Abstract:
A charge-neutralization system is provided. The charge-neutralization system includes a reservoir, a plasma generator and a flow-restricting transfer line. The reservoir is configured to hold matter to be ionized under pressure. The plasma generator is configured and arranged to create a neutral energy plasma on a vehicle from the matter to be ionized. The flow-restricting transfer line provides a fluid communication between the reservoir and the plasma generator.

Description:
BACKGROUND 
     Spacecraft that are in space are subject to a number of processes that result in a significant electrostatic charging. In the case of rendezvous and proximity operations of one or more spacecraft with one or more other bodies in space, a potential difference can exist between objects, which results in variable levels of electrostatic attractions and repulsions that can result in an electrostatic discharge. Electrostatic attraction and repulsion will result in non-linear forces that will increase the difficulty of the rendezvous and proximity operations. The electrostatic discharge due to the net charge difference between the spacecraft and the other body may or may not result in a larger discharge driven by the electrical system of one or more of the spacecraft. In either case, electrostatic discharge can result in problems with the spacecraft&#39;s electronics. Through operations on the ground, virtually all spacecraft quality assurance and safety requirements include protection against electrostatic discharge to prevent electrostatic discharge damage. In the case of the International Space Station, this issue was identified in a subsequent study that was carried out to investigate the risk to astronauts carrying out extra-vehicle activities. The result was that under certain conditions, the discharge of up to 10,000 Amps at 160 V was possible. In the testing of the impact of the size of discharge, an entire spacesuit was melted into a puddle on the bottom of the test chamber. As a result of this test, triply redundant microwave driven xenon plasma charge couplers were installed on the space station. 
     For the reasons stated above and for other reasons stated below, which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for an effective and efficient system for spacecraft to mitigate the charge differences between spacecraft, the surrounding plasma and another body. 
     SUMMARY OF INVENTION 
     The above-mentioned problems of current systems are addressed by embodiments of the present invention and will be understood by reading and studying the following specification. The following summary is made by way of example and not by way of limitation. It is merely provided to aid the reader in understanding some of the aspects of the invention. 
     In one embodiment, a charge-neutralization system is provided. The charge-neutralization system includes a reservoir, a plasma generator and a flow-restricting transfer line. The reservoir is configured to hold matter to be ionized under pressure. The plasma generator is configured and arranged to create neutral low-energy plasma on the vehicle with the matter to be ionized. The flow-restricting transfer line provides a fluid communication between the reservoir and the plasma generator. 
     In another embodiment, a spacecraft with a charge-neutralization system is provided. The charge-neutralization system includes a reservoir, a plasma generator, a flow-restricting transfer line and a ground plate. The reservoir is configured to hold matter to be ionized under pressure. The plasma generator is configured and arranged to generate neutral low-energy plasma with the matter to be ionized. The flow-restricting transfer line provides a fluid communication between the reservoir and the plasma generator. The ground plate is coupled to the spacecraft. The plasma generator is positioned to apply the generated neutral low-energy plasma to the ground plate of the spacecraft. 
     In still another embodiment, a method of neutralizing electrostatic discharge on a spacecraft is provided. The method includes: generating neutral low-energy plasma and applying the neutral low-energy plasma to the spacecraft to level an electrostatic charge difference between the spacecraft and another object. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention can be more easily understood and further advantages and uses thereof will be more readily apparent, when considered in view of the detailed description and the following figures in which: 
         FIG. 1A  is a diagram of a spacecraft with a charge-neutralization system of an embodiment of the present invention; 
         FIG. 1B  is a diagram of a spacecraft with another charge-neutralization system of an embodiment of the present invention; 
         FIG. 2  is a diagram of a spacecraft with yet a different charge-neutralization system of another embodiment of the present invention; 
         FIG. 3  is a short mission operational flow diagram of one embodiment of the present invention; and 
         FIG. 4  is a long mission operational flow diagram of an embodiment of the present invention. 
     
    
    
     In accordance with common practice, the various described features are not drawn to scale but are drawn to emphasize specific features relevant to the present invention. Reference characters denote like elements throughout the figures and text. 
