Hybrid electrostatic space tug

Space tug vehicles and methods for providing space tugs and moving target vehicles are provided. More particularly, a space tug utilizing electrostatic or Coulomb force for acting on target vehicles and for moving the target vehicles into new orbits or altitudes are provided. The space tug may establish an attractive electrostatic force by controlling the electrical potential of the space tug so that it is opposite the electrical potential of a target vehicle. The target vehicle may acquire an absolute electrical potential due to its interaction with the space plasma and photoelectrons or the space tug may impart additional charge to the target vehicle. After establishing the attractive electrostatic force, a propulsion system of the space tug is operated to provide thrust. Thrust is directed to change the orbit and/or position of the target vehicle that is being pulled towards the space tug by the electrostatic force.

FIELD

A space tug for altering the altitude of a target vehicle is provided. More particularly, a space tug that utilizes an electrostatic force to alter the altitude of a target vehicle is provided.

BACKGROUND

The Geostationary Earth Orbit (GEO) belt is becoming very crowded with communication and science satellites. If a satellite breaks down, or reaches its end of life without exiting the GEO belt, then the satellite continues to occupy a valuable GEO slot. In addition, without further orbit control, these satellites will drift due to lunar and solar radiation disturbances, allowing them to wander the GEO belt and interfere with other satellites.

The current practice is to have a satellite at its end of life exit the GEO belt to a super-geosynchronous orbit with its remaining fuel. However, this requires that the payload of the satellite include fuel for this purpose. As a result, the mass of the satellite for a given mission is increased. In addition, older satellites may not have made provisions for achieving a super-geosynchronous orbit, or through accident or the accumulated effect of maneuvers during the satellite's lifetime, there may otherwise be insufficient fuel to place the satellite in a super-geosynchronous orbit. There also is a need to remove malfunctioning satellites or debris from desirable orbits, to allow those orbits to be occupied by functioning satellites, or to move potentially dangerous debris to safer orbits.

In order to remove defunct satellites from desirable orbits, space tugs equipped with docking hardware have been proposed. Such vehicles would operate by physically grasping and holding a target satellite, and then moving that target satellite while it is attached to the space tug. Although simple in concept, a space tug that grabs hold of a target satellite is difficult to implement. In particular, it requires that the space tug maneuver into the immediate proximity of the target vehicle. The space tug must then grab hold of some component or section of the target vehicle that is sufficiently robust to allow the space tug to pull on that component to change the altitude of the target vehicle. This process is often complicated by the rotation or spin of the target vehicle and/or other motion of the target vehicle relative to the space tug. In addition, the space tug is required to precisely maneuver itself into very close proximity to the target vehicle.

SUMMARY

Embodiments of the disclosed invention are directed to solving these and other problems and disadvantages of the prior art. In particular, embodiments of the present invention provide a space tug that uses a hybrid blend of Coulomb forces to move a target vehicle. More particularly, a space tug in accordance with embodiments of the present invention uses active charge control to create an absolute electrical potential on the space tug that is opposite the absolute electrical potential of the target vehicle, creating an attractive force between the space tug and the target vehicle. This attractive force or electrostatic tractor force allows the space tug to pull the target vehicle to a desired location or orbit.

In accordance with further embodiments of the present invention, the space tug performs active charge control by emitting charged particles to control the electrostatic potential of the space tug. For example, if the electrical potential of the space tug is required to be more negative, the active charge control system can emit positive ions. If the electrical potential of the space tug needs to be more positive, the active charge control system can emit negative electrons. The target vehicle can acquire an absolute charge due to its interaction with the space plasma and the photoelectron effect. Alternatively and in accordance with further embodiments of the present invention, the space tug can impart additional charge to the target vehicle by using a wireless charge transfer, such as charge beaming to aim the space tug charge emission at the target vehicle so that the target vehicle acquires an absolute charge. In another alternative embodiment of the present invention, the space tug can impart additional charge to the target vehicle by using a wired charge transfer, such as a tether or other wired connection. In yet another alternative embodiment of the present invention, the target vehicle can acquire an absolute charge by docking with a free flying active charging craft. In accordance with still further embodiments of the present invention, the space tug will incorporate an efficient inertial thruster system such as the electric ion propulsion system. The ion propulsion system may share components with the active charge control system. Therefore, the hybrid electrostatic space tug may utilize electrostatic forces to pull on a target vehicle and inertial thrusters to change the orbit of the two-craft (space tug and target vehicle) system.

