Orbit separation device and orbit separation method

Abstract

An orbit separation device according to one embodiment of the present invention is attached to an object moving along a circular orbit in outer space, and comprises a tether that expands when gas is supplied to the inside and that forms a prescribed three-dimensional shape which receives atmospheric resistance for separating the object from the circular orbit.

Description

Deorbiting device and deorbiting method

[0001] The present invention relates to a deorbiting device and a deorbiting method.

[0002] Many spacecraft, such as artificial satellites and space stations, fly in Earth orbit, but so-called debris (space junk), including spacecraft that continue to fly in orbit even after their missions have ended and pieces of damaged spacecraft, has become a problem. In order to dispose of debris from orbit, a method called Post Mission Disposal (PMD) is used, in which a release device (release device) previously installed on the spacecraft is activated to remotely release the spacecraft from orbit.

[0003] For example, Patent Document 1 discloses a deceleration device having a structure in which a planar membrane member for receiving atmospheric resistance is deployed by a frame. Also, for example, Patent Document 2 discloses a technology in which a spacecraft extends a long, tape-like conductive tether in space and deorbits the spacecraft using the Lorentz force generated in the tether.

[0004] JP 2022-143395 A JP 2021-115713 A

[0005] A structure in which a planar membrane member is deployed by a frame, such as the reduction gear described in Patent Document 1, tends to be heavy and bulky. Furthermore, when the membrane member is deployed in a planar form, the rotational torque due to atmospheric resistance increases, making it difficult to maintain an attitude that is subject to atmospheric resistance. Furthermore, the long, tape-like tether described in Patent Document 2 requires advanced control of the deployment speed and the attitude of the device to ensure that it is deployed without insufficient deployment or tangling midway.

[0006] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an orbital de-orbiting device and orbital de-orbiting method that can easily deploy a tether and maintain an attitude that is subject to atmospheric resistance.

[0007] A de-orbiting device according to one aspect of the present invention is attached to an object moving in an orbit in outer space, and includes a tether that expands when gas is supplied to the inside and forms a predetermined three-dimensional shape that receives atmospheric resistance to de-orbit the object.

[0008] According to this aspect, the tether expands when gas is supplied to the interior thereof, forming a predetermined three-dimensional shape that receives atmospheric resistance to cause the target object to leave the orbit, making it easy to deploy the tether and maintain a posture that receives atmospheric resistance.

[0009] According to the present invention, a deorbiting device and a deorbiting method can be provided.

[0010] 1 is a schematic diagram showing the appearance of a de-orbit device 3 according to an embodiment when mounted on a spacecraft 2. FIG. 2 is a schematic diagram showing the appearance of a de-orbit device 3 according to an embodiment when mounted on a spacecraft in another state. FIG. 3 is a block diagram showing an example of the functional configuration of the de-orbit device 3 according to an embodiment. FIG. 4 is a diagram for explaining the dimensions of the tether 31 and the effect of the dimensions on the behavior of the tether 31. FIG. 5 is a graph showing the relationship between the altitude of the orbit and the threshold aspect ratio for tethers having various weights. FIG. 6 is a diagram showing an example of a cross-section of the tether 31 according to a modified example in a direction perpendicular to the longitudinal direction. FIG. 7 is a diagram showing another example of a cross-section of the tether 31 according to a modified example in a direction perpendicular to the longitudinal direction. FIG. 8 is a diagram showing yet another example of a cross-section of the tether 31 according to a modified example in a direction perpendicular to the longitudinal direction. FIG. 9 is a diagram showing yet another example of a cross-section of three tethers 31 according to a modified example in a direction perpendicular to the longitudinal direction. FIG. 10 is a diagram showing a cross-sectional shape parallel to the longitudinal direction of the tether 31 according to a modified example. FIG. 11 is a schematic diagram showing the appearance of two tethers 31 and a membrane portion 38 according to a modified example. FIG. 10 is a schematic diagram showing the appearance of six tethers 31 of a modified orbital departure device 3.

[0011] A preferred embodiment of the present invention will be described with reference to the accompanying drawings (in each drawing, the same reference numerals denote the same or similar configurations).

[0012] (1) Overall Configuration The overall configuration of the orbital de-orbit device 3 according to this embodiment will be described with reference to Figures 1 to 3. Figure 1 is a schematic diagram showing the appearance of the orbital de-orbit device 3 according to this embodiment when mounted on a spacecraft 2. Figure 2 is a schematic diagram showing the appearance of the orbital de-orbit device 3 according to this embodiment when mounted on a spacecraft in another state. Figure 3 is a block diagram showing an example of the functional configuration of the orbital de-orbit device 3 according to this embodiment.

