Chapter 10 9. Stellar Spaceship
Someday in the far future, we will spend our last good day on Earth.Eventually, billions of years from now, the sky will burn up.The sun will swell into a purgatory of misery, filling the entire sky and making everything in heaven seem insignificant.As temperatures rise rapidly across the planet, the oceans will boil and evaporate, leaving a scorched, parched landscape.Eventually, the mountains will melt and become liquid, creating lava flows where the vibrant metropolis once stood.
According to the laws of physics, this relentless future will be inevitable.The earth will eventually perish in tongues of flame and be swallowed by the sun.It's a law of physics.
This catastrophe will occur in the next 5 billion years.Within this cosmic time frame, the ups and downs of human civilization are but tiny ripples.One day we will have to leave Earth, or die, so how will humans - our descendants - cope when conditions on Earth become unbearable?
The mathematician and philosopher Bertrand Russell once lamented: "No spark, no heroism, no agitation of thought or feeling can transcend life and death and preserve life. Inspiration, the dazzling light of all human prowess, is doomed in the tragic demise of the solar system. The whole temple of human achievement will necessarily be buried in ruins beneath the wreckage of the universe..."
For me, this is one of the most thought-provoking passages in the English language.But Russell wrote this in a time when rockets and spaceships were considered impossible.Today, the imagination of one day leaving Earth is much less far-fetched.Carl Sagan once said that we should become a "two-planet species".Life on Earth is so precious, he says, that we should expand to at least one other habitable planet to prevent catastrophe.Earth operates in a "cosmic shooting range" of asteroids, comets, and other stray debris drifting near Earth's orbit, a single collision with any of them could be our demise.
The poet Robert Frost has asked whether the Earth will end in flames or in freezing, and using the laws of physics we can reasonably predict how the Earth will end in a natural catastrophe.
On a time scale of thousands of years, one of the crises facing human civilization is the appearance of a new ice age.The last ice age ended 10,000 years ago.When the next ice age arrives in the next 10,000 to 20,000 years, much of North America could be covered by half a mile of ice.Human civilization thrived in the period between two recent short ice ages, when the Earth was unusually warm, but such cycles don't last forever.
Over millions of years, a large meteor or comet colliding with Earth could have devastating effects.The last time a cataclysm fell from the sky was 65 million years ago, when a six-mile-diameter object slammed into Mexico's Yucatan Peninsula, creating a crater about 180 miles in diameter that wiped out the entire continent until then. The dominant life form on earth - dinosaurs.Another cosmic impact may have been of that magnitude.
Billions of years from now, the sun will gradually expand and engulf the earth.In fact, we estimate that the sun will heat up by about 10% over the next billion years, scorching the Earth.It will completely swallow the earth in 5 billion years, and our sun will transform into a huge red star.The Earth would actually be inside the sun's atmosphere.
Tens of billions of years from now, both the sun and the galaxy will die.When our Sun finally runs out of its hydrogen/helium fuel, it will shrink into a tiny white dwarf and gradually cool until it's a giant black pile of nuclear junk wandering the vacuum of space.The Milky Way will eventually collide with the neighboring Andromeda galaxy.Andromeda is larger than our Milky Way.The Milky Way will be torn apart and our Sun will be flung into outer space.The black holes at the centers of the two galaxies will perform a dance of death before the final collision and merger.
Since humanity must one day flee the solar system to survive, or perish, on a neighboring star, the question is: how do we get there?The nearest galaxy, the constellation Centaurus, is more than 4 light-years away.Conventional chemical booster rockets—the ponies used by space programs these days—can barely reach 40,000 mph.At this speed, it would take 70,000 years just to reach the nearest star.
Analyzing today's space program, there is a huge gulf between our meager technological capabilities today and a starship that would allow us to begin exploring the universe.Since exploring the Moon in the early 1970s, our human space program has sent astronauts to the Space Shuttle and International Space Station in orbit just 300 miles above Earth.However, NASA plans to phase out the space shuttle by 2010 to make way for the Orion spacecraft.The Orion crew spacecraft will return astronauts to the Moon by 2020 after a 50-year hiatus.The plan is to build a permanent, manned lunar base.After that, a human-operated mission will be carried out on Mars.
Clearly, new rocket designs will have to be developed if we want to someday reach other stars.Either we have to fundamentally increase the propulsion of the rocket, or we have to increase the time the rocket operates.For example, a large chemical rocket may have millions of pounds of propulsion but only burn for a few minutes.In contrast, other rocket designs, such as ion engines (detailed in the following paragraphs), may have weak propulsion that could operate in outer space for years.When it comes to rocketry, the tortoise can outwit the rabbit.
Unlike chemical rockets, ion engines don't produce the sudden, exciting blast of heat that propels conventional rockets.In fact, their propulsion is usually measured in ounces.If they were placed on a table on Earth, they would be too weak to move.But what they lack in propulsion, they more than make up for in endurance, as they can operate in the vacuum of space for years.
A typical ion engine looks like the inside of a picture tube.A fiery filament is heated by an electric current, creating a stream of ionized atoms, such as xenon, that shoot out from the bottom of the rocket.Instead of riding a hot, explosive airflow, an ion engine rides a thin but steady stream of ions.
