In the earlier blog article, we discussed the meaning of the term “Starship”; itself a joining of the two separate words “star” and “ship”. There are many types of Starships depending on whether they are robotic or crewed, slow or relativistic, precursor missions or full interstellar, small payloads or entire world ships. In this article we continue the examination of this term to help us to understand its meaning. This series of articles, “The Starship Travellers” is really a philosophical exploration of the meaning of the term “Starship”. Let’s briefly examine how the nautical world categorises its different types of vessels.
Firstly, there are different types depending on the type of waterway they are intended for. Some are designed for lakes, rivers, canals, oceans. Some are ferries, some are cargo carriers, and some are pleasure cruises. What all these vessels have in common is some form of hull, based on the simple requirement that in order for it to float, its weight must be less than that of the water that it displaces. The type of vessel will lead to the type of hull requirement, from a catamaran to a trimaran for example. They may have a wooden hull for pleasure craft, steel for commercial vessels, Aluminium for fast vessels and composite materials for some types of sailboats and pleasure craft. There are even concrete hulled vessels. These vessels also have an assortment of propulsion systems, which are usually either human propelled such as rowing, sailing such as by the use of a sail hoisted on an erect mast, and mechanical propulsion systems such as by the use of a motor or engine turning a propeller in steam driven engines or gas turbine engines. Recently, mechanical engines have also included nuclear propulsion systems, particularly for warships and icebreakers. There are also many variations on the types of propellers that a ship may have. The vessel would typically be steered by either some paddles or a rudder, which generates a lateral force to turn the craft. These can be manual or electro-hydraulic systems. Often the propulsion system and the steering are linked, such as in the use of an outboard motor and specific types of sails. All of these different elements will lead to the decision over whether to classify a water going vessel a boat or ship for example. In general a ‘ship’ is considered to be a large buoyant marine vessel, distinguished from boats based on its size and cargo or passenger capacity.
Is this the same way that ‘Starships’ should be categorised? Of course, you could break it down into the Starboat, Starship….There are already some types of Starships out there, such as Slow Boats (Enzmann) and the World Ships. In a recent paper submitted to JBIS three authors (Crowl, Long, Obousy) even invented a half-way definition called a Slow Ship. I it is important to dig into the issue of what is meant by a Starship because it helps us to argue the case for interstellar flight. So when someone says interstellar flight is not possible, they really have to be specific about what sort of ‘Starship’ they are talking about being possible to send. Is it a 1 kg probe, or a 1 million GTon World Ship? Having a first attempt at the question, you can define a Starship from various perspectives, the distance it goes to, the payload mass, mass ratio, mission type, whether it is robotic or human, what cruise velocity it attains. Following along these lines you can come up with some categories like the following:
I exosolar, 100-300 AU
II shallow precursor 300-1,000 AU
III deep precursor 1,000-65,000 AU (~1LY)
IV ISM 65,000-130,000 AU (~2.15LY, half way to Centauri)
VI Interstellar 130,000 – 644,953 AU (up to 100LY)
VI Superstellar >100LY
(i) Payload <0.1 ton
(ii) Payload 0.1-1 ton
(iii) Payload 1-10 tons
(iv) Payload 10-100 tons
(v) Payload 100-1,000 tons
(vi) Payload >1,000 tons
(a) <1% light, ultra slow
(b) 1-5% light, slow
(c) 5-10% light, fast
(d) 10-50% light, ultra fast
(e) 50-99.999% light, relativistic
(f) >c, FTL
(1) Flyby (no deceleration)
(3) Flyby return
(4) Rendezvous return
(5) Swing by (some deceleration)
(6) Swing by return
There are many types of Starships, and by and large it comes down to the means of propulsion chosen to propel the vehicle. There are solar sail craft, pushed simply by the pressure of the Sun. There are laser driven craft, which compensate for the 1/distance squared loss of radiation pressure by the emission of a collimating laser beam, thereby maintaining the pressure of the vehicle for longer. There are microwave beam driven systems, which use the power of microwaves to accelerate a probe up to high velocity before the efficiency begins to drop off. There are particle beaming systems, literally ejecting particles through space towards the rear of a vehicle, thereby transferring momentum to it. All of these systems avoid the need for taking on board propellant. Then there are the propellant carrying systems, usually consisting of multiple engines stages, which are based on either fission, fusion, antimatter or some hybrid combination of all of these, to generate the necessary “bang for the buck”. There is the external nuclear pulse driven systems, which involves the ejection of nuclear units out of the back of the vehicle, momentum transferred onto a rear pusher plate by the detonation of each unit. Other concepts get around all of this, by combining nuclear pulse without the need to carry on board propellant; this includes the Bussard interstellar ramjet – an attempt to find a “loop hole” in the Tsiolkovsky rocket equation.
When you look at all the concepts discussed above for Starship propulsion, they can be broadly split into one of three categories (1) solar ejectors; that is you start with a static system which stays back home and it ejects a payload towards the stars (2) staged release; that is the entire system is dynamic and moved with the journey, but parts are dropped off along the trajectory, with the payload being released either early or later into the mission (3) continuous structures; that is where the structure that leaves the solar system of origin is the same structure that arrives at the target destination. When you think about it you really just have (1) and (3) and (2) is just a mid-way design between those two extremes; static structure with ejected payload or dynamic continuous structure linked to the payload. This situation just describes rather reminds me of a wind tunnel in aeronautics. In the wind tunnel a scaled model of the aeroplane is held in a static position whilst a dynamic airflow is passed over the model. In the actual vehicle, the airplane is in motion, but moving through a near still air. The simultaneity of the two situations allows one to be studied so that you can learn something about the other. The different solutions for Starship propulsion are really just different ways of approaching the same problem, but subject to the same constraints.