[0013] 1 to 3, the direction of travel of the spacecraft 2 (the direction of the orbit around the Earth, etc.) is the X-axis, the horizontal direction perpendicular to the X-axis is the Y-axis, and the direction perpendicular to the X-axis and Y-axis is the Z-axis. In particular, the direction of the Z-axis facing away from the Earth (the positive Z-axis direction in FIG. 1) is sometimes referred to as the "zenith direction," and the direction of the Z-axis facing toward the Earth is sometimes referred to as the "geocentric direction."

[0014] The spacecraft 2 moves in an orbit around the Earth or other object in outer space. The spacecraft 2 is not particularly limited as long as it is an object that moves in an orbit, and may include an artificial satellite, a rocket, a space station, etc. The spacecraft 2 has, for example, a propulsion function and is configured to be able to move in the direction of the orbit (e.g., the positive direction of the X-axis). The mechanism of the propulsion function is not particularly limited, but can be achieved by a known mechanism such as an ion engine. The altitude of the orbit is not particularly limited, but may be, for example, approximately 200 to 1000 km. Note that the spacecraft 2 is not limited to the above-mentioned objects that move in an orbit, and may include objects that do not have a propulsion function, such as debris.

[0015] The orbital departure device 3 is a device attached to an object, such as a spacecraft 2, moving in an orbit in outer space, for causing the object to depart from the orbit. The orbital departure device 3 may include a main body 32 and a tether 31 configured to be tiltable with the main body 32 as a base point.

[0016] The tether 31 is configured to expand and form a predetermined three-dimensional shape, for example, when gas is supplied to the interior from a gas source 34 provided in the main body 32. In other words, the tether 31 according to this embodiment has an inflatable structure. Therefore, the orbital departure device 3 according to this embodiment can more easily deploy the tether 31 and maintain the deployed state than a tether that does not have an inflatable structure. Note that in this disclosure, "forming a predetermined three-dimensional shape" may include not only the case where the predetermined three-dimensional shape is completely formed, but also the case where twisting, bending, or the like occurs in the predetermined three-dimensional shape.

[0017] One end of the tether 31 may be configured as a fixed end 31T fixed to the main body 32, and the other end may be configured as a free end 31F not fixed to the main body 32. The main body 32 is configured to be able to store the tether 31 therein during the mission of the spacecraft 2, as shown in FIG. 2, for example. The main body 32 is also configured to be able to release the free end 31F of the tether 31 into space and supply a predetermined gas into the interior of the tether 31 at a predetermined timing, such as when the mission of the spacecraft 2 ends, thereby expanding the tether 31 and deploying it to form a predetermined three-dimensional shape. FIG. 1 shows the tether 31 deployed outside the main body 32 (in space). Note that while FIG. 1 depicts the direction of the tether 31 parallel to the Z axis, the tether 31 is configured to be tiltable at a predetermined angle with respect to the Z axis, with the main body 32 as the base point, depending on factors such as gravity gradient force and atmospheric resistance.

[0018] The tether 31 may be formed, for example, from a flexible thin film member. The material constituting the tether 31 may be, for example, one or more of resin, metal thin film (e.g., sputtered), adhesive, carbon fiber reinforced plastics (CFRP), carbon film, etc. The material constituting the tether 31 may also include a conductive material, such as metal fiber (e.g., aluminum) or conductive high-strength fiber. This allows current to flow through the conductive tether 31 due to induced electromotive force or an external power source, generating a Lorentz force through interaction with the Earth's magnetic field, allowing the Lorentz force to be used as part of the deorbital propulsion force. The tether 31 may also have a lapped portion formed by bonding multiple thin film members together. This lapped portion may also serve as a region subject to atmospheric resistance.

[0019] The predetermined three-dimensional shape formed by the deployed tether 31 is not particularly limited as long as it is a shape that can withstand the atmospheric resistance required to release the spacecraft 2 from its orbit.

[0020] The predetermined three-dimensional shape may be, for example, a three-dimensional shape with high isotropy about the longitudinal direction of the tether 31 (the Z-axis direction in FIG. 1 ) as its axis. The higher the isotropy about the longitudinal direction of the tether 31 (the Z-axis direction in FIG. 1 ), the smaller the variation in atmospheric resistance experienced when the tether 31 rotates around that axis. This makes it easier to ensure a projected area for receiving atmospheric resistance regardless of the orientation around the longitudinal direction. An example of a three-dimensional shape with high isotropy about the longitudinal direction of the tether 31 (the Z-axis direction in FIG. 1 ) as its axis may be a substantially cylindrical shape as shown in FIG. 1 . In this case, the longitudinal dimension of the tether 31 is not particularly limited and may be, for example, in the range of several meters to several kilometers.