NASA's NSTAR ion thruster was tested aboard Deep Space 1, which was successfully launched in 1998.The ion engine burned for a total of 678 days, a new record for an ion engine.The European Space Agency has also tested an ion engine on its Smart 1 probe.Japan's Hayabusa space probe, which flew past an asteroid, is powered by four xenon-ion engines.Although not very exciting, ion engines will be able to complete long-distance interplanetary missions (except for urgent ones).In fact, ion engines may one day serve as stage horses for interstellar transport.
There is a more powerful version of the ion engine - the plasma engine.For example, VASIMR (variable specific impulse magnetoplas marocket), uses a powerful beam of plasma to propel a rocket into space. Designed by astronaut/engineer Franklin Chang-Diaz, VASIMR uses radio waves and magnetic fields to heat hydrogen gas to 1 million degrees Celsius.Super-hot plasma is then ejected from the bottom of the rocket, creating enormous thrust.Although it has not yet been sent into space, prototypes of the engine have already been built on Earth.Some engineers hope plasma engines could be used to support missions to Mars, which could dramatically reduce the travel time to Mars to months.Some designs use solar energy to power the plasma in the engine, others use nuclear fission (which raises safety concerns because it involves sending large amounts of nuclear material into space on accident-prone craft).
However, neither ion engines nor plasma/VASIMR engines are powerful enough to get us to stars.To do this, we need a whole new set of propulsion designs.Designing a stellar spacecraft has a serious handicap: It takes a staggering amount of fuel to complete even one trip to the nearest star, and it takes a long time for the spacecraft to reach its distant destination.
One proposal that might solve these problems is solar sails.It takes advantage of the fact that sunlight can apply very small but very steady pressures, enough to propel giant rockets into space.The concept of solar sails is quite old, starting with the great astronomer Johannes Kepler's 1611 treatise "Sleepwalking".
While the physics of making a solar sail are fairly simple, progress toward creating a solar sail that actually can be sent into space has been hit and miss. In 2004, a Japanese rocket successfully launched two small prototype solar sails into space.In 2005, the Planetary Society, Cosmos Studios, and the Russian Academy of Sciences launched the Cosmos 1 spacecraft from a submarine in the Barents Sea, but The Volna rocket it was carrying failed and the spacecraft failed to reach orbit (a suborbital flight also failed back in 2001).However, in 2005, a 15-meter solar-sail spacecraft was successfully launched into orbit by a Japanese MV rocket, although the solar sail was not fully deployed.
While progress in solar sail technology has been painfully slow, solar sail proponents have another idea that might take them to the stars: building giant arrays of lasers on the moon that could fire at a solar sail. Intense laser light, making it sail towards the nearest star.However, the physics of such interstellar sails is really quite daunting.The solar sail itself would have to be hundreds of miles wide and fabricated entirely in space.We would have to create thousands of powerful laser beams on the moon, each capable of firing for years to decades (in one estimate, the lasers would have to be 1,000 times more powerful than the current total energy production of Earth).
In theory, a massive solar sail might be able to move at half the speed of light.Such a solar sail would take only about 8 years to reach a nearby star) The advantage of such a propulsion system is that it can be built using existing technology and does not require the discovery of new laws of physics.But the main problems are economic and engineering.Here's the engineering problem: Building a solar sail hundreds of feet wide and powered by thousands of powerful laser beams located on the moon is hard work and requires technology that may not be around for the next 100 years. (There is a problem with interstellar solar sails - recycling. We would have to build a second laser array on a distant planetary moon to push the ship down to Earth, or, the ship could rapidly orbit a star, turning it like a catapult to gain enough speed for the return trip. Lasers on the moon can then slow the sail down so it can land on Earth.)
My personal favorite for taking us to the stars is the ramjet fusion engine.There is abundant hydrogen in the universe, and ramjet engines obtain hydrogen during space travel, essentially giving them inexhaustible rocket fuel.Once collected, the hydrogen can then be heated to millions of degrees, hot enough for the hydrogen to fuse, releasing the energy of a thermonuclear reaction.
The ramjet fusion engine was proposed by physicist Robert E. Bussard in the 1960s and later popularized by Carl Sagan.Bassard calculates that a ramjet weighing about 1,000 tons might theoretically be able to maintain a steady thrust equivalent to 1 gram of force, the same force as standing on Earth.If a ramjet could sustain 1 gram of acceleration for a year, it could reach 77 percent the speed of light, enough to make interstellar travel truly possible.
The requirements for a ramjet fusion engine are easy to calculate.First, we know the average concentration of hydrogen gas throughout the universe.We can also calculate how much hydrogen must be burned to achieve 1 gram of acceleration.This calculation in turn determines how large the "spoon" used to collect the hydrogen should be.With some reasonable assumptions, we can see that we need a spoon with a diameter of about 160 km.While it would be impossible to make a spoon this large on Earth, it would be easier to make in space due to the weightlessness.
Basically, the ramjet is capable of self-propulsion indefinitely, eventually reaching distant star systems in the Milky Way.According to Einstein's theory, time would slow down inside the rocket, and it might be possible to reach astronomical distances without putting the crew in a life-suspended state.According to the internal clock of the spacecraft, after 11 years of acceleration of 1 gram, the spacecraft will reach the Pleiades star cluster 400 light-years away.In 23 years, it will reach the Andromeda galaxy, about 2 million light-years away from Earth.In theory, a spacecraft might be able to reach the limit of the visible universe within the lifetime of a crew member (although billions of years may have passed on Earth).
