![Conceptual artwork of the 'Ahad-Astercom' starship equipped with four 'strap-on' comets undergoing technology evaluations in high Earth orbit. [Copyright - Abdul Ahad]](http://www.astroscience.org/abdul-ahad/firstarktoalphacentauri/ahad-astercom-1.jpg)
Reaching Alpha Centauri using the resources of comets and planetoids.[A futuristic interstellar spaceflight concept by A. Ahad]
Copyright © 2004 by Abdul Ahad. All rights reserved.
Article finalised: 14 September 2004
But all of these concepts are still many many decades - if not centuries - away from reality and will require substantial success in their future interim evaluations in terms of the Physics and dynamics before we could accept them. My own view is that there is an important alternative possibility that has been missed out from our list of existing concepts, that could prove to be more realistic and achievable over the long term depending on how our technology evolves going forward. This concept (coined "Ahad's virtual bridge to Alpha Centauri"by a few colleagues) is outlined here.
My Own Concept of the Asteroid-Comet "Generation" Starship
The vision of a possible futuristic interstellar starship concept outlined here is essentially an optimistic one, based on a long duration, multi-generation voyage travelling at speeds and utilising technologies with which we are familiar in the current era. The possibility is presented here as very much of an "overview" based on our current astronomical understanding of the solar system and near interstellar environments, set against the background of manned and robotic spaceflight experience we have gained over the past 40 years. However, since the requirements in terms of new technology, the social dimension and financial resources for such a mission concept are steeply outside of realistic possibilities in the near future, many will argue that this is bordering on science fiction!
Above: Conceptual artwork of the 'Ahad-AsterCom' starship equipped with four 'strap-on' comets undergoing technology evaluations in high Earth orbit. - click to see a larger image. [Copyright: Abdul Ahad]
I do not claim that my concept of an asteroid-based generation starship, as depicted in the image above, is "rocket science" in the sense of it being totally *new*. A famous Star Trek episode, which was the brain work of Gene Roddenberry going as far back as in the 1960s, entitled "For the World is Hollow and I Have Touched the Sky" featured a colony living inside an asteroid. More recently, space artist David Hardy has painted stunning scenes of far future colonies arriving at nearby stars using an asteroid as a generation starship.
Where my concept is genuinely original is in the sense of exclusively relying on comets and icy planetoids as natural "resource banks", hopefully continually available along the voyage. The mission is envisioned to be intermittently docking with and mining icy bodies such as comets to provide essential needs like water, fuel and power on the long voyage to Alpha Centauri.
The success of such an Asteroid-Comet ("Aster-Com" for short) mission concept will be largely reliant on how much space debris actually exists between the outer edge of the Kuiper belt and the immensely distant shores of Alpha Centauri - some 272,000 astronomical units away.
As early as in the 1950s, the Dutch astronomer Jan Hendrik Oort first hypothesized the existence of the 'Oort cloud' - named after him - consisting of a swarm of comets beyond our solar system. Based on the frequency and directions of appearence of sporadic comets in our skies, the Oort cloud is thought to contain as many as a trillion (10^12) comets, which put together adds up to more mass than the giant planet Jupiter, and stretches outwards to as much as 3 light years around our solar system. Due to the enormous distance from the Sun ( > 30,000 AUs) where these cometary bodies are thought to reside and the minimal levels of light illumination provided by the Sun, as determined in my own paper on the interstellar night sky, the Oort cloud has so far evaded visual detection by telescopes like the Hubble.
The recent discovery of a small planetoid dubbed "Sedna" (official IAU designation - 2003 VB12), which is thought to have an orbit that takes it as far out as 1,000 astronomical units from the Sun (25 times the Sun-Pluto distance), has pushed the conventionally viewed edge of our solar system much further out than previously imagined. Some astronomers speculate that Sedna may be the first of a swarm of objects representing an inner belt which follows the Kuiper belt and precedes the Oort cloud, going out into interstellar space.
The discovery of Sedna is only the latest in the forever outward march of new and surprisingly distant physical bodies found around our Sun. At one time not so long ago, the Earth and the five naked eye planets as far out as Saturn were thought to be all that revolved around the Sun. Then, as our technological capabilities improved, came the discoveries of Uranus, the asteroids, Neptune, Pluto and eventually many smaller objects in the Kuiper belt. I am fully confident this trend will continue beyond Sedna well into the future.
From a dynamical perspective, as predicted by Isaac Newton's well known gravity equation:
F=G(m1*m2)/r^2
the Sun's sphere of influence reaches well beyond the distance of Alpha Centauri. Add to this the fact that the Sun has made some 20 revloutions around the Milky Way galaxy since its formation [4,500 million years (age) / 225 million years (galactic rotation period)] and within that time, it is certain to have passed through dense gas clouds, star and planet forming regions, brushing the outer edges of proto-planetary disks of other stars, attracting and shedding swarms of comets... There is every reason to be hopeful of swarms of comets existing much further out than those within current detection range of our instruments.
In summary, despite what people might say the concept that I am hypothesising in this article is essentially a vision based on some optimism and hope along with some genuine scientific underpinning.
Going to the Stars the "Slow" Way
No science authority on this planet has yet drafted a detailed blueprint for any manned interstellar mission, albeit using the "real" technology available to us today, and quite understandably so! The timescales involved are simply enormous, and we have yet to establish a proper foothold on nearby worlds at our doorstep like the Moon and Mars. Even travelling at the fastest possible propulsion speeds accessible to us, it will take at least 30,000 years to reach Alpha Centauri - the next nearest star beyond our solar system.
