On April 12, 2016, the famous British physicist Stephen Hawking and Russian businessman and philanthropist Yuri Milner announced the allocation of $100 million to finance the project Breakthrough Starshot. The goal of the project was to develop technologies for creating spacecraft capable of making an interstellar flight to Alpha Centauri.

Thousands of science fiction novels describe giant photon starships the size of a small (or large) city, leaving for interstellar flight from the orbit of our planet (less often, from the surface of the Earth). But, according to the authors of the project, Breakthrough Starshot, everything will happen completely differently: on one significant day two thousand of some year, not one or two, but hundreds and thousands of small spaceships the size of a fingernail and weighing 1 g will launch to one of the nearest stars, Alpha Centauri. And each of them will have a thin solar sail with an area of ​​16 m 2, which will carry the spaceship with ever-increasing speed forward - to the stars.

"Shot to the Stars"

The basis of the project Breakthrough Starshot was an article by UC Santa Barbara physics professor Philip Lubin, “A Plan for Interstellar Flight” ( A Roadmap to Interstellar Flight). The main stated goal of the project is to make interstellar flights possible within the lifetime of the next generation of people, that is, not in centuries, but in decades.

Immediately after the official announcement of the program Starshot The authors of the project were hit by a wave of criticism from scientists and technical specialists in various fields. Critical experts noted numerous incorrect assessments and simply “blank spots” in the program plan. Some comments were taken into account and the flight plan was slightly adjusted in the first iteration.

So, the interstellar probe will be a space sailboat with an electronic module StarChip weighing 1 g, connected by heavy-duty straps to a solar sail with an area of ​​16 m 2, a thickness of 100 nm and a mass of 1 g. Of course, the light of our Sun is not enough to accelerate even such a light structure to speeds at which interstellar travel will not last for millennia. Therefore, the main highlight of the project StarShot- this is overclocking using powerful laser radiation, which focuses on the sail. Lubin estimates that with a laser beam power of 50–100 GW, the acceleration will be about 30,000 g, and in a few minutes the probe will reach the speed of 20% of light. The flight to Alpha Centauri will last about 20 years.

Unanswered questions: a wave of criticism

Philip Lubin in his article provides numerical estimates of the points of the plan, but many scientists and specialists are very critical of these data.

Of course, to develop such an ambitious project as Breakthrough Starshot, it takes years of work, and $100 million is not such a large amount for work of this scale. This especially applies to ground infrastructure - a phased array of laser emitters. Installing such a capacity (50–100 GW) will require a gigantic amount of energy, that is, at least a dozen large power plants will need to be built nearby. In addition, it will be necessary to remove a huge amount of heat from the emitters over several minutes, and how to do this is still completely unclear. There are such unanswered questions in the project Breakthrough Starshot a huge amount, but so far the work has just begun.

“The scientific council of our project includes leading experts, scientists and engineers in various relevant fields, including two Nobel laureates,” says Yuri Milner. - And I have heard very balanced assessments of the feasibility of this project. In doing so, we certainly rely on the combined expertise of all members of our scientific council, but at the same time we are open to broader scientific discussion.”

Under the starry sails

One of the key details of the project is the solar sail. In the original version, the sail area was initially only 1 m 2, and because of this, it could not withstand heating during acceleration in the laser radiation field. The new version uses a sail with an area of ​​16 m2, so the thermal regime, although quite harsh, but, according to preliminary estimates, should not melt or destroy the sail. As Philip Lubin himself writes, it is planned to use not metallized coatings, but completely dielectric multilayer mirrors as the basis for the sail: “Such materials are characterized by a moderate reflection coefficient and extremely low absorption. Let’s say, optical glasses for fiber optics are designed for high light fluxes and have an absorption of about twenty trillionths per 1 micron of thickness.” It is not easy to achieve a good reflection coefficient from a dielectric with a sail thickness of 100 nm, which is much less than the wavelength. But the project's authors have some hope in using new approaches, such as monolayers of metamaterial with a negative refractive index.