The main wild card might be the fusion reaction.The ITER fusion reactor, due to be built in southern France, combines two rare forms of hydrogen (deuterium and tritium) to create an energy disk.In space, however, hydrogen's most abundant form consists of a proton surrounded by an electron.Therefore, ramjet fusion engines will have to utilize photon-photon fusion reactions.While physicists have studied the deuterium/tritium fusion process for decades, the poorly understood process of the photon-photon fusion reaction is much more difficult and produces far less energy.Therefore, mastering the more difficult photon-photon fusion reaction will be a major technical challenge in the next 10 years. feel suspicious).
Until the physics and economics of photon-photon fusion reactions are resolved, it is difficult to make precise estimates of the feasibility of ramjet engines.But it was on the short list of candidates when planning missions to the stars.
In 1956, the US Atomic Energy Commission (AEC) began serious work on nuclear rockets as part of Project Rover.In theory, a nuclear fission reactor would heat gases such as hydrogen to extremely high temperatures, which would then be ejected from one end of the rocket to create thrust.
Because of the risk involved of toxic nuclear fuel exploding in Earth's atmosphere, early nuclear rocket motors were placed horizontally on railroad tracks, where the rocket's performance could be carefully monitored.The first nuclear rocket engine tested in the rover program was Kiwi 1 (aptly named after Australia's flightless bird) in 1959.In the 1960s, NASA joined the AEC to create the Nuclear Engine for Rocket Vehicle Applications (NERVA), the first nuclear rocket to be tested vertically rather than horizontally.In 1968, the nuclear rocket was test-fired in an upside-down position.
The results of this study were mixed.Rockets are very complex and often fail.The violent vibrations of the nuclear engines frequently shattered the fuel bundles, causing the spacecraft to rip apart.Corrosion from burning hydrogen at high temperatures is also a recurring problem.The nuclear rocket program was finally terminated in 1972.
(There is another problem with these nuclear rockets: the danger of a runaway nuclear reactor, as in the case of small atomic bombs. Although today's commercial nuclear power plants run on diluted nuclear fuel and would not explode like the Hiroshima bomb, these nuclear rockets runs on highly enriched uranium, and is therefore capable of detonating in a chain reaction, creating a miniature nuclear explosion. As the nuclear rocket program was drawing to a close, scientists decided to conduct a final test. They removed the joystick [which was used for Inhibiting the nuclear reaction], the reactor then went supercritical and exploded into a flaming fireball. This spectacular curtain call of the nuclear rocket was even captured on video. The Russians were very displeased. They believed that this amazing move violated the Partial Nuclear Test Ban Treaty [Limited Test Ban Treaty], which prohibits the ground explosion of atomic bombs.)
Over the years, the military has periodically reconsidered nuclear rockets.There's a secret program called the Timberwind nuclear rocket, part of the military's "Star Wars" program in the 1980s (details of its existence were censored by the Federation of American Scientists). Scientist] was disclosed and abandoned).
The biggest concern about nuclear fission rockets is their safety.Although humanity has only been in the space age for 50 years, chemical booster rockets have suffered about 1% of catastrophic failures in that time (Challenger and Columbia Space Shuttles) The crash tragically killed 14 astronauts and further confirmed this failure rate).
But, over the past few years, NASA has restarted nuclear rocket research for the first time since the NERVA program in the 1960s.In 2003, NASA named a new project—Prometheus (Prometheus), named after the Greek god who brought fire to mankind.In 2005, the Prometheus Project received a $430 million grant, although that was cut to $100 million in 2006.The future of the plan is unclear.
There is also the less clear-cut possibility of using a series of mini-atom bombs to propel a starship.In Project Orion, mini-atom bombs are sequentially ejected from the bottom of the rocket so that the spacecraft can "ride" the shock wave created by these mini-hydrogen bombs.In theory, this design could allow a spacecraft to reach speeds close to the speed of light.The idea was conceived in 1947 by Stanislaw Ulam, who helped design the first hydrogen bomb, by Ted Taylor (one of the chief designers of nuclear warheads for the U.S. military) and the Princeton Institute for Advanced Study (Institute for Advanced Study) physicist Freeman Dyson (Freeman Dyson) took it a step further.
In the 1950s and 1960s, scientists performed sophisticated calculations on such interstellar rockets.It is estimated that such a spacecraft could fly to Pluto and back within a year, at a maximum speed of 10% of the speed of light.But even at that speed, it would take about 44 years to reach the nearest star.Scientists deduce that the ark in space propelled by this rocket must sail for centuries, and after several generations of crew members, their offspring will be born on the spacecraft and spend their lives on the spacecraft, so that their descendants can reach the nearest of stars.
In 1959, General Atomics published a report estimating the size of an Orion spacecraft.The largest, known as superOrion, weighs 8 million tons, has a diameter of 400 meters and is powered by more than 1,000 hydrogen bombs.
But the biggest problem with this project is that nuclear radioactive fallout may cause pollution during the launch process.Dyson estimated that the fallout from each launch could cause fatal cancer in 10 people.In addition, the electromagnetic pulses generated by the emissions are so intense that they can short-circuit adjacent electronic systems.