The starship concept outlined here is going to take that sort of timescale to reach Alpha Centauri, hence it will be home to hundreds of generations en-route and will therefore need to be a pretty large scale and complex structure in its design.
When we look at "natural" starships travelling across the cosmos like the comets, the asteroids, the Earth and the other planets, all gently describing their natural paths through space at moderate speeds, they are keeping to a natural balance in harmony with the "clockwork" of the universe. My starship concept depicted here is not at great variance or opposing to those natural and fundamental rhythms, effectively utilising a series of interim miniature worlds to bridge the gulf separating two larger worlds in neighbouring star systems.
The alternatives to this deal seem to me to be all "catch 22" situations: If you go at 10% light speed (the maximum most *theoretical* spaceflight concepts ever project) then at 30,000 km/sec you would need massive amounts of shielding against potential micro-meteoroids. You're covering a distance three times the diameter of the Earth in one second... even a grain of cosmic dust will pose a major hazard. At those kinds of speeds dodging any objects in the ship's path would be inconceivable, and the space between the stars is far from empty.
For example a recent detailed study of the Sun-like star Tau Ceti, only 12 light years from Earth, revealed it to have ten times the abundance of comets and asteroids that we find around our Sun, in a large halo around that star. In this regard, the tranquil environment in terms of debris abundance which we have been accustomed to in our solar neighbourhood, may be unusual.
If you hit a comet at anything higher than snail's pace, you're vaporised into oblivion. If you increase the shielding for micrometeroids, then you increase spacecraft mass, which would make it impossible to accelerate to those kinds of speeds. If you go at just 1% light speed (which will still make it impossible to dodge any interstellar debris), then you need 430 years or 17 generations to make the one way trip to Alpha Centauri. To accommodate 17 consecutive generations, you need a substantial biosphere on your starship. If you have a biosphere, then you're talking of a very large scale vessel and you will need re-fuelling, at least for water, if not for anything else.
In the light of this background, I think a gentle route to the stars could prove more acceptable and in agreement with nature in the long term. Besides, just imagine the adventure!
![Crossing a 'virtual' bridge to the stars. [Copyright - Abdul Ahad]](http://www.astroscience.org/abdul-ahad/firstarktoalphacentauri/ahad-astercom-route.JPG)
Above: Crossing a 'virtual' bridge to the stars. - click to see a larger image. [Copyright: Abdul Ahad]
The Challenges of Assembling a Starship of Viable Size
The rocky exterior of a small sized asteroid, such as the main belt asteroid #243 Ida which I have used in my concept artwork for the starship, could prove ideal in more ways than one. The crew habitat deep inside the asteroid would benefit from several miles of solid rock shielding all around and enjoy protection from cosmic rays and micrometeroid impacts en-route. A rocky body such as an asteroid would also be a vastly improved insulator of heat against the interstellar space temperatures of just 3 degrees Kelvin (-270 degrees Celsius), compared to any synthetic construction for starship framework. "Conceptually" it will be much like living inside a solid brick/concrete house on Earth as opposed to an wooden one which has the potential to rot or burn down in a fire. The other major advantage of using an asteroid in place of a synthetic construction, would be a huge saving on the cost and effort of ferrying up parts from Earth to build the large scale starship outer framework.
Asteroid #243 Ida has approximate dimensions of 56 x 24 x 21 kilometers (35 x 15 x 13 miles). Allowing 3 miles of rocky shielding from the exterior of the asteroid going inwards, we have 3,800 cubic miles of internal habitat space - enough to occupy a moderate sized city on Earth!
Of course hollowing out an asteroid of that size will have its own, massively steep up hill challenges. Based on the Galileo spacecraft's studies, asteroid 243 Ida has a density of 2.9 grams/cm^3 and its composition is probably the same as that of chondrite meteorites found on Earth, i.e. its largely composed of stony material. A nuclear-powered "bulldozer" excavating an asteroid by detonating miniature "bombs" and carving out a hole at the rate of (3 x 3 x 3 metres = 27 m^3) per day would take nearly 40 years to carve out a rectangular block of dimensions (15 km x 5 km x 5 km = 375 km^3), easily of sufficient capacity for a generation starship. That's not as un-viable as it may sound, considering the long term durability being offered, the "space city" being built inside the excavation for near-eternity... a fully secure, robust home to hundreds of future generations. We excavate tunnels on Earth through vast mountain ranges composed of hard granite, in order to build roads lasting perhaps only a few decades before erosion eats into them.
It may not always be necessary to "dig" an asteroid at all. Many asteroids have large craters and deep gorges and grooves across their surface. Take #433 Eros for instance, visited by NASA's Near Earth Asteroid Rendezvous (NEAR) mission in 2001. It has a large 3km diameter crater called Psyche which could easily be adapted to accommodate a surface colony of some kind. There are many many favourable alternative possibilities if we are willing to look for them!