[0021] A gravity gradient force acts on the tether 31. Here, the gravity gradient force is a force that acts to orient the attitude of an object in a predetermined direction due to spatial changes in the gravitational acceleration acting on the object. In particular, when an object that revolves around the center of gravity, such as the Earth, has a long shape like the tether 31, the gravity gradient force acts as a force that orients the longitudinal direction toward the center of gravity, that is, a force that reduces the angle between the longitudinal direction and the Z-axis direction.

[0022] Furthermore, the larger the longitudinal dimension of the tether 31, the more easily the gravity gradient force acts on it, and as a result, the more likely it is to point toward the zenith or the geocentric direction. The tether 31 may have dimensions such that the longitudinal direction of the deployed tether 31 is controlled toward the zenith or the geocentric direction by the gravity gradient force that the tether 31 receives. Here, the longitudinal direction of the tether 31 being controlled toward the zenith or the geocentric direction may include the tether 31, whose longitudinal direction is directed toward the zenith or the geocentric direction, not tilting up to a predetermined angle θ, which is the angle between the longitudinal direction and the Z axis, even when subjected to atmospheric resistance. In particular, the predetermined angle θ may be set to a right angle (90 degrees). Such dimensions of the tether 31 will be described further below. Note that, when the tether 31 is bent, the longitudinal direction of the tether 31 may be calculated based on the direction of each portion of the tether 31. Specifically, for example, the longitudinal direction of the tether 31 may be the average direction of the directions of each part of the tether 31, or the longitudinal direction of the tether 31 may be the direction of the straight line when the direction of the tether 31 is approximated by a straight line.

[0023] (2) Tether Storage Structure, etc. The tether storage structure of the orbital departure device 3 will be described with reference to FIGS. 2 and 3. FIG.

[0024] The main body 32 may include a storage section 33, a gas source 34, an electromagnetic valve 35, a control section 36, and a communication section 37, which are provided inside the housing. The main body 32 may further include a power source (not shown), such as a solar panel, provided on the surface of the housing.

[0025] The gas source 34 may hold a gas to be supplied to the tether 31. The gas source 34 may be configured as, for example, a gas cartridge. The type of gas to be supplied to the tether 31 is not particularly limited, but may be, for example, nitrogen gas.

[0026] The electromagnetic valve 35 may be connected to each of the gas source 34 and the tether 31. The electromagnetic valve 35 may adjust the opening and closing based on a control signal received from the control unit 36, thereby adjusting the amount of gas supplied from the gas source 34 to the tether 31.

[0027] The control unit 36 ​​performs overall control of the deorbital device 3. The control unit 36 ​​may be configured as a computer having a processor such as a CPU (Central Processing Unit) and memories such as a RAM (Random Access Memory) and a ROM (Read Only Memory). The control unit 36 ​​may control the opening and closing of an opening / closing lid (not shown) provided in the housing unit 33. When the opening / closing lid is opened, the tether 31 becomes capable of being released to the outside of the housing unit 33. Here, the gas source 34, the electromagnetic valve 35, and the control unit 36 ​​constitute a "gas supply unit." That is, the control unit 36 ​​may adjust the supply of gas from the gas source 34 to the tether 31 (such as the pressure and flow rate of the supplied gas) by controlling the opening and closing of the electromagnetic valve 35. This enables the control unit 36 ​​to control the deployment speed of the tether 31. The control unit 36 ​​may control the opening and closing of the electromagnetic valve 35 based on a control signal received from the communication unit 37. Alternatively, the control unit 36 ​​may control the opening and closing of the electromagnetic valve 35 based on various detection signals obtained from various sensors (such as an altimeter) (not shown) by executing a program stored in advance in a memory such as a ROM.

[0028] The communication unit 37 is a communication module for transmitting and receiving communication signals to and from any external device. The communication unit 37 may be configured to include an antenna, a communication circuit, etc. The communication unit 37 may receive signals from a ground management device or another spacecraft and supply the signals to the control unit 36. The communication unit 37 may also transmit signals supplied from the control unit 36 ​​to the ground management device or another spacecraft.

[0029] The storage section 33 may be configured to have, for example, a space therein capable of storing the tether 31. The manner in which the tether 31 is stored inside the storage section 33 is not particularly limited, but may be, for example, a manner in which the tether 31 is wound as shown in Fig. 3 or a manner in which the tether 31 is folded. Alternatively, the tether 31 may be stored inside the storage section 33 by being folded along a crease along the longitudinal direction of the tether 31 and then wound.