Solar sail

One of the main elements of the project is a solar sail with an area of ​​16 m2 and a mass of only 1 g. The sail material is multilayer dielectric mirrors that reflect 99.999% of the incident light (according to preliminary calculations, this should be enough to prevent the sail from melting in a radiation field of 100- GW laser). A more promising approach, which makes it possible to make the thickness of the sail smaller than the wavelength of reflected light, is to use a monolayer of metamaterial with a negative refractive index as the basis of the sail (such a material also has nanoperforation, which further reduces its mass). The second option is to use a material not with a high reflection coefficient, but with a low absorption coefficient (10 −9), such as optical materials for light guides.

“You also have to consider that the reflection from dielectric mirrors is tuned to a narrow range of wavelengths, and as the probe accelerates, the Doppler effect shifts the wavelength by more than 20%,” says Lubin. - We took this into account, so the reflector will be adjusted to approximately twenty percent of the radiation bandwidth. We designed such reflectors. If required, reflectors with larger bandwidths are also available.”

Laser machine

The main power plant of the spaceship will not fly to the stars - it will be located on Earth. This is a ground-based phased array of laser emitters measuring 1×1 km. The total laser power should be from 50 to 100 GW (this is equivalent to the power of 10–20 Krasnoyarsk hydroelectric power stations). It is supposed to use phasing (that is, changing the phases on each individual emitter) to focus radiation with a wavelength of 1.06 μm from the entire grating into a spot with a diameter of several meters at distances up to many millions of kilometers (the maximum focusing accuracy is 10 −9 radians). But such focusing is greatly hampered by the turbulent atmosphere, which blurs the beam into a spot approximately the size of an arcsecond (10 −5 radians). Improvements of four orders of magnitude are expected to be achieved using adaptive optics (AO), which will compensate for atmospheric distortions. Best systems adaptive optics in modern telescopes reduce blur to 30 milliseconds of arc, that is, there are still about two and a half orders of magnitude left to the intended target. “To overcome small-scale atmospheric turbulence, the phased array must be broken down into very small elements, the size of the emitting element for our wavelength should be no more than 20–25 cm,” explains Philip Lubin. - This is at least 20 million emitters, but this number does not scare me. For feedback In the AO system we plan to use many reference sources - beacons - both on the probe, and on the mother ship, and in the atmosphere. In addition, we will track the probe on its way to the target. We also want to use the stars as a buoy to adjust the phasing of the array when receiving the signal from the probe upon arrival, but will track the probe to be sure.”

Arrival

But then the probe arrived in the Alpha Centauri system, photographed the surroundings of the system and the planet (if there are any). This information must be somehow transmitted to Earth, and the power of the probe's laser transmitter is limited to a few watts. And five years later this weak signal must be accepted on Earth, isolating stars from the background radiation. According to the authors of the project, the probe maneuvers at the target in such a way that the sail turns into a Fresnel lens, focusing the probe signal in the direction of the Earth. It is estimated that an ideal lens with ideal focusing and ideal orientation amplifies a 1 W signal to 10 13 W isotropic equivalent. But how can we consider this signal against the background of much more powerful (by 13–14 orders of magnitude!) radiation from the star? “The light from the star is actually quite weak because the linewidth of our laser is very small. A narrow line is a key factor in reducing background, says Lubin. - The idea of ​​making a Fresnel lens out of a sail based on a thin-film diffractive element is quite complex and requires a lot of preliminary work to understand exactly how best to do this. This point is actually one of the main ones in our project plan.”

Interstellar flight is not a matter of centuries, but of decades

Yuri Milner ,
Russian businessman and philanthropist,
Founder of Breakthrough Initiatives:

Over the past 15 years, significant, one might say, revolutionary advances have taken place in three technological areas: miniaturization of electronic components, the creation of a new generation of materials, and also the reduction in cost and increase in laser power. The combination of these three trends leads to the theoretical possibility of accelerating a nanosatellite to almost relativistic speeds. At the first stage (5–10 years), we plan to conduct a more in-depth scientific and engineering study to understand how feasible this project is. On the project website there is a list of about 20 serious technical problems, without solving which we will not be able to move forward. This is not a definitive list, but based on the opinion of the scientific council, we believe that the first stage of the project has sufficient motivation. I know that the star sail project is subject to serious criticism from experts, but I think that the position of some critical experts is associated with a not entirely accurate understanding of what we are really proposing. We are not financing a flight to another star, but rather realistic multi-purpose developments related to the idea of ​​an interstellar probe only in a general direction. These technologies will be used both for flights in the solar system and for protection from dangerous asteroids. But setting such an ambitious strategic goal as interstellar flight seems 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 assumed, but rather of decades.