Further embodiments of the disclosed invention comprise methods for altering the location or altitude of a target vehicle or spacecraft. These methods may include approaching the target vehicle with a space tug, and controlling the electrical potential of the space tug to establish an attractive electrostatic force or electrostatic tractor force between the space tug and the target vehicle. The method further includes applying a propulsive force after establishing the electrostatic tractor force. The method can also include maneuvering the space tug to within the vicinity of a target vehicle, determining an electrical potential of the space tug, and determining an electrical potential of the target vehicle.

Additional features and advantages of embodiments of the disclosed invention will become more readily apparent from the following description, particularly when taken together with the accompanying drawings.

DETAILED DESCRIPTION

FIG. 1depicts a space tug104in accordance with embodiments of the present invention, and the relationship of that space tug104to a target spacecraft or vehicle108. More particularly, the space tug104has an electrical potential or voltage Vstthat is opposite the electrical potential or voltage Vtargetof the target vehicle108. These opposite voltages create an attractive electrostatic force or electrostatic tractor force112between the space tug104and the target vehicle108. As shown, the space tug104is generally following an orbit116that is different than the orbit120of the target vehicle108. Moreover, by applying a propulsive force Fp124, the space tug104can alter its orbit and the orbit of the target vehicle108. For instance, as illustrated inFIG. 2, over a series of orbital periods, the space tug104, applying a relatively small propulsive force Fp124, can pull the target vehicle108from a first orbit (e.g., a geosynchronous orbit (GEO))204to a second orbit (e.g., to a super-geosynchronous orbit)208.

In general, the magnitude of the electrostatic tractor force or Coulomb force Fc112is given by:

Fc=kc⁢q1⁢q2L2⁢ⅇ-Lλd⁡(1+Lλd)
where qiis the vehicle charge level, L is the separation distance, and λdis the plasma Debye length. As can be appreciated by one of skill in the art, Debye charge shielding causes the electrostatic interaction between two craft to be partially shielded due to the interaction with the local space plasma. However, at GEO, the Debye lengths average about 180 meters, with a range of between 100 and 1000 meters. Thus, a space tug104using active charge control as disclosed herein to establish and/or maintain an electrostatic tractor force112between the space tug104and a target vehicle108at ranges of, for example, 10 to 50 meters, is well within the Debye length of a charge at such altitudes.

FIG. 3Adepicts various charge flows with respect to the space tug104and the target vehicle108. As can be appreciated by one of skill in the art, the voltage or electrical potential of the space tug104and the target vehicle108can change as a result of an imbalance in charge flows. Moreover, there typically is a floating potential at which the electron current and ion current are balanced, resulting in zero net current with respect to the vehicle under consideration. This floating potential is the value that an isolated spacecraft, such as a target vehicle108that is not subject to active charge control, would assume in equilibrium. Moreover, the equilibrium voltage potential of a target vehicle108is typically negative in shaded orbit regions, and positive in solar illuminated regions. In particular, both the space tug104and the target vehicle108are subject to solar photons304. The photons impart a positive current by ejecting electrons from the vehicle (e.g., the space tug104or the target vehicle108) subject to the solar radiation304. The electrical potential of a vehicle such as a space tug104and a target vehicle108can also be influenced by a plasma current consisting of electrons312and/or a plasma current consisting of positive ions316. The net effect of the various naturally occurring current flows can result in the subject vehicle acquiring either a positive or a negative electrical potential. Accordingly, the target vehicle can acquire or change an absolute charge due to the target vehicle's interaction with the space environment.

A space tug104in accordance with embodiments of the present invention includes an active charge control mechanism that is capable of producing an active charge emission current320. The active charge emission current can include a flow of electrons, or can include a flow of positive ions. Accordingly, regardless of the voltage potential of the target vehicle108due to imbalances in the current flows with respect to the target vehicle108, and the voltage potential imparted by naturally occurring current flows to the space tug104, an active charge emission current320can be used to impart an electrical potential to the space tug104that is opposite the electrical potential of the target vehicle108, to establish an attractive electrostatic force112between the space tug104and the target vehicle108. In accordance with further embodiments of the present invention, the space tug can change the absolute charge of the target vehicle by imparting an additional charge to the target vehicle by using wireless or wired charge transfer mechanisms. The space tug104may change the absolute charge of the target vehicle by using wireless charge beaming to aim the space tug charge emission at the target vehicle so that the target vehicle changes or acquires an absolute charge. In accordance with further embodiments of the present invention, and as depicted inFIG. 3B, the space tug104may change the absolute charge of the target vehicle108by using a tether328or other wired connection to control the charge on the target vehicle108. An oppositely charged space tug104and target vehicle108can be obtained by using a physically conducting wire/tether328that extends between the target vehicle108and a voltage source (not shown) provided as part of the space tug104. In accordance with still further embodiments of the present invention, the absolute charge of the target vehicle108may be changed using other charge transfer mechanisms, such as by using a free flying charge control device324. As depicted inFIG. 3C, a free flying charge control device324with active charge emission capability can be launched from the space tug104or some other platform and approach and physically dock with the target vehicle108in order to change the absolute potential of the target vehicle108.