[0030] One end 31T of the tether 31 may be configured as a fixed end that is fixed to a predetermined position on the main body 32. The end 31T of the tether may be connected to the gas source 34 via an electromagnetic valve 35. The other end 31F of the tether 31 may be configured as a free end that is not fixed to the main body 32. The tether 31 may be stored inside the storage section 33, for example, during a mission of the spacecraft 2.

[0031] A slave unit may be provided at the end 31F of the tether 31. The slave unit may be configured as, for example, a sensor for detecting a predetermined parameter. The slave unit may also be configured as, for example, a communication module for transmitting and receiving communication signals to and from any external device, including an antenna and a communication circuit. The slave unit may also have a processor such as a CPU (Central Processing Unit) and memories such as RAM (Random Access Memory) and ROM (Read Only Memory). The slave unit may, for example, acquire position information of the spacecraft 2 and the slave unit itself (such as the distance and direction from the main body unit 32), acquire the deployment state of the tether 31, and transmit and receive this information to and from the control unit 36, a ground management device, or another spacecraft.

[0032] For example, when a start signal for starting tether deployment is transmitted from a ground control device or another spacecraft at a predetermined timing, such as after the spacecraft's mission is completed, the communication unit 37 receives the start signal and supplies the start signal to the control unit 36. In response to the start signal, the control unit 36 ​​opens an opening / closing lid (not shown) provided on the storage unit 33. In response to the start signal, the control unit 36 ​​also controls the opening and closing of the electromagnetic valve 35 to supply gas from the gas source 34 to the tether 31.

[0033] The speed at which the tether 31 is deployed by the supply of gas is not particularly limited, but may be, for example, a speed at which it takes several hours to deploy 1 meter, or a speed at which it takes several days to several months to deploy the tether 31. As the deployed tether 31 is subjected to atmospheric resistance, the spacecraft 2 carrying the orbital de-orbit device 3 decelerates, gradually departs from the orbit, and falls towards the atmosphere. The time from the start of de-orbit to the time the spacecraft 2 falls to an altitude at which it re-enters the atmosphere is not particularly limited, but may be several hours, weeks, months, years, decades, etc.

[0034] (3) Dimensions of the Tether, etc. The dimensions of the tether 31 and the influence of these dimensions on the behavior of the tether 31 will be described with reference to Fig. 4. The inventors of the present disclosure have found the dimensions of the tether 31 necessary for the longitudinal direction of the tether 31 to be stably controlled toward the zenith or the geocentric direction by the gravity gradient force while the tether 31 is affected by atmospheric resistance. The dimensions and the behavior of the tether will be described below.

[0035] First, based on the equation of motion of the tether 31, the work done on the tether 31 by the torque generated in the tether 31 is found.

[0036] The longitudinal dimension of the tether 31 is L (m), and the lateral dimension is W (m). Note that the longitudinal dimension and the lateral dimension of the tether 31 may both be set as approximate parameters based on the actual three-dimensional shape of the tether 31.

[0037] In this case, the equation of motion of the tether 31 simplified as a single rigid body is expressed by the following equation.

[0038]

[0039] where T is the disturbance torque [Nm] acting on the tether 31, and L ＡＭ is the angular momentum [Js] of the tether 31, and d / dt represents the time derivative.

[0040] As shown in FIG. 4, T is the torque (gravity gradient torque) T due to the gravity gradient force generated on the tether 31. Ｇ and the torque (aerodynamic torque) T due to atmospheric resistance acting on the tether 31.Ａ That is, the following equation holds:

[0041]

[0042] Here, θ is the angle [deg] that the tether 31 makes with respect to the Z axis as shown in FIG. 4, and ω is the angular velocity [deg / s] of the tether 31.

[0043] At this time, when the tether 31 is tilted from the main body 32 as the base point to the Z axis at an angle of 0 degrees to θ, the gravity tilt torque T Ｇ and aerodynamic torque T Ａ The work done by on the tether 31 is expressed by the following equation:

[0044]

[0045] Next, the gravity gradient torque T Ｇ is the work done on the tether 31.

[0046] If the angle between the longitudinal direction of the tether and the Z axis is θ, the gravity gradient torque T Ｇ is expressed by the following equation:

[0047]

[0048] where μ is the gravitational constant [m ３ / s ２ ] and R ０ is the distance [m] from the center of the Earth to the center of mass of the tether 31, and M is the mass [kg] of the tether 31.

