How to fly to Alpha Centauri - technical details
Not so long ago, Milner and Hawking made a noise by announcing their Breakthrough Starshot project. The project is worth $ 100 million, which will be spent on the study of the technical possibility of flying to Alpha Centauri. The engineering and research phase will last for a number of years, after which the development of the mission to Alpha Centauri itself will require the budget of the largest scientific experiment to date.
So, what is known at the moment from the developers of the project?
System concept, including laser emitter and light sail
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The Breakthrough Starshot project, according to the authors, is an attempt to approach space travel from Silicon Valley.
It involves the construction of an array of lasers in the high-altitude regions of the Earth, and the creation of special nanocraft — an array of cosmic femto satellites , which are accelerated by the radiation of these lasers.
Nanocraft is a robotic spacecraft of the mass of the order of grams, consisting of two parts:
1) StarChip Electronic Module: Moore's Law has significantly reduced the size of electronic components. This allows you to create gram devices that carry cameras, photon thrusters, power, navigation and communication equipment, which is a fully functional space probe. In this case, the cost of these probes for mass production will be equal to the cost of a smartphone.
2) Solar sail . Advances in nanotechnology have allowed the creation of incredibly thin and lightweight metamaterials. This promises the possibility of creating meter sails with a thickness of hundreds of atoms and a mass on the order of grams.
In recent years, the growth of laser power and the drop in their value is subject to Moore's law. This allows you to create special phased laser arrays ("light beamer"), with a capacity of up to 100 gigawatts.
Total Breakthrough Starshot project will require:
- Buildings in the high mountains of a kilometer array of laser emitters.
- Generate and store several gigawatt-hours of energy for each run
- Launch of the "mother ship", which will bring thousands of nanocrats to high orbit
- Using the capabilities of adaptive optics in real time, to compensate for atmospheric phenomena
- Focusing the light beam on the light sail for several minutes to disperse the nanocraft to the required speed (20% of the speed of light)
- Accounting for the effects of collisions with interstellar dust on the way
- Capturing the image of the planets, transmitting other scientific information to Earth using the on-board laser communication system
- The use of laser emitters to obtain data from nanocraft more than 4 years later
Some of these requirements are significant engineering challenges for the project team to address. The proposed laser motor system in scale far exceeds all working today analogues. The very essence of the project involves global cooperation and collaboration.
The concept of a system of nanocrafts, laser emitters and the StarChip electronic module is, for today, the most plausible and realistic way to launch the mission to Alpha Centauri in our generation. Key elements of the proposed system design are based on technologies that are already available, or will be available in the near future, with reasonable assumptions.
A team of scientific experts of the project does not see technically unrealizable things. But, as with any "flight to the moon," there are engineering challenges that need to be overcome.
The authors of the project listed ( eng ) problems and features of the mission:
1 Watt laser diodes weighing less than a gram are widely available today at a very low price. Production trends are such that there is a doubling of the laser power at the same mass every two years. It can be expected that this trend will continue for some time. This will help create efficient thrusters for nanocrafts.
The growth of laser power. From here
2 megapixel cameras weighing less than a gram are available at a low price. Their development is also subject to Moore's law, allowing you to double the number of pixels for the same matrix mass every two years.
Also of interest are the potential capabilities of cameras operating on the principle of a flat Fourier capture array ( PFCA ). They do not require mirrors, lenses and other moving parts. Consist of an array of semiconductor elements that react to light depending on its angle of incidence.
The volume of PFCA can be 100 thousand times smaller than the smallest focal camera. However, while this technology is at the start of its path.
Mona Lisa, shot with a PFCA camera.
Special coating is necessary to protect the structure of nanocrafts from collisions with particles in interstellar space. One of these materials is a beryllium-copper alloy.
The design of the battery is one of the most difficult technical challenges of the project.
At present, plutonium-238 or americium-241 is considered to be the main source of energy on board. The system is powered by 150 grams. This includes the mass of the radioisotope and supercapacitor, which will be charged by nuclear decay.