FIG. 4depicts components of a space tug104in accordance with embodiments of the present invention. In general, the space tug104includes a vehicle bus404, to which the various components of the space tug104are interconnected. For example, the space tug104can include components that are typical to satellites or other space vehicles generally. Examples of such components include a controller408, a thermal control system412, a telemetry, tracking and communications system416, an attitude control system420and a power supply424. In addition, a space tug104in accordance with embodiments of the present invention includes an active charge control mechanism or system428. The space tug104can also include a space tug charge sensor432, and/or a target vehicle charge control device436. However, this is not strictly required. The Coulomb force Fcis proportional to the charge product q1q2, which can be estimated by measuring the relative motion of the vehicles. In addition, the space tug104may include a target vehicle proximity detector440that can be used to determine the relative position of a target vehicle. The electrical potential of the target spacecraft may be estimated by analyzing the local space environment. Alternatively, the polarity and charge level of the target vehicle may be estimated by determining a relative motion response from a separation distance L between the space tug and the target vehicle. The space tug104also includes a propulsion system444.

As can be appreciated by one of skill in the art, the vehicle bus404comprises the physical or supporting structure of the space tug104satellite. Accordingly, the vehicle bus404can comprise mounting points or interconnections for various other components. The controller408may comprise a general purpose programmable processor, or an application specific integrated circuit (ASIC), alone or in combination with associated memory. The controller408may operate to control operation of the space tug104generally, for example by executing programming code or instructions. Moreover, the controller408may perform other functions, alone or in combination with various physical hardware, to provide or support for various functions performed by components of the space tug104. The thermal control system412generally operates to maintain the temperature of components of the space tug104within an acceptable operating range. The telemetry, tracking and communication system416supports the provision of telemetry and location information from the space tug104to a ground station or another spacecraft, and to receive control information. Accordingly, the telemetry, tracking and communication system416may comprise provision for radio frequency and/or optical communications, and location and pose determination sensors. The power system424may comprise sources of electrical power, including batteries and solar panels.

The active charge control system428generally functions to produce an active charge emission current320to alter the electrical potential Vstof the space tug104. For example, where the electrical potential of the space tug104is to be made more negative (or less positive), the active charge control system428may operate to produce an active charge emission current320consisting of positively charged ions. Conversely, where the electrical potential of the space tug104is to be made more positive (or less negative), the active charge control system428may be operated to create an active charge emission current320consisting of negatively charged electrons.

The optional target charge control device436may be used to alter the electrical potential Vtargetof the target vehicle108. For example, the target charge control device436may include wired charge transfer devices, such as a tether328or other wired connection that is capable of attaching to the target vehicle108and altering the absolute potential of the target vehicle108. Alternatively, the target charge control device436may include a free flying charge control device324having active charge emission capability. The free flying charge control device324may be operated to be deployed from the space tug104, approach the target vehicle108, and then dock with the target vehicle108. Once docked or attached to the target vehicle108, the free flying charge control device324, can perform active charge emission to change the absolute potential of the target vehicle108.

FIG. 5depicts components of an exemplary active charge control system428in accordance with embodiments of the present invention. In this exemplary embodiment, the active charge control system428includes an ion source504. The ion source504comprises a needle type liquid metal ion source. The ion source504effects a positive active charge emission320by producing a beam of gaseous ions or an ion beam508. More particularly, the ion source504includes a needle512mounted within a reservoir of an ion source charge material516. The ion source charge material is placed in a molten state, with a thin film of the ion source charge material coating the needle512. A relatively high voltage (e.g., about 6 kV) is applied between the liquid ion source charge material516and an accelerator electrode520. At the tip of the needle512, the local electric field reaches values on the order of volts per nanometer, resulting in the production of the ion beam508that functions as the active charge emission current320. Various materials may be used as the ion source charge material516. One example of a suitable ion source charge material516is Indium.

