When the first humans travelled into space in the early sixties, these men and women were carefully selected on their resistance to extreme physical and mental strain. This selection process was necessary because the first rockets carrying humans to space where not designed specifically for this purpose but derived from ballistic missiles developed for a quite different purpose. Maximum acceleration, for example, was the military requirement not its limitation to protect the humans on board from excessive g-loads. In addition, nobody knew about the real strain an astronaut or cosmonaut had to take before the first human flights had been conducted. Furthermore, the humans on board the first capsules had to be able to pilot their "spaceship" back to Earth, at least in case of emergencies. In the result, astronauts and cosmonauts were selected from the best air force pilots which had the manual skills, physical constitution, stress tolerance and health needed for the job.

With the advent of the space shuttle, the physical requirements for human space flight dropped with the reduction of g-loads but the psychological and educational requirements remained on a high level or even increased. The latter is connected to the longer stay times in orbit and the introduction of the "mission specialist" who not longer had to be able to pilot the spacecraft but to conduct complex research work on board the shuttle.

Even if the necessary level of skills and stress tolerance are lower then the actual performance of the astronauts and mission specialists, it is clear, that it still makes sense to select the most capable humans for the tasks in order to obtain a maximum probability of mission success. This is especially true with respect to the still tremendous cost of human space flight.

With the introduction of space tourism the described picture will change. Space tourists can be compared to flight passengers regarding the minimum requirements for safe transportation. They don't have to be able to pilot the vehicle or to perform complex tasks on board. This means, that any human may go to space providing that he/she is physically and mentally healthy enough to go and come back with a minimum risk to be hurt. A large percentage of the terrestrial population fulfils this minimum criteria, because a spaceship specifically designed for tourists can be limited to less than 3g's maximum acceleration and by thoroughly preparation of the tourists other physiological or psychological problems can be avoided.

As already stated, there is no principle reason which prevents any healthy human from going to space as a tourist. Thus, there is also not general upper limit for the age. As the astronaut John Glenn recently has proven, even an age in the late seventies may not cause a problem if the person has sufficient physical fitness.

On the lower end of the age scale the limit will be oriented at the ability of the persons to follow strict safety rules and to use the technology associated with space flight, e.g. using a space suit in case of emergencies. This will prevent children below a certain age and body size to go to orbit.

Since space is a totally unusual environment for humans, it is clear, that space tourists have to be prepared for their journey. The tourists must become familiar with the technical equipment on board the transportation systems and the orbital hotel. They also must be prepared for the experience of micro-gravity in terms of motion, perception and three-dimensional orientation. Most of this preparation should be aimed to avoid unnecessary stress as well as potentially risky situations imposed by unsafe behaviour.

Even the best training for the space tourists can not avoid health risks which are associated to the nature of space and which are therefor inherently present.

The inherent risks are associated with three characteristics of the space environment:

• Vacuum
• Micro-gravity
• High energy radiation

In the following, the possible influence of these characteristics on the health of the space tourists will be described in detail.

Since a spacecraft, a space hotel or a space suit will be designed for operation in vacuum, the remaining risk is that of an accidental major structural damage in the outer shell of the spacecraft space hotel or space suit. The possibility for such a damage comes from micro meteorites and space debris. Since all larger pieces of space debris are tracked and the smaller ones are unlikely to cause a damage, which can't be controlled, the associated risk can be compared with that in other human activities performed in a hostile environment such as diving.

A major health risk for space tourists as for every human going to orbit is the exposure to high-energy radiation. To evaluate this risk, firstly the types of radiation and their effect on the human body have to be discussed.

For the radiation load in orbit the following types of hard radiation are of importance:

• Solar Cosmic Radiation - (SCR)
• Solar Flares
• Galactic Cosmic Radiation - (GCR)

SCR consists to 99% of protons and 1% a-particles. The particle energies are in the order of 1 keV, the flux density in the vicinity of Earth amounts between 0.09 and 2.0¥109/cm2s depending on solar activity.

Solar Flares happen statistically and last between one and five days. The particle flux from a solar flare consists to 89% of fast protons with energies of >30 MeV. 10% are a-particles and 1% are particles with high charge numbers and high energies (HZE-particles) of 10 to 100 MeV, partly up to 1 GeV. At solar maximum, which occurs every 11 years, the probability of solar flares increases significantly.