In recent conversations, I4IS consultant Adam Crowl thinks that it is simple to just define a Starship in terms of the number of crew. He makes the point that the key distinction between different types of space vessels if whether is it a “Star-Probe” or a “Star-Ship. He goes onto say:
“Sending inert junk to other star systems is much easier than sending a ship with a crew, or a City with citizens, or a World with inhabitants. Thus three grades: (1) Star-ship (2) City-Ship (3) World-Ship. Seems “natural” to me. How you divide up between the three is arbitrary. It’s like the difference between a Town and a City. Numbers? Area? Services?”
During the same conversations, the writer Stephen Baxter had this to say about the matter:
“What is the common understanding of ‘Starship’? … A Starship: a large crewed cargo-carrying vessel that goes to the stars. If you look up ‘ship’ you get definitions about large ocean-going vessels. Voyager isn’t large, isn’t crewed or cargo-carrying, and although I understand the interstellar mission was always an objective, it isn’t really going to the stars, not in any meaningful sense. It’s a probe of interstellar space; it’s no more a Starship than a message in a bottle is a ‘ship’. I think I could accept ‘uncrewed’: a big unmanned cargo carrier could still be a ‘ship’. Is Daedalus a Starship? It has no cargo; you have to see the subprobes as an integral part of the design… I think most people would see it as a probe, not a ship. I would think a ‘Starship’ has to be at least a large cargo-carrying vessel which is heading intentionally for some star system. The city- and world-ships are just subsets of that. But Voyager isn’t meaningfully a Starship.”
In general, spacecraft concepts can be split into two types. First there are the Star-Probes, and then there are the Star-ships. The NASA/JPL Voyager Spacecraft is a “Space-craft” but perhaps we can consider it to be a “Star-Probe”, because it doesn’t carry any on board crew. This is debatable though, given that it has barely exceeded 100 AU distance. Can it rightfully be called a “Star-ship”? from a technical point of view, probably not, and this phrase should likely be reserved for crew carrying vessels, but from a philosophical point of view, given that it was one of the first spacecraft to leave the safety of our solar system, perhaps “Star-Ship can be justified with a little poetic license.
Whichever classification system we use however, there are likely to be some anomalies. Take these four examples: (1) a small probe carrying embryos (2) a vehicle that is crewed by non-biological Avatars (3) a vehicle that is crewed by a group of AIs (4) A living biological ship but which has no crew as such, the ship is just alive in itself. Ask yourself how you would categorise these – Are these “Star-Ships” or a “Star-Probes” or “World Ships” even?
In a recent conversation with I4IS researcher Stephen Ashworth, he made a good point about this:
“Regarding Voyager, I can’t help saying that from the point of view of an inhabitant of Sedna (aphelion 960 AU), the Voyagers are still well inside the Solar System! Furthermore, they are not actually targeted at any star. According to the Starflight Handbook, JPL have computed encounter distances of the four hyperbolic spacecraft with stars for the next million years. All those distances are greater than a light-year, except for one: in 358,000 years time, Voyager 2 will come within 0.8 light-years of Sirius (p.153-54). Does this count as a flyby? If Voyager is regarded as being already in interstellar space when still only 120 AU or so from the Sun, passing Sirius by at 50,000 AU hardly counts as a meaningful encounter (even setting aside the fact that it will be long defunct).”
On the distinction of sizes and classes of Starships Ashworth continues:
“Size: small, medium and large, corresponding very roughly to a small professional crew (as on the Space Shuttle), a town-sized community (thousands, living as one might on a cruise liner or a city housing estate) or a city-sized community (millions, living in approximately Earth-like conditions). Duration of voyage, or equivalently speed in uniform cruise: rapid (journey time of a few decades, within a human lifetime), medium (journey time of centuries) or millennial (journey time in millennia). I suggest that a more precise classification scheme is not really necessary at the present stage. This scheme does cover most of the options, and I think it makes the main important distinctions clear. Obviously any general scheme will miss things out, but the important thing is to get a broad overall sketch of the possibilities. ….Yes, a real Starship looks something like the Enterprise, but meanwhile the Pioneers and Voyagers are our first “Starships”.”
We should also consider that by referring to Voyager as our first Starship, this helps to promote an optimistic vision towards our ultimate aim – “the interstellar program has already began”. It is also a good nod to the achievements of NASA and JPL, which keeps them on the side of the interstellar agenda, and keeps the interstellar community from being marginalised to the fringe. i.e. we are merely trying to extend the program of work that already exists, rather than trying to do something totally speculative. Perhaps all this doesn’t matter, and our search to find definitions behind vague terms is subjugated by the simple achievement that eventually “we sent something to there”. Whatever the make-up of that vessel, it will represent a tremendous achievement for human kind.
Kelvin F. Long
Acknowledgments: I would like to thank Stephen Baxter, Adam Crowl and Stephen Ashworth for discussions that informed this blog article.