There are also ideas to take advantage of the heating of the frontal part of the surface of the nanocrats (due to interaction with interstellar dust). The heat source can supply 6mW for each square centimeter of its area during the cruise phase of the mission in interstellar space.
The light sail itself may be able to be covered with a thin film of photovoltaic material, as was done in the Japanese mission of the solar sail IKAROS . This can be very useful when approaching another star at a distance of 2 astronomical units. At a distance of 1 astronomical unit, such material, even with an efficiency of only 10%, will be able to provide 2 kW of power. This is more than 100 thousand times greater than the power of a radioactive energy source, and is likely to make it possible to achieve significantly higher data transmission rates via laser communications.
The search for the Earth is a fairly simple task, considering its proximity to the Sun — a very bright star, if viewed from the direction of Alpha Centauri.
Due to the diffraction limit, the angular diameter of a beam with a wavelength of 1 micron on a meter-class antenna will be about 0.1 angular seconds. The orientation of this accuracy can be achieved using photon thrusters.
Images of target planets can be transmitted by a single-watt laser on board, in pulsed mode. When approaching the target, the sail will be used to focus the laser signal.
For example, for a sail 4m in size, the diffraction limit of the spot size on Earth will be about 1000m 1 . Approximately the same scale is planned to do the receiving array of antennas. Using a sail as an optical system may require different forms of sail at the start of the mission (during acceleration) and during the communication phase. For more efficient transmission of information, when approaching the target, the sail can be shaped into a Fresnel lens. Due to the Doppler effect in the shift of nanocrafts relative to the Earth, it is necessary to use a laser wave shorter than that of the launch system - this will allow maintaining a high transmission rate through the atmosphere of our planet.
The recent successes of the MIL Lincoln Labs group and the Jet Propulsion Laboratory have shown the ability to detect single photons emitted by a laser from very large distances. Currently, the record holder is the LADEE system, which is capable of operating at lunar distances. It uses the technique of cryogenically cooled nanotubes. This allows you to transfer 2 bits per photon. The system uses 10cm optics on a spacecraft and a one-meter telescope on the ground.
The array of laser emitters involved in the acceleration of nanocrafts will be used in the inverse mode, as an array of receiving antennas.
The research phase assumes the use of a 100 gigawatt laser in the mission. How will such radiation affect the solar sail?
The most perfect reflective material for today is a dielectric mirror - a composite material with a layer thickness matched to the wavelength.
A dielectric mirror is able to reduce the amount of heat absorbed by 5 orders of magnitude, reflecting 99.999% of radiation.
For a 100-watt laser and a 4x4m sail, this means that every square meter of the sail will be heated with an energy of 60 kW. This is a lot - about 50 electric kettles at full power. It is difficult to dissipate such power by radiation. But, according to the developers, it will heat the sail, but will not melt it. It is assumed that using a fully dielectric sail with optimized materials it will be possible to reduce the absorption below 9 orders of magnitude from the incoming radiation.
Consider the use of new materials like graphene.
It is also possible to use materials with low absorption, even without high reflectivity (for example, glass). Such materials are used in fiber optics under high loads.
In addition to protection from the sail, the electronics of the StarChip module must be protected from the incoming flow. This can be achieved by a combination of geometry (orienting electronics "in profile", with a low cross section) and covering the most important components with special protection. Such coatings can be mentioned multilayer dielectric solutions that have already been prometrostimulated in laboratories. Weakly absorbing sail material, together with the limited use of highly reflective material to protect electronics, will protect StarChip without exceeding the gram scale of the module. For further production, a construction of silicon microcubes on a silicon dioxide substrate is being studied.
It is necessary to develop a sail skeleton that will hold the load when the device is accelerated, to be resistant to interaction with the interstellar medium, and will be able to change the shape of the sail. At present, a number of graphene-based composite materials are being considered, which are capable of changing their length depending on the voltage applied to them. It was previously shown that centrifugal acceleration of tiny masses at the edges can stretch the sail.
