Tuesday, June 23, 2015

Brakes on Alien Interstellar Expansion

What might slow down the expansion of an interstellar expansion by an alien civilization that had mastered the ability to travel between solar systems?  A simple calculation shows that if they achieved 10% of the speed of light capability, and it takes about as long to establish a civilization on each new planet as it does to get there, they can diffuse outward at about 5% of the speed of light.  This means that crossing the whole galaxy, about 200000 light years, should take 4 million years.  The age of the galaxy is of the order of 10 billion years, so there has been plenty of time for any civilization with the inclination and ability to diffuse to have finished the job. 

If there is a way to diffuse faster than the speed of light, and a civilization masters it, the argument is only stronger.  But what might go the other direction, and act as a brake on interstellar diffusion?  The most obvious one is travel times.  Suppose the mass/payload ratio gets too high as you increase your speed into the percentages of light speed.   If 1% is a limiting factor of the asymptotic technology, diffusion time is up to 40 million years, and 0.1% gives 400 million years, now a substantial portion of the age of the galaxy.  This would certainly serve as a brake.  If 0.1% is the practical limit, and desirable star systems are about 10 light years apart, it would be 10,000 years for each voyage. 

There are several distinct ways in which this type of diffusion might be done, based on what is included on the ship that makes the transit.  One way is to have an inhabited ship, with a crew or at least a group of passengers always present.  Another way is to have the ship autonomous, and the passengers brought into being or unfrozen after the trip concludes.  Reasons for having a crew include the necessity to respond to accidents, or to evade attacks, or simply because the civilization wants to do things that way.  Let us discuss crewed ships first.

This long travel time means that supplies are needed for an extended period, and the ship’s size must include this.  But then there has to be a large enough engine to propel all these supplies, and enough fuel to power the engine for the whole voyage.  To begin with, an interstellar colonization vessel has to be able to contain a large crew, materials to enable the construction of a habitat on the destination planet, and enough startup supplies to last until sustainability is reached, and then some more for a safety factor.  As travel times build up, the problem of materials might get acute.  It goes without saying that the ship is as closed an ecology as possible, but is 100% possible?  If there is a loss as large as 0.1% per year, due to perhaps mixture on an atomic scale, and separation costing too much, a long voyage like 10,000 years means that 10 times as many materials are needed, compared to the amount in circulation.  This recycling loss is added to the sustainability amount, which also needs to include a factor for recycling loss.  Thus, if asymptotic technology only allows a small fraction of light speed, ships grow and grow in size, meaning it is more of a drain on a planet to build and equip one. 

To do some estimating, assume that asymptotic technology has provided the aliens with a 500 year life span.  It could be 5 years with their type of organisms, or it could be 50,000 years, but to understand some tradeoffs, let’s assume 500 years.  For a trip of 100 years at 10% light speed, a group of aliens could be trained for their mission and accomplish it, all within their lifetime.  The ship is essentially the equivalent of an ocean-going vessel on Earth, able to feed and house a crew, but not much more.  For a trip of 10,000 years at 0.1% light speed, the ship has to become a miniature world, with several generations being born and trained, and then serving as crew for their lifetimes, and then dying.  All the infrastructure needed for multiple generations has to be added to the size and weight of the ship. 

Who exactly builds this ship?  On the home world, a civilization has been around for a long time, and can do the construction, but on a newly acquired world, the civilization has to establish itself and increase its numbers to the large amount needed to sustain the construction phase of the ship.  A much larger ship as needed by lower travel speeds would need not only longer construction time, but also longer civilization growth times, before it had the surplus resources necessary to build the next round of space ships.  Thus, travel speed limitations impose a large delay in diffusing, both in the time needed to arrive, but also in the delays needed to construct the next ships to go out.   

