It is widely understood that the growth of human activities in space is critically dependent on the cost of space transportation, which in turn is strongly influenced by the rate of traffic. To date, the demand for launches has derived mainly from the demand for satellites for communications, meteorology, surveillance, scientific research, etc.. However. the launch rate of around 100 launches per year (worldwide). required for these purposes has not hitherto been considered sufficient to justify the cost of development of a fully reusable launch vehicle, which is necessary if launch costs are to be reduced substantially below the present level of around $10,000/kg. Thus it is important to investigate any proposal that has the potential to generate much higher rates of launch traffic.

One potential use of space flight which arises directly from the wide popular interest in space but that has been considered in very little depth to date is space tourism, i.e. the taking of short pleasure trips in Low Earth Orbit ( LEO) by members of the public. This has begun to receive serious consideration only very recently (l, 2). Thus, in the discussions about possible uses of the space stations currently being planned in several countries. tourism has not been considered. A major reason why space tourism has been ignored is clearly because the cost of space travel today is so high that it is assumed that tourism is therefore not a serious possibility for the foreseeable future.

Nevertheless, the lack of interest in space tourism is surprising when it is considered that in many major countries the tourist industry is between ten and fifty times larger in revenue terms than the space industry; that it is a major creator of wealth and user of high technology (specifically of mass transportation, computers and telecommunications); and that holidays in space would have obvious attractions for the public. The aim of this paper is to stimulate further discussion of space tourism by illustrating the following possibilities:

The potential demand for space tourism is such that this could become the largest revenue-earning use of launch vehicles and space stations if the price could be reduced sufficiently; the technology exists today for reducing the cost of short holidays in space to a level affordable by a significant proportion of the population; and a flourishing space tourism industry could provide the high traffic rate for transport to and from LEO that is required to justify commercial development of low cost launch vehicles and infrastructure that will in turn lead to the everyday exploitation of space in other applications.

Before examining the potential market for space tourism services, and the possibility of supplying the required services at acceptable prices. the attractions of a visit to low Earth orbit are briefly considered.

Although there is considerable public interest in space travel and in the experience of weightlessness, there is much less public appreciation of the wide range of interesting and entertaining activities that are potentially available in an orbiting facility. Despite this, the potential demand for space holidays is already high: a recent opinion poll carried out for the American Express company in the U.K. showed that more than 50% of those under 45, and 65% of those under 25, would like a holiday in space (3). The U.K. is not likely to be exceptional in this respect, and it also seems probable that these figures would have been higher if those questioned had been given more information about the entertainment potentially available in LEO.

Table 1 lists a number of unique leisure activities that are possible in LEO, in the approximate order of their cost of provision. It is perhaps worth commenting briefly on some of these: first, astronauts from both the U.S.A. and U.S.S.R. have reported that observing the Earth and terrestrial phenomena from a porthole in LEO is quite fascinating, and does not pall even after several hours. This activity is clearly relatively straightforward to make available. although windows in spacecraft are of course more expensive than uninterrupted walls. Second, human-powered flight, using fabric wings attached to the arms, and tails attached to the ankles, will be possible in low gravity. Many recreational and sport activities could exploit this possibility in LEO given a fairly large chamber. Third, a slowly rotating, cylindrical swimming chamber would enable people to swim in low "gravity", but to propel themselves out of the water and "fly" in the central air-space. On a longer time-scale, and even more exotically, orbiting botanical gardens would hold enormous interest as they reveal how different plants adapt to a micro-gravity environment.

Thus, in addition to satisfying the public's general curiosity about space travel, and especially concerning the experience of weightlessness, it is clear that there is a wide range of interesting and absorbing occupations that could be provided for visitors to a LEO facility.