Harvesting an asteroid from the main belt of 243 Ida's mass and density in one go is likely to be next to impossible with current technology. However, there are certain to be more promising candidates in the 'Apollo' and 'Amor' classifications which approach and cross the Earth's orbit often. Take the near Earth visitor # 4179 Toutatis of approx. dimensions (4.5 x 2.4 x 1.9 km), during one of the regular near Earth fly by's of such an asteroid, applying sufficient retro-thrust would result in an initial 'capture' orbit around the Earth. An ideal asteroid for a starship design would be one of low density (easier to excavate) and low mass (easier to change its velocity vector and place it into a desirable alternative orbit), which at the same time offers a decent amount of internal volume. To harvest a 'main belt' asteroid, an engine mounted on its surface and powered using a large scale fission reactor, could be used to provide cumulative thrust over parts of its heliocentric orbit which gradually, over perhaps a few decades, works to alter the trajectory to eventually take the asteroid to a 'starship assembly' parking orbit around the Earth. I do not underestimate the size of the technical challenge and the potential risks in such operations. Asteroids have un-even mass distributions and often chaotic spin rates, which means any small errors in how they are propelled artificially toward a capture orbit around the Earth could, in the worst case, lead the body toward a direct collision course with the Earth or the Moon!
Here is a site detailing basic astrogeology of near Earth asteroids. Generally speaking, asteroids in the outer solar system are likely to be less dense and possess more chemicals and frozen water in their surface compositions, useful for "mining". It is possible that the resources of such an asteroid could be processed in-situ on its surface and used to propel the body towards an alternate orbital or even solar system escape trajectory out towards the stars.
As my concept is genuinely "futuristic" in its timescales, based on the economics and technology advancements of the era in which the starship is designed and built, we may have access to materials and alloys which could rule out the need for an asteroid altogether and build the whole thing like a giant rocket ship of the future. But for the foreseeable future, I still see an asteroid as the most robust, most durable framework that can withstand the stresses of a voyage of such cosmic proportions, bridgeing enormous light years across the millennia of time.
The idea behind capturing comets within our own solar system first and strapping them onto the main ship prior to departure for Alpha Centauri (as depicted in my 'aeroplane like' artwork at the opening of this article) is merely to enable technology evaluation and refinements "in house" before embarking on the journey to Alpha Centauri. Throughout the mission, any *physical* association with comets is only momentary and purely for mining operations where this proves absolutely necessary, hence comets will be jettisoned immediately after their resource extraction, to reduce any excess mass being carried by the ship.
Comet-chasing spacecraft could fly to periodic comets within our solar system and using advanced and large-scale robotic arms (like the Canada Arm 2 example currently operating onboard the International Space Station, as illustrated below) grip onto their solid nuclei, and apply thrust from nuclear-powered engines cumulatively over several years with the objective of altering their orbits.
![An artist's impression of the robotic arm attached to the International Space Station [Credit: NASA]](http://www.astroscience.org/abdul-ahad/firstarktoalphacentauri/arm-large.jpg)
Above: An artist's impression of the robotic arm attached to the International Space Station [Credit: NASA]
Changing the heliocentric orbits of massive comets like P/Encke and P/Halley is going to be no easy feat, requiring vast amounts of energy. But given extended timescales for the project going into many decades, with evolving technology, the task will certainly not be outside the reach of of possibility. The comets might be hurled in, initially towards the inner solar system and then towards a high Earth orbit, where they eventually dock with the central asteroid habitation module as part of a high Earth orbit "mining evaluation" program.
Once all the technology is thoroughly evaluated and the ship is ready for departure to Alpha Centauri, escaping out of Earth's gravity well will call for vast amounts of fuel and have its own challenges. The starship's escape trajectory to begin with would be a highly elliptical orbit around the Earth (the initial 'capture orbit' when the asteroid was first brought in) which, over several orbits and application of thrust on a cumulative basis at each perigee passage, gradually extends towards an interplanetary escape hyperbola which eventually puts the ship into orbit around the Sun. A similar process will then be repeated, using gravity assisted fly bys of giant planets like Jupiter and Saturn, to escape from the Sun's gravity into a final interstellar, out of ecliptic trajectory, towards Alpha Centauri.
The concept outlined here is left flexible to the possibility that a more suitable asteroid of favourable (shape-mass-volume-density-orbit) mix is found in the outer solar system, where it is fully worked into a generation starship and from where it eventually departs for Alpha Centauri without any need for bringing to near Earth space. Such an outer solar system project on the scale necessary for construction of a starship like the one conceptualised here, will require a substantial base to have been already established on a nearby moon like Titan or Triton or a planetoid like Pluto or Quaoar in the Kuiper belt. There will be a massive advantage in building a starship in the outer solar system where the escape velocity, dictated by the equation:
V = (2GM/r)^1/2
[ where V = escape velocity, M = mass of Sun, r = heliocentric distance, G = universal constant of gravitation = 6.67 x 10^-11 N m^2 / kg^2 ]
will be substantially lower and present less of a steep climb out of the Sun's gravity well than that encountered in a ship departing from the inner solar system.
Upon departure, attaining too high an initial velocity boost immediately beyond the gate of our solar system's front door would introduce problems when it comes to docking with comets further out, should their mining become necessary. Too high a speed of encounter will not permit sufficient reaction time for the ship's capture mechanisms to grip onto potential comets, and will also affect its ability to swerve past any oncoming debris. An optimum balance will therefore be required between speed and journey time on this voyage, which will make the journey immensely long perhaps stretching over tens of thousands of years. Hence the need for this to be a "generation" starship concept and going to the stars the "slow" way.
