Intro
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| Going To Proxima Centauri |
That of one day reaching out and touching the stars is a relatively recent desire of our species...We can perhaps date its appearance to 1838, the year, that is, when the German astronomer Friedrich Bessel first succeeded in determining the distance to a star and clarifying the extent of the problem we faced. A dream, we said,,,,, but with recent advances in technological development it seems that dream is slowly becoming a reality... hand in hand with our growing awareness of the limited resources and environmental problems we face here on Earth.
Indeed, stars are far, far too far away... But, there you go... If there were an exoplanet with some Earth-like qualities within a radius of a few light-years (not a hundred or a thousand, as is the norm) then yes, this could work! It would do a bit like the Moon, which served as a first step, a spur to get us to the other planets... What a pity! But what's going on there among you who are listening?
But yes there is this planet! We've been assured of it since 2016! Well done... It's true! We have an extrasolar planet very close to us, discovered in 2016, but confirmed absolutely only recently. And that's incredibly lucky since the average distance from Earth of the other 5,000 extrasolar planets is about 300 light-years! A stroke of double luck, since the planet orbits the closest star to Earth, namely Proxima Centauri!
A fortune that many are now realizing. So even NASA has apparently decided to go and have a look there by the end of this century; with a project that is finding strong resistance in certain scientific circles, but which the American space agency believes it will be able to launch by 2069. A date obviously not coincidental, remembering what happened exactly 100 years earlier... That would indeed be an exhilarating feat, would it not?
But where then does anyone's skepticism come from? Well...the immensity of the distance that separates us from Proxima is scary anyway...how can you blame them? Moreover, it is enough to look a little closer at the destination of this trip to immediately realize that going to the Moon was like climbing the first step of a staircase that has more than a hundred million steps...
Alpha Centauri System
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| Alpha Centauri System |
A hundred million, yes...because Proxima Centauri is exactly one hundred million times farther away than the Moon! Alpha Centauri is a system composed of three stars, Alpha Centauri A (officially Rigil Kentaurus), Alpha Centauri B (officially Toliman), and Alpha Centauri C (officially Proxima Centauri). Alpha Centauri A and B form a binary pair of Sun-like stars revolving around their common center of gravity; Proxima Centauri is a red dwarf revolving around the two main ones at a distance of about 0.2 light-years and is only 4.25 light-years away from us, or about 40 trillion kilometers.
The extrasolar planet we are talking about, called Proxima b, revolves around Proxima at a distance of about 0.049 Astronomical Units, more than 20 times closer than Earth is to the Sun, but still within the habitability zone. Compared to some other depths of the cosmos we are basically talking about our own backyard, yet a trip to Proxima with current technology would take about 78,000 years!
An enormous amount of time, calculated based on the maximum speed reached by our fastest probe, New Horizons, which is 59,000 km per hour. If, on the other hand, we wanted to reach Proxima with a journey of the reasonable duration of 20 years, a spacecraft would have to travel at an average speed (including acceleration and deceleration) of more than 300 million kilometers per hour, or 28 percent of the speed of light. Quite a difference, wouldn't you say?
In short, NASA will henceforth have the difficult task of finding a faster way and more effective technology to reach at least the stars closest to us. It would require the development of a propulsion system that would reach at least 10 percent of the speed of light to reach the Alpha Centauri system in an acceptable time. The main difficulty lies in storing the fuel load necessary to enable the spacecraft to be able to accelerate steadily to the expected speed threshold.
Despite the seemingly insurmountable difficulties, several interstellar ship concepts have been proposed in the past decades. An early example dates back to studies and experiments conducted in America in the 1960s as part of the Orion project. Basically, the idea was to harness the shock wave produced by the sequential explosion of many small nuclear bombs to accelerate a spacecraft to high speed. The project was originally designed to put large masses into orbit or to enable human missions to Mars, but someone realized its potential for interstellar travel as well.
In the original version, the Orion was to eject nuclear fission bombs in rapid succession, which, detonating just behind the vehicle, would give it a substantial boost through the impact of the shock wave thus generated on a special shield, capable of absorbing the shock without damage. A more powerful version, with fusion bombs, was later suggested by British physicist Freeman Dyson, but it was soon overtaken by technological advances and the changing political climate.
However, this concept, which would solve several of the problems discussed above, remains suggestive and could be applied to launching small probes to the stars. In those same years, American physicist Robert Bussard proposed an original way to solve the mass problem by avoiding carrying the necessary propellant. Thus was born the concept of the interstellar ramjet.
Interstellar Ramjet
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| Interstellar Ramjet |
A vehicle capable of collecting the hydrogen present between the stars as it traveled and harnessing it in a nuclear fusion engine. But apart from the problems presented by making the actual engine, collecting interstellar hydrogen would be anything but simple.
In fact, assuming an average density of one atom per cubic centimeter, to collect 1 kg of hydrogen at one-tenth the speed of light would require a huge funnel with a mouth over 5,000 km in diameter. And the matter collected would still be negligible compared to the needs of propulsion. This is why interstellar ramjets, in their various proposed versions, are not considered physically feasible.