In 1963, the Partial Test-Ban Treaty sounded the death knell for the program.Ultimately, the main driver behind the project, atomic bomb engineer Ted Taylor, gave up (he once confided to me that when he realized that the physics contained in the mini-bomb could also be used by terrorists to create a portable atomic bomb) At that point, he finally felt his dream was shattered. Although the program was eventually terminated because it was considered too dangerous, its name was carried on the Orion spacecraft, which NASA chose to replace the 2010 Space Shuttle [Space Shuttle in 2010]).
The nuclear-powered rocket concept was briefly revived by the British Interplanetary Society between 1973 and 1978. Project Daedalus is a preliminary study into the possibility of building an unmanned spacecraft capable of reaching Barnard's Star, a star 5.9 light-years from Earth. spacecraft. (Barnard's star was chosen because it was supposed to host a planet. Since then, the astronomers Jill Tarter and Margaret Turnbull have compiled neighboring A list of 17,129 stars that may have planets suitable for life. Among them, Epsilon Indi A is the most popular star [Epsilon Indi A], 11.8 light-years away from Earth.)
The rocket ship planned for the Daedalus project is so large that it will have to be built in space.It will weigh 54,000 tons, almost all of its weight in rocket fuel, be capable of traveling at 7.1 percent the speed of light, and carry a payload of 450 tons.Unlike Project Orion, which used miniature fission bombs, Project Daedalus would have used a deuterium/isotope helium-3 mixture ignited by electron beams.The Daedalus project was also shelved indefinitely due to insurmountable technical difficulties it faced, as well as anxiety over its nuclear propulsion system.
Engineers sometimes speak of "specific impulse," which allows us to rank the efficiency of various engine designs. "Specific impulse" is defined as the change in momentum per mass unit of propellant, so the more efficient an engine is, the less fuel is needed to propel a rocket into space.Momentum, in turn, is the product of a force acting on it for some time.Although chemical rockets have very high thrust, they only run for a few minutes, so the specific impulse is very low. Because ion engines can run for several years, they can have high specific impulse and very low thrust.
Specific impulse is measured in seconds.A typical chemical rocket has a specific impulse of 400-500 seconds.The specific impulse of the space shuttle engine is 453 seconds (the highest specific impulse ever achieved by a chemical rocket is 542 seconds, using a propellant mixed with hydrogen, lithium and fluorine). The engine specific impulse of the "Smart One" ion engine is 1640 seconds, and the specific impulse of the nuclear rocket can reach 850 seconds.
The largest specific impulse possible would come from a rocket capable of reaching the speed of light, which would have a specific impulse of about 30 million.The following table lists the specific impulse of different types of rocket motors.
(In principle, laser sails and ramjets have infinite specific impulse since they don't have any rocket propellant at all, though they have their own problems.)
The most serious objection to many of these rockets is that they are too large and heavy to ever be built on Earth, which is why some scientists have proposed building them in space.There, weightlessness might allow astronauts to lift impossibly heavy objects with ease.But critics now point out that the cost of assembling them in space is prohibitively large. For example, the International Space Station requires more than 100 space shuttle launches to complete the assembly, and the cost has gradually accumulated to 100 billion US dollars.It is the most expensive science project in history.It costs many times more to build a starship or ramjet scoop in space.
But, as sci-fi author Robert Heinlein likes to say, if you can get your spacecraft 160 kilometers above Earth, you're already halfway there to travel the solar system at will.Because, within the first 160 kilometers of any kind of launch, the rocket struggles to escape gravity, which is obviously the most expensive part.After that, rocket ships can almost easily sail to Pluto and beyond.
In the future, there is a way to significantly reduce costs, and that is the space elevator.The idea of climbing up to heaven by a rope is very old.Like in the fairytale Jack and the Beanstalk, for example, but it might become a reality - if the rope could be sent high into space.After that, the centrifugal force generated by the earth's rotation will be enough to counteract the force of gravity, and the rope will never fall.The rope will magically rise vertically into the air and disappear into the clouds (imagine a sphere spinning on an axis. It appears to defy gravity because centrifugal force pushes it away from the center of rotation. Similarly, a very Long ropes can "hung" in the air due to the rotation of the earth. Nothing is needed to hold the rope apart from the rotation of the earth. In theory a person could climb on the rope and go up into space. We sometimes learn from people who take physics classes at the City University of New York Students asked them to calculate the tension on such a rope, and it was not difficult to find that the tension on the rope would be enough to make the steel wire rope snap. This is why space elevators have long been considered impossible.
The first scientist to delve into space elevators was the Russian visionary and scientist Konstantin Tsiolkovsky.In 1895, inspired by the Eiffel Tower, he imagined a tower that could rise into space, linking the earth with a "castle in the sky" in space.It will be built from the bottom up, starting at the ground, and engineers will slowly extend the elevators to the sky.
In 1957, Russian scientist Yuri Artsutanov came up with a new solution: building space elevators in reverse order — top down, starting in space.He imagined a satellite in space in a geostationary orbit 36,000 miles away, from where a cable could be lowered to Earth.Subsequently, the cable will be anchored to the ground.But the gauge of a space elevator must withstand a tension of about 60-100gpa (gigapascals).Steel breaks at about 2gpa, making this idea out of reach.