On the other hand, a phased array of optical emitters/radiation receivers with a total aperture of a kilometer is an instrument capable of seeing exoplanets from distances of tens of parsecs. Using tunable wavelength receivers, the composition of the atmosphere of exoplanets can be determined. Are probes needed at all in this case? “Certainly, using a phased array as a very large telescope opens up new possibilities in astronomy. But, adds Lubin, we plan to add an infrared spectrometer to the probe as a longer-term program in addition to the camera and other sensors. We have a great photonics group at UC Santa Barbara that is part of the collaboration.”

But in any case, according to Lubin, the first flights will be made within the solar system: “Because we can send a huge number of probes, this gives us many different possibilities. We can also send similar small ( wafer-scale, that is, on a chip) probes on conventional rockets and use the same technologies to study the Earth or the planets and their satellites in the solar system."

The editors thank the newspaper “Troitsky Option - Science” and its editor-in-chief Boris Stern for their assistance in preparing the article.

And left the solar system; Now they are used to study interstellar space. At the beginning of the 21st century, there are no stations whose direct mission would be to fly to the nearest stars.

The distance to the nearest star (Proxima Centauri) is about 4,243 light years, that is, about 268 thousand times the distance from Earth to the Sun.

Starship projects driven by the pressure of electromagnetic waves

In 1971, in a report by G. Marx at a symposium in Byurakan, it was proposed to use X-ray lasers for interstellar travel. The possibility of using this type of propulsion was later investigated by NASA. As a result, the following conclusion was made: “If the possibility of creating a laser operating in the X-ray wavelength range is found, then we can talk about real development aircraft(accelerated by the beam of such a laser), which will be able to cover distances to the nearest stars much faster than all currently known systems with rocket engines. Calculations show that using the space system considered in this work, it is possible to reach the star Alpha Centauri... in about 10 years."

In 1985, R. Forward proposed the design of an interstellar probe accelerated by microwave energy. The project envisaged that the probe would reach the nearest stars in 21 years.

At the 36th International Astronomical Congress, a project for a laser starship was proposed, the movement of which is provided by the energy of optical lasers located in orbit around Mercury. According to calculations, the path of a starship of this design to the star Epsilon Eridani (10.8 light years) and back would take 51 years.

Annihilation engines

The main problems identified by scientists and engineers who analyzed the designs of annihilation rockets are obtaining the required amount of antimatter, storing it, and focusing the flow of particles in the desired direction. It is indicated that the current state of science and technology does not even theoretically allow the creation of such structures.

Ramjet engines powered by interstellar hydrogen

The main component of the mass of modern rockets is the mass of fuel required by the rocket for acceleration. If we can somehow use the environment surrounding the rocket as a working fluid and fuel, we can significantly reduce the mass of the rocket and thereby achieve high speeds.

Generation ships

Interstellar travel is also possible using starships that implement the concept of “generation ships” (for example, like O’Neil’s colonies). In such starships, a closed biosphere is created and maintained, capable of maintaining and reproducing itself for several thousand years. The flight occurs at low speed and takes a very long time, during which many generations of astronauts manage to change.

FTL propulsion

Notes

see also

Sources

• Kolesnikov Yu. V. You should build starships. M., 1990. 207 p. ISBN 5-08-000617-X.
• http://www.gazeta.ru/science/2008/01/30_a_2613225.shtml?4 Lecture on interstellar flights, on acceleration of 100 km/sec near stars

Let's say the Earth is ending. The sun is about to explode, and an asteroid the size of Texas is approaching the planet. Large cities are inhabited by zombies, and in the countryside farmers are planting corn intensively because other crops are dying. We urgently need to leave the planet, but the problem is that no wormholes have been discovered in the Saturn region, and no superluminal engines have been brought from a galaxy far, far away. The nearest star is more than four light years away. Will humanity be able to achieve it with modern technology? The answer is not so obvious.

