft power. In addition to the use of this technology to provide orbit transfer of payloads and upper stages from LEO to higher orbits it may also be an attractive option for future missions to Jupiter and any other planetary body with a magnetosphere.

The totally gastrectomized (TGX) rat is a new experimental model with which to produce widespread spongy vacuolation in spinal cord (SC) white matter, strongly reminiscent of that observed in subacute combined degeneration (SCD) of human SC.We did in long-term experiments combined biochemical and histologic studies on SCs from both TGX-rats and rats fed a cobalamin-deficient (Cbl-D) diet. We also investigated the effects of single in vivo administration of some neurotrophic growth factors on the activity of L-ornithine decarboxylase (ODC) (the key-point in the polyamine biosynthetic pathway) in rat SC.Biochemically, ODC activity was still induced 3 and 6 months after total gastrectomy (TG), while it did not change significantly even after 9 months of feeding a Cbl-D diet. Both TG and feeding the Cbl-D diet greatly decreased the cobalamin level in both serum and SC, although these decreases occurred more slowly in rats fed a Cbl-D diet. Nerve growth factor did not induce ODC in either Cbl-D myeloneuropathy; epidermal growth factor induced ODC in both Cbl-D myeloneuropathies. Basic fibroblast growth factor induced SC ODC only in TGX-rats. Histologically, spongy vacuolation was still widespread 3 and 6 months after TG, while it was spotty even after 9 months of feeding a Cbl-D diet. There was massively increased staining of astrocytes positive for glial fibrillary acidic protein, mainly in the gray matter, in both Cbl-D myeloneuropathies. Finally, repeated in vivo injections of cobalamin to TGX rats only partially reduced ODC induction, the severity of spongy vacuolation, and the increase in glial fibrillary acidic protein-positive astrocytes.These results suggest: (a) ODC induction is a persistent and inherent feature in the TG-induced SCD of rat SC; (b) an increase in glial fibrillary acidic protein positive astrocytes in rat SC is not mandatorily connected with an increase in polyamine biosynthesis; (c) a mere deficiency of Cbl seems to be not the only key-point in the pathogenesis of the ODC induction and of the SCD-like lesions, both brought about in rat SC by TG.

Totally gastrectomized rats have been used to induce a spongy demyelination in the white matter of the spinal cord (SC) which is strongly reminiscent of that observed in subacute combined degeneration of human SC. Totally gastrectomized rats are deprived of intrinsic factor and thereafter become deficient in cobalamin. Morphologically, the spongy demyelination of the white matter of the rat SC, was evident 2 months after total gastrectomy. Biochemically, we investigated the hypothesis that polyamine biosynthesis might be deranged in the rat SC with experimental subacute combined degeneration, since polyamines are well known to be bound to myelin in the mammalian central nervous system. We measured the levels of both the polyamine biosynthetic decarboxylases, L-ornithine decarboxylase (ODC) and S-adenosyl-L-methionine decarboxylase, the key points in the polyamine biosynthetic pathway, in these SC. There was a sharp increase in ODC activity in SC 2 months after total gastrectomy, without significant changes in S-adenosyl-L-methionine decarboxylase activity. The increase in ODC activity seems to be organ-specific and was not due to a proliferation of neuroglial cells. Interestingly enough, the same morphologic and biochemical features found in SC of 2-month-totally-gastrectomized rats were present also in SC of newborn rats, which indeed showed incomplete myelination, vacuolated appearance, and an ODC activity level higher than that of adult SC. Therefore, total gastrectomy seems to induce a type of regression in the SC of totally gastrectomized rats toward neonatal life, at least in terms of the degree of myelination and of ODC activity level. Biochemically, no changes in ODC activity were observed in SC of rats fed a cobalamin-deficient diet for 3 months. Morphologically, only a proliferation of neuroglial cells with a moderate demyelination was observed in SC of these rats maintained on a cobalamin-deficient diet for 3 months.