The Propulsion System
The primary means of propulsion on initial departure from our solar system will be nuclear fission. Further out into deep space, as the need arises for additional fuel, there will be a large choice from water-ice and methane-ammonia resources mined from comets and planetoids. The frozen resources extracted from them could be processed, refined and channelled through inter-vessel pipe lines to feed the propulsion systems at the back of the main asteroid habitat module.
Secondary engines would burn these additional fuels much like the chemical rockets in use here on Earth today. However, with chemical rocket motors, there will be a requirement to contain the wear and tear and for the ship to maintain onboard factories to build replacement parts for engines, as indeed for all other starship hardware, as they are needed.
In the era in which the ship is launched, it is just possible that we may have found ways to contain fusion reactors, in which case they could be added on board for ever lasting power and further propulsion. There is also nothing to say that a "Project Orion" type of pulse-detonation could not be used for part of the journey where more thrust is needed or where comets or planetoids prove to be sparse in their availability. Emergency, nuclear backup reserves would be an essential requirement for the mission.
The Crew Habitation Module within the Asteroid
Whatever the choice of asteroid as final framework for the starship, the crew habitat carved out of its interior is envisioned to be a true "cosmic ark", having people, plants and animals all living happily side by side, effectively representing a small section of planet Earth's own biosphere. It will be totally self supporting via extensive recycling of all resources. Where necessary, additional life support ingredients such as water and oxygen will be mined from comets on the voyage for ensuring an overall healthy, natural environment similar to that enjoyed on Earth.
![An artist's rendition of a highly developed 'controlled ecological life support system' (CELSS). I envision a similar kind of biosphere operating inside the Aster-Com starship's asteroid habitation module. [Image Credit: Don Davis]](http://www.astroscience.org/abdul-ahad/firstarktoalphacentauri/oneill.jpg)
Simulating "sunshine" across the vast biosphere will be a huge challenge. Across most latitudes on Earth, the total power received from the Sun equates to approximately 1.39 kilo-Watt / metre^2 (the "solar constant"). It will call for enormous amounts of electricity to match daily solar outputs on this scale across the entire surface area of the ship's biosphere, where trees and vegetation are freely growing. Hence the need to mine resources from comets and planetoids to supplement power requirements in addition to the nuclear reserves taken from Earth at departure. Full spectrum sunlight could conceivably be produced using hundreds of lights calibrated to solar wavelengths. But growing plants under electric light bulbs requires vast amounts of water, as I have found from my own simple experiments. As anyone who's ever tried growing indoor vegetables will know, the water evaporates rather quickly. Water, being the most essential ingredient for all life, will be *the* biggest and most highly prized of all commodities extracted from icy bodies such as comets encountered along the journey.
It is unlikely that we could ever simulate Earth conditions exactly no matter how large the ship's ecosystem; clouds and rain for instance just may not "happen". Hence the environment will require continuous sampling, with computers monitoring conditions against a pre-defined environmental model of some kind and applying / recommending periodic corrective adjustments. Control of viruses and harmful pathogens will call for large-scale water and air purification drivers to be operated continuously, which will be large consumers of electricity.
The successful operation of advanced 'controlled ecological life support systems' (CELSS) on the Moon, in low Earth orbit and on Mars over many decades will be important precursors to habitat design on my Aster-Com starship.
Nuclear power is used in many countries to support the electricity needs of entire populations on Earth. In France for example nearly 80% of electricity generation is from nuclear fuels. Similar nuclear processes could be used to power the massive lighting and heating needs of the starship community. The overall power requirements would be substantial, to keep the ship's extensive biosphere and ecosystem 'alive', 24 hours per day, 365 days per year... over thousands of years.
In addition to taking reserves from our own solar system on departure, beneath the surface ice of comets and planetoids encountered along the way, there could exist ores of Uranium 235/238. This could be mined and processed from the larger bodies encountered en-route to provide nuclear fuels for electrical power generation as the need arises on the immensely long voyage. In the absence of nuclear fuels, power for the crew and internal life support systems could be provided by large scale Hydrogen/Oxygen fuel cells drawing on the comet's icy surface resources. These fuel cells will be much more advanced over the ones used on the Space Shuttle today.
Fundamental to habitat design for this voyage will be the need to generate artificial gravity, in even measure, all around the ship's cylindrically shaped interior biosphere. A slow rotation of the asteroid could easily achieve this. Thrusters mounted at appropriate locations around the body of the asteroid would maintain the spin at a given rate. The main propulsion engines could be mounted in a favourable geometry so that thrust is applied along the spin-axis of the ship, thereby not affecting its spin stabilisation whenever propulsive thrust is applied. Maintaining asteroid spin will be especially crucial through phases of rendezvous operations with comets/planetoids, so as to not affect the delicate gravity equilibrium between the ship and its internally contained biosphere.
The shape of the asteroid will be an important criterion in its selection for an Aster-Com starship design. Most bodies have non-uniform mass distribution relative to their centers of gravity, which will cause chaotic and de-stabilised spin rates. It may therefore be necessary to perform some "cosmetic surgery" work on the asteroid's outline prior to embarking on any excavation projects into its interior.
![An engineering schematic of the Ahad-AsterCom starship [Copyright: Abdul Ahad]](http://www.astroscience.org/abdul-ahad/firstarktoalphacentauri/astercom-blueprint2.jpg)
Above: An engineering schematic of the Ahad-AsterCom starship [Copyright: Abdul Ahad]
Depending on the dimensions of the asteroid and the size of central habitat carved out within its interior, it will be possible to work out the spin rate required in order to generate a one "g" of gravity (i.e. equivalent to Earth's gravity, g = 9.81 metre/sec^2) from centrifugal force arising from such rotations.