In accordance with further embodiments of the present invention, the active charge control system428may additionally or alternatively include a cathode524for producing an active charge emission current320comprising a stream of electrons528. The cathode524may, for example, comprise a field emission cathode.

FIG. 6depicts a propulsion system444in accordance with embodiments of the present invention. In this exemplary embodiment, the propulsion system444is an ion engine or thruster604. The ion engine604includes an impulse cathode608, a propellant inlet612, magnets616, and an ion optic system620. The ion engine604may additionally include a neutralizer cathode624. The ion engine604may include or be associated with a power supply628.

In operation, the impulse cathode608produces electrons632that impact atoms636introduced by the propellant inlet612to the interior of the ion engine604. As can be appreciated by one of skill in the art, the inlet612receives propellant from a propellant tank (not shown). The impact of the ions632with the gas atoms636ionizes the atoms632, producing ions640in a diffused plasma. An anode644collects electrons632, raising the positive electrical potential of the plasma. The magnets616act to inhibit electrons and ions from leaving the plasma. The ions640are ejected from the ion engine604through the ion optic system620, as a result of the voltage difference between the grids of the ion optic system620, generating thrust. Electrons collected at the anode644can be injected into the ion beam exhaust of the ion engine604via the neutralizer cathode624.

The optional space tug charge sensor432operates to determine the electrical potential of the space tug104. While not strictly required, it can simplify the control algorithm by providing direct knowledge of the space tug charge level. Various devices or device configurations may be used in implementing the space tug charge sensor432. For example, the charge on an electrically isolated charge plate can be measured. In such a configuration, an externally mounted surface charge detector is used to determine the electrical potential of the exterior of the space tug104, while an internal charge plate or monitor is used as a reference voltage. As another example, a photo-emission based charged sensor that uses the photo-emission from a photo-emitting conductive plate is used as a reference point.

A target vehicle proximity detector or sensor440measures the separation distance between the space tug104and a target vehicle108. As an example, the target vehicle proximity detector440may comprise a laser range finder or light detection and ranging (LIDAR) system.

FIG. 7depicts aspects of a method for altering an orbit of a target vehicle using a space tug in accordance with embodiments of the present invention. Initially, the space tug is brought into proximity with the target vehicle (step704). Bringing the space tug into the vicinity of the target vehicle can include placing the space tug into a slightly higher orbit than the target vehicle, and coming to within a range of about 10 to 50 meters of the target vehicle. In accordance with embodiments of the present invention, the space tug104is maneuvered into proximity with the target vehicle using the onboard propulsion system444, which can comprise an ion engine or thruster604. Accordingly, the space tug104will have already been placed into orbit, for example by a conventional chemical rocket. In addition, placing the space tug104into proximity with the target vehicle108can include obtaining range information from a target vehicle proximity detector440, in order to determine when the space tug104is at a desired distance from the target vehicle108. As can be appreciated by one of skill in the art after consideration of the present disclosure, the desired distance is one that is close enough to establish an attractive electrostatic force112of sufficient strength to effect the desired altitude change of the target vehicle108, while maintaining separation between the space tug104and the target vehicle108, for instance to avoid an electrostatic discharge between the vehicles104and108, and to avoid direct physical contact between the vehicles104and108. Placing the space tug104into proximity with the target vehicle108can additionally include operation of the telemetry, tracking and communications system416, and the attitude control system420, to ensure that the space tug104is properly aligned with respect to the target vehicle108, and to impart a force to the target vehicle108in a desired direction.