A proviso that is perhaps worth mentioning is that it will be necessary to ensure that passengers do not suffer from space adaptation syndrome at least no more than very temporarily. This will probably require the availability of effective anti-nausea drugs. Considerable thought will also have to be given to the design of passenger facilities, and to the organization of passengers' activities. These will all be "indoors", and it will therefore be necessary to take steps to prevent both sensations of claustrophobia, and staleness of the atmosphere. In this context it is interesting to note that the Skylab crew members found that the upper deck of the workshop (a cylindrical chamber some 12 m in length by 6.5 m in dia, which they used for acrobatic exercises) was large enough to provide them with a feeling of freedom and release from the confinement of the living quarters (4).

The ultimate scope for development of orbital hotel facilities is enormous. An example of an advanced facility has been described by Kraft Ehricke (5), and would include all of the amenities described in Table 1, at a correspondingly high manufacturing cost. However, it is notable that an early phase, short-stay hotel facility could be assembled using only existing Spacelab and Space Shuttle hardware: Spacelab structural modules, lengthened to take up the entire Space Shuttle cargo bay, could form the basic building blocks; four such modules, connected by tunnels, could provide space for up to 50 people - say, 40 tourists and 10 staff. Power could be provided by existing solar array modules. Life support, attitude control, navigation and communication systems could be derived from those in the Space Shuttle and Spacelab. Although some development work would be required to combine these systems appropriately, the requirements for a space "hotel" are far less technically demanding than those of Spacelab. Thus, if required. a limited but useful tourist facility suitable for near term utilization could readily be built from existing hardware or derivatives.

From the point of view of a business assessing the feasibility of providing space tourism services, market information derived from market surveys is essential in order to estimate the potential revenues that may be earned. By combining such market research with estimates of the probable cost of providing the services in question, it is then possible to estimate the potential profitability of such a venture. To date, the information that is available on the potential market for space tourism is very limited (though perhaps more extensive than some may realise). Before discussing these data, it is interesting to examine briefly some theoretical considerations relevant to this subject.

By analogy with commercial developments in the past, one might reasonably expect the demand for space tourism services to evolve through several broad phases, about which it is possible to make a number of general observations:

The market for space tourism services in this phase comprises individuals who are prepared to pay a very high price for a trip into orbit. Customers would not necessarily require a high degree of comfort, elaborate facilities, or a prolonged stay in orbit, perhaps remaining within the launch vehicle (at least in the catty stagcs). Due to the high price, the market may be expected to consist of very wealthy individuals with an interest in space.

In this phase the service would become available on a regular basis. The price would remain at a high level (by comparison with the average level of expenditure on leisure pursuits) so that customers would belong primarily to high income groups. The service would be more comfortable, and facilities would be more extensive than in the first phase. The quality of the service provided (in terms of comfort, food and entertainment) would be more important to customers, and appropriate changes in the marketing of the service would be necessary. As the price of the service fell over time. that part of the demand that was dependent on the perceived "exclusivity" of the service would presumably fall, and would shift to more exotic trips as these were made available.

In this phase costs would have fallen (through scale economies, learning curve effects and technological advances) far enough to bring the service within reach of a significant proportion of the population. Facilities would be available on a large scale, and turnover would be much higher than in the previous phases. There would be price competition between different suppliers of services, leading to a continuing decline in prices, and corresponding growth in the total market.

This is the ultimate evolution of space tourism, and represents the phase that international air travel has already reached in the industrialized countries - it is available to a large proportion of the population, at least at some stage in their lives. Although clearly far in the future, it is nevertheless possible to envisage an industry on a scale approaching that of present-day air travel to tourist centres, with many tens of millions of passenger movements per year. However, it is sufficient for the present discussion to confine the consideration to the first three phases.

