- Part 1 -
If you use existing technology, it will take a very, very long time to send scientists and astronauts on an interstellar mission. The journey will be painfully long (even by cosmic standards). If we want to make such a journey in at least one life, well, or a generation, we need more radical (read: purely theoretical) measures. And if wormholes and subspace engines are absolutely fantastic at the moment, there have been other ideas for many years that we believe in.
Nuclear power plant
A nuclear power plant is a theoretically possible "engine" for fast space travel. The concept was originally proposed by Stanislav Ulam in 1946, a Polish-American mathematician who took part in the Manhattan Project, and preliminary calculations were made by F. Reines and Ulam in 1947. The Orion project was launched in 1958 and existed until 1963.
Led by Ted Taylor of General Atomics and physicist Freeman Dyson of the Institute for Advanced Study at Princeton, Orion would harness the power of pulsed nuclear explosions to deliver enormous thrust with very high specific impulse.
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In a nutshell, Project Orion includes a large spacecraft that picks up speed by supporting thermonuclear warheads, ejecting bombs behind and accelerating as a blast wave escapes into a rear-mounted pusher, a push panel. After each push, the force of the explosion is absorbed by this panel and converted into forward motion.
Although this design is hardly elegant by modern standards, the advantage of the concept is that it provides a high specific thrust - that is, it extracts the maximum amount of energy from a fuel source (in this case, nuclear bombs) at the lowest cost. In addition, this concept can theoretically accelerate very high speeds, according to some estimates, up to 5% of the speed of light (5.4 x 107 km / h).
Of course, this project has inevitable downsides. On the one hand, a ship of this size would be extremely expensive to build. In 1968, Dyson estimated that the Orion spacecraft, powered by hydrogen bombs, would weigh between 400,000 and 4,000,000 metric tons. And at least three-quarters of that weight will come from nuclear bombs, each weighing about one ton.
Dyson's conservative estimate showed that the total cost of building Orion would have been $ 367 billion. Adjusted for inflation, this amount is $ 2.5 trillion, which is quite a lot. Even with the most conservative estimates, the device will be extremely expensive to manufacture.
There is also a small problem of radiation that it will emit, not to mention nuclear waste. It is believed that it was for this reason that the project was canceled under the partial test ban treaty of 1963, when world governments sought to limit nuclear testing and stop the excessive release of radioactive fallout into the planet's atmosphere.
Nuclear fusion rockets
Another possibility of using nuclear energy is thermonuclear reactions to generate thrust. Under this concept, energy must be created by inertial confinement igniting pellets of a mixture of deuterium and helium-3 in a reaction chamber using electron beams (similar to what is done at the National Ignition Complex in California). Such a fusion reactor would detonate 250 pellets per second, creating a high-energy plasma, which would then be redirected into a nozzle, creating thrust.
Like a rocket that relies on a nuclear reactor, this concept has advantages in terms of fuel efficiency and specific impulse. The estimated speed should reach 10,600 km / h, well above the speed limits of conventional rockets. Moreover, this technology has been extensively studied over the past several decades, and many proposals have been made.
For example, between 1973 and 1978, the British Interplanetary Society undertook a feasibility study for Project Daedalus. Drawing on modern knowledge and technology of thermonuclear fusion, scientists called for the construction of a two-stage unmanned scientific probe that could reach Barnard's Star (5.9 light years from Earth) over the span of a human life.
The first stage, the largest of the two, would run for 2.05 years and accelerate the craft to 7.1% the speed of light. Then this stage is discarded, the second is ignited, and the apparatus accelerates to 12% of the speed of light in 1.8 years. Then the second stage engine is turned off, and the ship has been flying for 46 years.
Project Daedalus estimates that it would take the mission 50 years to reach Barnard's Star. If to Proxima Centauri, the same ship will reach in 36 years. But, of course, the project includes a lot of unresolved issues, in particular unsolvable with the use of modern technologies - and most of them have not yet been resolved.
For example, there is practically no helium-3 on Earth, which means that it will have to be mined elsewhere (most likely on the Moon). Second, the reaction that drives the craft requires the energy emitted to be much greater than the energy expended to trigger the reaction. And although experiments on Earth have already surpassed the "break-even point", we are still far from the amount of energy that can power an interstellar vehicle.
Third, there remains the question of the cost of such a vessel. Even by the modest standards of a Project Daedalus unmanned vehicle, a fully equipped vehicle would weigh 60,000 tons. Just so you know, the gross weight of the NASA SLS is just over 30 metric tons, and the launch alone will cost $ 5 billion (2013 estimates).