The beam shape and light sail devices should be optimized for stability during the launch phase. During this period of about 10 minutes, the sail receives 1 terajoule of light energy. For this reason, even small differences in sail properties or beam irregularities will shift the center of pressure from the center of mass of the sail, and shift its thrust vector.
The modern industry of optical coatings for mass production of smartphones and telescopic optics is already at an acceptable quality level for the mission. But the final sail material does not yet exist and must be developed.
Estimation of the estimated cost of the laser array on Earth is based on the extrapolation of the last two decades, as well as on the prospects of cheaper prices for mass production.
The cost of laser amplifiers decreases exponentially from 1990 to 2015, halving every one and a half years. If the trend continues, the construction of a large radiator in the coming decades will cost several orders of magnitude cheaper.
While developers compare the cost with the largest research project in the world. This could be, for example, the ISS (worth $ 157 billion) or an experimental ITER fusion reactor ($ 15 billion).
To test the capabilities of the system, a case with a meter scale sail was studied. For example, to focus a beam of light on a 4x4m sail at a distance of 200 thousand kilometers, a focus angle of 2 nanoradians (0.4 angular milliseconds) is required. This is the diffraction limit for a kilometer-long laser emitter operating at a wavelength of 1 micron.
Interferometry for Event Horizon Telescope has demonstrated the ability to achieve sub-nanoradian accuracy at a wavelength of 1mm.
The atmosphere introduces two effects:
- absorption (violation of transmission integrity)
- reduction in the quality of the beam (erosion of the beam)
The transmitting ability of the atmosphere at a wavelength of 1 micron is very good - more than 90% for objects located high in the mountains. With such an arrangement, this will reduce the erosion of the beam in the atmosphere, which will allow adaptive optics to approach the diffraction limit as close as possible. Atmospheric turbulence, which blurs the beam, is about 4 times lower at an altitude of 5 km than at sea level. Even more neutralize the action of the atmosphere can be a correction of the mode of operation of laser emitters with the help of a beacon in space.
The Breakthrough Starshot project wants to achieve a diffraction limit for optical laser systems of 0.2–1 km. This is 1-2 orders of magnitude better than existing solutions, but there are no fundamental limitations in achieving this goal.
The laser emitter should be focused on a spot on a sail smaller than the size of the sail itself in orbit 60,000 km above the ground.
Laser targeting must be coordinated with the position of the Alpha Centauri star system so that the span of the system passes within two astronomical units. The use of photon thrusters will correct the course by 1-2 astronomical units.
In the problem of beam positioning, the main problem is to keep the sail on the beam. It depends on the size of the sail and the distance to it. For a meter sail, the working distance for launch can reach several million kilometers. The aiming accuracy required at this distance is a few angular milliseconds. There are several ways to solve this problem.
The model of the atmosphere is calibrated using radar, a laser beam and real-time optical measurements. This will achieve the required positioning accuracy.
Most terrestrial telescopes (for example, the Keck telescope) have an accuracy on the order of several arc-seconds and are limited to tracking objects in a mode of 100 arc milliseconds. For mission objectives, significant improvement in accuracy is required.
Nevertheless, the generation of a laser beam by a system with a phased array, with a tracking system of a beacon signal (for correcting the influence of the atmosphere) of a spacecraft can make it possible to achieve the required accuracy.
There are a number of effects that make this task difficult. These are beam instability, laser modes of operation, forces acting on the sail, heating of the sail, atmospheric heterogeneities caused by the energy of the emitters.
The problems described above can be solved by rotating the sail and regulating the shape of both the sail and the beam of rays that arrive at it. Feedback will help the operation of laser emitters, but a short flight time requires self-stabilization of the system.
One promising approach is to give the sail a special shape that stabilizes its position on the beam. That is, during rotation, the torque will be affected by such torques and forces that will seek to restore its orientation. High-frequency shudder will reduce the total amount of energy transmitted to the sail, but a good sail dynamics can reduce its susceptibility to interference above a certain frequency.