[0049] At this time, when the tether 31 is tilted from the main body 32 as a base point to the Z axis at an angle of 0 degrees to θ, the gravity tilt torque T Ｇ The work done by on the tether 31 is expressed by the following equation:

[0050]

[0051] In addition, when the tether 31 is made of a conductive material and a Lorentz force is generated in the tether 31, the gravity gradient torque T Ｇ can be expanded to include the Lorentz force and then work can be calculated.

[0052] Next, the aerodynamic torque T Ａ is the work done on the tether 31.

[0053] If the angle between the longitudinal direction of the tether 31 and the Z axis is θ, the aerodynamic torque T Ａ is expressed by the following equation:

[0054]

[0055] Here, r Ａ is the distance [m] from the center of mass of the tether 31 to the center of wind pressure, and ρ is the atmospheric density [kg / m ３ ], V is the velocity [m / s] of the orbital deorbiter 3 (spacecraft 2) in the tangential direction of the orbit, and C Ｄ is the resistance coefficient [-].

[0056] Therefore, when the tether 31 is inclined from the main body 32 as a base point to the Z axis at an angle of 0 degrees to θ, the aerodynamic torque T Ａ The work done by on the tether 31 is expressed by the following equation:

[0057]

[0058] Next, the work done by each torque on the tether 31 is found.

[0059] By substituting the formulas (2) and (3) into the formula (1), the formula (1) is expressed as follows:

[0060]

[0061] Here, in order for the longitudinal direction of the tether 31 to be controlled toward the zenith or the center of the earth, that is, for the tether 31 whose longitudinal direction is oriented toward the zenith or the center of the earth to not tilt up to a predetermined angle θ, which is the angle between the longitudinal direction and the Z axis, even when subjected to atmospheric resistance, the work Work expressed by equation (1) must be a positive value, that is, the following equation must be satisfied.

[0062]

[0063] When this is solved for the aspect ratio (L / W), which is the ratio of the longitudinal dimension L to the lateral dimension W of the tether 31, the following equation is obtained.

[0064]

[0065] From the above, the threshold of the aspect ratio (L / W) of the longitudinal dimension L to the lateral dimension W of the tether 31, in order to control the longitudinal direction of the tether 31 to the zenith direction or the geocentric direction, is

[0066]

[0067] This becomes:

[0068] In other words, if the aspect ratio (L / W) is greater than the threshold, the tether 31 will not tilt until its longitudinal direction forms the predetermined angle θ with the Z axis. Then, tension is generated in the tether 31 due to the gravity gradient force, and the tether 31 passively maintains its orientation in which it is subjected to atmospheric resistance, making it possible for it to withstand large atmospheric resistance for deorbiting. Therefore, the longitudinal direction of the tether 31 is stably controlled toward the zenith or geocentric direction by the gravity gradient force.

[0069] In particular, when θ is set to a right angle, i.e., when θ=90 degrees,

[0070]

[0071] Therefore, equation (4) becomes

[0072]

[0073] In other words, if the aspect ratio is equal to or greater than this value, the tether 31, whose longitudinal direction faces the zenith or the geocenter, will not tilt up to 90 degrees even when subjected to atmospheric resistance. Here, if the angle θ between the longitudinal direction of the tether 31 and the Z axis exceeds 90 degrees, the effects of the gravitational gradient force on the tip portion (the portion of the free end 31F) and the base end portion (the portion connected to the main body 32) of the tether 31 are reversed, and the base end portion of the tether 31 attempts to face the zenith or the geocenter with the tip portion as the base point, increasing the possibility that the tether 31 will become entangled.

[0074] In the orbital de-orbit device 3 according to this embodiment, by designing the tether 31 based on the above dimensions, tension is generated due to the gravity gradient force even after the tether 31 has completed deployment, and the attitude in which it is subjected to atmospheric resistance is passively maintained, making it possible to withstand large atmospheric resistance for de-orbit. Furthermore, even though large atmospheric resistance for de-orbit is obtained, disturbances caused by that atmospheric resistance that disrupt the attitude in which it is subjected to atmospheric resistance and entanglement of structures around the spacecraft are reduced, making it possible to maintain the performance of the orbital de-orbit device 3 over a long period of time.

[0075] (4) Relationship Between Altitude and Aspect Ratio Threshold The relationship between altitude and aspect ratio threshold will be explained with reference to Fig. 5. Fig. 5 is a graph showing the relationship between the orbit altitude and the aspect ratio threshold for tethers 31 having various masses. The horizontal axis represents the spacecraft altitude (km), and the vertical axis represents the aspect ratio (L / W) threshold.

