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  • The year is 2154. Our planet has been ruined by environmental catastrophe.

  • In the movie Avatar, greedy prospectors from Earth descend on the world of an innocent

  • hunter-gatherer people called the Na'vi.

  • Their home is a lush moon far beyond our solar system called Pandora.

  • Could such a place exist?

  • And could our technology... and our appetite for exploration... one day send us hurtling

  • out to reach it?

  • In fact, the supposed site of this fictional solar system is one of our most likely interstellar

  • targets, until a better destination turns up.

  • Pandora orbits a fictional gas planet called Polyphemus.

  • Its home is a real place... Alpha Centauri... the brightest star in the southern constellation

  • of Centaurus.

  • At 4.37 light years away, it's part of the closest star system to our sun.

  • Alpha Centauri is actually two stars, A and B, one slightly larger and more luminous than

  • our own sun, the other slightly smaller.

  • The two stars orbit one other, swinging in as close as Saturn is to our Sun... then back

  • out to the distance of Pluto.

  • This means that any outer planets in this system... anything beyond, say, the orbit

  • of Mars... would likely have been pulled away by the companion and flung out into space.

  • For this reason, Alpha Centauri was not high on planet hunters' lists... until they began

  • studying a star 45 light years away called "Gamma Cephei." {SEF-ee-eye}

  • It has a small companion star that goes around it every 76 years. Now, it seems... it also

  • has at least one planet.

  • That world is about the size of Jupiter, and it has planet hunters excited. Perhaps two-thirds

  • of all the stars in our galaxy are in so-called binary relationships.

  • That means there could be many more planets in our galaxy that astronomers once assumed.

  • At least three teams are now conducting long-term studies of Alpha Centauri... searching for

  • slight wobbles in the light of each companion star that could indicate the presence of planets.

  • If they find a planet that passes in front of one of the stars, astronomers will begin

  • intensive studies to find out what it's like.

  • One of their most promising tools will be the James Webb Space Telescope, scheduled

  • for launch in 2014 or 2015. From a position a million miles away from Earth, it will deploy

  • a sun shield the size of a tennis court, and a mirror over 21 feet wide.

  • The largest space telescope ever built, it will offer an extraordinary new window into

  • potential solar systems like Alpha Centauri.

  • With its infrared light detectors, this telescope will be able to discern the chemical composition

  • of a planet's atmosphere... and perhaps whether it harbors a moon like Pandora.

  • One prominent planet hunter predicted that if a habitable world is found at Alpha Centauri,

  • the planning for a space mission would begin immediately.

  • Here's that star duo as seen by the Cassini spacecraft just above the rings of Saturn.

  • To actually get to this pair of stairs, you have to travel as far as the orbit of Saturn,

  • then go another 30,000 times further.

  • Put another way, if the distance to Alpha Centauri is the equivalent of New York to

  • Chicago, then Saturn would be just... one meter away.

  • So far, the immense distances of space have not stopped us from launching missions into

  • deep space.

  • In 1977, the twin Voyager spacecraft were each sent on their way aboard Titan 3 Centaur

  • rockets.

  • After a series of gravitational assists from the giant outer planets, the spacecraft are

  • now flying out of the solar system at about 40,000 miles per hour.

  • They are moving so quickly that they could whip around the Earth in just 45 minutes,

  • twice as fast as the International Space Station.

  • Voyager I has now traveled over 110 astronomical units. That's 110 times the distance from

  • Earth to the Sun... or about 10 billion miles. But don't hold your breath...

  • If it was headed in the right direction, it would need another 73,000 years to travel

  • the 273,000 astronomical units to Alpha Centauri.

  • When it comes to space travel, we've yet to realize the dream forged by rocketeers a century

  • ago.

  • A Russian school teacher, Konstantin Tsiolkovsky, inspired generations of space visionaries

  • with sophisticated ideas about multi-stage launch vehicles.

  • He imagined the construction of space stations in Earth orbit, and eventually vast permanent

  • space colonies.

  • In time, he predicted, we'd evolve into a whole new species... Homo Cosmicus.

  • Since then, rocketry's greatest advances have centered on ways of containing explosive propellants...

  • and methods of maintaining stable flight at high speeds.

  • The problem is that chemical rockets are just not efficient for long distance space travel.

  • To reach the speed needed to escape Earth's gravity, 17,000 miles per hour, the space

  • shuttle must carry fifteen times its weight in fuel. And that's efficient, compared to

  • some other rocket systems. And you'd need to travel more than 25,000 miles per hour

  • to break free of Earth's orbit and go anywhere else.