     DETAILED DESCRIPTION 
     In the following detailed description, reference is made to the accompanying drawings, which form a part hereof and in which is shown by way of illustration, specific embodiments in which the inventions may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that changes may be made without departing from the spirit and scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the claims and equivalents thereof. 
     Embodiments of the present invention provide a radioisotope electrostatic charge neutralizer. Matter to be ionized, such as volatile gas, is directed into an ionizing source such as a plasma generation source volume (generally referred to herein as a “plasma generator”). Ionizing radiation from a radioisotope in the plasma generator ionizes the volatile gas creating a neutralizing plasma. The radioisotopic electrostatic charge neutralizer provides a source of neutral low-energy plasma for leveling the electrostatic charge between the spacecraft and another object. Charged particles are attracted to electrostatic fields based upon the relative charge difference, a positively charged spacecraft approaching a negatively charged space object results in electrons from the radioisotopic-generated plasma to be attracted to the positively charged spacecraft, and the positively charged ions will be attracted to the negatively charged other body. This results in a small current that will safely decrease the charge difference between the bodies. This process is self-controlling due to the inherent attraction of electrostatic charge particles being attracted to the one body and repelled from the other body in the case of a charge difference. Depending upon the duration of the mission, the amount of charge that will need to be dissipated and the cost of the gas being used to neutralize the charge at the rate desired, it may or may not be necessary to provide a dynamic control of gas flow rate. If the rate of charge dissipation is achieved with a gas flow on the order of 1 E-3 standard cubic centimeters per second or less, it may be advantageous to provide gas flow through a fixed-rate leak, which would not require any controls. If higher flow rates result in gas flow rates that would require too much gas to be used, valve control may be required to conserve gas. In either case, flowing gas through the radioisotope-based ionizer (plasma generator) will result in charge neutralization between closely surrounding bodies. 
     Referring to  FIG. 1A , a block diagram illustrating a first embodiment of a passive charge-neutralization system  100  that mitigates electrostatic discharge associated with a spacecraft  110  is provided. This embodiment would be used when the duration of the flight is not long and the cost of gas is not prohibitive in letting the system constantly run during the flight. The charge-neutralization system  100  includes a volatile liquid/gas  104  (matter to be ionized) such as, but not limited to, xenon stored in a pressurized reservoir  105 . The liquid/gas  104  flows through a restricting transfer line  106 . In the restricting transfer line  106 , any remaining liquid evaporates to gas  107 . The restricting transfer line  106  delivers the volatile gas  107  to a plasma generator  102 . The plasma generator  102  (plasma generation source volume) generates a neutralizing plasma with the volatile gas  107  (matter to be ionized). That is, the ionizing radiation of the radioisotope in the plasma generator  102  ionizes the volatile gas  107  to generate the neutralizing plasma. The plasma generator  102  is electrically bonded to a ground plate  108  of the spacecraft  110 . This embodiment also illustrates an electrostatic charge sensor  111  that detects electrostatic charge on the spacecraft  110 . The electrostatic charge sensor  111  is in communication with a controller  120 . The controller  120  may be a guidance navigation control system of the spacecraft. The controller  120  monitors the electrostatic charge of the spacecraft  110  through the electrostatic charge sensor  111  and uses the information during operations.  FIG. 1B  illustrates another charge-neutralization system  150 . In this system, the reservoir  105  that contains the matter to be ionized  104  is mounted outside the spacecraft  110 . Moreover, it is possible to mount the entire system outside the spacecraft. 