With the introduction of Arthur C. Clarke's novel "The Fountains of Paradfae" (1979) and Robert Heinlein's novel "Friday" (1982), the space elevator's Concepts communicated to a wider audience.However, without any progress, the concept withered.
The equation changed dramatically when chemists developed carbon nanotubes.In 1991, the research results of Sumio Iipna of Nippon Electric suddenly aroused widespread interest in the academic community (although the origin of carbon nanotubes can be traced back to the 1950s, a fact that is now unknown). is ignored).Remarkably, the nanotubes are stronger than steel wire ropes, while also being lighter.In fact, they exceed the strength required to sustain a space elevator.Scientists believe that a carbon nanotube fiber can withstand a pressure of 120gpa, which is well above the breaking point.This discovery reignited enthusiasm for building space elevators.
In 1999, a NASA study took the space elevator seriously, imagining a belt about 1 meter wide and 47,000 kilometers long, capable of delivering a payload of about 15 tons into Earth orbit.Such a space elevator could transform the space economy overnight.The construction expenditure can be reduced to one ten thousandth of the original, which is an amazing and revolutionary change.
Currently, it costs more than $10,000 (roughly the price of an ounce of gold) to send a 1-pound object into orbit around the Earth.A space elevator, for example, could reduce the cost of a space shuttle mission to as much as $700 million per mission to as little as $1 a pound.Such a dramatic reduction in space program spending could revolutionize the way we view space.Simply pressing the elevator button, we could theoretically ride an elevator into space for the price of a plane ticket.
But before we can build a space elevator in which to ride into space, formidable practical hurdles must be addressed.Currently, pure carbon nanotube fibers produced in the laboratory are no more than 15 millimeters long.To build a space elevator, we have to create thousands of miles of carbon nanotube strands.Although this is nothing more than a technical problem from a scientific point of view, it is a stubborn and difficult problem that must be solved if we are going to build a space elevator.However, many scientists believe that within a few decades we will have the technology to make long cables of carbon nanotubes.
Second, tiny impurities in the carbon nanotubes can make the long cables a problem.Nicola Pugno of the Polytechnic of Turin in Italy estimates that an error in even one molecule in a carbon nanotube can reduce the strength of the carbon nanotube cord by 70%, making the It cannot achieve the minimum number of gigapascals necessary to support a space elevator.
To incentivize original ideas on space elevators, NASA has funded two separate awards (modeled after the Ansari X-prize, which has successfully inspired enterprising inventors to make A commercial rocket that carries passengers to the edge of space. The X Prize went to Spaceship One in 2004). NASA offers prizes called the Beam Power Challenge and the Tether Challenge.In the Beam Power Competition, teams must send a mechanical device weighing at least 25 kg at a speed of at least 1 meter per second up a tether (hanging from a crane) for at least 50 meters.This may sound easy, but the difficulty is that the mechanism must not use fuel, batteries, or wires. Instead, the mechanism must be powered by a solar array power system, solar concentrators, lasers, or microwaves, which are suitable for use in space. to provide motivation.
In the tether competition, teams must create a 2-meter-long tether weighing no more than 2 grams and must be able to carry 50% more weight than the previous year's best tether.The purpose of the competition is to encourage research to develop materials lightweight enough to suspend 100,000 kilograms in space.Prizes are $150,000, $40,000, and $10,000 (emphasizing the difficulty of conquering this competition: in 2005, the competition's first year, no one won the prize).
While a successful space elevator could revolutionize the space program, such machines come with their own set of hazards.For example, the orbits of near-Earth satellites are constantly changing as they orbit the Earth (due to the rotation of the Earth beneath them).That means the satellites would eventually collide with the space elevator at 18,000 miles per hour hard enough to snap the tethers.In order to prevent such disasters, in the future, either artificial satellites must be designed with small rockets that can swim around the space elevator, or the tether of the elevator must be equipped with small rockets to avoid passing artificial satellites.
Also, collisions with meteorites are a problem because the space elevator is well above Earth's atmosphere, which normally protects us from meteors.Since meteorite impacts are unpredictable, the elevator compartment must have additional defensive shields and perhaps a failsafe system.Extreme weather on Earth can also cause problems, such as hurricanes, tidal waves and storms.
There is also a novel way to hurl objects close to the speed of light by using the "slingshot" effect.When sending space probes to planets in space, NASA sometimes spins them rapidly around neighboring planets so they can use the slingshot effect to gain speed. NASA saves valuable rocket fuel this way.This is how the Voyager spacecraft managed to reach Neptune, which lies on the edge of the solar system.
Physicist Freeman Dyson of Princeton University has proposed that in the distant future, we may find two neutron stars orbiting each other at high speed.By getting very close to one of the two neutron stars, we can orbit it at high speed and then be flung into space at nearly 1/3 the speed of light.In fact, we will use gravitational force to give extra boost to approach the speed of light.In theory, this is only possible.
