Nuclear Propulsion for Interstellar Travel

Interstellar travel is difficult because it may take decades or centuries, and it requires a colossal amount of energy.

The nearest star system is Alpha Centauri, which is 4.3 light-years away. If we travel at the speed of light it would takes 8.6 years to reach it and to get the information back to Earth. If we assume a maximum velocity of 0.1 c for the spaceship in the acceleration and deceleration phases, the mission would take almost 43 years with an additional time of 4.3 years for the return of the data

An interstellar probe requires gigawatts of energy. The actual energy requirements depend on the propulsion system efficiency in converting input energy into kinetic energy. A vehicle with a mass of 1000 kg travelling at the tenth of the speed of light would need 1018 J, which represents the total energy consumption of the United States over almost three weeks.

Laser Beam Propulsion for Interstellar Travel

Some research on laser-beamed propulsion was done as part of the Strategic Defense Initiative. This technology relies in photonic pressure from a solar-pumped laser array to push a vehicle with a large lightsail. The problem is that the transmitted laser power must be available for the vehicle at any time during the mission over interstellar distances. So, it would involve large diameter sails (on the order of 1000 km) 1000 km and large amounts of laser power , (on the order of 10 5 TW).

This idea was first proposed by Robert L. Forward in 1984. Russian scientists have flown a spinning 20 m diameter polymer solar reflector called Znamya to demonstrate providing extra winter illumination in northern Russia. American laboratory tests could measure the properties of candidate laser-sail materials for missions to Mars, the Kuiper Belt, and beyond with the military megawatt-class chemical laser at White Sands Missile Range in New Mexico.

Although the task remains daunting, this method at least allows us to conceptually consider interstellar travel. Let us keep the hope that dreams can become reality.

Human Spaceship Colonies

To prepare for interstellar flight, the concept of a human colony ship exploring our own Solar System may be considered.

The concept is to build a spaceship able of travel in a fully autonomous manner throughout the Solar System, transporting a human colony and using regenerative life support systems and in-situ resources from bodies such as asteroids and the moons of the gas giants.

The spaceship would travel without route constraints, exploring the planets for scientific purposes, tourism, comparative planetology, and in-situ resource utilization. This autonomous spaceship may allow the space colony to travel out to Pluto, the border of our planetary system.

Here is a description of the hypothetical spaceship. It would be designed to meet the following requirements:

• Provide habitation, power, and a livable environment for at least a thousand people.
• Travel within the Solar System from Mercury to Pluto (40 Aus) , i.e. 40 in less than twenty years.
• Facilitate the exploration of any celestial body.
• Allow resource extraction from any relevant solar system body (minerals, materials, and water) for infrastructure building.

The above requirements may be met by the following spaceship design:

• The most stringent parameter when considering such an endeavor is the infrastructure. The mass of a spaceship housing one thousand people is so huge that it would have to be constructed in space. In order to constrain the building requirements, we have considered the direct use of near-Earth objects (NEOs) not only as a material resource but also as the shell of the habitat itself. A NEO such as Castalia (see Figure 2-21) may serve to house human habitats. Castalia is an irregularly shaped body roughly 1.8 km long which appears to consist of two bodies in contact. The mass has been estimated to be about 5.1011 kg. Castalia has an orbital period of 1.1 years, and makes a close approach to Earth every 11 years.

• Castalia's natural two-body configuration is used to design a rotating system, which is the most feasible way of generating artificial gravity. However, in a rotating system there are forces acting other than the centrifugal force that supplies the pseudo-gravity. Thus, although the inhabitant at rest in the rotating system would only feel the sensation of weight, when he or she moves the Coriolis force is also felt. Consequently, motion sickness can result even at low rotation rates, although people may eventually adapt to rates below 3 rpm after prolonged exposure. At 1 rpm, a rotating radius of 600 meters is required to obtain an artificial gravity of 0.5 g. These values are fully compatible with the size of Castalia.
   
• Food is provided thanks to an artificial biosphere that allows agriculture activities. 100 square meters of agricultural land are be allocated per inhabitant. Water would be largely recycled. The remaining needs would be met by resupply at different Solar System bodies like Europa that contain water.
   
• Environmental psychologists and behavioral scientists have pointed out that variety, diversity, flexibility and motivation can make apparently deficient environments quite satisfactory to their inhabitants. There are ways to offset the undesirable effects of artificiality. One is to provide large-scale area and vistas. Taking into consideration space for travel, rest, tourism, manufacturing, and agriculture, an average value of 700 square meters per colonist is assumed. A projected surface of 1200m x 400m plus 800m x 350m would support a population of more than 1000 people. The allocated volume with a height of 100 m is assessed to 760 105 cubic meters. This proposed infrastructure could house all the elements required for an appropriate standard of living:
   
  • Residential units, pedestrian access, and transportation.
  • Commercial business.
  • Public open space and parks.
  • Forestry and agriculture.
  • A tourism zone that includes artificial seas and beaches.
  • Food and water storage.
  • Manufacturing and power plants.
  • Propellant storage and propulsion system.

Figure 2-22 and Figure 2-23 present the proposed concept.

The preceeding discussion of the asteroid-derived interplanetary spaceship is not original, but based largely on concepts that have been described in science fiction for decades. While it is certainly possible to contemplate voyages aboard such spaceships, these missions will certainly not be feasible within the near- to mid-term future.

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