It is unlikely that anyone would argue that a global environmental disaster that would threaten the existence of all life on Earth can only happen in the movies. Mass extinctions have occurred more than once on our planet, during which up to 90% of existing species died. The Earth experienced periods of global glaciation, collided with asteroids, and went through bursts of volcanic activity.

Of course, even during the most terrible disasters life never completely disappeared. But the same cannot be said about the dominant species at that time, which died out, making way for others. Who is the dominant species now? Exactly.

It is likely that the opportunity to leave your home and go to the stars in search of something new can someday save humanity. However, we should hardly hope that some cosmic benefactors will open the way to the stars for us. It’s worth calculating what our theoretical capabilities are to reach the stars on our own.

Space Ark

First of all, traditional chemical traction engines come to mind. At the moment, four earthly vehicles (all of them were launched back in the 1970s) have managed to develop a third escape velocity, sufficient to leave the solar system forever.

The fastest of them, Voyager 1, has moved away from Earth to a distance of 130 AU in the 37 years since its launch. (astronomical units, that is, 130 distances from the Earth to the Sun). Each year the device travels approximately 3.5 AU. The distance to Alpha Centauri is 4.36 light years, or 275,725 AU. At this speed, the device will take almost 79 thousand years to reach the neighboring star. To put it mildly, it will be a long wait.

Photo of the Earth (above the arrow) from a distance of 6 billion kilometers, taken by Voyager 1. The spacecraft covered this distance in 13 years.

You can find a way to fly faster, or you can just resign yourself and fly for several thousand years. Then only the distant descendants of those who went on the journey will reach the final point. This is precisely the idea of ​​the so-called generation ship - a space ark, which is a closed ecosystem designed for a long journey.

There are many different stories about generation ships in science fiction. Harry Garrison (“Captured Universe”), Clifford Simak (“Generation That Achieved the Goal”), Brian Aldiss (“Non Stopping”), and among more modern writers Bernard Werber (“Star Butterfly”) wrote about them. Quite often, distant descendants of the first inhabitants completely forget about where they flew from and what the purpose of their journey was. Or even begin to believe that all existing world comes down to a ship, as, for example, in Robert Heinlein's novel Stepsons of the Universe. Another interesting plot is shown in the eighth episode of the third season of the classic Star Trek, where the crew of the Enterprise tries to prevent a collision between a generation ship, whose inhabitants have forgotten about their mission, and the inhabited planet to which it was heading.

The advantage of the generation ship is that this option will not require fundamentally new engines. However, it will be necessary to develop a self-sustaining ecosystem that can survive without external supplies for many thousands of years. And don’t forget that people can simply kill each other.

The Biosphere 2 experiment, conducted in the early 1990s under a closed dome, demonstrated a number of dangers that can await people during such travel. This includes the rapid division of the team into several groups hostile to each other, and the uncontrolled proliferation of pests, which caused a lack of oxygen in the air. Even the ordinary wind, as it turns out, plays vital role- without regular rocking, trees become fragile and break.

Technology that immerses people in long-term suspended animation will help solve many of the problems of long-term flight. Then neither conflicts nor boredom are scary, and a minimal life support system will be required. The main thing is to provide it with energy for a long time. For example, using a nuclear reactor.

Related to the theme of the generation ship is a very interesting paradox called Wait Calculation, described by scientist Andrew Kennedy. According to this paradox, for some time after the first generation ship departs, new, faster modes of travel may be discovered on Earth, allowing later ships to overtake the original settlers. So it is possible that by the time of arrival the destination will already be overpopulated by the distant descendants of the colonizers who went later.

Installations for suspended animation in the film "Alien".