This expected evolution through a series of hypothetical phases can be illustrated on a graph, as in Fig. 1. Note that the price bands quoted above are not exact, and the horizontal scale has been intentionally left blank. Before considering the likely levels of demand for space tourism at the different price levels, it is interesting to survey the published literature that exists on the subject. Although there has been only one serious commercial investigation to date of the market for space tourism (2), many authors have referred to the possibility of space tourism in the future, and several authors have given estimates of the level of demand. These are discussed in chronological order below:

Kraft Ehricke discussed the subject of space tourism at an American Astronautical Society conference in 1967 (5). Without attempting to estimate the probable level of demand as such, he considered the economics of a large orbiting hotel with 1100 beds. In his example Ehricke assumed fortnight-long stays in LEO, and transportation costs of $10/lb ($22/kg) (i.e. approx. $33.50/lb ($74/kg) in 1985$). Adjusting his 1967 figures for inflation up to 1985, and for the higher nominal interest rates today, one obtains trip prices of $7800-$11,200, for which he assumed that there would be a demand of 28,600 customers per year. If these figures are further adjusted to the case of 3-day stays at the orbiting hotel, a demand of 114,000 customers per year would be obtained at a trip price of $4300-$7700.

In 1984 David Ashford looked at some of the implications of assuming that the availability of 3-day trips to a small orbiting hotel at £5000 per head would generate a demand equal to 5% of the current expenditure on transatlantic tourism (1). This would represent a market of 100,000 customers per year at approx. $7000 each. It is clear that both Ashford and Ebricke were considering the market in the "mature" phase, which together with the assumed launch costs of some $30/lb ($66/kg) is well in the future.

In 1985 the travel company Society Expeditions of Seattle, Wash., which specializes in the provision of exotic holidays (such as voyages through the North-West Passage), published an estimate of the demand for space tourism at the higher priced end of the demand curve (Table 2). Society Expeditions' concept is somewhat different from that of Ehricke or Ashford. The customers would stay for some 7-10 days tn a hotel/training facility on Earth, where they would participate in familiarization and training courses for space flight, followed by a short trip to LEO. In the initial stages at least, there would be no orbital hotel. This service is clearly aimed towards the "pioneering" and "exclusive" phases described above. These figures constitute a rather more bullish view of the market than the assumptions of Ehricke and Ashford, but they are perhaps likely to be more accurate due to the company's commercial interest in the matter.

Also in 1985, Jesco von Puttkamer of NASA quoted an estimate of the near-term demand for space tourism that is even more optimistic than that of Society Expeditions (6). A demand of 300-350 passengers per year at a price of $2-3 million clearly represents the extreme "pioneer" end of the market. While this figure appears to be out of line with the previous estimates, it may reflect the number of passengers that might be paid for by national governments if the appropriate service was available.

Table 2. Estimate of demand for LEO space tourism

Source: Society Expeditions (2)

Finally, and perhaps most interestingly, in September 1985 Society Expeditions announced the signing of a contract between them and Pacific American Launch Systems, a private company planning the development of a low-cost, reusable launch vehicle, the Phoenix (7, 8). This comprised an agreement for Pacific American to provide launch services to Society Expeditions over 5 years from 1992. The proposed service comprises a 12 h flight in polar orbit, in a vertical take-off, vertical landing ( VTOVL) craft carrying 20 passengers. These trips are priced at $50,000 per head, and they will operate weekly, representing a demand of 1000 passengers per year. This figure is somewhat below Society Expeditions earlier estimates, but it is perhaps natural to be more cautious when "putting one's money where one's mouth is". Nevertheless these figures clearly represent real market estimates: while Pacific American Launch Systems bears the technical and economic risks involved in the development of the launch vehicle, Society Expeditions bear the risk of failing to generate $280 million of revenues over 5 years. It is interesting to note that, as of June 1986, some 250 people had placed deposits of $5000 with Society Expeditions to book places for 1992, which suggests that the demand does indeed exist at the level estimated by Society Expeditions (9).