[0076] For example, for a tether 31 having a mass of 100 kg, when the altitude of the orbit is 300 km, the threshold aspect ratio is approximately 14.6. That is, for a tether 31 having a mass of 100 kg, when the altitude of the orbit is 300 km, if the aspect ratio is approximately 14.6 or more (when the longitudinal dimension L of the tether 31 is approximately 14.6 times or more the lateral dimension W of the tether 31), the longitudinal direction of the tether 31 can be stably controlled toward the zenith or the geocentric direction by the gravity gradient force.

[0077] Furthermore, when the altitude of the orbit is 300 km, the threshold value of the aspect ratio decreases as the mass of the tether 31 increases. Therefore, if the longitudinal dimension L and the transverse dimension W of the tether 31 are set so that the threshold value of the aspect ratio is approximately 14.6 (if the longitudinal dimension L of the tether 31 is set to be approximately 14.6 times the transverse dimension W of the tether 31), the longitudinal direction of the tether 31 can be stably controlled toward the zenith or the geocentric direction by the gravity gradient force at an altitude of 300 km, even if the mass of the tether 31 is 100 kg or more.

[0078] Furthermore, if the tether 31 with a mass of 100 kg is deployed in orbit, for example, at an altitude of 500 km, atmospheric resistance increases as the altitude decreases to 400 km and then 300 km. Therefore, in order to continue to receive atmospheric resistance stably even when the spacecraft 2 reaches an altitude of 300 km, the aspect ratio required for the longitudinal direction of the tether 31 to be stably controlled in the zenith direction or the geocentric direction by the gravity gradient force, i.e., the longitudinal dimension relative to the lateral dimension of the tether 31, must be greater than the threshold value of 14.6 at an altitude of 300 km.

[0079] In addition, by increasing the aspect ratio of the tether 31, it becomes possible to drop a spacecraft into the atmosphere even if the spacecraft has a light mass or a short target number of years for its fall.

[0080] (5) Modifications Modifications of the orbital departure device will be described with reference to Figures 6A to 6D. Figures 6A to 6C are diagrams showing examples of cross sections of the modified tether 31 in a direction perpendicular to the longitudinal direction. Figure 6D is a diagram showing yet another example of cross sections of the three tethers 31 of the modified orbital departure device 3 in a direction perpendicular to the longitudinal direction.

[0081] 6A , for example, the cross-sectional shape of the tether 31 perpendicular to the longitudinal direction may be elliptical. In an elliptical shape, the isotropy of the tether 31 with respect to the longitudinal direction as its axis can be evaluated, for example, by the ratio (minor axis / major axis) of the length of the minor axis ds (minor axis) to the length of the major axis dl (major axis), i.e., the ellipticity. In the cross-sectional shape of the tether 31 according to this embodiment perpendicular to the longitudinal direction, the ellipticity is preferably 0.5 or more, more preferably 0.6 or more, even more preferably 0.7 or more, even more preferably 0.8 or more, even more preferably 0.9 or more, and even more preferably 0.95 or more.

[0082] As shown in FIG. 6B , for example, the cross-sectional shape of the tether 31 perpendicular to the longitudinal direction may be a polygon. FIG. 6B illustrates, as an example, a case where the cross-sectional shape of the tether 31 perpendicular to the longitudinal direction is a regular octagon. In a regular polygon, for example, the isotropy of the tether 31 with respect to the longitudinal direction as an axis can be evaluated based on the number of axes of line symmetry. In the case of a regular octagon, as shown by the dotted lines a and b in FIG. 6B , the lines connecting two opposing vertices become axes of line symmetry, and four axes of line symmetry can be provided. The cross-sectional shape of the tether 31 perpendicular to the longitudinal direction according to this embodiment is not limited to a regular octagon and may be another polygon. For example, the cross-sectional shape preferably has one or more axes of line symmetry, more preferably two or more axes of line symmetry (e.g., a rectangle), even more preferably three or more axes of line symmetry (e.g., an equilateral triangle), and even more preferably four or more axes of line symmetry (e.g., a square).

[0083] As shown in Fig. 6C , for example, the cross-sectional shape of the tether 31 perpendicular to the longitudinal direction may be a five-pointed star. In the case of a pentagram, as shown by dotted lines a and c in Fig. 6C , the lines connecting the vertices of opposing convex and concave portions become axes of line symmetry, and five axes of line symmetry can be provided. Note that the cross-sectional view of the tether 31 is not limited to a so-called five-pointed star as shown in the figure, and may be other star-shaped polygons or pointed star shapes, such as a six-pointed star.