Since a phased array array will be used to form the beam, the beam profile can be shaped to maximize the ability of the sail to maintain its own position on the beam, even without a feedback mechanism.
Energy production and storage is a technological challenge.
The generation of 100 GW of power and its delivery within a few minutes is quite achievable at the modern level of technology. Natural gas power plants can generate energy at a cost of $ 0.1 per kilowatt-hour.
Currently, batteries and supercapacitors are also available, which are able to provide the necessary storage capacity at a reasonable price.
In order to deliver a nanocraft to an exoplanet with an accuracy of 1 astronomical unit, it may be necessary to accurately account for all massive bodies near the flight path.
Some of the information can be collected by the first missions of the project and taken into account in subsequent launches. Efforts are also being made to better understand the ephemeris - the orbital positions of large objects at specific points in time that can affect the trajectory of motion. This includes collaboration with the largest telescopes in the southern hemisphere, including Very Large Telescopes and Gemini.
Based on estimates of the density of dust in the interstellar medium closest to us, during the journey to Alpha Centauri, each square centimeter of the frontal cross-sectional area of ​​the StarChip electronic module and the light sail will collide with approximately 1000 dust particles from 100 nanometers and above. However, the probability of collision with a particle of 1 micrometer for the entire flight time is about 10%. And the probability to meet larger particles is insignificant.
A dust particle with a size of 100 nanometers, moving at a speed of 20% of the speed of light, will penetrate the electronic module to a depth of about 0.4 mm. To estimate the effect, calculations are given for the module, with dimensions of 10 cm x 0.1 mm. The cross-sectional area of ​​such a module is 0.1 cm 2 . A protective coating of beryllium bronze applied to the front of such a module can protect it from dust and erosion. If necessary, the StarChip geometry can be changed (for example, in the form of a “needle”) to further reduce the cross-sectional area.
The sail itself, to minimize damage, can be collapsed into a more streamlined configuration during the cruising phase of flight.
The impulse from the impact of a particle with a size of 100 nm is relatively small, and can be compensated by photon thrusters.
The influence of interplanetary dust inside the solar system is insignificant compared to interstellar dust. Little is known about the presence of dust in the Alpha Centauri system.
The mean free path and the Larmor radius of the particles of the interstellar plasma are much larger than the size of the nanocraft. This means that such particles will affect the walls independently of each other, without forming shock shock.
The protons from the interstellar plasma at a speed of 20% of the speed of light will affect the nanocraft with a kinetic energy of 18 MeV, and the electrons will have an energy of 10.2 keV. It does not matter whether the proton and the electron are combined into a hydrogen atom, or they arrive separately. Erosion of the nanocraft surface due to spraying will occur. The number of atoms sputtered in this way will be about 1000 per cm 2 . The total mass loss of the front surface of the device will be only a few layers.
Protons at 18 MeV will penetrate to a depth of the order of a few millimeters. Therefore, a protective layer capable of stopping such particles will be necessary to avoid damage to the electronics.
Cosmic rays are much less rare than interstellar protons, and therefore can be ignored. Collisions with heavier elements should be mitigated by a protective coating: helium nuclei have energies of the order of 72 MeV and their number is about 10% of the number of free protons. The nuclei of carbon, nitrogen and oxygen carry 200-300 MeV of energy and are present in an amount of 0.01% of the total.
To develop protection technologies, it is necessary to conduct laboratory experiments for ions moving at a speed of 20% of the speed of light and colliding with a solid.
Collisions with interstellar ions and electrons, theoretically, can have their advantages: they could give the nanocraft potential up to 10 kV (kinetic energy per electron). The front surface of the nanocrats will be heated at a speed of 6 mW per cm 2 , which will give a small thermoelectric source of energy when traveling in interstellar medium.
So, what is known at the moment from the developers of the project?
System concept, including laser emitter and light sail
')
The Breakthrough Starshot project, according to the authors, is an attempt to approach space travel from Silicon Valley.