Figure 2 is a demand curve derived by plotting these different estimates (using logarithmic axes). It is perhaps not too far-fetched to claim that three phases are distinguishable. The "pioneer" phase approximately corresponds to the cost of taking passengers on the Space Shuttle - if it were to carry fare-paying passengers. (This possibility was in fact explicitly ruled out by NASA (10)). While the Society Expeditions Pacific American project would seem to represent something between the first and second phases, Ehricke's and Ashford's projections clearly relate to the "mature" phase. However, if Society Expeditions' figures turn out to be correct, the level of demand towards the right-hand side of the graph is likely to be significantly higher than Ehricke and Ashford assumed - particularly in view of the fact that the latters' figures assumed that guests would spend several days in hotel accommodation, by contrast with the very limited facilities to be offered initially by Society Expeditions.

It is also interesting to draw the graph of total revenue earned against the price of the service, as in Fig. 3. The figure quoted by Puttkamer is clearly out of line with the other estimates. Towards the right-hand side of the graph (which. as mentioned above. is not derived from market surveys but would not appear inconsistent with the other figures), the market is potentially very large indeed, if the demand could be satisfied profitably.

On the basis of the figures quoted above, the shaded areas in Fig. 4 represent the region within which future demand for space tourism services might be expected to lie, on both optimistic and pessimistic assumptions.

However, there is no substitute for real market research data, and it is to be hoped that more extensive market surveys will be performed in the near future. In the meantime the following conclusions would seem reasonable for present purposes;

For the earlier phases of space tourism. a price of $55,000-$l00,000 will generate significant business. An orbiting hotel facility is not necessary for this phase, as sufficient novel activities can be carried out in the launch yehicle in orbit.

For the mature phase, at a price of $10,000 or less, the annual demand could be of the order of 1,000,000 tourists. This figure will be used in the economic analysis which follows.

The preceding discussion of the demand for space tourism has a major weakness in that it made no allowance for the differences in the facilities that would be offered to passengers. The two studies referring to the mature phase of the market included hotel facilities, while the others referred to trips only in the launch vehicle. Clearly the more elaborate and expensive the facilities provided, the greater the price of the trip must be, and so the higher the turnover that is required in order to pay for the investment. In the following section some illustrative cost figures are considered in this context.

In order to approach the question of the potential profitability of providing space tourism services, it is necessary to estimate the likely level of costs that will be involved, even if only approximately. and to assess the likelihood of meeting the target levels discussed in the previous section. We will do this by looking at two different cases, one representing an early phase of space tourism and one in the "mature" phase.

The first task is made simpler by the availability of the projected figures for the Society Expeditions Pacific American project. Phoenix has been designed deliberately to achieve the lowest possible costs (8). It is a single-stage, ballistic vehicle. as were the designs of Philip Bono (l l), and Dietrich Koelle (12); it uses off-the-shelf technology as far as possible, in order to minimize development costs; and it incorporates a number of novel design features. Although not designed primarily as a passenger launch vehicle, Phoenix consists of a propulsion mainstage and a family of add-on modules to permit maximum versatility of operations, including automatic, unpiloted satellite launches; piloted operations; fuel "tanker" operations; and a passenger version.

The development costs of Pacific American's Phoenix launch vehicle are expected to be of the order of $200 million (8, 13). The selling price of the basic vehicle is projected at some $50 million, and the cost per launch (carrying 20 passengers) is expected to fall below $1 million once operations become routine, giving the cost per head quoted above of $50,000. The achievement of this target would represent a reduction in current launch costs by more than 95%, which is clearly a sufficiently large improvement to alter the cost of space operations radically. Society Expeditions are not currently planning the provision of orbiting hotel facilities (though it can be envisaged that if their initial service is profitable they will move towards this, using cargo versions of the Phoenix for launching the system components).

In the mature phase of space tourism, the large volume of traffic will have a dominant influence on the costs of operations due to scale economies and learning curve effects. both in the manufacture of vehicles and facilities, and in the provision of the services involved. In order to investigate the potential for cost reduction by these means we will consider the level of activity discussed above at which one million tourist trips are made to orbiting hotels per year. (It is perhaps relevant for comparison to remember that many individual companies today provide holidays for more than one million tourists per year.)