Daedalus
In the 1970s the British Interplanetary Society conducted a detailed study of a variant of the Orion design, called Daedalus, under the leadership of British aerospace engineer Alan Bond. It is perhaps the most realistic interstellar ship design conceived to date. Instead of nuclear bombs, the Daedalus was to make use of a series of micro-explosions produced by nuclear fusion of small deuterium and helium-3 tablets.
The fully automated, two-stage spacecraft would have been able to reach 0.1c (ten percent of the speed of light), with a mass at the departure of 50,000 tons (93 percent of which was propellant). This would have enabled it to reach Proxima in just over 30 years, then transmit the information gathered during the flyover back to Earth. Another promising proposal is the Starwisp laser sails, where the problems of mass and heat are solved by eliminating the engine, replaced by the pressure exerted by a beam of radiation directed onto a very light sail.
However, the problem of energy and also that of travel time must be addressed with highly innovative solutions. Since the original idea was formulated in the first half of the 1980s, the interstellar sail concept has undergone several reworkings and improvements, mainly to limit the energy requirements of the voyage and to make its operation more plausible.
In any case, the main elements of the system would consist of a very powerful laser or microwave transmitter, probably placed in space; a giant lens, used to concentrate the radiation into an extremely collimated beam even at a very great distance; and the actual interstellar sail, capable of intercepting the radiation and being significantly accelerated by the pressure thus exerted. And when we say sail, we do not mean a racing spinnaker, but of something at least 6 km in diameter, and weighing only 30 grams! Accelerated to 0.2c in just two weeks, Starwisp could reach the alpha Centauri system in as little as 21 years...
Antimatter Propulsion
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| Antimatter Propulsion |
Another possible solution is that of antimatter propulsion. We know that one gram of antimatter when converted into energy generates an immense amount, about 70 times that produced by nuclear fusion, and four billion times the energy produced by burning oil. Such an engine would enable us to reach about 40 percent of the speed of light, and the duration of a trip to Alpha Centauri would be reduced to just over a decade.
However, little antimatter exists in nature now, and it is not easily captured. It is therefore necessary to produce it, which is done in infinitesimal quantities in large particle accelerators. Making engines specifically for space exploration, however, would require several grams of it. This sounds like nothing, but it is a vastly greater amount than has been synthesized to date. Just think that an accelerator would have to run its Antimatter Factory for about a billion years to produce a single gram of antimatter.
Even if we imagined technological innovations such that this immense time would be cut down, the conventional energy required to produce a couple of grams of antiprotons per year would be equivalent to a total energy consumption corresponding to about 50 billion watts, equal to the power output of all U.S. nuclear power plants. Here, this is currently our situation. We want to go and see the planet Proxima b, but we don't know how yet! Our technology is practically still in its infancy, although it is progressing rapidly.
No one can predict what humans will be able to do by the fateful date of 2069, but if development proceeds at the current rate, along with many new problems to be solved we will also have greatly increased our capabilities in precisely those fields crucial to interstellar travel. In the beginning, the missions directed to the stars will probably be conducted with intelligent robotic probes, miniaturized beyond any currently conceivable limits, capable of traveling at 0.1c.
But one need only extrapolate a little to the extraordinary progress being made in the fields of biotechnology, genetic and molecular engineering, embryonic development, and cell physiology, to realize that even with simple probes such as those, humans themselves could rapidly propagate around the Galaxy, as proposed by physicist Frank J. Tipler and described by Arthur C. Clarke in the fascinating novel The Songs of the Far Earth.
In practice, this would involve the creation of so-called von Neumann probes, i.e., intelligent automatons that, upon arriving at their destination, would be able to construct, from available raw materials, copies of themselves and habitats for eventual living beings (including humans), synthesized in situ from the information contained in the genetic code and instructions for the generation and development of embryos.
Having successfully initiated the colonization of a world, von Neumann's probes could then build new interstellar spaceships and automatons, destined to take the flight to other star systems to repeat the feat. Of course, we are talking about small probes... Interstellar travel with humans on board, and NASA will have to take this into account, would be enormously more complex, involving mass and energy scales thousands to millions of times larger. What can be conceived with current knowledge points us back to the idea of interstellar arks, giant ecologically self-sustaining habitats capable of roaming the stars for thousands of years as countless generations of humans, animals and plants follow one another within them.
These arks could be built on the model of the giant space colonies studied by physicist Gerald O'Neil in the 1970s, and then launched into interstellar space even at modest speeds of less than 0.01c. But it is difficult to imagine under what conditions and for what reasons human beings should undertake a journey knowing already for certain that they would never reach their destination. Probably, only the progressive uninhabitability of our own and other Solar System planets, colonized in the meantime, could induce many to that extreme choice.
The discourse would change drastically if it became possible to significantly extend human life (or slow down metabolic processes during travel) while being able to reach, with the large spaceships required, at least speeds between 0.1 and 0.3c. But for this to materialize it will probably have to take several more centuries unless physics still holds some incredible surprises in store for us. How then will NASA make good on its promises? May it have an ace hidden up its sleeve? Don't worry, we'll just have to wait until 2069 to find out!