Others propose that we spin rapidly around the sun to accelerate to nearly the speed of light.In fact, this approach was used in Star Trek IV: Saving the Future. The crew of the Enterprise hijacks a Klingon ship and races toward the sun to break the light barrier and travel back in time.In the film When Worlds Collide, Earth is threatened with an asteroid impact, and scientists build a giant roller coaster to escape Earth.A rocket spaceship slides off a roller coaster, gaining extreme speed, then spins around the bottom of the coaster and blasts off into space.
然而，在事实上，这些利用引力将我们推进到太空中的方法没有一个是可行的（由于能量守恒定律，从过山车上滑下和驶回的过程中，我们最终达到的速度与初始的一样，因此我们无论如何都不会获得能量。同样，绕着静止不动的太阳转动，我们最终达到的速度与最初开始时相同）。戴森使用两颗中子星的方式可能有效的原因是中子星转动得极快。一艘利用弹弓效应的宇宙飞船从一颗恒星的行星运动中获取能量。如果它们是静止的，那就根本不存在弹弓效应。
尽管戴森的提议或许可行，但它对如今被束缚在地球上的科学家们没有帮助。因为我们将需要一艘宇宙飞船专门用以到达转动的中子星。
另有一种将物体以梦幻般的速度掷入太空的绝妙方法——轨道炮（railgun）。亚瑟·C.克拉克和其他人在自己的科幻小说中对其大加描绘，在“星球大战”的导弹防御系统中它也作为其一部分得到了认真的评估。
轨道炮不使用火箭燃料或是火药将炮弹推进到高速，而是采用电磁的力量。
轨道炮最为简单的形式由两根平行的导线或轨道构成，一颗炮弹横跨在两根导线上，组成了一个U形结构。甚至连迈克尔·法拉第也知道，当一束电流被放置在磁场中的时候会遭遇力（这其实是所有电动机的基础）。通过将数百万安培的电力送过这些导线，并通过炮弹，轨道周围形成了巨大的磁场。这一磁场随后会以巨大的速度将炮弹推下轨道。
轨道炮已经成功将金属物体以极高的速度射出非常短的距离。非比寻常的是，理论上，一门简单的轨道炮应该能够将一颗金属炮弹以每小时18000英里的速度发射，如此它将进入地球周围的轨道。基本上，NASA的整个火箭战队都可以用一门轨道炮替代，它可以将所有荷载物从地球发射入轨道。
相比化学火箭和枪炮，轨道炮具备极大的优势。在一杆来福枪里，膨胀的空气推动子弹所能达到的极限速度被冲击波的速度所限制。尽管儒勒·凡尔纳在他的经典小说（From the Earth to the Moon）中使用火药把宇航员发射到了月球上，但我们可以计算出使用火药所能获得的极限速度仅及将人送上月球所需速度的一小部分。但是，轨道炮不被冲击波的速度所限制。
可是，轨道炮也存在问题。它极快地将物体加速，使它们通常会在空气的冲击之下被压扁。荷载物在被射出轨道炮炮筒的过程中会遭到严重的扭曲变形，因为炮弹撞上空气，就好像撞上一堵砖墙一样。此外，荷载物沿轨道产生的巨大加速度也足以使它们变形。由于炮弹引起的损毁，轨道不得不定期更换。并且，一位宇航员承受的加速度力足以导致他死亡，能轻易压碎他体内的所有骨头。
有人提议在月球上安装一台轨道炮。在地球大气层之外，一颗轨道炮的炮弹可以不费吹灰之力疾行过太空的真空，但是一门轨道炮产生的巨大加速度就可能毁坏荷载物。从某些意义上来说，轨道炮是激光帆的对立面。激光帆经过一长段时间温和地获得自己的速度；轨道炮是有限制的，因为它们将巨大的能量填充进了小小的空间内。
能够将物体发射到附近恒星上的轨道炮将相当昂贵。一项提议认为，轨道炮应该在太空中制造，延伸达地球至太阳的2/3距离之长。它储存来自太阳的太阳能，随即猛地将那些能量排放入轨道炮，以1/3光速送出10吨的荷载，有5000克的加速度。不出意料的是，只有最强壮的机器荷载才能在如此巨大的加速度之下幸存。
太空旅行不是星期天的野餐，巨大的危险恭候着去往火星或更远处的载人飞船。地球上的生命已经被庇护了数百万年：地球的臭氧层保护地球免受紫外线侵袭，它的磁场对抗太阳耀斑和宇宙射线，它厚厚的大气层保护地球免遭流星撞击，使流星在一进入大气层的时候就被烧毁。我们将地球上温和的气温与气压视作理所当然。但是，在外太空中，我们必须面对这样的事实：宇宙的大部分处于混乱之中，有危险的辐射带和大群致命的流星。