If one million guests per year were to stay for one week at an orbiting hotel. simultaneous accommodation for 20,000 guests would be required. as well as facilities to transport 20,000 passengers to and from LEO per week, The question to be addressed is therefore, given the scope for obtaining substantial (and, for the space industry, unprecedented) economies of scale and learning curve benefits, how near is it possible to approach the target cost of $10,000 per passenger, and hence profitability? That is, if we assume that there is an assured market on this scale, how low can we realistically project the costs of providing such a service to be? The provision of the required transportation capacity and of the necessary accommodation are considered in turn.

The transportation system required for space tourism in the mature phase involves passenger ferries, and cargo vehicles for launching the hotel facilities. In considering a level of activity of one million passengers per year, it is clear that we are discussing a stage of development at which the business has matured to the extent that launches are "airline" type operations, i.e the vehicles are proven, and are produced and operated in substantial numbers. In order to attempt to reach the target cost figures discussed above for the mature phase of space tourism, it is necessary to consider dedicated, fully reusable launch vehicles, using existing technology to the maximum possible extent.

The passenger launch vehicle clearly needs to be a great improvement on any existing launcher in terms of its operating costs, while also providing customers with the comfort of mass air transport. In order to be conceivable as a regular passenger ferry the launch vehicle must satisfy a number of requirements which can be summarized as follows:

Design and certification to full airliner airworthiness standards (which are much stricter than current space flight standards).

Moderate acceleration levels during boost and re-entry.

Capability to operate from conventional airports with minimal additional facilities.

Direct operating cost per seat of less than $5000, in order to meet the target trip cost of $10,000.

Of various configurations which can be envisaged to meet these requirements. perhaps that requiring the least development of new technology is a piloted. two-stage, horizontal take-off, horizontal landing ( HTOHL) vehicle, using existing rocket engines on the orbiter, and a combination of turbojet and rocket engines on the booster. as described by Ashford (14, 15). This vehicle has been called "Space-bus" to denote its primary function of carrying fare-paying passengers.

The booster stage of Spacebus is a large, supersonic aeroplane, with twice the take-off weight of Concorde. It has turbo-jet engines for take-off. accelleration to Mach 3-4, and flyback and landing. Four rocket engines of J2S or HM6O performance are installed in the aft fuselage. These provide acceleration to the separation speed of Mach 6. which occurs at the top of a semi-ballistic climb to an altitude where the dynamic pressure is low (reducing both air loads on the orbiter, and heating loads on the booster). The orbiter stage of Spacebus consists of a conventional airliner fuselage with a capacity of some 40 passengers. The liquid oxygen/liquid hydrogen propellants are carried in a large, thick, pressure-stabilized wing. The orbiter's engines are two J2S/HM6O motors. Leading data for Spacebus, derived from first order sizing calculations. are given in Table 3. A concept sketch of Spacebus is shown in Fig. 5.

The key design features of Spacebus, intended to enable it to meet the stated requirements. are as follows:

Full reusability. Spacebus is fully reusable. This feature is clearly mandatory for any launcher hoping to come near to the target cost per launch. Hitherto all launchers have involved large throw-away elements.

Two stages. Single stage to orbit ( SSTO) winged vehicles are at the margins of feasibility (viz. the U.S. NASP and U.K. HOTOL projects). However, such vehicles require ultra-light design and advanced propulsion, which militate against the design margins needed for long life and robust, low maintenance structure, engines and systems. A two-stage vehicle is less technically demanding: state-of-the-art engines can be used, and the weight margins are such that airliner-like maintenance costs should be achievable. By comparison. a reusable SSTO design attempts to take two major steps at once: step one being a fully reusable vehicle: and step two being a single-stage reusable vehicle. Since the first step is adequate to achieve dramatic cost reductions. it is commercially sensible to defer the second (and much more difficult) step until space transportation technology and operations have matured to the extent that the advantages of a single-stage vehicle offer significant economic benefits.

