Under the "star sail" to Alpha Centaur

Pre -analysis of flight

In the Triv-Hunger No. 8 dated April 19 of this year, the article “Two in Physics Milner with Hawking” was published, where the author sharply criticized the “Star Sail” project (or “Star shot”, Starshot , which sounds worse in Russian). The research part of the project was funded by Yuri Milner. The article caused many responses, and one of the first to react to Yuri Milner himself. He proposed to deploy a public discussion in the TRV about the project, to which we proceed.

No matter how fantastic the project may seem, there is an undoubted educational benefit from it: let's begin to disassemble it by bones and as a result we learn a lot of new things (I have already learned). At the same time, we will probably talk about the study of exoplanets and the search for life in general.

I have already made my own opinion about the project, now I am changing my role and I will act not as a critic, but as a moderator of discussion. It will be organized as follows: we publish the main provisions of the project, then an interview with Yuri Milner, where we will mainly affect strategic aspects, finally, an interview with the main enthusiast of the “star sail” by Philip Lyubin, in which we will consider some of the specific problems of the project. This interview is clearly not completed, but it will be better if the second portion of questions is born from a public discussion.

Briefly about the project

The previous version of the project is presented in the preprint of Philip Lyubin https://arxiv.org/abs/1604.01356 .

Subsequently, a number of project parameters were changed, the new version in a very thesis is described here: www.breakthroughhinitives.org/Challenges/3 .

Scenario

Nanozond weighing 2 g (1 g - sail and 1 g - the probe itself) is accelerated by a laser beam with a capacity of 50-100 GW to a speed of 20% light in the direction of the alpha centaur system. Acceleration of the probe during irradiation is 30,000 g, the acceleration path is 6 million km. After 20 years, he flies to the goal, takes pictures and transfers them to Earth.

Laser installation

The ground -based phased grille of laser emitters is used in size kilometers per kilometer (the emitters are packed back to each other). The total capacity is 50–100 gigavatts (10–20 Krasnoyarsk hydroelectric power stations), the wavelength is 1.06 μm. It is supposed to focus the radiation from the entire grate in a spot with a diameter of a few meters at distances up to many millions of kilometers (maximum focus accuracy-λ/d ~ 10-9). Such a focus is strongly interfered with a turbulent atmosphere, blurring a beam in a spot size in an angular second (10-5). Four -order improvements are supposed to be achieved using adaptive focusing. The best achievements of adaptive optics of modern telescopes reduce the blur of to 30 corner milliseconds. Two and a half orders remain. Theoretically, such advance is possible if individual emitters are quite small, and their number is huge (see question 4 in an interview with Lyubin). As a supporting source for adaptive phasing, it is supposed to use the probe itself, the ship that launched it, and the “bacons” in the atmosphere (see the 5th question in an interview with Lyubin).

Sail

The sail should weigh 1 g with an area of ​​16 m2. The corresponding thickness is less than 100 nanometers. At the same time, it should pull the probe with a force of 30 kg (1 g with an acceleration of 30,000 G) and have appropriate strength (there will also be skeletal threads and slings, which should also meet 1 g of sails). It is interesting that if you triple such a sail, then it can serve as a parachute for a person. The parachute weighing three grams! In addition, it should be exclusively transparent so as not to burn. It is not easy to achieve a decent reflection from the dielectric with thickness of the thickness, for this it is assumed a certain trick with holes that I admit, I did not understand (see www.breakthrouginitiatives.org ). There are quite few about the material. When transparency is required, the glass used for fibers is mentioned. In other cases, something associated with graphene.

Probe

The probe weighing 1 g should be flat, include a power source (radioisotopic + sunny), camera, processor. When accelerated, the probe is behind the sail and enters the accelerating radiation field. It is supposed to protect it from radiation with a multi -layer dielectric mirror with a reflection coefficient of 0.99999. It should be oriented all the way to the direction of movement and is protected along the front face with a layer of substance that absorbs particles of the interstellar medium. With regard to the coefficient of reflection of the surface of the probe, problems may appear with the Doppler frequency shift arising from acceleration (see Lyubin's 2nd question).

Acts on arrival

At the target, the sail turns into a Frell lens, focusing the probe signal in the direction of the earth. The probe photographs the surroundings and planets, if they are there, and transfers images to the Earth. Signal power - 1 watts. Formally, with divergence of the L/D bundle (l is the wavelength), the perfect lens with perfect focus and ideal orientation enhances the signal up to 1013 watts in isotropic equivalent.

Interview with Yuri Milner

- The Star Sail project is seriously criticized by experts. I did not see the official reaction of the scientific community in the form of publications in the media, but I heard many unofficial responses, very tough. And he himself is extremely skeptical. Indeed, the project has several frightening requirements for technologies involving radical advancement in those areas where it is difficult. Some of the accompanying problems with a high probability may be fatal for technological or financial reasons. How do you evaluate the chances of such a project? If you believe in the possibility of its implementation, then when and at what cost?