At step708, the electrical potential of the space tug104is set. More particularly, the electrical potential of the surface of the space tug104is controlled such that it is opposite the electrical potential of the target vehicle108. For example, if the target vehicle108has a negative electrical potential, the active charge control system428may be operated to impart an overall positive electrical potential to the space tug104. Operation of the active charge control system428may be in cooperation with a target vehicle proximity detector440that may be used to determine the relative motion of the vehicles and consequently the relative position of the target vehicle108. That is, the target vehicle proximity detector440may be used to estimate the polarity and charge level of a target vehicle108by determining the separation distance L between the space tug104and the target vehicle108over time. In addition, a determination can be made as to whether control of the electrical potential of the space tug104alone will establish a sufficient attractive electrostatic force112between the space tug104and the target vehicle108(step712). If an additional attractive force is required, an electrical charge can be imparted to the target vehicle108(step714). For example, if the electrical potential of the target vehicle108is only slightly negative, it can be made more negative by directing a beam of electrons from the space tug104to the target vehicle108. In particular, by directing a stream of electrons at the target vehicle108, the target vehicle108will acquire a more negative charge. The stream of electrons may be supplied by a cathode, for example by a cathode provided as part of the active charge control system428or the propulsion system444of the space tug104. At the same time, absent any counteracting current, the space tug104will become more positive, enhancing the attractive force112between the space tug104and the target vehicle108. Alternatively, where the electrical potential of the target vehicle108is to be made more positive, a stream of ions may be directed towards the target vehicle108. Where a beam of positively charged ions is desired, such a beam may be provided by the active charge control system428and/or the propulsion system444of the space tug104. In another example, the target vehicle charge control device436may be used to change the absolute charge of the target vehicle108. For example, the target vehicle charge control device436can impart an additional electrical charge to the target vehicle108by using a tether328or other wired connection or by using a non-tethered, free flying charge control device324. As such, the target vehicle charge control device436may be used to control the absolute charge on the target vehicle108to maintain or to help maintain a desired electrostatic attractive force112.

After controlling the electrical potential of the space tug104, and/or imparting charge to the target vehicle108, the propulsion system of the space tug104may be operated to produce thrust Fp124(seeFIG. 1) (step716). In general, by applying thrust the altitude of the space tug104and the target vehicle108can be altered. Specifically, the thrust from the propulsion system444may operate on the space tug directly404, while the thrust124from the propulsion system444may operate on the target vehicle108via the attractive electrostatic force112. In accordance with embodiments of the present invention, the application of thrust using the propulsion system444is done in a way that increases the altitude of the space tug104and the target vehicle108gradually, such that the target vehicle108is brought to a desired altitude over a number of orbital periods.

At step720, a determination is made as to whether a desired separation distance between the space tug104and the target vehicle108is being maintained. If the desired separation distance is not being maintained, remedial action can be taken (step724). For example, the attractive electrostatic force Fc112can be altered by altering the electrical potential of the space tug104and/or the target vehicle108. Alternatively or in addition, the thrust Fp124provided by the propulsion system444can be altered or discontinued.

After determining that a desired separation distance between the space tug104and the target vehicle108is being maintained, or after making adjustments or otherwise taking remedial action to maintain or reestablish a desired separation, a determination may be made as to whether the target vehicle108has reached the desired altitude (step728). If the desired altitude has not been reached, the process may return to step716. If the target vehicle108has reached the desired altitude, the active charge control system428of the space tug104may be operated to place the space tug104at or near the electrical potential of the target vehicle108, to remove the attractive electrostatic force Fc112(step732). The space tug104can then use its propulsion system444to move away from the target vehicle108(step736).

At step740, a determination may be made as to whether there is another target vehicle108to be moved by the space tug104. If another target vehicle108that needs to be moved has been identified, the process may return to step704. If no additional target vehicles are identified, the process may end. For example, the space tug104can remain in orbit, until a next target vehicle108is identified. Although the aspects of the method discussed in connection withFIG. 7present steps in a particular order, it should be appreciated that the illustrated exemplary order is not required. In addition, at least some of the illustrated steps can be performed continuously or during tug operations. For example, monitoring whether a desired separation distance is being maintained can be performed continuously while the space tug104is towing a target vehicle108. Moreover, any necessary remedial action to maintain or reestablish a desired separation distance can be performed continuously.

Although a target vehicle108may comprise a defunct satellite or satellite at its end of life that is to be removed to a new orbit, embodiments of the present invention are not so limited. For example, a space tug104in accordance with embodiments of the present invention may be operated to reposition satellites equipped with charge control to new orbits. In addition, a target vehicle108is intended to include debris that is to be moved to a new orbit. For instance, pieces of a satellite that for one reason or another has broken up, or components that have served their function and are no longer operable, such as spent booster rocket sections, can be target vehicles108that are moved to a new orbit by a space tug104in accordance with embodiments of the present invention. The ability of the space tug104is disclosed herein to move target vehicles108comprising debris, even debris that is small in size but nonetheless presents a hazard to other spacecraft, highlights one of the advantages of the space tug104of the present invention. In particular, the ability to act on and move a target vehicle108without coming into direct physical contact with the target vehicle108allows the space tug104to maintain a safe separation distance from the target vehicle108, and to operate on the target vehicle108even if that target vehicle108is in an uncontrolled spin, is devoid of features that can easily be grasped, and/or is broken into multiple pieces.