Aeroplane-like booster. The Spacebus booster is built like an aeroplane. and its cost per flight should therefore be comparable to that of a large commercial passenger aeroplane, which is <1% of that of the Space Shuttle. Horizontal take-off is the obvious means of meeting the requirements both of using existing airports and facilities, and of full airliner airworthiness.

Winged, horizontal-landing orbiter. A winged reentry vehicle has a larger landing footprint than a ballistic one, and lower re-entry deceleration. It can also be designed to land at existing airports. The re-entry decelerations of a ballistic vehicle (about 3 g) may not be acceptable for mass tourism. The design of the Spacebus upper stage is also easier than that of an SSTO vehicle: the required fuel fraction is only about 77% compared with about 87%. and hence the weight fraction available for structure, systems, engines, furnishing and payload is nearly doubled. If a reusable single-statte vehicle is on the margins of feasibility, then a two-stage vehicle is well within them, and virtually state-of-the-art.

Buried upper stage. The upper stage is partially buried in the booster. It is thereby protected from air and heating loads during the boost phase. and can therefore be optimized for re-entry.

Table 3 Spacebus leadine data

Fig. 5. The Spacebus passenger ferry design concept.

5. SPACEBUS COSTS

The most demanding of the design requirements of the passenger ferry for space tourists is the cost target. While it is clearly necessary to assume that LEO ferry flights will have become routine in the mature phase, there remains considerable scope for making different assumptions about the level of maturity of the technology. A turnover of 20,000 passengers per week would require some 500 flights per week of vehicles with a capacity of 40 passengers. At a time when ferry trips to LEO are assumed to have become operationally routine, each orbiter vehicle might perform 7 flights per week, and the boosters 14 launches (The annual numbers of hours of flight per vehicle would still be much lower than those of commercial aircraft). The minimum number of launch vehicles that would be required to handle the assumed traffic load is therefore 72 orbiters and 36 boosters. Table 4 gives estimates of the costs per flight of Spacebus. based on a certain set of assumptions. These assumptions are discussed in the following notes, and, in order to illustrate in more detail approximately what assumptions are necessary to meet the suggested cost target, the sensitivity of the result to changes in the assumptions is examined.

Table 4. Spacebus operating cost estimates

The assumptions made in Table 4 are in no sense intended to be definitive. They are made with the intention of illuminating the question of the level of technological maturity that is required in order for a mature space tourism industry to be feasible. It is interesting in this context to consider the sensitivity of the overall cost figure to changes in the major cost components:

Propellant costs. At some 30% of the total, propellant costs could be significantly reduced only by increasing the number of passengers per flight. The maximum value is limited, however, by the maximum all-up weight that is feasible for horizontal take-off.

Development costs. A further 40% of the total cost represents capital recovery costs, of which approx. one-third (or some $1200 per passenger) are accounted for by development costs. Thus if, for example, development costs were higher by 50% (at $4500 million), trip costs would increase by some $600 per head.

Rate of return. If the effective rate of return required was 10% (instead of the 15% assumed), the trip cost would fall by $900.

Numbers of vehicles built. If the total production runs for the booster and orbiter were twice those assumed above (and spread over 10 years), the average production cost would fall by 15%, which, together with the smaller share of development costs borne by each vehicle, would reduce the cost of a passenger trip by some $800.

Vehicle utilization. If the launch vehicle technology was rather more advanced than the level assumed above, orbiters could fly twice daily, and boosters four times (while retaining 10 year lifetimes). The number of vehicles required would then fall by 50%, and the total capital cost per vehicle would increase. However, the overall effect would be to reduce flight costs by $1050 per passenger.

