Basaed on Sid Deutsch's article "Why Visiting Alien Spaceships are Impossible" in Skeptical Briefs, June 2008, page 4, with much additional material gathered from Couper & Henbest's Encyclopedia of Space (Dorling Kindersley, London 2003), Kaufmann & Freedman's Universe 5th edition (Freeman, New York 1999), Swinerd's How Spacecraft Fly (Springer, New York 2008), Millis and Davis's Frontiers of Propulsion Science (American Institute of Aeronautics and Astronautics 2009), and Wikipedia (June 2009). Sid Deutsch is a professor of engineering retired from the Polytechnic University in Brooklyn, New York.
Unintelligently designed to increase air resistance
Believers in UFOs should get acquainted with E = 0.5mv2. The E stands for kinetic energy, the energy required to get a stationary spaceship (or anything else) of mass m moving at velocity v. When v approaches 20% light speed, the required E increases to roughly 0.5mv2 + 0.375mv4/c2, where c is the speed of light 3 x 108 metres per second (beyond 20% light speed the E given by this equation is increasingly too small). Specifically, the kinetic energy required to reach 10% or 50% of light speed is increased by 0.8% or 26% over that indicated by E = 0.5mv2, so the increase can be ignored for speeds up to one-tenth light speed. At 100% light speed the increase is infinite, which is why light speed cannot be exceeded.
As we shall see, kinetic energy explains why visiting alien spaceships are impossible, or at least (even if not theoretically impossible) not realistically achievable. Just do the sums!
Doing the sums
Suppose the spaceship travelled at one-tenth light speed. Its velocity v would be 30,000,000 metres per second, enough to get it from London to Sydney in 1/20th of a second. If it came from the nearest stars 10 light-years away, the journey to Earth would take a century. If it came from one of the brightest stars the journey might take fifty centuries. Beyond this we could be talking thousands or millions of centuries. ETs would need either suspended animation or much patience.
What might be a realistic value for mass m? The Mars Surface Module, in which a human crew could live for 260 days, will weigh around 150,000 kg -- and that's just for existing on Mars. To get the crew to Mars and then back to Earth will each require another spacecraft of around 150,000 kg (this is with crew and spacecraft starting from Earth orbit), making the total around 450,000 kg, same as the international space station. So an alien spaceship assembled in orbit around its home planet might have to weigh at least 500,000 kg, including fuel, if its inhabitants (even tiny ones) are to survive journeys of many years or centuries. Even this weight is only a quarter of what a NASA space shuttle weighs at takeoff, so it is nothing if not optimistic.
Substituting the numbers
There is more. After the first second the kinetic energy of the 500,000-kg ship would be 1012 joules, which when equated to 0.5 x 500,000 x v2 gives v = 2000 metres per second. Which means that during the first second the ship is accelerating at 2000 / 9.8 = 204 g. A 50-kg super model would weigh in at ten tonnes before being squashed flat.
Finding the energy
For comparison, 1012 joules per second is equivalent to pushing more than 3000 NASA space shuttles, each with a takeoff weight of 2,000,000 kg, simultaneously into orbit. It would get a typical heavy payload of 20,000 kg into orbit in two-thirds of a second at an acceleration of 3500 g. If applied to an average 1000-kg car, it would do 0-60 mph in 0.000,0004 seconds, be close to light speed in five minutes, and be past Mars in another ten, always assuming it could survive the acceleration and wasn't destroyed by hitting a speck of dust. Yes, that sort of power corrupts absolutely. But what if less power is applied for longer periods? Would that get around the problem?
Life in the slow lane
Contrary to what we might expect, tiny accelerations permanently applied can have surprisingly big results. For example a permanent acceleration and then deceleration of 10-6 g (meaning an average person would weigh little more than a postage stamp) would reach stars 10 or 100 light-years away in about 65 or 200 centuries at a maximum of 1/300th or 1/100th light speed. The thrust of 0.5 kg needed by our 500,000-kg spaceship could be obtained from the sun's radiation pressure near the Earth by a reflector 0.5 sq km in area, roughly 60 football fields. But to maintain thrust the reflector would need to quadruple in area (and therefore weight) each time the distance from the sun doubled, meaning 60,000 football fields when level with Pluto, and indefinitely more in deep space.
Alternatively the spaceship could start as close to its home star as would be possible without frying the reflector, tack sideways to maximise the starshine and build up speed, then swing by a suitable planet to use the planet's speed to slingshot itself out of the system. In this way a 500,000-kg spaceship with 1000 sq metres of reflector for every kg of mass (60,000 football fields) could leave our solar system at about 140,000 metres per second, enough to reach Proxima Centauri in 95 centuries. To merely escape from the solar system would need 600 square metres of reflector for every kg of mass, which just for the reflector is equivalent to a film of plastic about 1/10,000 mm thick, less if a payload is to be carried, but even this would be so fragile that it could not be assembled except in the weightlessness of space.
Running on empty
Nothing lasts forever
But suppose some ETs were willing to have a go, and could persuade their stay-at-home others to channel resources into an expedition with no hope of the stay-at-homes learning anything, as might apply if the home planet was dying and migration of the fittest was the only option. Indeed, the required resources might be so extreme that only a situation like this could justify them. For example when finished the international space station will have cost $US130,000 million without actually going anywhere, and sending men to Mars is likely to cost ten times this, or roughly $US200 for every person on Earth. Unless there is no alternative, interstellar travel seems unlikely to be popular with the masses.
From the Australian skeptic magazine the Skeptic
Travelling vs arriving
But bumper shields would quickly burn up during atmospheric entry, so a lander with heat shields would be needed. In which case entering the Earth's atmosphere would be a trip of no return unless the lander carried enough brute rocket power for a safe landing and takeoff back into orbit, and once again the numbers are against it, especially if no remnants (like the descent part of the lander) are to be left behind. Alternatively, if entry was designed to be one-way on the offchance that the Earth was suitable and free of fundamentalist hostility (but how could ETs know this without landing?), our satellite surveillance should have noticed the landing party by now.
Nevertheless scientists have for many years been working on the problems of interstellar travel, which essentially boil down to the problems of propulsion. Their latest findings are collected in Frontiers of Propulsion Science, a 739-page anthology of highly technical articles for graduates and engineers edited by M Millis and E Davis (American Institute of Aeronautics and Astronautics 2009, $US129.95, a version for general readers is planned). The specialist authors survey every currently conceivable way to reach the stars including warp drives, gravity control, and faster-then-light travel. They conclude that rockets are fundamentally inadequate. Plausible alternatives may be achievable in 20-50 years, while others such as worm holes and warp bubbles, even if theoretically possible, will be very difficult and may never be achievable.
In the meantime, whenever someone claims to have spotted an alien spaceship, your response should be: "Whatever you saw couldn't realistically be an alien spaceship." At the end of the day, you need to decide which is the more likely -- an alien spaceship or someone making a mistake.