Any electrodynamic tether working in an inclined orbit is affected by a dynamic instability generated by the continuos pumping of energy from electromagnetic forces into the tether attitude motion. To overcome the difficulties associated with this instability, two control schemes have been analyzed. In both cases the background strategy is the same: we add appropriate forces to the system with the aim of converting an unstable periodic orbit of the governing equations into an asymptotically stable one. The idea is to take such a stabilized periodic orbit as the starting point for the operation of the electrodynamic tether. In the first case, the unstable periodic orbit is taken as a reference orbit. In the second one, we use a delay feedback control scheme that has been used successfully in problems with one degree of freedom. To obtain results with broad validity, some simplifying assumptions have been introduced in the analysis. Thus, we assume a rigid tether with two end masses orbiting along a circular, inclined orbit. We also assume a constant tether current that does not depend on the attitude and orbital position of the tether, and the Earth’s magnetic field is modeled as a dipole aligned with the Earth’s rotation axis.

The International Space Station (ISS) will require periodic reboost due to atmospheric aerodynamic drag. This is nominally achieved through the use of thruster firings by the attached Progress M spacecraft. Many Progress flights to the ISS are required annually. Electrodynamic tethers provide an attractive alternative in that they can provide periodic reboost or continuous drag cancellation using no consumables, propellant, nor conventional propulsion elements. The system could also serve as an emergency backup reboost system used only in the event resupply and reboost are delayed for some reason. The system also has direct application to spacecraft and upper stage propulsion. Electrodynamic tethers have been demonstrated in space previously with the Plasma Motor Generator (PMG) experiment and the Tethered Satellite System (TSS-1R). The advanced electrodynamic tether proposed for this application has significant advantages over previous systems in that higher thrust is achievable with significantly shorter tethers and without the need for an active current collection device, hence making the system simpler and much less expensive. Propellantless Reboost for the ISS: An Electrodvnamic Tether Thruster The need for an alternative to chemical thruster reboost of the ISS has become increasingly apparent as the station nears completion. We propose a system to utilize ISS electrical power to generate thrust by means of a new type of electrodynamic tether attached to the station (Fig. 1). A flexible system could be developed to generate an average thrust of 0.5 to 0.8 N for 5 to 10 kW of electrical power. By comparison, aerodynamic drag on 755 is expected to average from 0.3 to 1.1 N (depending upon the year). The proposed system uses a tether with a kilometers-long uninsulated (bare) segment capable of collecting currents greater than 10 A from the ionosphere. The new design exhibits a remarkable insensitivity to electron density variations, allowing it

Simulation of the dynamics of an electrodynamic tether on a circular inclined orbit shows a very complex motion driven by the electrodynamic forces acting on the conductive tether. These forces depend on the current flowing in the wire, the Earth magnetic field, the orbital velocity and the tether position. In this paper we use a simple model to describe the dynamic effects of these forces. The tether is modeled as a rigid rod with point masses at the ends. We also adopt a non-tilted dipole model for the Earth magnetic field, and we assume that the tether current is constant. When the current is null, the system has a stable equilibrium position with the tether aligned along the local vertical. When the current is different from zero, a periodic motion appears. A nonlinear analysis of the motion shows that the periodic solutions are always unstable (within the limitation of the model considered in the paper). The physical reason for the instability is that the electrodynamic forces pump energy continually into the system. The net energy increase per orbit for the periodic solution (or state space trajectory) is zero. However, any nonperiodic trajectory in its neighborhood has a positive net energy flux per orbit so that after several orbits the in-plane libration becomes a rotation. The mechanism responsible for this instability depends on the orbital inclination. Unlike other destabilizing mechanisms found in electrodynamic tethers, this one is present in any kind of tether system with either a flexible or a rigid tether, operating in the generator or thruster mode and utilizing a bare tether or a large spherical termination to collect the ionospheric electrons. The instability described in this paper is independent of the presence of resonant force components that may be generated by the magnetic and plasma fields.

This paper investigates the dynamics and acceleration levels of a new tethered system for micro- and variable-gravity applications. The system consists of two platforms tethered on opposite sides to the Space Station. A fourth platform, the elevator, is placed in between the Space Station and the upper platform. Variable-g levels on board the elevator are obtained by moving this facility along the upper tether, while microgravity experiments are carried out on board the Space Station. By controlling the length of the lower tether the position of the system center of mass can be maintained on board the Space Station despite variations of the system's distribution of mass. The paper illustrates the mathematical model, the environmental perturbations and the control techniques which have been adopted for the simulation and control of the system dynamics. Two sets of results from two different simulation runs are shown. The first set shows the system dynamics and the acceleration spectra on board the Space Station and the elevator during station-keeping. The second set of results demonstrates the capability of the elevator to attain a preselected g-level.