Due to the assumption that operations have become routine, the other cost components are relatively unimportant. The above sensitivity figures illustrate the effects of changes in cost components occurring singly; in order to achieve a substantial reduction in transportation cost per passenger, it is necessary to combine two or more of these more optimistic assumptions.

The hotel accommodation required for 20,000 guests might be provided in the form of 80 hotels with an average of 250 beds each. Such hotels rnight comprise some 30 Spacelab-type structural modules, plus perhaps four much larger modules, (such as Space Shuttle external tanks, suitably converted). Thus 80 such hotels would require some 2400 smaller modules. and some 320 larger chambers to be manufactured. These production runs are far longer than have been achieved with space hardware hitherto.

It is important to note that the manufacture of the habitation modules required for an orbiting hotel facility would be much less technically demanding than Spacelab modules themselves: There is no requirement for state-of-the-art laboratory hardware, computing equipment or telecornmunications facilities. Nor are large amounts of power required; nor are accurately controlled attitude or gravitational environments required in the facility. The sole requirements are for agreeable living accommodation and leisure facilities. These require only standard structural modules, with a range of lightweight interior partitioning and suitable furnishings. Environmental control and life support, power, thermal control, attitude control, communications and other systems would need little adaptation from systems that either already exist, or are under development. The only significant new developments required would seem to be the need for multiple windows in every module, and the semi-autonomous environmental control systems needed to provide multiple redundancy in the facility. Even for the windows the basic technology is already fully established, while the latter requires the development of a fairly straight-forward hierarchical control system.

On the proposed scale of manufacture, learning curve effects would cause very significant reductions in unit prices: the habitation modules would have an average unit cost of only one-sixth of the first unit cost, and the larger modules approx. one-quarter. Thus it would perhaps not be unreasonable to assume average manufacturing costs of $5 million per small module, and $25 million per large, giving an average total cost of $250 million for the 30 habitation modules plus 4 large modules required for each hotel. If spread over a 20-year lifetime, the repayment of this cost would represent some $800,000 per week, (assuming 15% discount rates), or $3200 per guest.

However, this does not include the launch costs for the hotel. Spacebus is a dedicated passenger vehicle, and is not suitable for large cargo loads. If launch costs were to add to the hotel cost by no more than 50%, say, the launch and assembly cost would have to be no more than $125 million (for some 350 tons), or $360,000 per ton. This represents an order of magnitude reduction in cargo launch costs below existing levels, which could be achieved by utilizing a reusable, unpiloted, VTOVL orbiter craft such as Phoenix (8) or Beta (12). It is notable that the development of such vehicles would be commercially justified by the requirement to launch the hotel components alone.

$600 per guest per week would allow for operation and maintenance costs representing some 3% of the capital cost of the hotel per year. Staff costs of $400 per guest per week would generate $4000 per week for each ofl say, 25 staff. This would cover salaries for two crews working on a 2-months-on-orbit/2-months-on-Earth rota, as well as crew transportation costs and some management overheads. A cost of some $5800 per guest is therefore obtained.

In round figures a total trip cost of some $16,000 would therefore seem broadly feasible on the assumptions made. relating to a level of activity of one million space tourists per year. It is interesting to note that if the demand was not as great as this. and only a single launcher and hotel facility (of the types discussed above) were constructed, the cost per trip would be some $70,000 (in round figures), requiring a turnover of 12,500 passengers per year at full capacity. This figure lies well above the market estimates discussed above. In order to reduce the cost per trip by a further 40% so as to reach the target figure of $10,000 per passenger. it would be necessary to make a number of more optimistic assumptions, for instance concerning the turnaround times of the booster and orbiter, or concerning the tax treatment or availability of government subsidies for R&D costs, as seen above. These cost estimates would therefore seem to imply that it is at least not obvious that, given this level of demand, cost figures of the right order of magnitude could not be achieved with more or less existing technology. It need hardly be said that from a commercial point of view, a proposition utilizing mainly existing technology is much more attractive than one requiring substantial new developments.