In short, a fusion rocket will not only be too expensive to build, but it will also require a fusion reactor level far beyond our capabilities. Icarus Interstellar, an international organization of civilian scientists (some of whom have worked for NASA or ESA), is trying to revitalize the concept with Project Icarus. The group assembled in 2009 hopes to make the fusion movement (and others) possible for the foreseeable future.
Thermonuclear ramjet
Also known as the Bussard ramjet, the engine was first proposed by physicist Robert Bussard in 1960. At its core, it is an improvement on the standard thermonuclear rocket, which uses magnetic fields to compress hydrogen fuel to the point of fusion. But in the case of a ramjet engine, a huge electromagnetic funnel sucks in hydrogen from the interstellar medium and pours it into the reactor as fuel.
As the vehicle picks up speed, the reactive mass enters the confining magnetic field, which compresses it before fusion begins. The magnetic field then directs energy into the rocket nozzle, accelerating the ship. Since no fuel tanks will slow it down, a thermonuclear ramjet can reach speeds of the order of 4% light and go anywhere in the galaxy.
Nevertheless, this mission has many possible disadvantages. For example, the problem of friction. The spacecraft relies on high fuel collection rates, but it will also collide with large amounts of interstellar hydrogen and lose speed - especially in dense regions of the galaxy. Secondly, there is not much deuterium and tritium (which are used in reactors on Earth) in space, and the synthesis of ordinary hydrogen, which is abundant in space, is still beyond our control.
However, science fiction has grown to love this concept. The most famous example is perhaps the Star Trek franchise, which uses the Bussard Collectors. In reality, our understanding of fusion reactors is nowhere near as perfect as we would like.
Laser sail
Solar sails have long been considered an effective way to conquer the solar system. In addition to being relatively simple and cheap to make, they have a big plus: they don't need fuel. Instead of using rockets that need fuel, the sail uses the pressure from the radiation from the stars to propel ultra-thin mirrors to high speeds.
However, in the case of an interstellar flight, such a sail would have to be propelled by focused beams of energy (laser or microwaves) to accelerate to near light speed. The concept was first proposed by Robert Forward in 1984, a physicist at the Hughes Aircraft Laboratory.
His idea retains the advantages of a solar sail in that it does not require fuel on board, and also that laser energy is not scattered over a distance in the same way as solar radiation. Thus, while the laser sail will take some time to accelerate to near-light speed, it will subsequently be limited only by the speed of light itself.
According to a 2000 study by Robert Frisbee, director of advanced propulsion research at NASA's Jet Propulsion Laboratory, a laser sail would hit half the speed of light in less than ten years. He also calculated that a sail with a diameter of 320 kilometers could reach Proxima Centauri in 12 years. Meanwhile, a sail of 965 kilometers in diameter will arrive in just 9 years.
However, such a sail will have to be built from advanced composite materials to avoid melting. Which will be especially difficult given the size of the sail. The cost is even worse. According to Frisbee, lasers will need a steady stream of 17,000 terawatts of energy - roughly how much the entire world consumes in one day.
Antimatter engine
Science fiction lovers are well aware of what antimatter is. But if you forgot, antimatter is a substance made up of particles that have the same mass as ordinary particles, but with the opposite charge. An antimatter engine is a hypothetical engine that relies on interactions between matter and antimatter to generate energy, or create thrust.
In short, an antimatter engine uses particles of hydrogen and antihydrogen colliding with each other. The energy released in the annihilation process is comparable in volume to the energy of the explosion of a thermonuclear bomb accompanied by a stream of subatomic particles - pions and muons. These particles, which travel at one-third the speed of light, are redirected into the magnetic nozzle and generate thrust.
The advantage of this class of rockets is that most of the mass of the matter / antimatter mixture can be converted into energy, which provides a high energy density and specific impulse that is superior to any other rocket. Moreover, the annihilation reaction can accelerate the rocket to half the speed of light.
This class of missiles will be the fastest and most energy efficient possible (or impossible, but proposed). If conventional chemical rockets require tons of fuel to propel a spacecraft to its destination, an antimatter engine will do the same job using a few milligrams of fuel. Mutual destruction of half a kilogram of hydrogen and antihydrogen particles releases more energy than a 10-megaton hydrogen bomb.
It is for this reason that NASA's Advanced Concepts Institute is investigating this technology as possible for future missions to Mars. Unfortunately, when looking at missions to nearby star systems, the amount of fuel needed grows exponentially, and the costs become astronomical (and this is not a pun).
