Space travel with solar power and a dyson sphere
Is there any chance for humanity to survive our Sun's impending natural evolution toward a Red Giant stage star? One attempt to answer this vital question was made by Leonid Mikhailovich Shkadov at the 38th Congress of the International Astronomical Federation meeting in Brighton, UK during 10-17 October 1987.
There, he reported on the real possibility of controlling our intact Solar System's motion within the Milky Way Galaxy, allowing mankind to migrate to another star without leaving Spaceship Earth or, indeed, the Solar System itself.
In plain words, he proposed a technical means to render the term interstellar travel utterly meaningless since humanity could direct our Sun, along with its cortege of planets and other orbiting objects, towards another Sun-type star located elsewhere in the galaxy!
Dr. Shkadov's thruster consisted of a mirror placed some distance from the Sun. His solar sail-like mirror would cause the central symmetry of the solar radiation in the Sun-mirror system to be violated and, as a consequence, a certain thrust force will thereby be instigated.
For a certain mirror of a superficial mass density, where the mirror-Sun distance remains constant, a balance exists between the gravitational force and the force due to solar radiation pressure. Shkadov proved that in the case of a well-designed system, the thrust force could permit a significant deviation of the Solar System's normal expected trajectory in our Galaxy.
The energy radiated by a Sun-type star is due to the nuclear fusion reactions occurring in its nucleus; a steady-state star is characterised by a permanent balance between the energy flux that is generated by its interior nuclear reactions and the energy flux emitted at its surface. The star's energy flux is emitted almost isotropically.
In the instance of Shkadov's 1987 thruster, the star is prevented from losing energy on the solid angle opturated by the mirror, as the energy emitted in that direction is returned to the Sun's surface together with the reflected radiation. If exactly backscattered toward the Sun, for example, only radial momentum is gained by the scattering unit; however, if deflected at any angle less than pi (the still mysterious 3.14), then it will also impart angular momentum to the scatterer: the Poynting-Robertson Effect.
As the energy output does not fluctuate (steady-state literally means no change of some special quality) the same energy flux has to be dissipated in space, but this time, quite unnaturally, from the effective (non-opturated) star surface only. Consequently, the photosphere temperature will increase and it is expected that the star will change gradually to a new and different steady-state. This effect was neglected previously, but was recently considered by us.
It was proved that for the mirror ram angle of 30 degrees first considered by Shkadov, both the Sun's photosphere temperature and the absolute bolometric magnitude remain quite close to the present-day measured values. Also, detailed calculations performed in our report lead to the important conclusion that the lateral deviation during one orbital period of the Sun, estimated in to be about 4.4 parsec, is probably an overly cautious underestimate. (A parsec equals about 206,000 astronomical units, or 206,000 times the distance from the Sun to our Spaceship Earth.)
But, the example above is not the only possible stellar engine! In a recent paper published in the Journal of the British Interplanetary Society (September-October 2000), we defined a stellar engine as a device that uses a significant part of a star's resources to produce energy.
A Class A stellar engine (for example, Shkadov's 1987 engine) uses the impulse of the radiation emitted by any appropriate star to produce a vectored thrust force; when acting at a distance this thrust force generates work. A Class B stellar engine uses the energy emitted by a star to generate mechanical power.
The Class B stellar engine proposed by Badescu consists of two concentrically spherical surfaces englobing a star. The inner (or Dyson) shell surface acts as a solar energy collector. (A Dyson Sphere is an aggregation of Sun-orbiting space habitats first proposed in 1960 by Dr. Freeman John Dyson, the most recent winner of the Templeton Prize for Progress in Religion.) Its outer shell surface is a thermal radiator.
The result is that the two shells have different but uniform temperatures; the existing difference of temperature determines a heat flux from the inner towards the outer shell and this flux can enter a thermal engine used for ordinary power generation purposes. So, one can see that small radii increase the feasibility of a Class B stellar engine as the amount of component material required is proportional to the radius.
A totally new type of stellar engine was first proposed by us: it is a blend of Class A and Class B stellar engines. We have named it a Class C type stellar engine. The Class C stellar engine uses the impulse and the energy of a star's radiation to provide both a thrust force and mechanical power as well. This sort of physical configuration could provide mankind with both power and the possibility of interstellar travel.
The efficiency of a Class C stellar engine increases by lengthening its radius and decreasing the mirror ram angle. And, there is a rigorously calculable minimum radius for such an engine to provide useful power. The really important fact, however, is that there is an optimum stellar engine radius as far as the provided power density is concerned. For values adopted in our report, this optimum radius is around 450 million kilometres (or, approximately, 3 astronomical units).
In sum, interstellar travel seems entirely possible for mankind, either by staying aboard Spaceship Earth (and, perhaps, future terraformed planets like Mars and Venus) OR following the construction of a Class C stellar engine.
Should future unfavourable local changes in the Milky Way Galaxy provoke a desire to move our Solar System, whatever its physical form-that is, whether as an intact Solar System OR as a Dyson Sphere-as a system of various kinds of solid objects, this can be done quite effectively utilising stellar engines!
Since it has been postulated by SETI proponents that some non-terrestrial Aliens may have become self-absorbed after enclosing themselves within an isolating Dyson Sphere, our proposal means that it ought to be unlikely that mankind becomes so inwardly fixated. Further, the Sun's heliosphere, a volume wherein the solar wind plasma has its greatest influence on the dynamics and the energetics of various Sun-created particles, may become a 'walled' astrophysical laboratory for particle acceleration experiments of a kind not easily or inexpensively done on a planetary surface.
 L.M. Shkadov, Possibility of controlling solar system motion in the galaxy, 38th Congress of IAF, October 10-17, 1987, Brighton, UK. Paper 1AA-87-613.
 V. Badescu and R.B. Cathcart, Stellar engines for Kardashev's Type II Civilisations, Journal of the British Interplanetary Society, 53: 297-306 (September-October 2000).
 V. Badescu, On the radius of the Dyson's Sphere, Acta Astronautica, 36: 135-138 (1995).
Authors: Viorel Badescu* and Richard B. Cathcart**
(*) Candida Oancea Institute of Solar Energy, Faculty of Mechanical Engineering, Polytechnic University of Bucharest, Spl. Independentei 313, Bucharest 79590, Romania.
(**) Geographos, Glendale, California, USA.
Author: Viorel Badescu