* Interstellar travel

Many science-fiction movies depict a colonized Galaxy, with space ships touring throughout it. Stars are incredible distant, yet many people have no doubt that in the future we will reach them. Let’s try to separate reality from fantasy.

Traveling to the stars is not the same task as traveling to the planets. Current space vehicles need three days to reach the Moon, about 10 months to reach Mars and 15 years for Pluto, but they need 80 000 years for the nearest star system. But let’s look at other possibilities.

1) AT THE SPEED OF LIGHT OR EVEN FASTER

A trip to the nearest star system will last hours or days. But at the speed of light (one billion km/h) time stops and mass becomes infinite, so we would need infinite energy. The “light barrier”, different from the sound barrier, can not be broken.

Because of that, Michael Morris, Kip Thorne y Ulvi Yurtsever proposed in 1988 to try to leave the Universe (!) at one place and enter again at another place very far away, as if going through a “wormhole”. And in 1994, Miguel Alcubierre proposed to create a “wrap in space-time”, compressing “terrain” in front of a spaceship to get it closer to its destination.

Or you could travel at light speed via radio waves: teleportation. What would go from one place to the other would be the information to reconstruct, atom by atom, another similar astronaut at the place of destination. Reloading his or her memories, personality and identity, as well as what to do with the original, are yet-to-be-solved issues. In 1993, Charles Bennett and others proposed to manipulate certain very tiny particles in order to transmit information faster than light. Several experiments were performed, but as today it was not possible to teleport a complete atom.

All of the above continue to be total fantasy.

2) ALMOST AT THE SPEED OF LIGHT

We would arrive at destination after some years. At these speeds time inside the ship goes slower: at 90% of the speed of light 1 year onboard the ship equals 2,3 years on Earth, at 99,9% it equals to 22 years and at 99,999 999 999% it equals to 223 609 years.

On departure, if we accelerate at a value no higher than what Earth’s gravity presses on us, we would be at almost the speed of light after one year. If we accelerate continuously through halfway to destination and then decelerate, in 20 years (ship time) we would have traveled 28 000 light-years (the distance light could travel in 28 000 years), in 30 years, 2 800 000 light-years and in 44 years we will reach the edge of the observable Universe. Thereafter we will need an extra year for a full stop.

But the Space Shuttle (in order to reach an altitude of just 400 km) needed to carry along an external tank with 730 tons of hydrogen and oxygen that it burned away in just 8 minutes. Even if we use the perfect fuel, antimatter, should we ever travel in a spaceship as big as the International Space Station we would need 1274 external tanks to reach 99,9% of the speed of light, 1 249 292 tanks to reach the center of our galaxy and 5 552 407 932 for the Andromeda galaxy. As Nobel laureate Edward Purcell would fancy, if this seems absurd to you, you are right.

Between 1996 and 2002, NASA investigated systems that would need no fuel. Marc Millis published that exotic things would be necessary, such as a negative-gravity towing vehicle, or a non-toweable gravitational towing vehicle, or generating gravity without mass, or distorting a gravitational field. It is possible to imagine space sail vehicles that would be blown by exotic phenomena, like the so-called virtual pairs, or the zero-point energy, or “dark matter”. All of these continue to be fantasy. However, a hypothetical sail pushed by the microwave radiation left over from the big explosion from which the Universe emerged is compatible with known science.

3) AT A FRACTION OF THE SPEED OF LIGHT

We will travel for decades or centuries. At 90% the speed of light we will arrive at the next star system after 3,2 years (5,7 on Earth); at 70%: after 5,3 years (6,9 on Earth) and at 50%: after 8 years (9,1 on Earth). At 90% the speed of light we would use only 11,6 external tanks of antimatter; 3,1 tanks at 70% and just 1,4 tank at 50% the speed of light. Inside this kind of rocket, antimatter (“reversed” atoms, with negative protons and positive electrons) annihilates in contact with normal matter producing a stream of gamma rays, capable of accelerating the ship to more than 90% the speed of light. It is compatible with known science, even though the current price of antimatter (in production at laboratories) is several million dollars… per microgram.