“I can’t agree with you, as well as with your“ two in physics ”. The Scientific Council of our project includes leading specialists, scientists and engineers in various relevant fields, including two Nobel laureates. And for my part, I heard very balanced assessments of the implementation of this project. I think your position, as well as the position of some other critical experts, is associated with a not quite accurate understanding of what we really offer. And we speak actually the following. Over the past 15 years, significant, one might say, revolutionary advancements in three technological areas, namely: miniaturization of electronic components, the creation of a new generation of materials, also reduce the cost and increase in the power of lasers. The combination of these three trends leads to the theoretical opportunity to disperse the nanosonomic to almost relativistic speeds. At the first stage, the horizon of which is 5-10 years, we plan to conduct a more in-depth scientific and engineering study of the possibility of practical implementation of this project. At the same time, we certainly rely on the total examination of all members of our Scientific Council (its full list is published on our website) and at the same time open to a wider scientific discussion. Moreover, at the same time with the announcement of the project, we ourselves published a list of about 20 serious technical problems, many of which you also mentioned in your publication. I believe this is not a final list, but we, based on the opinion of the scientific council, we believe that the first stage of the project has sufficient motivation.

- You have financed research that will most likely have useful and interesting applications, regardless of sending the probe to the alpha Centaurus - flights in the solar system, the effect on asteroids, etc. In this sense, the money will definitely not disappear. Question: Is the use of such a “banner” as an alpha Centaurus probe for the propaganda of the project? On the one hand, this attracts the attention of the wide masses, on the other hand, the fund discredits the fund in the eyes of professionals due to the fantastic of the project. Wasn't it worth making a statement like a disclaimer that you are not financing a flight to another star, but quite realistic multi -purpose developments associated with the idea of ​​an interstellar probe only by a common direction?

- We actually announced this. But the setting of a strategic goal seems to us justified in the sense that the development of technology over the past 10–20 years, probably, makes the implementation of such a project not a matter of centuries, as many have expected, but rather decades.

- In my opinion, the “road to the stars” lies through cosmic interferometers that can see the planets of the earthly type in close stars and remove their spectrum. Including register oxygen in the atmosphere, which is equivalent to the detection of extraterrestrial life. It may be highly likely that there is no need to send a probe to the alpha Centaurus, that an interesting goal is somewhere else. Giant ground telescopes and JWST may be insufficient for the study of land -like exoplanets - only interferometers in space can probably cope with this. Similar projects exist on paper, but stuck without financing. They are very complex, but much less fantastic than the “star sail”. There, KSTATI, laser technologies can also play an important role. Maybe it makes sense for your fund to push one of these projects?

- Yes, and no. Calculations show that the equivalent size of the telescope, located in the solar system, should be hundreds of kilometers. The design of a similar scale in open space is unlikely to be implemented in the foreseeable future. However, our fund is already negotiating to finance a number of initiatives with a horizon of several years to increase the capabilities of ground telescopes, in particular the Coronaraphic Infrand Camera installation. These are quite practical steps that can be taken in the near future. In addition, a laser installation on Earth, which is part of our proposal, can find other useful applications, in particular, a deviation of potentially dangerous asteroids. Do not forget that it itself will be a powerful telescope, although in a narrow frequency range.

- The last question concerns your previous project - the search for extraterrestrial civilizations. It is very risky, in the sense that with a very high probability there are no signals. Nevertheless, in my opinion, someone should do this, and it is a pity that the Seti program lost state support. The risk of not finding a signal can be compensated by side data, the openness of radio -observation data and the public collection of observation applications within the framework of the program. This will increase interest in the project, attract researchers, make it more meaningful and give a good educational effect. Do you make efforts in this direction? Is there any observation data on your public domain?

- I absolutely agree with you. Moreover, a month ago we announced that we are completely opening information about all observations. The data archive is posted in the public domain on the project website, as well as the observation plan for the next six months. We are also open for any reasonable proposals in this area. In general, from my point of view, it does not matter who finances such risky projects - the state or private individuals. The main thing is that experiments of this kind are not stopped, because technologies are constantly being improved. Now no one will remember who financed the Columbus Expedition, but everyone remembers the discovery of America.

Interview with Philip Lyubin

- The dielectric mirror, designed to reflect radiation from the sail and probe * , works thanks to the quarterwave layers with different refractive index. Radiation nevertheless penetrates a couple of outer layers. Can this radiation at a density of a few gigavatts per m2 evaporate (or abbrease) the outer layer? Then the turn of the next layer will come and so on. Did you consider such a problem?

- Yes. As a basis, we use a completely dielectric mirror with an extremely low absorption and a moderate reflection coefficient for a sail. Existing materials absorb 20 trillion thicknesses. (See section 4 and Fig. 18 in preprint.)