GCR consists of high-energy particles from outside our solar system. The flux density of about 10 particles/cm2s is much lower than that of the SCR but it is almost impossible to shield these particles. 85% of the GCR are protons, 14% are a-particles and the remaining are HZE-particles, positrons and electrons which are travelling almost with the speed of light. The energies are about 10 GeV average, but a spectrum of 0.1 GeV to 1011 GeV has been measured. In the space station orbit about 5 to 10% of the radiation is GCR. This portion of the radiation is called "background radiation".

The energy dose D, measured in Gy (Gray) describes the released energy per unit mass of matter. 1 Gy equals a released energy of 1 J/kg radiated matter.

The dose equivalent De, measured in Sv (Sievert) describes the biological effectiveness of radiation, because different types of radiation have different biological effects. With the relative biological effectiveness RBE the dose equivalent becomes De=RBE¥D. RBE has a value between 1 and 20 depending on the kind of radiation. The highest biological effectiveness (RBE=12Ö20) have a-particles and neutrons in the energy range around 1 MeV and protons with energies ²0.1 MeV.

Biological tissue can be damaged either by a sudden short-term radiation event or by long duration exposure to relatively small dose equivalents. Table 1 lists possible damages by short duration exposure to high-energy radiation. Radiation loads of up to 0.5 Sv can be tolerated in this case. The observation and interpretation of long term radiation exposure effects is more complicated because typically results are syndromes like cancer which can also be caused by a number of other environmental effects. The tolerable radiation exposure limits according to German laws are listed in Table 2 /2,3,4/.

People on the Earth get an average natural radiation exposure of about 1.7 mSv per year (Germany: 2.4 mSv/year) at sea level. In addition, a civilisation dependent dose of about 1.6 mSv per year must be added mainly coming from x-ray examinations. Values in this order of magnitude to not impose an increased health risk. The variation of the exposure is in the order of 0.3 mSv/year. In the Space Shuttle, energy doses between 0.007 and 0.026 mGy/day have been measured (see Figure 1, /1/) depending on orbital altitude. In the worst case (max. RBE, max. Altitude), this would accumulate to a yearly dose equivalent of 190 mSv, which is 112 times the natural exposure on Earth. At minimum exposure (maximum altitude, minimum RBE) the load accumulates to a dose equivalent of 2.555 mSv, which is in the order of magnitude of tolerable additional long duration radiation loads (1.8 mSv) according to German laws.

In case of sudden high radiation events (e.g. solar flares), only dose equivalents of less than 0.5 Sv are tolerable with respect to the effect on humans according to Table 1. This value will be significantly exceeded in a Space Shuttle during a typical solar flare.

Background radiation will not be a particular risk for space hotel guests who will come to orbit once in a few years for a few days. Assuming a radiation shielding similar to the Space Shuttle, the maximum does equivalent (14 days stay time, max. altitude, and max. RBE) will be 7.3 mSv. The average load will be much lower, because the average RBE is lower, the average stay time will be shorter and the radiation shielding will be much better than in the Space Shuttle.

The space hotel personnel will take a much higher radiation load than the guests considering their much longer time in orbit. For a half year stay, the average radiation load (RBE=5, D=0.11 mGy/day) will be 10 mSv assuming Shuttle-like shielding.

Figure 2 shows the dose equivalent as a function of in orbit time for the minimum and maximum case. The 1.8 mSv limit is shown for radiation shields of 30g/cm2 and 250 g/cm2. It is shown, that a the larger shield will allow a stay time in orbit of at least 8 days even in the worst case regarding background radiation.

Personnel and guests of a space hotel must be protected from the background radiation as well as from short duration radiation events of high intensity like solar flares.

The radiation shielding of the outer skin of the hotel depends strongly on the kind of material and the mass per area of the skin. An outer surface of material with a low nucleon number such as hydrogen combined with an inner layer of a high nucleon number material (e.g. lead) represents a favourable design. Current spacecraft and space stations (Space Shuttle, MIR) provide a shield of 30g/cm2. A state of the art space suit provides only 3g/cm2. Therefor, EVA activities of space tourists should be very limited and only conducted during phases of low radiation intensity.

Figure 3 shows the influence of the shield mass per area on the radiation dose. A significant reduction of the background radiation requires more than 100g/cm2 of shield mass. With a shield of 250g/cm2 the in orbit stay time for the hotel personnel can be extended to 6 month at average radiation level.