The situation gets worse if the maximum travel speed drops another decade, to 0.01% of light speed.  Light speed is 300000 km/sec, and 0.01% of this is 300 km/sec.  For comparison, Earth’s orbital velocity is 30 km/sec, and the sun’s orbital velocity in the galaxy is around 300 km/sec, so if this cannot be achieved, there is no capability of interstellar voyaging.  However, in the most extreme case where the speed limit is 0.01% of light speed, the mass/payload value again takes a large jump, due to recycling losses and generational infrastructure.  It may well be that if asymptotic technology cannot provide 0.1% of light speed, interstellar travel is non-existent.

Engine size controls the acceleration of a ship, and acceleration times burn time provides maximum velocity.  But engine size requirements do not only come from maximum speed.  It is also necessary to make a successful arrival at the destination planet, and this means going into orbit around it.  First, the engine must be able to put the behemoth ship into orbit around the star of the solar system, and then it can gradually work down to where it can transition to planetary orbit.  Once in planetary orbit, it can gradually lower the orbit to wherever the final parking orbit is chosen to be. 

Just to get some numbers, suppose the parking orbit is like the moon’s orbit, and the planet is about the size of Earth.  This provides an orbital velocity of about 1 km/sec, and if the ship has to do the transition in about 10 times the moon’s radius, which is around 300,000 km, to the nearest half an order of magnitude, the ship would have to be able to provide an acceleration of about 1 km/sec in 3,000,000 sec, or essentially 0.3 mm/sec2.  A thousand years is about 30 billion seconds, so this gives a speed of 10,000 km/sec, which is well over 0.1% of light speed.  So having an engine able to maneuver into orbit means that the engine could, if fuel were adequate for 1000 years, get to 0.1% of light speed.  This is another indication that if 0.1% light speed cannot be achieved with asymptotic technology, interstellar diffusion of a civilization is not very likely.

To summarize, there is a very significant tradeoff in cost and time that is controlled by the maximum velocity achievable in an interstellar spaceship, and that if it does not reach 0.1% of light speed, interstellar diffusion is unlikely, and if it does not achieve 1% of light speed, costs go way up to build a slow, multigenerational ship and stock it with enough materials to enable both the transit and the development of sustainable life on the destination planet. 

Under the assumption that the ship is self-propelled, there are three categories.  One is where the ship carries its own propellant and energy source, another where it only carries the energy, and the last where it harvests the energy and propellant as it goes.  A sailing ship is an example of the third and a steamship of the second.  Apollo is an example of the first.  Aircraft are hybrids of the first and second. 

For the first category, if you use your propellant efficiently, you shoot it out in the opposite direction to your destination at light speed, using particles or photons.  To a ballpark estimate, you have to throw out 10% of your total mass to get up to 10% of the speed of light.  The energy you need is proportional to the mass you use for propellant.  Assuming engine mass is proportional to propellant mass flow, the whole ship is roughly proportional in mass and size to the payload mass.  For the second category, collecting propellant has to be done across a larger cross section for a larger ship, but unfortunately, it does not scale up.  Cross-section of a ship goes only as the two-thirds power of the total weight.  This means category two and three ships get harder to design as maximum speed gets lower.  In short, there is no obvious savings of size and cost for slower speeds to compensate for the increase in size and cost caused by making the transition to intergenerational ships, and depending on your choice of propulsion design, it may go the other way.

For autonomous ships, the same arguments may or may not hold.  If there is a transition time when reliability of systems, even using asymptotic technology, degrades, perhaps 1000 years, then instead of multigenerational ships, one would have multiply tandem or self-repairing systems needed.  A multigenerational ship would probably need several times the payload mass of a non-generational one, and a long-term reliable ship might also need several times the payload mass of a short-term reliable one.  Just as the longevity of the individuals of a diffusion alien civilization is not guessable, neither is the period for a transition in reliability requirements.

This means that discussions of interstellar dispersion of an alien species should concentrate on expansion speeds of 0.1% to 10% of the speed of light.  Slower means grave difficulties and faster means some unexpected technology has to be invented.  We all know technology produces surprises all the time, but for the purpose of current-day discussions, this range will serve nicely.

No comments:

Post a Comment