- The reflection of Bragg (dielectric mirrors) works in a narrow wavelength range, since the layers are adjusted to a certain wavelength in thickness. The Doppler effect as the probe accelerates will shift the wavelength by more than 20%. At the same time, the reflection coefficient will catastrophically fall if you do not adjust the frequency of lasers. Are there powerful lasers with a rebuilding frequency in this range? If so, how much can they cost?

- The reflector will be tuned to about twenty percent width of the strip. We designed such reflectors, and, if necessary, reflectors with a larger width are available.

- Suppose you can perfectly control the profile of an accelerating beam. But it is impossible to control the probe due to a large temporary delay. It may turn out that in the ray, which seems to ensure static balance, the dynamic instability of the type of motor calbing will develop. Are there any assessments about this?

- I agree, this may become a problem. We need a self -stabilizing system with passive stabilization.

- Now the main doubt about the accelerating beam. To defeat small -scale atmospheric turbulence, the phased grille must be divided into very small elements. The size of the radiating element should be smaller than the radius of Frida, that is, 20–25 cm for micron wavelength. With in mind a tightly packed lattice of 1 × 1 km, we get a minimum of 20 million emitters. Does this quantity not scare you?

- Yes, it is for this reason that we plan small emitters. Their number will be large, but it does not scare me.

- A typical variable time for small -scale turbulence is milliseconds. This means that to manage the phases with feedback, the supporting “bacin” should be no further than several hundred kilometers and exactly on the Line of the Radiff - probe. How can this be done?

- This is a complex part of the feedback control system and direct control. We plan to use a lot of bucks both on the probe, and on the maternal ship, and in the atmosphere.

- What will serve as a backed for phasing the lattice when receiving a signal from the probe upon arrival? Alpha Centaurus? But this is a double star. Is there a problem?

- The laser communication system is really complicated. We will track the probe on the way to the goal, so we will know where it is. We also have the opportunity to expand the bundle in order to easier to track the probe. This will not change the rate of data acceptance, but will increase the noise, so we will focus the beam as much as possible. We also want to use stars as a backed for configuration, but for reliability we will monitor the probe.

- Suppose that the probe has reached the alpha Centaurus. Suppose that the sail somehow turned into a perfect lens of Frenel (by the way, a good idea is to make a parabolic mirror from the sail would be much harder). Assuming the focus of ideal, we get a winning 13 of orders, that is, the equivalent of an isotropic emitter of 1013 watts. But here you are faced with the problem of Galo. With the size of the emitter/receiver module 20 cm, in a halo 2 ”there will be almost half of the light of the star, which is 13-14 orders brighter. You can win some orders of the size due to the narrowness of the transmitter line (and what is its achievable width?). But still the problem of signaling the signal against the background of the star is almost bad to use the cosmic interferometer for receiving the signal?

- The thin -fed -free diffraction element is the same - the same sail. It is difficult and requires a lot of work to understand how to do it correctly. The corresponding point is one of the main in the roadmap. The light from the star is actually weak, since the width of the line of our laser is very small. (See Preprint section 7 - 5.10.) A narrow line is a key factor in the background reduction.

- If you eventually can receive a signal from the probe, then you do not need a probe, since you will have a tool that can see exoplanets on dozens of parsecs. In principle, (having a receiver with a rebuilding wavelength), you can see if there is oxygen in the planet’s atmosphere, and then you can determine whether this is a goal for sending a probe. Of course, a similar tool can be made cheaper and better than a terrible huge laser grille. Maybe such an approach to the study of extra -headed systems is more logical?

- We plan to add an infrared spectrometer to the probe as a more long -term program in addition to the camera and other sensors. We have an excellent group of photonics at the University of Santa Barbara, which is part of the collaboration. The use of the phased lattice as a very large telescope opens up new opportunities in astronomy.

— Вероятно, у вас есть ряд гораздо более реалистичных подпроектов по изучению Солнечной системы с лазерными парусами? Было бы интересно услышать о некоторых из них.

— Первые полеты будут в пределах Солнечной системы. Поскольку мы можем посылать огромное количество зондов, это дает нам много разных возможностей. Мы также можем посылать маленькие (wafer-scale) зонды на обычных ракетах и использовать те же технологии для изучения Земли, Солнечной системы и т. д.

Conclusion

Этим материалом дискуссия лишь начинается. Количество непроясненных вопросов, оставшихся за ее рамками, огромно. Предлагаем продолжить дискуссию в комментариях к онлайн-версии статьи, где можно не только высказать свое мнение, но и задать вопросы, наиболее адекватные из которых мы переадресуем Филипу Любину. На сей раз дискуссия будет модерироваться. Премодерации не предполагается, но малозначимые посты будут через некоторое время удаляться. Наоборот, грамотные утверждения и вопросы будут сведены в следующую онлайн-публикацию.

* Первые два вопроса подразумевали первую версию проекта с более толстым парусом. Что касается второй версии, они остаются актуальными для самого зонда и, вероятно, для строп.