Riding a nuclear bomb

Suppose we are not satisfied that the descendants of our descendants will reach the stars, and we ourselves want to expose our face to the rays of someone else’s sun. In this case, one cannot do without a spaceship capable of accelerating to speeds that will deliver it to a neighboring star in less than one human lifetime. And here the good old nuclear bomb will help.

The idea of ​​such a ship appeared in the late 1950s. The spacecraft was intended for flights within the solar system, but it could also be used for interstellar travel. The principle of its operation is as follows: a powerful armored plate is installed behind the stern. Low-power nuclear charges are uniformly ejected from the spacecraft in the direction opposite to the flight, which are detonated at a short distance (up to 100 meters).

The charges are designed in such a way that most of the explosion products are directed towards the tail of the spacecraft. The reflective plate receives the impulse and transmits it to the ship through the shock absorber system (without it, overloads will be detrimental to the crew). The reflective plate is protected from damage by light flash, gamma radiation streams and high-temperature plasma by a coating of graphite lubricant, which is re-sprayed after each detonation.

The NERVA project is an example of a nuclear rocket engine.

At first glance, such a scheme seems crazy, but it is quite viable. During one of the nuclear tests on Enewetak Atoll, graphite-coated steel spheres were placed 9 meters from the center of the explosion. After testing, they were found undamaged, which proves the effectiveness of graphite protection for the ship. But the Treaty Banning Tests of Nuclear Weapons in the Atmosphere, Outer Space and Under Water, signed in 1963, put an end to this idea.

Arthur C. Clarke wanted to equip the Discovery One spaceship from the movie 2001: A Space Odyssey with some kind of nuclear explosion engine. However, Stanley Kubrick asked him to abandon the idea, fearing that audiences would consider it a parody of his film Dr. Strangelove, or How I Stopped Being Scared and Loved the Atom Bomb.

What speed can be achieved using a series of nuclear explosions? Most information exists about the Orion explosion project, which was developed in the late 1950s in the USA with the participation of scientists Theodore Taylor and Freeman Dyson. The 400,000-ton ship was planned to accelerate to 3.3% of the speed of light - then the flight to the Alpha Centauri system would last 133 years. However, according to current estimates, in a similar way it is possible to accelerate the ship to 10% of the speed of light. In this case, the flight will last approximately 45 years, which will allow the crew to survive until they arrive at their destination.

Of course, building such a ship is a very expensive undertaking. Dyson estimates that Orion would cost approximately $3 trillion in today's dollars to build. But if we find out that our planet is facing a global catastrophe, then it is likely that a ship with a nuclear pulse engine will be humanity’s last chance for survival.

Gas giant

A further development of the Orion ideas was the project of the unmanned spacecraft Daedalus, which was developed in the 1970s by a group of scientists from the British Interplanetary Society. The researchers set out to design an unmanned spacecraft capable of reaching one of the nearest stars during a human lifetime, conducting scientific research and transmitting the information received to Earth. The main condition of the study was the use of either existing or foreseeable technologies in the project.

The target of the flight was Barnard's Star, located at a distance of 5.91 light years from us - in the 1970s it was believed that several planets revolved around this star. We now know that there are no planets in this system. The Daedalus developers set their sights on creating an engine that could deliver the ship to its destination in no more than 50 years. As a result, they came up with the idea of ​​a two-stage apparatus.

The necessary acceleration was provided by a series of low-power nuclear explosions occurring inside a special propulsion system. Microscopic granules of a mixture of deuterium and helium-3, irradiated with a stream of high-energy electrons, were used as fuel. According to the project, up to 250 explosions per second were supposed to occur in the engine. The nozzle was a powerful magnetic field created by the ship's power plants.

According to the plan, the first stage of the ship operated for two years, accelerating the ship to 7% the speed of light. The Daedalus then jettisoned its spent propulsion system, removing most of its mass, and fired its second stage, which allowed it to accelerate to a final speed of 12.2% lightspeed. This would make it possible to reach Barnard's Star 49 years after launch. It would take another 6 years to transmit the signal to Earth.