It involves the construction of an array of lasers in the high-altitude regions of the Earth, and the creation of special nanocraft — an array of cosmic femto satellites , which are accelerated by the radiation of these lasers.
System components
Nanocraft is a robotic spacecraft of the mass of the order of grams, consisting of two parts:
1) StarChip Electronic Module: Moore's Law has significantly reduced the size of electronic components. This allows you to create gram devices that carry cameras, photon thrusters, power, navigation and communication equipment, which is a fully functional space probe. In this case, the cost of these probes for mass production will be equal to the cost of a smartphone.
2) Solar sail . Advances in nanotechnology have allowed the creation of incredibly thin and lightweight metamaterials. This promises the possibility of creating meter sails with a thickness of hundreds of atoms and a mass on the order of grams.
Laser emitters
In recent years, the growth of laser power and the drop in their value is subject to Moore's law. This allows you to create special phased laser arrays ("light beamer"), with a capacity of up to 100 gigawatts.
Total Breakthrough Starshot project will require:
- Buildings in the high mountains of a kilometer array of laser emitters.
- Generate and store several gigawatt-hours of energy for each run
- Launch of the "mother ship", which will bring thousands of nanocrats to high orbit
- Using the capabilities of adaptive optics in real time, to compensate for atmospheric phenomena
- Focusing the light beam on the light sail for several minutes to disperse the nanocraft to the required speed (20% of the speed of light)
- Accounting for the effects of collisions with interstellar dust on the way
- Capturing the image of the planets, transmitting other scientific information to Earth using the on-board laser communication system
- The use of laser emitters to obtain data from nanocraft more than 4 years later
Some of these requirements are significant engineering challenges for the project team to address. The proposed laser motor system in scale far exceeds all working today analogues. The very essence of the project involves global cooperation and collaboration.
Technical features of the project
The concept of a system of nanocrafts, laser emitters and the StarChip electronic module is, for today, the most plausible and realistic way to launch the mission to Alpha Centauri in our generation. Key elements of the proposed system design are based on technologies that are already available, or will be available in the near future, with reasonable assumptions.
A team of scientific experts of the project does not see technically unrealizable things. But, as with any "flight to the moon," there are engineering challenges that need to be overcome.
The authors of the project listed ( eng ) problems and features of the mission:
StarChip electronic module
4 photon thruster
1 Watt laser diodes weighing less than a gram are widely available today at a very low price. Production trends are such that there is a doubling of the laser power at the same mass every two years. It can be expected that this trend will continue for some time. This will help create efficient thrusters for nanocrafts.
The growth of laser power. From here
4 cameras
2 megapixel cameras weighing less than a gram are available at a low price. Their development is also subject to Moore's law, allowing you to double the number of pixels for the same matrix mass every two years.
Also of interest are the potential capabilities of cameras operating on the principle of a flat Fourier capture array ( PFCA ). They do not require mirrors, lenses and other moving parts. Consist of an array of semiconductor elements that react to light depending on its angle of incidence.
The volume of PFCA can be 100 thousand times smaller than the smallest focal camera. However, while this technology is at the start of its path.
Mona Lisa, shot with a PFCA camera.
Protective covering
Special coating is necessary to protect the structure of nanocrafts from collisions with particles in interstellar space. One of these materials is a beryllium-copper alloy.
Battery
The design of the battery is one of the most difficult technical challenges of the project.
At present, plutonium-238 or americium-241 is considered to be the main source of energy on board. The system is powered by 150 grams. This includes the mass of the radioisotope and supercapacitor, which will be charged by nuclear decay.
There are also ideas to take advantage of the heating of the frontal part of the surface of the nanocrats (due to interaction with interstellar dust). The heat source can supply 6mW for each square centimeter of its area during the cruise phase of the mission in interstellar space.