There is one very important proviso to all the foregoing, namely that the demand for space tourism services will be critically dependent on one particular factor: The entire service will clearly have to be demonstrably safe. both in the perception of potential customers, and in order for insurance companies to be prepared to underwrite the risk of accidents. Such safety requires assurance on three different aspects of the operation:

1. the vehicle and facilities will have to be safe;
2. there will have to be no significant health risks; and
3. the probability of damage from collisions with other spacecraft or debris will have to be insignificant.

1. This will require the vehicles and facilities to be inherently safe against mechanical failures, as proven by the prior performance of a very extensive flight test programme to civil aviation standards. In addition, the achievement of adequate safety levels will require the permanent availability of comprehensive safety and rescue facilities and vehicles.
2. This will require further research on the biological effects of space flight, both short-term and long-term, and the provision of storm-shelters against solar flare particles.
3. This will require further research in quantifying the risks of collisions with orbital debris, and the use of appropriate levels of design safety. It is also possible that international legal agreements concerning orbital traffic systems may become desirable in order to reduce the risk of collisions between spacecraft to an acceptable level (17).

At present, in the absence of more precise information. it is perhaps reasonable to conclude that the level of demand for LEO space tourism services that is postulated above is not inconsistent with published market estimates. The provision of services to one million passengers per year would provide the opportunity to achieve very substantial economies of scale, reducing the overall cost per passenger by approximately a factor of four, and approaching a price at which this level of demand might be achieved. Thus, if potential demand on such a scale does indeed exist, it would appear that this could propel the commercial development of cheap space transportation and habitation facilities. Great interest must therefore attach to any detailed market surveys that are performed to determine the likely level of demand for space tourism services of various kinds, particularly at prices of between $10,000 and $20,000, as well as to the results of further feasibility studies of the transportation and accommodation systems discussed above.

1. D M Ashford, 1984, " Space tourism - key to the universe?", Spaceflight 26, 123-129
2. Society Expeditions, 1985, " Space tourism could drive space development", Proc. L5, NSI, AAS, SEDS Space Development Conference
3. American Express Co., Personal communication (l986).
4. H S Cooper, 1978, " A House in Space", pp.68-69. Granada, London
5. K Ehricke, 1967, "Space tourism (P)". AAS 23, 259-291
6. J Puttkamer, 1985, " Space: the long-range future", Spaceflight 27, 348-354
7. (anon), 1985, " Travel agency reach space tour pact", Aviation Week Space Technol. 123, No.13, 24
8. G C Hudson, 1985, " Phoenix: a commercial, reusable single-stage-to-orbit launch vehicle", AAS Preprint 85444
9. T E Bell, 1986, " Space tour company books 250 passengers", The Institute (IEEE) 10 No.6, 9
10. T A Brosz, 1985, " NASA administrator rules out shuttle for space tourism", Commerc. Space Rep. 9, No.8, 6
11. P Bono, 1967, " The reusable booster paradox - aircraft technology or operations?", Spaceftight 9, 379-387
12. D Koelle, 1971, "Beta: a single-stage reusable ballistic space shuttle concept", Proc. 2/st JAF Congress, pp. 393AO8. North-Holland, Amsterdam
13. T A Brosz, 1984, " Update: Phoenix launch system", Cornmerc. Space Rep. 8, No. 10.1-5
14. D M Ashford, 1986, " Spacecab II: a low-cost small shuttle for Britain", Aerospace 13, 3340 (1986).
15. D M Ashford, 1965, " Boost-glide vehicles for long range transport", Jl R. Aeronaut. Soc. 69, 448-458
16. British Oxygen Co., Personal communication (1986).
17. P Q Collins and T W Williams, 1986, "Towards traffic systems for near-earth space", 37th JAF Congress, IISL-86-31