In 1960 Robert Bussard proposed another kind of rocket, equipped with some sort of electromagnetic “vacuum cleaner” in order to capture hydrogen atoms, present in space, for fuel; through nuclear fusion (joining the atoms to form bigger atoms, a process that yields enormous amounts of energy), a controlled jet rushes away and it might accelerate the spaceship to 70% the speed of light. If the gigantic funnel can not be constructed, then the entire supply of fuel might be carried onboard; in this case a speed of 30% the speed of light might be reached.

Other systems would use photons (light) from a gigantic laser in order to push a space sail; the laser sail proposed in 1984 by Robert L. Forward, with a diameter of 1000 km, could reach 20% the speed of light. However, its laser would need to drain electricity from hundreds of electrical power plants. A year later Forward (after a conversation with Freeman Dyson) proposed a less energy-demanding system, using microwaves instead of laser. 

Freeman Dyson believes (personal communication, 24 January 2013) that the development of nanotechnology could result in a radical shrinking of payload mass, facilitating acceleration.

All of the above proposals are compatible with known science.

Some projects to reach 10% the speed of light are already compatible with known technology. The Daedalus spaceship, with a mass of 50 000 ton, proposed in 1978 by Alan Bond and his colleagues at the British Interplanetary Society, would use nuclear-fusion explosive microcharges (250 per second) in order to propel itself forward. Other projects similar to this are Orion, from 1965, led by Freeman Dyson, and Longshot, published by the U.S. Navy and NASA in 1988.

Besides these proposals, 1% of the speed of light might be attained by launching a powerful jet from a nuclear fission device (splitting large atoms, a process that yields huge amounts of energy). But it would work only if the engine is a million-degree hot, a temperature that would vaporize the machine. However, it is compatible with known science.

Even though we already know how to build a Daedalus-class spaceship, a couple of centuries may pass before the space engineering community could succeed in raising the money required. That reminds us that NASA’s Johnson Space Center former director Chris Kraft once remarked that if he had a blank check he would go to [the neighbouring star] Alpha Centauri. (probably broadcast in the series "Journey through the Solar System", episode "Uranus, Neptune, Pluto and beyond", Lewis Research Center Educational Services Office, NASA, Cleveland, Ohio, 1984.)

4) MUCH SLOWER THAN THE SPEED OF LIGHT

This is our current capability. In this way we would spend thousands or millions of years for getting to the stars.

There is the plasma engine, which uses electromagnetism to heat and push a gas; it has a very low thrust, but uses little fuel, and so it avoids the heavy tanks.

The ion rocket is similar, but this does not heat the gas, just pushes it out by electrostatic forces (negative repels negative, etc.); it could reach the next star system in 8000 years. This technology was proven in the space probe Deep Space 1.

A nuclear reactor at a more normal temperature (solid) can heat and push a gas jet with twice the efficiency of the Space Shuttle’s chemical engines.

A sail that uses only the Sun’s photons could be four times faster than current spaceships.

All these technologies already exist.

Ark spaceships, the size of cities (similar to those proposed by Gerard O’Neill in 1974), could be built, and could travel slowly through space with all their inhabitants onboard; the people that would arrive at destination would be then the initial crew’s remote descendants. Some problems: the spacecraft’s sheer size and the complexities of maintaining an entire ecosystem inside, without any external resource, not even the Sun’s light.

Sending computers might be less complicated, but they should last for 80 000 years.

Even though, these latter projects are compatible with known science.

Finally, we have the “bottle-thrown-into-the-ocean-with-a-message-inside”-type of space probes; in 1972 and 1974, Pioneer 10 and Pioneer 11 were launched with identification plates which tell in scientific language their home planet and year of launch. In 1977 Voyager 1 and Voyager 2 departed with discs containing images and sounds of the Earth. Even though these spacecraft will stop operating soon, they will continue to wander through space with enough impulse to reach the stars. These tiny spaceprobes are the first interstellar ships from the human species.

CONCLUSION

An interstellar trip in a matter of hours or days is fantasy. Those lasting years are compatible with known science. Interstellar travel lasting decades or centuries is already compatible with known technology. Those involving thousand or million of years are already real, unmanned for the time being.

