Dilithium crystals, warp propulsion, ion engines… Science fiction has solved one of the great problems, if not the great problem, of space travel through new and magical technologies: the method of propulsion.
If there is one thing that worries scientists concerned about the future of humanity in space, it is thrusters. Because one thing is clear: it is neither profitable nor useful to travel through space with current chemical propellants. New ideas are absolutely necessary.
Pulses from nuclear explosions
At the end of the 1970s, the British Interplanetary Society culminated a proposal by the mathematician Stanislav Ulam in 1946. Ulam proposed a nuclear pulse propulsion. Imagine a flat surface covered by a thin layer of graphite. In space, a nuclear bomb is detonated a few tens of meters from the “dish”; Just enough not to destroy it. Instead, the debris from the explosion will collide with the plate and bounce off of it. Due to the shock, the plate will come forward. If the interval between explosions is well calculated, a ship can be propelled through space as well as being a useful way of ending the world’s nuclear arsenal… The British Interplanetary Society took up Ulam’s idea and developed the Daedalus project , an automatic probe which should reach the star Barnard, located 50 light-years away by internal thermonuclear micro-explosions.
But the Golden Age of nuclear fission pulse propulsion was 15 years before Daedalus. It all happened between 1958 and 1965, when a team of 40 people with a budget of 11 million dollars investigated theoretically and practically how to build a ship that would work: the Orion project . A test vehicle named Put-Put was built, which used five charges of high explosive chemical to ascend to a height of 60m, just the same distance as Goddard’s first liquid-fueled rocket launched on March 16. 1926. In 1968 the physicist Freeman Dyson proposed the use of nuclear fusion bombs to reach speeds of the order of 1,000 to 10,000 km/s. A short time later, the Blascon system was proposed by A. P. Fraas. Its operation was also simple: to cause nuclear micro-explosions using a high-intensity laser or a jet of electrons to fuse small balls of plasma.
Obviously, a variant of this type of spacecraft is the one with a built-in nuclear fusion engine. However, the problem they have is the same as conventional ships: they have to carry fuel, and that means added weight. Couldn’t engines be developed that, paradoxically, didn’t need to transport fuel?
solar sails
The wind has always been an inseparable companion of sailors. Traveling by sea brings to mind the image of sails in the wind propelling ships in search of other lands. The cosmic ocean awaits us up there, but there is no wind there, is there? Our Sun, in addition to light, emits what astrophysicists know as solar wind : a flow of charged particles that are responsible, among other things, for the polar lights. However, solar navigation or solar sailing has nothing to do with this. The one who will one day sail the new spaceships is sunlight. In fact, light exerts an effective pressure on objects. Something at least disconcerting, because no one has been hit by a ray of sunlight when going out. However, it is so. A solar sail is nothing more than a mirror with a high power of reflection : when the photons of light hit the mirror, they transmit the necessary impulse to move the vehicle. If the candlestick is small, the momentum provided will also be small. Huge sails are needed: to carry a weight of one kilo at an acceleration of 30 km/h per second, a sail of 1 square km would be needed.
Solar navigation was first proposed by Carl Wiley in 1951 , in an article titled Clipper Ships of Space in Astounding Science Fiction magazine. Seven years later, physicist Richard Garwin published the first technical paper on the subject, ending with these prophetic words: “The propulsion method is negligible in cost and perhaps more powerful than many competing schemes.” Today NASA considers that solar navigation may be viable . Not only because in 20121 it deployed a solar sail in orbit with the NanoSail-D satellite, but because it already demonstrated it on August 12, 1960 with the Echo-1 communications satellite. The demonstration was obviously inadvertent, but the pressure of sunlight on this 30m diameter highly reflective mylar ball brought its closest point in orbit to Earth – perigee – some 500km to the surface of Earth. our planet.
The basic components of solar navigation are in our kitchens: aluminum foil and that thin plastic we use to wrap food. Aluminum is the reflective material; the plastic, the resistant structure on which it is mounted. For example, aluminized mylar is a good “low-tech” material for solar navigation. The only problem is that, being immensely large sails, they cannot be assembled on Earth, but in space. It is the only way to be free from the deformations and tensions that the force of gravity induces. Also due to its extreme size, the thickness must be reduced as much as possible so that it is not difficult to move a body with an excessive mass, especially considering that the solar pressure is more like a slight breeze than a hurricane wind. Thus, a width whose obtaining does not represent an excessive technological problem is one thousandth of a micron: so small that four aligned aluminum atoms fit. With it, and starting at a tenth of the Earth-Sun distance, a thousand-ton ship with a circular sail of more than 2,000 kilometers in radius would reach a speed of 2,300 km/s, almost one percent of the speed of light. But there is a problem: aluminum becomes transparent when its thickness drops below a hundredth of a micron. To get a candle similar to the previous one, it would have to be drilled with holes of a size of, at most, half a micron. Only in this way could sunlight be reflected in the same way that a mesh used to enclose chickens in a pen reflects radio waves.
Solar sails and lasers
The other misfortune is that this type of propulsion is useless at long distances from a star. In order to use it as an interstellar thruster, Robert Forward proposed in 1962 to use lasers in orbit that would fire directly onto the solar sail . In this way, a ship weighing a thousand tons and with a sail with a radius of 242 kilometers would be accelerated to 15% of the speed of light in just two months. To keep the laser focused on the target sail, a no less large Fresnel-type lens – 100 m in radius – must be placed in solar orbit. The truth is that this type of navigation is big. The laser, to be effective, must have a formidable power: 240 TeraWatts, equivalent to 20 times the electrical power generated by all of humanity in a year. Now, at 15% the speed of light, the spacecraft would reach the nearest star in 29 years.
References:
Mallove, E. F. (1989) The Starflight Handbook: A Pioneer’s Guide to Interstellar Travel, Wiley
Gilster, P. (2004) Centauri dreams, Copernicus