  • A NASA study showed that to send a space shuttle sized craft to Alpha Centauri in 900 years

  • would take an unbelievable amount of fuel: 10 to the 137th kilograms of rocket propellant.

  • Suffice it to say... it's more mass than is in the entire visible universe.

  • While only a very tiny percentage of NASA's budget goes to advanced propulsion, there

  • are some promising ideas on the drawing board.

  • Rockets powered by nuclear fuel...

  • Or plasma... a supervolatile gas.

  • Huge sails pushed along by the pressure of photons from the Sun.

  • Ion drives.

  • To reach Alpha Centauri within a human time scale, we'll have to go with most potent fuel

  • in nature that we current know...

  • It's the science fiction fuel of choice...

  • Anti-matter.

  • In James Cameron's Avatar, hybrid nuclear fusion and antimatter engines power a mile

  • long interstellar spaceship. At a speed of 670 million miles per hour, this vehicle makes

  • the journey to Alpha Centauri in just six years.

  • Anti matter really does exist... as the mirror image of the universe we know. It consists

  • of electrons and protons, but with their electrical charges reversed. Whenever it comes into contact

  • with normal matter, the two annihilate each other in a ferocious blast of energy.

  • Large amounts of antimatter were created and destroyed in the fiery dawn of our universe,

  • the Big Bang. But somehow, in one of the great mysteries in science, we were left with a

  • universe whose visible substance is almost all normal matter.

  • The universe still produces antimatter through powerful collisions... such as a jet from

  • a black hole slamming into a cloud of gas.

  • When matter and antimatter obliterate one another, they emit gamma radiation that we

  • can then detect with instruments such as the Fermi Gamma Ray Space Telescope.

  • Fortunately, black holes aren't the only way to generate antimatter. In giant labs like

  • the Large Hadron Collider, scientists accelerate atoms to nearly the speed of light... and

  • blast them together to expose their fundamental constituents.

  • Small amounts of antimatter can be made this way, but it's incredibly expensive. With a

  • dedicated facility, the cost of producing it might come down far enough to produce usable

  • amounts.

  • And that's the hope of one researcher...

  • Dr. Gerald Smith has been working for over a decade to find a way to trap this volatile

  • substance... and store it in isolation from the rest of the universe.

  • Smith and his colleagues have designed a trap the size of a cigar case.

  • It sits within a tank filled with liquid nitrogen and liquid helium designed to cool it down

  • to 270 degrees below zero.

  • Once injected into this trap, antimatter particles are suspended by magnetic fields, within a

  • vacuum as empty as deepest space.

  • But the problem is that anti-electrons, called positrons, tend to repel each other... explosively.

  • That makes it tough to store more than a few at a time.

  • This team now believes it may have discovered a pathway to storing large amounts over longer

  • periods of time.

  • Their solution lies in combining positrons with electrons, forming an element called

  • positronium. In theory, with the right magnetic fields, these electrically neutral atoms might

  • be held indefinitely.

  • When released under controlled conditions, ultra high-energy antimatter beams could turn

  • out to be ideal cancer killers... or lead to revolutionary industrial applications...

  • Or perhaps, one day... they could power long distance space flight.

  • It wouldn't take much. Antimatter is so potent that it defies common sense: A chunk the size

  • of a small coin could propel the space shuttle into orbit.

  • Smith estimates that once in low Earth Orbit a human mission to Mars would take as little

  • as 10 milligrams worth.

  • The basic idea of an anti-matter rocket engine is simple. A beam of positrons is released

  • into the engine core... where it annihilates the surface of a metal plate. That creates

  • an explosion that propels the craft forward.

  • Another design uses a sail. A cloud of antimatter particles reacts explosively to its surface...

  • propelling it forward.

  • Short of traveling to another solar system, there may be good reasons to contemplate developing

  • antimatter propulsion.

  • A preliminary mission would speed beyond the orbit of Pluto, sending back close-up images

  • at dark planet-like objects that ring the solar system out in the Kuiper Belt.

  • A longer distance probe could reveal new details about the Oort Cloud, a vast realm of comets

  • that envelopes the solar system.

  • Once out there, it could sample particles that make up the interstellar medium... or

  • send back unique data sets on dark matter - the invisible stuff that makes up the overwhelming

  • portion of our Universe.