Living inside a giant asteroidal starship with, say a 6 mile wide *cylindrically* shaped biosphere, would afford its occupants some stunning and highly dream like views! When one looks directly overhead from the edge of a pine forest on this idyllic miniature world inside the Aster-Com, a river called "Eridanus" (perhaps named after the celestial river constellation by the same name) could be seen to meander its away across the "sky", flowing from horizon to horizon... Such a "river in the sky" would be supported by the outward pushing artificial gravity generated by the asteroid's spin.
Mining Comets and Planetoids in Interstellar Space
On a voyage spanning tens of millennia, accommodating the day to day needs of hundreds of consecutive generations it is dead certain that vital resources like water, air, fuel, power and raw materials will require topping up periodically along the voyage, no matter how good the recycling process on the starship. The availability of miniature worlds, acting as "stepping stones" for resource mining along the journey, will be a crucial requirement in this mission concept.
As outlined at the outset, it is just possible that the Oort cloud may extend from the outer Kuiper Belt all the way to perhaps as far as half way to Alpha Centauri. With even more luck, Alpha Centauri could have its own Oort cloud of comets stretching out like an extended arm reaching towards us, beckoning us to cross the interstellar gulf with far more ease than we might have previously imagined.... With an integrated mass of 2.0 x solar mass, the combined gravitational strength of Alpha Centauri's two principal stars (A and B) could meet us more than half way along the journey, with the system's own Oort cloud of comets.
In the depths of interstellar space, away from the bright neighbourhoods of either the Sun or Alpha Centauri, the total flux (the "Ahad's constant of universal flux" ) is going to be a microscopic 14 milli-Watt / metre^2 of star light illuminating any comet or icy planetoid. That's very dark indeed and it will take some skill and effort to detect upcoming comets and planetoids in the forward path of the ship. This is one of the reasons why I feel we might never be able to visually detect debris floating around in the distant Oort cloud using solar system based telescopes, unless the optical sensitivity of current CCD detectors improves beyond all proportions.
However, since we are talking futuristic technology here, a generation starship could be kitted up with powerful, narrow beam lasers mounted on its exterior body, which could be used to temporarily act as long range "flash lights" that selectively light up the AsterCom starship's forward path and illuminate any oncoming target comets / ice balls. Since these objects are likely to be "icy" in their compositions at that distance, away from any star (with high albedos) they would show up for hundreds of thousands of miles up a ahead in the torch beam, lighting up the ship's path like an airport runway lit at night for an aircraft coming in to land.
The strength of optical sensing of localised comets and planetoids in the 3D space surrounding the ship described here, can be further augmented using good old fashioned radar scanners. The ship's navigation team will be expertly equipped with a full suite of astronomical instruments that determine in advance the (trajectory, ship-relative velocity, surface composition) mix of any oncoming objects in order to determine refuelling rendezvous feasibilities.
A further option would be to launch a series of remote sensing, robotic "observation platforms" which operate at say, t+10 days, t+50 days, t+200 days,... ahead of the Aster-Com's current position (where 't' is the current ship time). They would serve as effective early warning alert systems for upcoming cometary bodies (or lack of them). The information relayed by these robotic platforms could be used to weigh up potential risks to projected resource requirements.
The heliocentric orbital velocity, V, of an object is given by:-
V = [G(m1+m2)*(2/r - 1/a)]^1/2
[ where m1 = mass of Sun (1.9891 x 10^30 kg), m2 = mass of comet/planetoid, r = heliocentric distance, a = semi-major axis of orbit, G = universal constant of gravitation = 6.67 x 10^-11 N m^2 / kg^2 ]
Assuming a circular heliocentric orbit and a negligible mass in relation to the mass of the Sun, for a comet at the distance of Pluto (39 AUs), V = 4,743 metres/sec. This slows down to just 172 metres/sec at 30,000 AUs and even further to just 94 metres/sec at 100,000 AUs solar distance. This means any docking operation will demand slowing down to match the low orbital speeds of objects at that distance from the Sun. Typically, upon any physical encounter the comet/planetoid captured will add "reaction mass" and cause the ship's trajectory and speed to alter slightly during the mining operations. However, the resources recovered from such an object are expected to more than offset any set backs resulting fom such an encounter, and as soon as the mining process is complete the object will be jettisoned, with the ship resuming its course and speed via propulsive burns.
Through the mid-range of the voyage (c. 70,000 - 120,000 AUs solar distance), where the individual gravitational spheres of influence of the Sun and Alpha Centauri intersect and merge into each other, it will be necessary to slow right down in order to successfully dock with any prospective comet/planetoid. Should refuelling become necessary, the resources extracted from one such body would be used to propel the ship toward the next. However, applying the standard practice of docking first and then mining using the robotic ship-to-comet attachment mechanism is not going to be feasible for a "mid-range refuelling strategy", as the ship would need to slow down for one comet, then speed up, then slow down for the next comet, and so forth. See "Alternative Mining and Refuelling Strategy" below.
The design of this mission leaves both the total journey time to Alpha Centauri and adhereing to a precisely *forward* directed course line totally flexible and of subsidiary importance. The Aster-Com starship's "Mission Management Committee" (MMC) will operate to a prime directive of keeping the ship's biosphere 'alive' by mining cometary bodies as often as proves necessary.