Even though reaching for the stars is extremely difficult, it is not impossible. It will not be as we watch in television series, in which the heroes visit a different stellar system per week. It will be more like those great voyages of past centuries: Magellan’s crew spent three years to go around the Earth, Darwin’s expedition was five-year-long, and Marco Polo spent 25 years to go and come between Venice and China, on foot. A spaceship like Daedalus would reach Alpha Centauri in four decades. Humankind can achieve it.

CONTRIBUTIONS FROM THE BIOLOGICAL SCIENCES

Cryogenics, or cryonics, which is the conservation of bodies at below-zero temperatures, is practiced since 1967. To date some 150 people have been frozen and a further 1000 are in waiting lists, with the hope that the Medicine of the future will bring them back to life. But nobody knows if this is possible; it continues to be fantasy.

On other fronts, Matthew Andrews discovered in 1998 that human beings also have the genes that control hibernation, a natural process in many mammals. If we could activate those genes, that kind of deep sleep might stretch human lifespan by centuries, making missions like Daedalus easier.

COLONIZATION OF THE GALAXY

Even though technologies like that of Daedalus would still be some very slow way of travel, this would not prevent human beings from colonizing the whole Galaxy some day, as seen on the movies. For, as Lao-Tzu used to philosophy back in the sixth century B. C. E., the longest journey begins with the first step. 

A. L.

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Based on a lecture given at USP, on 21 June 2003. First published in ABC Color, on 29 May 2006. A slightly retouched version of this article, joined to many other related articles from this website making a compilation titled "Do you believe in UFOs?", is available for sale in electronic book format at http://www.amazon.com/dp/B00GF0REFI. Photograph: The Pioneer F (later renamed Pioneer 10) spacecraft, destined to be the first human-made object to escape from the Solar System into interstellar space, carries this pictorial plaque, as well as its sister the Pioneer G (Pioneer 11). It is designed to show scientifically educated inhabitants of some other star system, who might intercept it millions of years from now, when Pioneer was launched, from where, and by what kind of beings. (Hopefully, any aliens reading the plaque will not use this knowledge to immediately invade Earth.) The design is etched into a 0,15 m by 0,23 m gold-anodized aluminum plate, attached to the spacecraft's antenna support struts in a position that helps shield it from erosion by interstellar dust. The radiating lines at left represents the positions of 14 pulsars, a cosmic source of radio energy, arranged to indicate our sun as the home star of our civilization. There are "1-" symbols at the end of the lines: these are binary numbers that represent the frequencies of each pulsar at the time of launch of Pioneer F, relative to the frequency of the hydrogen atom, shown at the upper left, which carries a "1" unity symbol. The hydrogen atom is thus used as a "universal clock", and the regular decrease in the frequencies of the pulsars will enable another civilization to determine the time that has elapsed since Pioneer F was launched. The hydrogen is also used as a "universal yardstick" for sizing the human figures and the outline of the spacecraft shown on the right. The hydrogen wavelength, about 0,21 m, multiplied by a binary number representing "8" (hidden from view) next to the woman gives her height, 1,68 m. The figures represent the type of creature that created Pioneer. The man's hand is raised in a gesture of goodwill. Across the bottom are the planets, ranging outward from the Sun (hidden at left), with the spacecraft trajectory arching away from Earth, passing Mars, and swinging by Jupiter. Credit: Great Images in NASA Collection.

A scientific, very respectful and well-thought reply to the popular question "Do you believe in UFOs?"  This book evolved as a reply to one of the most frequent questions that I used to hear from the public when I was working in an astronomical observatory: "Do you believe in UFOs?". That seems an odd question to ask to scientists, but after researching conscientiously for about a full year, I discovered, to my surprise, that mainstream Science has a few things to say about the topic.  This book is not about conspiracy theory, "NASA is hiding the truth", or much less, that flying saucers have already landed on the lawn of the White House. Rather, it is a book about what is the most rational reply that a scientist, or in my case, a science writer, can offer when people insist on asking that question.  Of course, "Do you believe in UFOs?" is, understandable, one of the most popular questions that common people ask (even if silently, to themselves) when they raise their eyes and look at the stars. So it has to be treated respectfully, and why not, given a well-thought reply.

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