[0084] FIG. 6D is a diagram showing the cross-sectional shapes of three tethers 31 in an orbital departure device 3 having three approximately cylindrical tethers 31. In this manner, the orbital departure device 3 may have multiple tethers 31. By having multiple tethers 31, even if a hole is formed in one of the tethers 31 and gas leaks from the hole, the function of the orbital departure device 3 can be ensured by the other tethers 31. The radii of the cross-sectional shapes of the tethers 31 may be the same or different. The arrangement of the tethers 31 is not particularly limited, but may be, for example, as shown in FIG. 6D , such that the centers of the tethers 31 are disposed near the vertices of a triangle. The multiple tethers 31 may be independently tiltable with respect to the Z axis, with the main body 32 as the base point. Alternatively, at least two of the multiple tethers 31 may be connected to each other. Even when multiple tethers 31 are provided, as long as axes of line symmetry can be established, the isotropy of the tethers 31 along the longitudinal direction can be evaluated based on the number of axes of line symmetry.

[0085] 6B, 6C, and 6D, the tether 31 has a larger number of side surfaces than when the tether 31 forms a substantially cylindrical three-dimensional shape. Increasing the number of side surfaces of the tether 31 improves the rigidity of the tether 31, making it easier to maintain the three-dimensional shape formed by the deployment of the tether 31.

[0086] The isotropy of the tether 31 about its longitudinal axis may be evaluated using an index other than the number of axes of symmetry in the cross-sectional shape of the tether 31 perpendicular to its longitudinal direction. For example, the isotropy may be evaluated using an index based on the projected area (including the projected area per unit length in the longitudinal direction) obtained by projecting the tether 31 onto a predetermined plane parallel to the longitudinal direction of the tether 31. The projected area makes it possible to specify the minimum area required for the tether 31 to experience atmospheric resistance when the tether 31 rotates about its longitudinal axis. For example, when the tether 31 is rotated around its longitudinal axis, a function of the angle of rotation about the axis and the projected area is obtained. In this case, the difference or ratio between the maximum and minimum values ​​of the projected area may be used as an index of the isotropy of the tether 31 about its longitudinal axis. Alternatively, the ratio of the maximum or minimum value of the projected area to the average or median value of the projected area may be used as an index of the isotropy of the tether 31 about its longitudinal axis. When the ratio of the minimum value to the maximum value of the projected area is used as an index, for example, the ratio is preferably 0.5 or more, more preferably 0.6 or more, even more preferably 0.7 or more, even more preferably 0.8 or more, even more preferably 0.9 or more, and even more preferably 0.95 or more.

[0087] FIG. 7 is a diagram showing a cross-sectional shape of a modified tether 31 taken along a line parallel to the longitudinal direction. As shown in FIG. 7 , the tether 31 may have multiple compartments 31c separated by partition members 31s and connected via gas supply channels 31g. This makes it easier to maintain the structure of the tether 31 even when the longitudinal dimension of the tether 31 is increased. The partition members 31s may be formed integrally with the tether 31 or as separate members from the tether 31. The number of compartments 31c is not particularly limited and may be two, three, four, or more. The dimensions of the compartments 31c may be the same or different between different compartments. The gas supply channel 31g may be provided with a valve, for example, at a position separating the multiple compartments 31c, allowing gas to flow in one direction. Even if a hole is created in the tether 31 due to a collision with other flying objects such as debris, causing a gas leak, the interior of the tether 31 is divided into multiple compartments 31c, making it possible to prevent poor deployment due to gas leakage.

[0088] 8A and 8B, a modified example of the deorbiting device 3 will be described.

[0089] FIG. 8A is a schematic diagram showing the appearance of two tethers 31 and a membrane portion 38 of a modified orbital departure device 3. As shown in FIG. 8A , the orbital departure device 3 may have a structure in which a membrane portion 38 is provided between two substantially cylindrical tethers 31. The membrane portion 38 may be formed of the same material as the tethers 31, or may be formed of a different material. In particular, the membrane portion 38 may be made of a thin material to reduce weight. Each of the two substantially cylindrical tethers 31 may have an inflatable structure, while the membrane portion 38 may not have an inflatable structure. The structure shown in FIG. 8A makes it possible to obtain greater atmospheric resistance compared to a configuration having only one substantially cylindrical tether 31.