When we are drifting in the dark waters of this endless interstellar ocean where the shores reach out to near eternity in every direction we care to look, navigation will be yet another big challenge. With no magnetic fields, no bright planets, no "GPS" for relative referencing, triangulation by the minute positional shifts of nearby stars in the surrounding cosmic night sky may be the only means of interstellar navigation on this grandest of all voyages.
In the extreme event where resources prove sparse in availability in the forward direction of the ship, it may be necessary to 'back track' to previously encountered bodies on emergency reserves. Radio transmitters planted on comets following the mining operation, can be used to track their positions relative to the ship in such an eventuality. The ability to go as fast or as slow as you like and turning back when you cannot see islands up ahead for meeting projected resource requirements, are some of the fundamental "safety" attractions with a voyage of this kind.
The "mid-range" which I refer to above, is a *gravitational* mid range centered on approx. 90,000 AUs solar distance - a third of the total linear distance separating our Sun from Alpha Centauri. At that distance, an object's orbital period, given by Kepler's 3rd law:-
a^3 = k * T^2
[ where a = semi-major axis of orbit, T = orbital period and k = Gaussian gravitational constant ]
would be no less than 27 million years! Since this territory is effectively a "no man's land", where an object would be equally gravitationally perturbed by both systems, it is likely that more comets and icy debris would be encountered on a voyage going towards Alpha Centauri compared to all other directions outward into the vast interstellar ocean surrounding our solar system.
![The expected increase in distribution of comets and planetoids going towards the mid-range territory. [Image - Abdul Ahad]](http://www.astroscience.org/abdul-ahad/firstarktoalphacentauri/sun-alphacentauri-midrange.JPG)
Above: The expected increase in distribution of comets and planetoids going towards the mid-range territory (sizes and distances not drawn to scale). - click to see a larger image. [Image: Abdul Ahad]
At the *precise* point of equilibrium between the individual gravitational forces emanating from each system there will exist a "knife edge" scenario, where bodies would be perturbed into a rapid motion towards either star system (whichever star system 'wins' the object so to speak with its greater gravitational might). This precisely pivotal point would likely be devoid of any objects for mining, and the starship "command" must plan the journey accordingly.
However, leading up to that precise point of equilibrium, I would expect orbits of objects around each star system (Sun versus Alpha Centauri A+B+C) to be pulled and stretched toward a middle ground, causing the respective orbital ellipses to have a commonly oriented direction for their major-axes lines, which lines up with a line connecting the Sun and the barycentre of the Alpha Centauri triple star system. This is by virtue of a "three body" orbital scenario, as illustrated above. By the dynamics of elliptical motion, since more time is spent by orbiting bodies towards the aphelia points of their orbits (by Kepler's second law), it is within reason (from a theoretical point of view) to expect a higher frequency of encounter with comets and planetoids when travelling towards Alpha Centauri, compared to all other directions going outward from our solar system.
The above scenario is set to strengthen further going forward, since Alpha Centauri is still gradually edging closer towards us. A future generation of Earth will face a challenging choice of either taking this window of opportunity to cross a "virtual" bridge to Alpha Centauri or declining the offer in anticipation of another star passing by the Sun. But that's gonna be a long, long time coming...
![The nucleus of Comet Halley, photographed by ESA's Giotto spacecraft during its close fly-by encounter on March 13, 1986. [Credit: NASA - National Space Science Data Center]](http://www.astroscience.org/abdul-ahad/firstarktoalphacentauri/giotto_halley2.jpg)
Above: The nucleus of Comet Halley, photographed by ESA's Giotto spacecraft during its close fly-by encounter on March 13, 1986. [Credit: NASA - National Space Science Data Center]
Since its first description by Dr. Fred Whipple back in the 1950s, a cometary nucleus like the one shown above is generally regarded as a "dirty snowball", composed of frozen gases and dust, which is held together in a relatively weak but stable structure. Hence the art of docking with comets successfully would be to do so fairly gently, by matching their speed and applying a measured and incremental grip. Based on robotic spacecraft fly-bys of periodic comets like Halley and Wild 2, the nucleus is thought to be quite small, typically just a few kilometers across.
The Aster-Com starship's giant, fully retractable robotic arms could be used to grip onto comets in-flight, carry out the mining process and jettison them as the ship travels through cometary swarms in the denser parts of the Oort cloud (I use the term "swarms" lightly here, individual bodies will likely be separated by vast distances). As our robotic technology is certain to be much more advanced in the future, much of the physical effort and co-ordination of the robotic arm, tightness of its grip on the comet, the processing and inter-vessel channeling of resources into fuel, power, water, etc. could all be handled autonomously by the starship's onboard computers. Manual interventions and out of ship EVA requirements will be minimal in any refuelling rendezvous operation via this scenario.
A planetoid of larger size and mass compared to comets will require a different tactical rendezvous approach. Depending on its size and gravity, the starship may need to achieve a temporary orbit around a larger body and rely on manned shuttle craft to carry out the mining via short term excursions down to its surface. The mined resources will then be ferried up to the ship, hopefully without too much effort in escaping the weak gravity of such a mini-world.