The light sail itself may be able to be covered with a thin film of photovoltaic material, as was done in the Japanese mission of the solar sail IKAROS . This can be very useful when approaching another star at a distance of 2 astronomical units. At a distance of 1 astronomical unit, such material, even with an efficiency of only 10%, will be able to provide 2 kW of power. This is more than 100 thousand times greater than the power of a radioactive energy source, and is likely to make it possible to achieve significantly higher data transmission rates via laser communications.
Communication
Transmitter orientation to Earth
The search for the Earth is a fairly simple task, considering its proximity to the Sun — a very bright star, if viewed from the direction of Alpha Centauri.
Due to the diffraction limit, the angular diameter of a beam with a wavelength of 1 micron on a meter-class antenna will be about 0.1 angular seconds. The orientation of this accuracy can be achieved using photon thrusters.
Sending images with a laser using a sail as an antenna
Images of target planets can be transmitted by a single-watt laser on board, in pulsed mode. When approaching the target, the sail will be used to focus the laser signal.
For example, for a sail 4m in size, the diffraction limit of the spot size on Earth will be about 1000m 1 . Approximately the same scale is planned to do the receiving array of antennas. Using a sail as an optical system may require different forms of sail at the start of the mission (during acceleration) and during the communication phase. For more efficient transmission of information, when approaching the target, the sail can be shaped into a Fresnel lens. Due to the Doppler effect in the shift of nanocrafts relative to the Earth, it is necessary to use a laser wave shorter than that of the launch system - this will allow maintaining a high transmission rate through the atmosphere of our planet.
Acquire images using an array of laser emitters
The recent successes of the MIL Lincoln Labs group and the Jet Propulsion Laboratory have shown the ability to detect single photons emitted by a laser from very large distances. Currently, the record holder is the LADEE system, which is capable of operating at lunar distances. It uses the technique of cryogenically cooled nanotubes. This allows you to transfer 2 bits per photon. The system uses 10cm optics on a spacecraft and a one-meter telescope on the ground.
The array of laser emitters involved in the acceleration of nanocrafts will be used in the inverse mode, as an array of receiving antennas.
Sunny sail
Sail Integrity Underneath
The research phase assumes the use of a 100 gigawatt laser in the mission. How will such radiation affect the solar sail?
The most perfect reflective material for today is a dielectric mirror - a composite material with a layer thickness matched to the wavelength.
A dielectric mirror is able to reduce the amount of heat absorbed by 5 orders of magnitude, reflecting 99.999% of radiation.
For a 100-watt laser and a 4x4m sail, this means that every square meter of the sail will be heated with an energy of 60 kW. This is a lot - about 50 electric kettles at full power. It is difficult to dissipate such power by radiation. But, according to the developers, it will heat the sail, but will not melt it. It is assumed that using a fully dielectric sail with optimized materials it will be possible to reduce the absorption below 9 orders of magnitude from the incoming radiation.
Consider the use of new materials like graphene.
It is also possible to use materials with low absorption, even without high reflectivity (for example, glass). Such materials are used in fiber optics under high loads.
In addition to protection from the sail, the electronics of the StarChip module must be protected from the incoming flow. This can be achieved by a combination of geometry (orienting electronics "in profile", with a low cross section) and covering the most important components with special protection. Such coatings can be mentioned multilayer dielectric solutions that have already been prometrostimulated in laboratories. Weakly absorbing sail material, together with the limited use of highly reflective material to protect electronics, will protect StarChip without exceeding the gram scale of the module. For further production, a construction of silicon microcubes on a silicon dioxide substrate is being studied.
Device
It is necessary to develop a sail skeleton that will hold the load when the device is accelerated, to be resistant to interaction with the interstellar medium, and will be able to change the shape of the sail. At present, a number of graphene-based composite materials are being considered, which are capable of changing their length depending on the voltage applied to them. It was previously shown that centrifugal acceleration of tiny masses at the edges can stretch the sail.
Hold on the beam
The beam shape and light sail devices should be optimized for stability during the launch phase. During this period of about 10 minutes, the sail receives 1 terajoule of light energy. For this reason, even small differences in sail properties or beam irregularities will shift the center of pressure from the center of mass of the sail, and shift its thrust vector.