The total mass of the Daedalus was 54 thousand tons, of which 50 thousand were thermonuclear fuel. However, the supposed helium-3 is extremely rare on Earth - but it is abundant in the atmospheres of gas giants. Therefore, the authors of the project intended to extract helium-3 on Jupiter using an automated plant “floating” in its atmosphere; the entire mining process would take approximately 20 years. In the same orbit of Jupiter, it was planned to carry out the final assembly of the ship, which would then launch to another star system.

The most difficult element in the entire Daedalus concept was precisely the extraction of helium-3 from the atmosphere of Jupiter. To do this, it was necessary to fly to Jupiter (which is also not so easy and fast), establish a base on one of the satellites, build a plant, store fuel somewhere... And this is not to mention the powerful radiation belts around the gas giant, which additionally would make life more difficult for technology and engineers.

Another problem was that Daedalus did not have the ability to slow down and enter orbit around Barnard's Star. The ship and the probes it launched would simply pass by the star along the flyby path, covering the entire system in a few days.

Now an international group of twenty scientists and engineers, operating under the auspices of the British Interplanetary Society, is working on the Icarus spacecraft project. “Icarus” is a kind of “remake” of Daedalus, taking into account the knowledge and technology accumulated over the past 30 years. One of the main areas of work is the search for other types of fuel that could be produced on Earth.

At the speed of light

Is it possible to accelerate a spaceship to the speed of light? This problem can be solved in several ways. The most promising of them is an antimatter annihilation engine. The principle of its operation is as follows: antimatter is fed into the working chamber, where it comes into contact with ordinary matter, generating a controlled explosion. The ions generated during the explosion are ejected through the engine nozzle, creating thrust. Of all possible engines, annihilation theoretically allows one to achieve the highest speeds. The interaction of matter and antimatter releases a colossal amount of energy, and the speed of the outflow of particles formed during this process is close to that of light.

But here the question of fuel extraction arises. Antimatter itself has long ceased to be science fiction - scientists first managed to synthesize antihydrogen back in 1995. But it is impossible to obtain it in sufficient quantities. Currently, antimatter can only be produced using particle accelerators. Moreover, the amount of substance they create is measured in tiny fractions of grams, and its cost is astronomical. For one billionth of a gram of antimatter, scientists from the European Nuclear Research Center (the same one where they created the Large Hadron Collider) had to spend several hundred million Swiss francs. On the other hand, the cost of production will gradually decrease and in the future may reach much more acceptable values.

In addition, we will have to come up with a way to store antimatter - after all, upon contact with ordinary matter, it is instantly annihilated. One solution is to cool the antimatter to ultra-low temperatures and use magnetic traps to prevent it from coming into contact with the walls of the tank. The current record storage time for antimatter is 1000 seconds. Not years, of course, but taking into account the fact that the first time antimatter was contained for only 172 milliseconds, there is progress.

And even faster

Numerous science fiction films have taught us that it is possible to get to other star systems much faster than in a few years. It is enough to turn on the warp engine or hyperspace drive, sit back comfortably in your chair - and within a few minutes you will find yourself on the other side of the galaxy. The theory of relativity prohibits travel at speeds exceeding the speed of light, but at the same time leaves loopholes to circumvent these restrictions. If they could tear apart or stretch space-time, they could travel faster than light without breaking any laws.

A gap in space is better known as a wormhole or wormhole. Physically, it is a tunnel connecting two remote regions of space-time. Why not use such a tunnel to travel into deep space? The fact is that the creation of such a wormhole requires the presence of two singularities at different points in the universe (this is what is beyond the event horizon of black holes - in fact, gravity in its purest form), which can tear apart space-time, creating a tunnel that allows travelers to “ shortcut through hyperspace.

In addition, to maintain such a tunnel in a stable state, it must be filled with exotic matter with negative energy, and the existence of such matter has not yet been proven. In any case, only a supercivilization can create a wormhole, which will be many thousands of years ahead of the current one in development and whose technologies, from our point of view, will be similar to magic.

The second, more affordable option is to “stretch” the space. In 1994, Mexican theoretical physicist Miguel Alcubierre proposed that it was possible to change its geometry by creating a wave that compresses the space in front of the ship and expands it behind. Thus, the starship will find itself in a “bubble” of curved space, which itself will move faster than light, thanks to which the ship will not violate fundamental physical principles. According to Alcubierre himself, .