Depending on the extreme good fortunes of the particular "generation" then enjoying the cosmic adventures of the mission, it is easily within the realm of possibility they could encounter a dark, rocky ice world drifting through interstellar space that turns out to be of similar size to our own planet Earth! In such an eventuality, the Aster-Com starship might get docked into an orbital encounter around this exotic world for an extended stop over, where mining expeditions can be despatched down to its surface using shuttle craft carried onboard "Ahad AsterCom-1":-
![A mining expedition operating on the surface of a frozen ice world drifting in the Oort Cloud, might get to see a vista of our distant solar system from inside a cave. [Image Credit: David A. Hardy]](http://www.astroscience.org/abdul-ahad/firstarktoalphacentauri/iceworld.jpg)
Above: A mining expedition (despatched from the Aster-Com mother ship) operating on the surface of a frozen ice world drifting in the Oort cloud, might get to see a vista of our distant solar system from inside a cave. [Image Credit: David. A. Hardy]
Alternative Mining and Refuelling Strategy
Up to this point, I have described a robotic ship-to-comet attachment mechanism as the standard method of extracting resources from the exterior surfaces of comets. Another alternative strategy I mentioned would be to land on the body using shuttle craft by way of EVA excursions and manually draw the resources that way.
A third possibility would be to perhaps disintegrate objects directly ahead of the ship using long range missiles and collect the resulting icy fragments around the body of the ship as it travels through the debris cloud. As described above, switching to this strategy will be especially necessary towards the mid-range of the voyage, where orbital speeds of comets and planetoids approaches an absolute minimum and successive rendezvous operations will require fuel-inefficient speed ups and slow downs.
The missiles would be much like conventional ballistic missiles we have for military use on Earth today, and they would be launched from a bay towards the front of the ship. A cluster of such missiles could be precision guided to simultaneously impact around the icy outer edges of oncoming bodies, so as to maximise the release of frozen chemicals coating their exterior.
For collection, a retractable array of funnels of some kind could be erected around the body of the asteroid in a plane which is perpendicular to the velocity vector of the ship. The resources collected by these funnels on the asteroid surface could then be channeled inward into processing and refinement plants housed further into the body framework of the starship. It is likely the front half of the ship will become coated by a thick layer of icy material accumulating over time, which is going to add to its mass. In such an alternative mining and refuelling strategy of disintegration/collection, a periodic exterior 'dusting' mechanism of some kind will be necessary as part of the ship's ongoing maintenance.
The Challenges of Managing Generation after Generation
They might forget the purpose and destination of the mission and end up going far far astray...
A "military" and strictly imposed solution would be to program the ship's computer system at initial launch from our solar system, so as to rigidly adhere to strict destination co-ordinates. This might prove much more difficult to enforce in practice, since the onboard crew and their own levels of intelligence and IT education will invariably be evolving and advancing through the immensely long duration of the mission. As a precursor to implementation of my mission concept envisioned here, the "World Space Agency" (WSA established through the unifications of NASA/ESA/RSA/ISRO/JAXA/CSA/... by then!?) will probably assemble a brainstorming workshop session of some kind to determine solutions to this issue, which may not even be such a high priority after all.
A full discussion of the social dimension and how to go about maintaining cohesion and stability on the miniature world of the Aster-Com across successive generations, is outside the scope of my technical document here. The election of a starship "Mission Management Committee" (MMC) through a democratic system of voting as one committee reaches old age only to be succeeded by another, will need to be managed using perhaps statutory frameworks maintained by robust computerised means. The challenges of keeping law and order and managing all other social functions like school, work, past times, etc. will mirror exactly what we find on Earth today, except it will be on a miniature scale.
Other obvious issues include the control of population growth, an ability to manage hereditary diseases passed on from one generation to the next, and a readiness to tackle the myriad of unknown health and psychological disorders that could develop en-route. A full team of onboard medical experts equipped with ongoing, advanced research facilities will be a core mission requirement in this voyage. Self-evolving 'expert systems' will record the medical advances and pass them on from one generation to the next - much the same as what we are doing on "our" own generation starship of planet Earth.
The Shores of Alpha Centauri...
With the vast immensity of interstellar oceans separating us from the stars, even with the most optimistic, *theoretical* fractions of light speed mission concepts, few envision a journey where the person departing from Earth will be the one who gets to enjoy the end goals of the mission. Almost invariably, it will be your children who benefit from any hardships you encounter along the voyage. With my "generation" starship concept, I question the wisdom that if you are not going to be the one to see the light of day on the other end of the interstellar tunnel, then provided conditions onboard the starship are nice and comfortable, does it really matter whether its the generation after you... or the 300th generation after you that eventually gets greeted by New Earth?!
Having made it through the immense tunnel of darkness stretching over 270,000 astronomical units, what manner of alien landscapes and exotic skies can our distant descendants expect to see burst forth before their unaccustomed eyes?
Well, that's entirely open to one's own imaginations as no human eye has ever seen what those panoramic scenes might look like. First and foremost Aster-Com starship dwellers will be warmly greeted by sunshine coming from not one, but *two* or possibly *three* suns... as if its nature's way of compensating them for having been totally deprived of any sunlight across a cosmic night lasting 30,000 years or more, the longest night ever endured by humanity...
And looking back homeward towards our own distant Sun and solar system in some forgotten corner of the night sky, there will shine a rather bright, yellowish looking star. At present that star is set against the backdrop of constellation Cassiopeia, although by the time a generation starship actually reaches Alpha Centauri, in many thousands of years from now, the galactic proper motion will have caused it to wander across a couple of neghbouring constellations by then.