According to a report prepared for the 39th AIAA / ASME / SAE / ASEE Joint Propulsion Conference and Exhibit, a two-stage antimatter rocket will require more than 815,000 metric tons of fuel to reach Proxima Centauri in 40 years. It's relatively fast. But the price …
Although one gram of antimatter produces an incredible amount of energy, producing one gram alone would require 25 million billion kilowatt-hours of energy and would amount to a trillion dollars. Currently, the total amount of antimatter that has been created by humans is less than 20 nanograms.
And even if we could produce antimatter cheaply, we would need a massive ship that could hold the required amount of fuel. According to a report by Dr. Darrell Smith and Jonathan Webby of Embry-Riddle Aviation University in Arizona, an antimatter-powered interstellar ship could pick up 0.5 light speed and reach Proxima Centauri in a little over 8 years. However, the ship itself would weigh 400 tons and would require 170 tons of antimatter fuel.
A possible way around this is to create a vessel that will create antimatter and then use it as fuel. This concept, known as the Vacuum to Antimatter Rocket Interstellar Explorer System (VARIES), was proposed by Richard Obausi of Icarus Interstellar. Building on the idea of on-site reprocessing, the VARIES ship would use large lasers (powered by huge solar panels) that create antimatter particles when fired into empty space.
Similar to the concept with a thermonuclear ramjet engine, this proposal solves the problem of transporting fuel by extracting it directly from space. But again, the cost of such a ship will be extremely high if built with our modern methods. We simply cannot create antimatter on a massive scale. The radiation problem also needs to be addressed, since the annihilation of matter and antimatter produces bursts of high-energy gamma rays.
They not only pose a danger to the crew, but also to the engine, so that they do not fall apart into subatomic particles under the influence of all this radiation. In short, an antimatter engine is completely impractical with our current technology.
Alcubierre Warp Drive
Science fiction lovers are no doubt familiar with the concept of the warp drive (or Alcubierre drive). Proposed by the Mexican physicist Miguel Alcubierre in 1994, this idea was an attempt to imagine instantaneous movement in space without violating Einstein's special theory of relativity. In short, this concept involves stretching the fabric of spacetime into a wave, which theoretically would cause the space in front of the object to contract and behind it to expand.
An object inside this wave (our ship) will be able to ride on this wave, being in a "warp bubble", at a speed much higher than the relativistic one. Since the ship does not move in the bubble itself, but is carried by it, the laws of relativity and space-time will not be violated. In fact, this method does not involve movement faster than the speed of light in the local sense.
It is "faster than light" only in the sense that the ship can reach its destination faster than a ray of light traveling outside the warp bubble. Assuming the spacecraft will be equipped with the Alcubierre system, it will reach Proxima Centauri in less than 4 years. Therefore, if we talk about theoretical interstellar space travel, this is by far the most promising technology in terms of speed.
Of course, this whole concept is extremely controversial. Arguments against, for example, include that it does not take quantum mechanics into account and can be refuted by a theory of everything (like loop quantum gravity). Calculations of the required amount of energy also showed that the warp drive would be prohibitively voracious. Other uncertainties include the safety of such a system, space-time effects at the destination, and causality violations.
However, in 2012, NASA scientist Harold White said that he and his colleagues began to explore the possibility of creating the Alcubierre engine. White stated that they have built an interferometer that will capture the spatial distortions produced by the expansion and contraction of the spacetime of the Alcubierre metric.
In 2013, the Jet Propulsion Laboratory published the results of warp field tests, which were conducted under vacuum conditions. Unfortunately, the results were considered “inconclusive”. In the long run, we may find that the Alcubierre metric violates one or more fundamental laws of nature. And even if its physics turns out to be correct, there is no guarantee that the Alcubierre system can be used for flight.
In general, everything is as usual: you were born too early to travel to the nearest star. Nevertheless, if humanity feels the need to build an "interstellar ark" that will house a self-sustaining human society, it will take a hundred years to get to Proxima Centauri. If, of course, we want to invest in such an event.
In terms of time, all the available methods seem extremely limited. And if we spend hundreds of thousands of years traveling to the nearest star, we may be of little interest, when our own survival is at stake, as space technology advances, the methods will remain extremely impractical. By the time our ark reaches the nearest star, its technologies will become obsolete, and humanity itself may no longer exist.
So unless we make a major breakthrough in fusion, antimatter, or laser technology, we will be content with exploring our own solar system.
Based on materials from Universe Today
- Part 1 -