True, the scientist himself considered that it would be impossible to implement such a technology in practice, since this would require a colossal amount of mass-energy. The first calculations gave values ​​exceeding the mass of the entire existing Universe; subsequent refinements reduced it to “only” Jupiterian.

But in 2011, Harold White, who heads the Eagleworks research group at NASA, carried out calculations that showed that if you change some parameters, then creating an Alcubierre bubble may require much less energy than previously thought, and it will no longer be necessary to recycle the entire planet. Now White's group is working on the possibility of an "Alcubierre bubble" in practice.

If the experiments yield results, this will be the first small step towards creating an engine that allows travel 10 times faster than the speed of light. Of course, a spacecraft using the Alcubierre bubble will travel many tens, or even hundreds of years later. But the very prospect that this is actually possible is already breathtaking.

Flight of the Valkyrie

Almost all proposed starship projects have one significant drawback: they weigh tens of thousands of tons, and their creation requires a huge number of launches and assembly operations in orbit, which increases the cost of construction by an order of magnitude. But if humanity nevertheless learns to obtain large amounts of antimatter, it will have an alternative to these bulky structures.

In the 1990s, writer Charles Pelegrino and physicist Jim Powell proposed a starship design known as Valkyrie. It can be described as something like a space tractor. The ship is a combination of two annihilation engines connected to each other by a super-strong cable 20 kilometers long. In the center of the bundle there are several compartments for the crew. The ship uses the first engine to reach near light speed, and the second to reduce it when entering orbit around the star. Thanks to the use of a cable instead of a rigid structure, the mass of the ship is only 2,100 tons (for comparison, the ISS weighs 400 tons), of which 2,000 tons are engines. Theoretically, such a ship can accelerate to a speed of 92% of the speed of light.

A modified version of this ship, called the Venture Star, is shown in the film Avatar (2011), one of the scientific consultants of which was Charles Pelegrino. Venture Star sets off on a journey, propelled by lasers and a 16-kilometer solar sail, before stopping at Alpha Centauri using an antimatter engine. On the way back the sequence changes. The ship is capable of accelerating to 70% the speed of light and reaching Alpha Centauri in less than 7 years.

No fuel

Both existing and future rocket engines have one problem - fuel always makes up the majority of their mass at launch. However, there are starship projects that will not need to take fuel with them at all.

In 1960, physicist Robert Bussard proposed the concept of an engine that would use hydrogen found in interstellar space as fuel for a fusion engine. Unfortunately, despite the attractiveness of the idea (hydrogen is the most abundant element in the Universe), it has a number of theoretical problems, ranging from the method of collecting hydrogen to the estimated maximum speed, which is unlikely to exceed 12% of light speed. This means that it will take at least half a century to fly to the Alpha Centauri system.

Another interesting concept is the use of a solar sail. If a huge, super-powerful laser was built in Earth orbit or on the Moon, its energy could be used to accelerate a starship equipped with a giant solar sail to fairly high speeds. True, according to engineers’ calculations, in order to give a manned ship weighing 78,500 tons half the speed of light, a solar sail with a diameter of 1000 kilometers will be required.

Another obvious problem with a starship with a solar sail is that it needs to be slowed down somehow. One of its solutions is to release a second, smaller sail behind the starship when approaching the target. The main one will disconnect from the ship and continue its independent journey.

***

Interstellar travel is a very complex and expensive undertaking. Creating a ship capable of covering space distance in a relatively short period of time is one of the most ambitious tasks facing humanity in the future. Of course, this will require the efforts of several states, if not the entire planet. Now this seems like a utopia - governments have too many things to worry about and too many ways to spend money. A flight to Mars is millions of times simpler than a flight to Alpha Centauri - and yet, it is unlikely that anyone will dare to name the year when it will take place.

Work in this direction can be revived either by a global danger threatening the entire planet, or by the creation of a single planetary civilization that can overcome internal squabbles and wants to leave its cradle. The time for this has not yet come - but this does not mean that it will never come.