![The night sky of an imaginary planet in the Alpha Centauri system. Notice a strange, bright lit star to the left of Cassiopeia? That's our Sun! It would shine at a magnitude of +0.4 (similar in brightness to the star Capella in our night sky) - [Credits: Night sky - Abdul Ahad, landscape - Dan Durda]](http://www.astroscience.org/abdul-ahad/firstarktoalphacentauri/startwinkle.gif){kind=small}
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I HOPE TO BE CONTINUING MY ADVENTURE WITH THIS *FACTUAL* SPACEFLIGHT CONCEPT INTO A FUTURE SCIENCE FICTION MOVIE AND NOVEL.
WATCH THIS SPACE!
EPILOGUE: The Imperatives for Launching a Cosmic Ark to the Stars
I have no wish to add myself here to the ever growing list of prophets of doom, but there are some underlying imperatives which demand our launching for the stars sooner rather than later.
Here in early years of the 21st century, few people reading this article will oppose the wisdom established by science with regards to the likely evolutionary route our species has taken since the humble beginnings of life on our planet as micro-organisms in a primitive ocean some 4 billion years ago. We eventually crawled out of that ocean, developing in stages to dominate the planet as conscious, self aware, intelligent beings with hopeful futures and pleasant dreams. We looked up at the night sky in wonder and saw billions of stars with potentially habitable zones around each of them and noted so many extra-solar planets in our journals. We developed technology to take us to the Moon and had every potential to reach for the planets and, ultimately, the immensely distant stars.
Suppose now, through one of the many well known dangers facing our planetary cradle, our species is destroyed right here on Earth where it was born, before we had the opportunity to launch a single part of human society to safety on one of the countless far off worlds glittering across our night sky. How much of a tragedy would that be from a universal perspective?
There are incurable diseases like AIDS, there's the Yellowstone National Park "Super Volcano" waiting to go off, giant meteorites drifting in the darkness of space with our names written on them... the list goes on. Those who saw the recent Hollywood release "The Day After Tomorrow" will have had a graphic illustration, albeit fictitiously screen played, of the sudden and catastrophic impacts that an ice age could inflict upon our world.
![Ice age scene fom the Hollywood movie 'The Day after Tomorrow' [Courtesy: Twentieth Century Fox]](http://www.astroscience.org/abdul-ahad/firstarktoalphacentauri/dayaftertomorrow.jpg)
Heaven forbid, a disaster such as this can strike at any time and with very little warning, with the result of putting our technology and spaceflight capabilities back by hundreds if not thousands of years.
As I write, I am peering into a crystal ball that has been rather clouded by the debates raging around the globe on the future of manned spaceflight over the next few decades. My own passionately held view is that the political, financial and social constraints we face must all be overcome fast, with a push for permanent manned bases on the Moon and Mars in the next couple of decades and then a firm plan to initiate a manned interstellar voyage in the next 100 to 150 years.
But there are very profound religious and ethical questions that must be answered which were outside the scope of my scientific article here. Did God ever *intend* us to cross the huge interstellar distances? Did He create vast distances separating solar systems to keep species apart?
"He it is who has made the earth a cradle for you, and has provided for you ways [of livelihood] thereon, so that you might follow the right path." - Qur'an (43:10)
"... If you have the power to pass beyond the zones of the heavens and the earth, then pass beyond them! But you will never be able to pass them, except with authority (from God)!" - Qur'an (55:33)
95% of people on this planet believe in some kind of a supreme authority who is in charge and who determines all this, and they quite rightfully, should be the ones who get to decide in the end.
I am optimistic that interstellar travel, along similar lines to the concept I have detailed here, will become a reality in the next few hundred years irrespective of whether the more "theoretical" pursuits like suspended animation, matter/anti-matter propulsion, solar sailing and controlled nuclear fusion ever take off in the end.
Peering even further ahead, through the mists of the uncertain near future, one day crossing a "virtual" bridge to the stars by utilising the resources of comets and planetoids encountered along the way could become the de facto method of safe and secure interstellar travel. I can then claim to be the world's FIRST to have put forward such a scientific proposal in clear and concrete terms!
Credits & Acknowledgements
I would like to personally thank Alfred Aburto Jr, Zahid Lal, Ian Stirling, Rob Dekker, David A. Smith, Phillip Thorne, Bill McHale... and everyone else on the usenet article link below under "Discussion Articles on this Topic", who contributed their own thoughts and ideas to my spaceflight concept.
The world's *FIRST* human spaceflight proposal for reaching the nearest star beyond our solar system using the resources of comets and planetoids, could not have happened without the support and encouragement from so many people scattered across so many countries around the globe!
"First Ark to Alpha Centauri" - A motion picture to be!
How might the "Aster-Com" starship begin its life?
About me
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My paper on the Interstellar Night Sky
Discussion Articles on this Topic!
First Ark to Alpha Centauri - A novel by A. Ahad
"An epic fantasy voyage carrying the hopes and dreams of everyone who ever was, everyone who ever is and everyone who ever will be... on a journey spanning 2,000 generations and lasting for no less than 50,000 years into the future!"
Click on the book's cover image for a fuller, more condensed version of the synopsis for this first novel in the series. You will also find information there on how to order your own copy!
Copyright Notice
Copyright © 2004 by Abdul Ahad. All intellectual property rights on this article and the spaceflight concept depicted herein are fully reserved for a fictional movie and novel. Any sharing of this work is only permitted for personal, non-profit and educational use.
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