延长太空旅行首先要解决的问题是失重。俄罗斯科学家对失重的长期研究表明，在太空中人体流失宝贵的矿物质和化学元素的速度比预想中快得多。尽管经过严格的训练，但在空间站度过一年以后，俄罗斯太空人的骨骼和肌肉仍然严重萎缩，他们在刚回到地球的时候只能像婴儿一样爬行。肌肉萎缩、骨骼恶化、红细胞产量减少、免疫反应低下以及心血管系统功能减弱，看来是长时间在太空中失重带来的不可避免的后果。
去往火星的任务或许要花上数月到一年，它将推进宇航员忍耐力的极限。对于飞往近处恒星的长距离任务而言，这个问题将是致命的。未来的宇宙飞船或许不得不旋转、通过离心力制造出人造重力以维持人们的生命。这—调整将大大增加未来宇宙飞船的花费和复杂性。
其次，宇宙中存在以每小时数万英里速度飞行的流星，这或许会要求宇宙飞船必须装备额外的防御盾。对航天飞机机身的详细检査显示了几次微小的、但有致命可能的小型流星撞击。在未来，宇宙飞船可能必须为船员配备—个特别双重加固的舱室。
外太空中的辐射强度比过去所认为的要强很多。例如，在11年的太阳黑子周期中，太阳耀斑发出巨量的致命等离子体，向地球奔腾而来。在过去，这一现象迫使空间站上的宇航员们寻找特殊的保护，对抗亚原子颗粒组成的可能致命的火力网。在这样的太阳爆发期间进行太空行走是致命的（举例来说，哪怕是从洛杉矶到纽约作一次简单的横跨大陆旅行也会使我们接受在每小时飞行1毫笛姆辐射的照射。在整个旅程中，我们被暴露在几乎相当于一台牙科X光机的辐射之下）。在外太空，地球的大气层和磁场不再保护我们，辐射照射会成为一个严重的问题。
到目前为止，我介绍的火箭设计有一项始终存在的非议。那就是，哪怕我们能制造出这样的恒星飞船，也要花上数十年到数百年才能到达附近的恒星。这样的任务需要数代船员参与，他们的后代将到达最后的目的地。
《异形》（Alien）和《人猿星球》（Planet of the Apes）等电影提出了一个解决方法，让太空旅行者们接受暂停生命，也就是说，他们的体温会被小心翼翼地降低，直到身体功能几乎停顿。冬眠的动物每年冬季期间都这么做，某些鱼类和蛙类可以在冰块中冻得一动不动，但当温度上升时又能解冻。
研究这一奇特现象的生物学家们发现，这些动物具备创造天然“抗冻功能”的本领，能够降低使水结冰的凝固点。这一天然抗冻功能由鱼体内的蛋白质和蛙体内的葡萄糖构成。通过使血液中充满这些蛋白质，鱼可以在N极-2℃的气温下生存，蛙类进化出了维持高葡萄糖水平的能力，因此可以阻碍冰晶形成。尽管它们的身体也许会被从外面冻僵，它们的身体内部却没有冻结，这使它们的身体器官能够继续运转，虽说速度会减缓。
然而，使这一能力适合人类是有问题的。当人体组织被冰冻，冰晶就开始从细胞内部形成。随着这些冰晶变大，它们能够穿透和摧毁细胞壁（希望在死后将自己的头部和身体冷冻在液氮中的名人们或许会重新考虑）。
虽然如此，近期还是在不会自然冬眠的动物如老鼠和狗身上取得了有限暂停生命的进展。在2005年，匹茨堡大学（University of Pittsburgh）的科学家们成功地在狗的血液流干并使用特殊冰冻液体作为替代后将狗复活。临床死亡3小时后，狗在心脏复跳后重获生命（尽管大多数狗在这一程序后很健康，但有几只遭受了一些大脑损伤）。
同一年，科学家将老鼠放入含有氢化硫的房间中，并且成功地将它们的体温减为13℃长达6小时。老鼠的代谢率下降到了原来的1/10。在2006年，波士顿马萨诸塞州综合医院（Massachusetts General Hospital）的医生使用氧化硫使猪和老鼠进入了暂停生命状态。
在未来，这样的步骤或许可以拯救发生严重意外或数着秒数过日子的心脏病患者。生命暂停允许医生“冻结时间”，直到患者有法可医。但将这样的技术应用于人类宇航员或许还需要数十年以上，因为他们可能需要暂停生命几个世纪。
还有一些方法能让我们通过更先进、未经验证、接近科幻小说的科技到达其他恒星。最有希望的提议是使用以纳米技术为基础的无人驾驶探测器。在本篇讨论中我自始至终都假设恒星飞船必须是巨大的装置，消耗巨量能源，能够将大批人类船员带去恒星，类似于《星舰迷航》中的“企业号”。
更合适的途径可能是首先以接近光速的速度发送一架微型无人驾驶探测器到遥远的恒星，正如我们早先提到的那样，在未来，有了纳米科技可以制造出微型宇宙飞船，它们利用的是原子和分子大小的器械的力量。例如，离子，由于它们很轻，因此能够使用实验室中的普通电压轻易加速到接近光速。或许可以使用强大的电磁场以接近光速的速度将它们送入太空，而非使用巨大的助推火箭。这意味着，如果一台纳米机器人被电离，并且放入一个电场中，它将毫不费力地被提速到接近光速。这台纳米机器人随即会向恒星们滑翔而去，因为太空中没有摩擦力。通过这种方法，许多困扰大型恒星飞船的问题就都立刻迎刃而解了。无人操控的智能纳米机器人宇宙飞船或许仅需花费制造和发射一艘巨型载人恒星飞船所需开支的一小部分就可到达近处的恒星系统。