       FIG. 2  illustrates an embodiment of a passive charge-neutralization system  200  that would be applied to longer missions or where it is important to conserve the volatile liquid/gas  104 / 107 . In this embodiment, instead of delivering a constant flow of volatile gas  107  from the pressurized reservoir  105  to the plasma generator  102 , a valve  202 , positioned in the restricting transfer line  106 , selectively turns on and turns off the flow. In this embodiment, the controller  120  is operationally coupled to the valve  202  to activate the valve  202  between an open and closed position. The controller  120  may control the valve  202  to conserve the volatile material  104 / 107  based on different trigger systems. The controller  120  may be part of the guidance and navigation control system of the spacecraft  110 . Moreover, the controller  120  may be a simple switch or a processing device that dynamically controls the valve  202  based on events. For example, one or more electrostatic charge sensors  111 - 1 ,  111 - 2  and  111 - n  may be used to detect the electrostatic charge on the spacecraft  110 , or between the spacecraft  110  and another body  148  as the other body  148  gets close to the spacecraft  110 . When an electrostatic charge level is detected that is beyond a set limit, the controller  120 , based on the measured electrostatic charge by the one or more sensors  111 - 1 ,  111 - 2  or  111 - n , opens the valve  202  until an electrostatic charge level is below the set limit. In another example, the passive charge-neutralization system  200  may include a timer  132 . The controller  120 , in this embodiment, would periodically open and close the valve  202  based on the timer  132 . In still another embodiment, the passive charge-neutralization system  200  would include an input system  134  that would provide a signal to the controller  120  that an event (such as docking with another object or a spacewalk by an astronaut) is about to occur. Based on a signal from the input system  134 , the controller  120  would control the valve  202 . 
       FIG. 3  illustrates a short mission operational flow diagram  300 . As illustrated, the process starts by delivering volatile gas  107  (matter to be ionized) through a restricting transfer line to plasma generator  102  ( 302 ). As discussed above, the volatile liquid/gas  104  that is held in a pressured reservoir  105  is delivered as a gas  107  to the plasma generator  102  through the restricting transfer line  106 . The plasma generator  102  then creates a neutralizing plasma from the material to be ionized, volatile liquid/gas  104  ( 304 ). The plasma generator  102  uses ionizing radiation from the radioisotope, ionizing the matter to be ionized to create the neutralizing plasma that is applied to the ground plate  108  of the spacecraft  110 . The neutralizing plasma of the plasma generator  102  creates a charge-neutralizing current on the spacecraft ( 306 ). In particular, as discussed above, as charged particles are attracted to electrostatic fields based upon the relative charge difference, a positively charged spacecraft approaching a negatively charged second object will result in electrons from the radioisotopic-generated plasma to be attracted to the spacecraft, and the positively charged ions will be attracted to the negatively charged other body. This results in a small current that will safely decrease charge difference between the bodies. 
       FIG. 4  illustrates a long mission operational diagram  400  of one embodiment. In this embodiment, the spacecraft  110  is monitored ( 402 ). In one embodiment, the spacecraft  110  is monitored for the buildup of electrostatic charge. In another embodiment, the spacecraft  110  is monitored for a future event such as docking with another object (such as another spacecraft, satellite, etc.) or a planned space walk by an astronaut. If it is determined that a neutralization of charge is not needed ( 404 ), the process continues monitoring at step ( 402 ). If it is determined that a neutralization of charge is needed ( 404 ), valve  202  is opened ( 406 ). Once the valve  202  is opened, gas  107  is delivered through the restricting transfer line  106  ( 408 ). The plasma generator  102  then creates a neutralizing plasma with the volatile gas  107  ( 410 ). The neutralizing plasma is applied to the spacecraft  110  to create a charge-neutralizing current ( 412 ). It is then determined if the event is done or the electrostatic charge on the spacecraft has been neutralized ( 414 ). For example, the system may monitor for an indication that the event has been completed (i.e., the object has completed the docking process or the astronaut has returned to the spacecraft) or whether the electrostatic charge has been neutralized. If the event is not completed or the spacecraft  110  has not been neutralized ( 414 ), the valve  202  remains open so the gas continues to be delivered to the plasma generator at step ( 408 ). If the event has been completed or it is determined that the spacecraft has been neutralized ( 414 ), the valve  202  is closed ( 416 ). Closing the valve  202  conserves the liquid/gas source  104  in the reservoir  105 . Once the valve  202  is closed at step ( 416 ), the process continues by monitoring for an event or buildup of electrostatic charge at step ( 402 ). 
     The above-described charge-neutralization system is relatively inexpensive to build while its minimal size, weight and power consumption is ideal for implementation in a spacecraft. Moreover, because the radioisotope electrostatic charge neutralizer is primarily passive and is based on the fundamental properties of two materials (i.e., the ionizing radiation from the radioisotope of the plasma generator and the volatile gas), there are very few possible failure opportunities. 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement, which is calculated to achieve the same purpose, may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the present invention. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.