The modern industry of optical coatings for mass production of smartphones and telescopic optics is already at an acceptable quality level for the mission. But the final sail material does not yet exist and must be developed.
Laser emitter
Cost of
Estimation of the estimated cost of the laser array on Earth is based on the extrapolation of the last two decades, as well as on the prospects of cheaper prices for mass production.
The cost of laser amplifiers decreases exponentially from 1990 to 2015, halving every one and a half years. If the trend continues, the construction of a large radiator in the coming decades will cost several orders of magnitude cheaper.
While developers compare the cost with the largest research project in the world. This could be, for example, the ISS (worth $ 157 billion) or an experimental ITER fusion reactor ($ 15 billion).
Phase
To test the capabilities of the system, a case with a meter scale sail was studied. For example, to focus a beam of light on a 4x4m sail at a distance of 200 thousand kilometers, a focus angle of 2 nanoradians (0.4 angular milliseconds) is required. This is the diffraction limit for a kilometer-long laser emitter operating at a wavelength of 1 micron.
Interferometry for Event Horizon Telescope has demonstrated the ability to achieve sub-nanoradian accuracy at a wavelength of 1mm.
Atmosphere
The atmosphere introduces two effects:
- absorption (violation of transmission integrity)
- reduction in the quality of the beam (erosion of the beam)
The transmitting ability of the atmosphere at a wavelength of 1 micron is very good - more than 90% for objects located high in the mountains. With such an arrangement, this will reduce the erosion of the beam in the atmosphere, which will allow adaptive optics to approach the diffraction limit as close as possible. Atmospheric turbulence, which blurs the beam, is about 4 times lower at an altitude of 5 km than at sea level. Even more neutralize the action of the atmosphere can be a correction of the mode of operation of laser emitters with the help of a beacon in space.
The Breakthrough Starshot project wants to achieve a diffraction limit for optical laser systems of 0.2–1 km. This is 1-2 orders of magnitude better than existing solutions, but there are no fundamental limitations in achieving this goal.
Run:
Accuracy of guidance on meter sail
The laser emitter should be focused on a spot on a sail smaller than the size of the sail itself in orbit 60,000 km above the ground.
Laser targeting must be coordinated with the position of the Alpha Centauri star system so that the span of the system passes within two astronomical units. The use of photon thrusters will correct the course by 1-2 astronomical units.
In the problem of beam positioning, the main problem is to keep the sail on the beam. It depends on the size of the sail and the distance to it. For a meter sail, the working distance for launch can reach several million kilometers. The aiming accuracy required at this distance is a few angular milliseconds. There are several ways to solve this problem.
The model of the atmosphere is calibrated using radar, a laser beam and real-time optical measurements. This will achieve the required positioning accuracy.
Most terrestrial telescopes (for example, the Keck telescope) have an accuracy on the order of several arc-seconds and are limited to tracking objects in a mode of 100 arc milliseconds. For mission objectives, significant improvement in accuracy is required.
Nevertheless, the generation of a laser beam by a system with a phased array, with a tracking system of a beacon signal (for correcting the influence of the atmosphere) of a spacecraft can make it possible to achieve the required accuracy.
Hold sails on the beam
There are a number of effects that make this task difficult. These are beam instability, laser modes of operation, forces acting on the sail, heating of the sail, atmospheric heterogeneities caused by the energy of the emitters.
The problems described above can be solved by rotating the sail and regulating the shape of both the sail and the beam of rays that arrive at it. Feedback will help the operation of laser emitters, but a short flight time requires self-stabilization of the system.
One promising approach is to give the sail a special shape that stabilizes its position on the beam. That is, during rotation, the torque will be affected by such torques and forces that will seek to restore its orientation. High-frequency shudder will reduce the total amount of energy transmitted to the sail, but a good sail dynamics can reduce its susceptibility to interference above a certain frequency.