这样的纳米飞船可以用于飞往近处的恒星，或者像一位退休的美国空军航天工程师杰拉德·诺德利（Gerald Nordfey）建议的那样，用于向一艘太阳帆施加压力，以便将其推进太空。诺德利说：“如果有一群针头大小的恒星飞船排成队形飞行，并且相互联系，你就可以实际上用一束闪光推动它们。”
但是，纳米恒星飞船面临挑战。太空中飞过的电场或磁场改变方向的同时可能会改变飞船的方向。为了对抗这些力量，我们需要在地球上将纳米飞船的电压增强到极高的水平，这样它们就不会轻易改变方向。其次，我们或许不得不送出数百万艘的一大群这样的纳米机器人恒星飞船，以保证有少量能够真正成功到达目的地。向最近的恒星送出大群恒星飞船或许看起来很奢侈，但这样的恒星飞船很廉价，并且可以数以十亿计地大批生产，这样它们中只要有一小部分到达目的地就行了。
这些纳米飞船会是什么样的？NASA的前领导人丹·古德林（Dan Goldin）想象了一个可乐罐大小的宇宙飞船舰队，其他人则谈论针那么大小的恒星飞船。五角大楼已经在调査开发“智能尘埃”（smart dust）的可能性，尘埃大小的粒子内部装有微型探测器，能够喷洒遍整个战场，给予指挥官实时信息。在未来，可以想象“智能尘埃”或许会被送往近处的恒星。
尘埃大小的纳米机器人的电路系统将使用半导体产业应用的蚀刻技术制造。这一技术能够制造出小至30纳米、或者约150个原子宽的元件。这些纳米机器人能从月球使用轨道炮或者甚至是粒子加速器发射，粒子加速器一般能将亚原子颗粒发射到接近光速。这些装置非常便宜，可以被数百万计地发射入太空。
一旦它们到达某个附近的恒星系统，纳米机器人可以在一颗荒无人烟的衍星卫星上着陆。由于行星卫星的引力小，一台纳米机器人可以毫不困难地着陆和起飞。由这样的一颗行星卫星所提供的稳定环境，它会是理想的运行基地。纳米机器人可以建立一家纳米工厂，使用在行星卫星上发现的矿物，以建造一个能将信息发送回地球的强大的无线电台。或者纳米工厂可以被用于制造数百万个纳米机器人复制品，以探索那个恒星星系和去其他附近的恒星探险，重复这一过程。由于这些飞船是机器人化的，因此它们不必在使用无线电发送回信息后飞回地球。
我刚刚描述的纳米机器人有时被称为“冯·诺依曼探测器”（von Neumann probe），以著名数学家约翰·冯·诺依曼（John von Neumann）的名字命名，他解出了能够自我复制的图灵机的数学公式。原则上，这样自我复制的纳米机器人恒星飞船或许能够探索整个银河系，而不仅是附近的恒星。最终，或许会产生一个由数万亿个这样的纳米机器人组成的球体，它们越来越快地增加，同时变大、以接近光速的速度扩张。这一扩张中的球体中的纳米机器人可以在数十万年内将整个银河系开拓为殖民地。
一位电气工程师非常认真地思考着纳米飞船这一概念——密歇根大学（University of Michigan）的布莱恩·吉尔克莱斯特（Brain Gilchrist）。他不久前从NASA的先进概念研究所（Institute for Advanced Concepts）获得了50万美元拨款，探究建造发动机不大于细菌的纳米飞船这一构想。他想象使用半导体行业的蚀刻技术来建造数百万纳米飞船舰队，它们会喷射直径仅数十纳米的纳米粒子来自我推进，这些纳米粒子靠通过一个电场来获得能量，就如在离子发动机内部一样。由于每一个纳米微粒都比一个离子重数千倍，这样的发动机将携带比一台典型离子发动机多得多的推力。这样，纳米发动机将具备与离子发动机相同的优势——除了它们具备更大的推进力之外。吉尔克莱斯特已经开始蚀刻这些纳米飞船的某些部件。迄今为止，他已能在一块1厘米宽的硅芯片上蚀刻1万个独立推进器。最初，他想把他的纳米飞船舰队送到整个太阳系中测试它们的能力，但最终这些纳米飞船或许会成为首先到达恒星上的舰队中的一部分。
吉尔克莱斯特的提议是NASA所考虑的几个新颖提议之一。在几十年的停滞状态之后，NASA不久前对各种各样的星际旅行提议给予了认真的考虑——这些提议从脚踏实地到奇异荒诞，应有尽有，自20世纪90年代早期开始，NASA主办了一年一度的先进太空推进研究研讨班（Advanced Space Propulsion Research Workshop）。在研讨班期间，这些技术被认真的工程师和物理学家小组批驳得体无完肤。更为野心勃勃的是“突破推进物理”（Breakthrough Propulsion Physics）项目，它探索与星际旅行相关的神秘的量子物理世界。尽管两者没有共同观点，但是它们的活动有许多都集中在该领域的领先者——激光帆和各种类型的聚变火箭身上。
由于宇宙飞船设计方面的进展缓慢但稳定，因此假设第一艘某种类型的无人驾驶探测器或许将在本世纪后期或下个世纪早期被送上近处的恒星是合理的。这使它成为一项“一等不可思议”。
但恒星飞船最强有力的设计或许要涉及反物质。尽管它听起来像科幻事物，但反物质巳经在地球上被制造出来，而且或许某天会为可行的载人恒星飞船提供最有前景的设计方案。
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