  • To make it all the way to Alpha Centauri within 50 years, an antimatter probe would have to

  • gradually accelerate to around ten percent the speed of light... that's 67 million miles

  • per hour.

  • It would then gradually decelerate as it approached its destination.

  • At those speeds, hitting even a grain of dust could destroy the spacecraft. So it might

  • be best to slow the journey down to a century or more.

  • It's safe to assume for now that we'd only send a probe there if we discovered a habitable

  • world.

  • There may be other choices in our solar neighborhood. They include Proxima Centauri, a red dwarf

  • star 4.2 light years away that may be gravitationally bound to Alpha Centauri.

  • Beyond that, not quite 6 light years away, is Bernard's Star.

  • Or there's Lalande 21185, a red dwarf 8.3 light years away. We already know it has two

  • Jupiter-sized planets.

  • There are at least 22 stars within 12 light years of Earth.

  • And anyway you look at it, the first interstellar voyage will be a quantum leap for humanity.

  • The urge to reach out to distant horizons... to climb the highest peaks... to push ourselves

  • past our perceived limits... seems to be a vital part of what makes us human.

  • Yet explorers of old set off not just because "it was there." At times it was greed, hunger,

  • fear, and despair that propelled them from their homelands... and allowed them to endure

  • their long journeys.

  • Whether we attempt to make a leap to the stars... may come to depend on how we regard this planet.

  • To the physicist Stephen Hawking, the journey is imperative.

  • "I don't think the human race," he said, "will survive the next thousand years unless we

  • spread into space. There are too many accidents that can befall life on a single planet."

  • Indeed, we can't foresee the impact of wars... social upheaval... or the course of human

  • civilization in coming centuries.

  • But today we can see the often conflicting trends today that could one-day propel us

  • out into the interstellar void.

  • On one hand, the technological advances that might make such a mission possible could revolutionize

  • many other aspects of life on this planet.

  • The ever-increasing rate at which numbers of transistors can be placed inexpensively

  • on a computer microchip has become a metaphor for the advance of all technologies in this

  • century.

  • From a few thousand transistors on the first printed circuits of the 1970s... computer

  • chips now have billions etched onto their surfaces.

  • Even that number could seem amazingly small in another few decades.

  • Many observers forecast a steep rise - even an acceleration - in the pace of invention

  • and basic research... and for whole of new solutions to the problems of energy, food

  • production, health, and more.

  • On the other hand, major periods of scarcity may loom.

  • In the 20th century, the world saw the largest increase in its human population, from less

  • than two billion up to six billion.

  • The world's population is now around 6.8 billion.

  • It's expected to reach 9 to 10 billion by the year 2040, with the biggest gains in Asia

  • and Africa.

  • According to a recent UN report, the world will have to produce 70% more food by the

  • year 2050... and at least that much more energy... to sustain its population.

  • The scarcity of simple clean water in some regions is already frightening.

  • Now throw in environmental impacts like rising sea levels or the spread of deserts linked

  • to a gradually warming climate. The culprit, to most scientists, is rising emissions of

  • greenhouse gases like carbon dioxide since the start of the industrial revolution.

  • This map charts rising temperature readings from the year 1885 through to the present.

  • In some places, they've gone up by as much two and a half degrees Fahrenheit.

  • Computer models project the trend out to the end of this century. Depending on population

  • growth, energy use, and conservation... temperatures could rise anywhere from two to eleven degrees

  • more.

  • Will technological advancements allow us to halt the degradation of our natural environments

  • and increase the carrying capacity of our planet?

  • Will we find ways to mitigate the impacts of war, natural catastrophes, or political

  • upheavals?

  • No doubt, if or when we launch our first mission beyond this solar system, the occasion will

  • spur reflection on who and what we have become as a people... as a planet... just as the

  • first missions to the Moon and our neighboring planets once did.

  • At first, we'll send a probe designed to relay basic information on what's there... on a

  • world whose light we have only studied from afar.

  • As this cosmic emissary makes its way across the void, we on Earth will continue to struggle

  • in our pursuits of happiness, prosperity... and mere survival.

  • When it arrives, we'll scan the data for evidence of a world like our own... one that may harbor

  • life.

  • How will our perspectives on that world - and upon our own - have changed?

  • 8

The year is 2154. Our planet has been ruined by environmental catastrophe.

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前往潘多拉的航行。第一次星際空間飛行 (Voyage to Pandora: First Interstellar Space Flight)

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    Wonderful 發佈於 2021 年 01 月 14 日
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