Since a phased array array will be used to form the beam, the beam profile can be shaped to maximize the ability of the sail to maintain its own position on the beam, even without a feedback mechanism.
Energy production and storage
Energy production and storage is a technological challenge.
The generation of 100 GW of power and its delivery within a few minutes is quite achievable at the modern level of technology. Natural gas power plants can generate energy at a cost of $ 0.1 per kilowatt-hour.
Currently, batteries and supercapacitors are also available, which are able to provide the necessary storage capacity at a reasonable price.
Exact determination of the orbital position of exoplanets
In order to deliver a nanocraft to an exoplanet with an accuracy of 1 astronomical unit, it may be necessary to accurately account for all massive bodies near the flight path.
Some of the information can be collected by the first missions of the project and taken into account in subsequent launches. Efforts are also being made to better understand the ephemeris - the orbital positions of large objects at specific points in time that can affect the trajectory of motion. This includes collaboration with the largest telescopes in the southern hemisphere, including Very Large Telescopes and Gemini.
Cruise stage:
Interstellar dust
Based on estimates of the density of dust in the interstellar medium closest to us, during the journey to Alpha Centauri, each square centimeter of the frontal cross-sectional area of ​​the StarChip electronic module and the light sail will collide with approximately 1000 dust particles from 100 nanometers and above. However, the probability of collision with a particle of 1 micrometer for the entire flight time is about 10%. And the probability to meet larger particles is insignificant.
A dust particle with a size of 100 nanometers, moving at a speed of 20% of the speed of light, will penetrate the electronic module to a depth of about 0.4 mm. To estimate the effect, calculations are given for the module, with dimensions of 10 cm x 0.1 mm. The cross-sectional area of ​​such a module is 0.1 cm 2 . A protective coating of beryllium bronze applied to the front of such a module can protect it from dust and erosion. If necessary, the StarChip geometry can be changed (for example, in the form of a “needle”) to further reduce the cross-sectional area.
The sail itself, to minimize damage, can be collapsed into a more streamlined configuration during the cruising phase of flight.
The impulse from the impact of a particle with a size of 100 nm is relatively small, and can be compensated by photon thrusters.
The influence of interplanetary dust inside the solar system is insignificant compared to interstellar dust. Little is known about the presence of dust in the Alpha Centauri system.
Interstellar medium and cosmic rays
The mean free path and the Larmor radius of the particles of the interstellar plasma are much larger than the size of the nanocraft. This means that such particles will affect the walls independently of each other, without forming shock shock.
The protons from the interstellar plasma at a speed of 20% of the speed of light will affect the nanocraft with a kinetic energy of 18 MeV, and the electrons will have an energy of 10.2 keV. It does not matter whether the proton and the electron are combined into a hydrogen atom, or they arrive separately. Erosion of the nanocraft surface due to spraying will occur. The number of atoms sputtered in this way will be about 1000 per cm 2 . The total mass loss of the front surface of the device will be only a few layers.
Protons at 18 MeV will penetrate to a depth of the order of a few millimeters. Therefore, a protective layer capable of stopping such particles will be necessary to avoid damage to the electronics.
Cosmic rays are much less rare than interstellar protons, and therefore can be ignored. Collisions with heavier elements should be mitigated by a protective coating: helium nuclei have energies of the order of 72 MeV and their number is about 10% of the number of free protons. The nuclei of carbon, nitrogen and oxygen carry 200-300 MeV of energy and are present in an amount of 0.01% of the total.
To develop protection technologies, it is necessary to conduct laboratory experiments for ions moving at a speed of 20% of the speed of light and colliding with a solid.
Collisions with interstellar ions and electrons, theoretically, can have their advantages: they could give the nanocraft potential up to 10 kV (kinetic energy per electron). The front surface of the nanocrats will be heated at a speed of 6 mW per cm 2 , which will give a small thermoelectric source of energy when traveling in interstellar medium.
Source: https://habr.com/ru/post/372165/
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