[0090] FIG. 8B is a schematic diagram showing the appearance of six tethers 31 of a modified orbital de-orbit device 3. As shown in FIG. 8B , the orbital de-orbit device 3 may have a configuration in which tethers 31 having a substantially cylindrical inflatable structure are connected in a row. In particular, FIG. 8B shows six tethers 31 connected in a row. The structure shown in FIG. 8B can provide greater atmospheric resistance compared to a configuration having only one substantially cylindrical tether 31. In particular, for example, if the orbital de-orbit device 3 continues to rotate around the longitudinal direction of the tether 31 due to atmospheric resistance, greater atmospheric resistance can be achieved compared to a configuration having only one substantially cylindrical tether 31. Furthermore, with this configuration, even if gas leakage occurs in one tether 31, the gas leakage does not cause gas leakage in the other tethers 31, thereby improving resistance to gas leakage. Furthermore, the amount of gas required per unit area subjected to atmospheric resistance can be reduced.

[0091] As described above, the tether 31 according to this embodiment has an inflatable structure. Therefore, in the de-orbiting device 3 according to this embodiment, it is easier to deploy the tether 31 and maintain the deployed state compared to a tether without an inflatable structure. Furthermore, in the de-orbiting device 3 according to this embodiment, by designing the tether 31 based on the above dimensions, tension is generated by the gravity gradient force even after the tether 31 has completed deployment, and the attitude in which it is subjected to atmospheric resistance is passively maintained, making it possible to withstand large atmospheric resistance for de-orbiting. Furthermore, despite the large atmospheric resistance required for de-orbiting, disturbances caused by the atmospheric resistance that disrupt the attitude in which it is subjected to atmospheric resistance and entanglement of structures around the spacecraft are reduced, making it possible to maintain the performance of the de-orbiting device 3 over a long period of time.

[0092] Furthermore, with a string-like or band-like long tether, there is a high possibility that the distal end of the tether will rewind toward the proximal end when the tether is fully extended after deployment. In contrast, the tether 31 according to the present embodiment has an inflatable structure, and the interior is filled with gas, which makes it easier to maintain a predetermined three-dimensional shape, making it possible to suppress the rewinding described above.

[0093] The above-described embodiments are intended to facilitate understanding of the present invention and are not intended to limit the present invention. The elements of the embodiments, as well as their arrangement, materials, conditions, shapes, sizes, etc., are not limited to those illustrated and can be modified as appropriate. Furthermore, configurations shown in different embodiments can be partially substituted or combined with each other.

[0094] 2...spacecraft, 3...orbital deorbiting device, 31...tether, 32...main body, 33...accommodation section, 34...gas source, 35...electromagnetic valve, 36...control section, 37...communication section, L...longitudinal dimension of tether, W...transverse dimension of tether, 31c...compartment, 31s...partition member, 31g...supply flow path

Claims

1. An orbital departure device that is attached to an object moving in an orbit in outer space, comprising a tether that expands when gas is supplied thereto and forms a predetermined three-dimensional shape that receives atmospheric resistance to depart the object from the orbit.

2. The orbital departure device of claim 1, wherein the tether is formed of a flexible thin film.

3. An orbital departure device as described in claim 1 or 2, wherein the longitudinal direction of the tether is controlled to the zenith direction or the geocentric direction by the gravity gradient force acting on the tether.

4. The orbital departure device of claim 3, wherein the aspect ratio (L / W), where L is the longitudinal dimension of the three-dimensional shape and W is the transverse dimension of the three-dimensional shape, is set to be equal to or greater than a predetermined threshold value such that the tether, whose longitudinal direction is oriented toward the zenith or the center of the earth, does not tilt to the horizontal direction even due to atmospheric resistance.

5. The orbital departure device of claim 4, wherein the predetermined threshold is defined by the following formula: Here, R ０ is the distance from the center of the Earth to the center of mass of the tether, and r Ａ is the distance from the center of mass of the tether to the center of wind pressure, ρ is the atmospheric density, V is the speed of the tether in the tangential direction of the orbit, M is the mass of the tether, and θ is the angle between the zenith or geocentric direction and the longitudinal direction of the tether.

6. The orbital departure device of claim 5, wherein said θ is set to a right angle.

7. An orbital departure device as claimed in any one of claims 1 to 6, further comprising a housing for housing the tether.

8. The deorbital device of any one of claims 1 to 7, wherein the tether comprises a conductive material.

9. The orbital departure device of any one of claims 1 to 8, further comprising a gas supply that supplies the gas to the interior of the tether.

10. The orbital departure device of claim 9, wherein the gas supply controls the pressure or flow rate of gas supplied to the tether.

11. A method for de-orbiting an object moving in an orbit in outer space using the orbital de-orbiting device according to any one of claims 1 to 10, comprising: a step of supplying gas to the tether to expand it, thereby causing the tether to form the predetermined three-dimensional shape.

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