UFO reports have been evaluated in terms of the supposed reliability of eyewitness accounts and questionable photographic evidence. The constraints that interstellar distances, time and the conservation of energy impose on interstellar space travel for these supposed alien craft seem never to be considered by UFO proponents. Since they do provide descriptions of spacecraft of circular disks, cylinders and triangles that move strangely and rapidly and vary in size from 50 feet in diameter to 300 feet long, I undertake here to apply these constraints to the design of a hypothetical spacecraft in order to determine the feasibility of such craft and their use for interstellar travel. As a physicist and astronomer I think it important to consider not just the accounts of alien contact, but the physics of such a possibility as well.

For my model I have chosen a spacecraft with a crew of six that will leave its planet for a planet in the habitable zone of a star 10 light years away. It will be accelerated at a rate of 10 m/s^{2} (10 meters per second squared) to a velocity of 0.5 times the velocity of light (0.5c, where c is the velocity of light). The time for it to reach this velocity is given by this equation:

t = v/a = 1.5×10^{8}/10 = 3.06×10^{7}s = 174 days

*(a = acceleration in meters/second squared; v = velocity in meters/second; s = time seconds)*

This is remarkably short compared to the nonrelativistic time of 20 years for the trip to the destination star. I have chosen 0.5c to minimize the relativistic mass increase of the spacecraft and to minimize travel time. The acceleration rate is approximately equal to the gravity the crew would experience on an earth-like home planet.

The spacecraft would be constructed in orbit from components delivered by shuttles. It would include, in addition to engines and fuel, an internal power supply for all the operational systems as well as life support systems and sustenance for the crew. For a 20-year trip this would necessarily be a small nuclear reactor. A mechanism for rotating the crew’s quarters to provide artificial gravity would be essential. I have chosen a live crew rather than robots or androids because all of the alien encounter and abduction stories indicate their presence. A shuttle for transporting the crew to the surface of the destination planet would also have to be on board.

Our current space shuttles have an unloaded mass of 10^{5} kg. Consequently, considering all of the requirements, a mass of 10^{7} kg is not unreasonable for our model. The kinetic energy of the spacecraft, defined as the energy any object has by virtue of its motion, at 0.5c is

E = ½mv2 = 0.5×10^{7}×2.25×10^{16} = 1.13×10^{23} joules

*(m is the mass of the spacecraft and v is the velocity equal to 0.5c).
*

This is the energy that must be supplied by engine thrust to reach 0.5c

The only source that can supply energy of this magnitude is thermonuclear fusion. […] This energy would be expended over the 174 days of acceleration and is equal to 1.8 megatons per second during acceleration. […] For propulsion of the hypothetical spacecraft the blast energy would have to be converted, with near 100% efficiency, to a constrained unidirectional particle beam with thrust pulses of 1.8 megatons per second for 174 days. For a round trip to a star 10 light years distant this rate of energy expenditure would be needed for slowing down at the destination, leaving, and slowing down again when returning to the home planet after a 40 year expedition.

A lesser source than thermonuclear fusion would be inadequate to provide the required energy for traveling at 0.5c. A lower velocity would mean travel times of hundreds to thousands of years. A lower acceleration rate would greatly increase the time to reach the desired velocity. […]

There is no possible material construction that can constrain and direct the thermal and blast energy of the nuclear fusion rate required for interstellar travel. Consequently, I conclude that alien spacecraft cannot exist.

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Any spacecraft, whether from present or future technology, would have a significant inertial mass. Ten thousand years from now conservation of energy will apply anywhere in the galaxy as well as it does today. […]

In point of fact we do have proof of the effects of two megaton unconstrained nuclear fusion reactions, and because of the maximum cohesive force that electrons can create between protons no substance will remain solid above 5000ºC.

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