The high radiation intensity during an event like a solar flare does not allow a cost and mass effective shielding of the whole orbital hotel. Since the advance notice time for a solar flare is in the order of hours the hotel guests either can go to special radiation protected rooms where they can stay save for a few days or they can be evacuated to earth. The shielding requirement for solar flare protection is in the order of 500g/cm2.

The effect of µg on human health must be taken into account very seriously when planning space tourism. Because of the different stay times in orbit, the µg-imposed risks must be considered differently for the space tourists and space hotel personnel.

Generally, two categories of µg-imposed problems must be considered:

• Medical aspects
• Comfort aspects

Micro gravity has short duration and long duration medical effects on a human body. A short duration effect is the "space sickness syndrome" which is very similar to seasickness or general travel sickness on Earth. Space sickness is caused by an upset of the sense of balance caused by the micro gravity. In most cases it begins shortly after reaching micro gravity and results in dizziness, increased perspiration and nausea. The symptoms of space sickness normally disappear within a time span of a few hours up to five days. As far as the affected person doesn't have to do work requiring a high level of concentration, space sickness can be treated medically.

A much more severe medical problem is the loss of bone and muscle mass during longer stays under µg conditions, which is based on an adaptation of the body to the lack of gravity. As a result, the physical and mental performance of the person in orbit drops and the ability to fast readapt again to gravity decreases. Furthermore, the decrease of performance is accompanied by cardiac arrhythmia, especially during extra vehicular activities.

So far, investigations have shown, that humans loose about 10% bone mass during a one year stay on a space station under µg. The decrease is linear with time. After a stay of more than half a year, the loss is not fully reversible. Normally, the performance of the cardiovascular system can be fully rebuilt. Bone and muscle loss seem not to stem from lack of use in micro gravity only but there are obviously additional mechanisms of direct influence of µg because training can avoid the problem only partly. It is not clear so far, which level of gravity is sufficient to prevent the described problems.

For typical space tourists, who will not stay in orbit longer than two weeks, the health risks caused by micro gravity can be almost neglected.

Thus the problem of space sickness and how to deal with is the primary issue in this context. Unfortunately, the susceptibility for the pace sickness syndrome can't be tested and predicted under normal gravity very well. Since also the flight to an orbital hotel featuring artificial gravity does include µg phases, the problem can't be fully avoided.

For the crew on board a space hotel the loss of bone mass together with the radiation exposure are the limiting factors for the stay time. For in-orbit times of more than half a year artificial gravity is mandatory to avoid severe health problems. If artificial gravity is not available, physical fitness must be maintained through daily exercise even for shorter working periods.

Besides the addressed medical issues, passenger comfort is an important reason to provide artificial gravity in a space hotel. One key issue in this context is personal hygiene, which imposes a lot of problems and loss of comfort in µg. Even a low artificial gravity level avoids the associated problems.

Since artificial gravity is possible only in a rotating space station, another question is how the centrifugal acceleration influences passenger comfort. Theodore W. Hall /6/ has addressed the design problems of orbital hotels with artificial gravity. His analysis shows that "comfort" in artificial gravity is defined by minimising the disturbance of the sense of orientation and balance caused by the coriolis acceleration. Four parameters are of importance in this context:

• The "apparent" gravity felt by the passenger and resulting from the effective total acceleration
• The gravity gradient in radial direction of the spinning hotel
• The angular velocity of the housing area
• The tangent velocity of the housing area

The resulting comfort limits are described very differently by different sources as listed in Table 3. A comparison of different artificial gravity environments by Hall /6/ led to a "comfort diagram" (Figure 3). The "comfort zone" within this diagram is an area of angular velocities and rational radiuses of the hotels habitation area, which fit. Hotel angular velocity and radius should be selected in order to get a design point, which lies within the "comfort zone".

Thus, to provide healthy working conditions for the space hotel personnel and a comfortable stay for the space tourists, an orbital hotel must feature artificial gravity, preferably in the habitation area.

Author	Year of Publication	Min. Apparent Gravity	Max. Apparent Gravity	Max. Apparent Gravity Gradient per Meter	Max. Angular Velocity of Habitat	Maximal Tangential Velocity of Habitat
		A/9.91	A/9.81	dA/dAref	/(2/60)	Vt= . r
Clark & Hardy	1960	-	-	-	0,1 rpm	-
Hill & Schitzler	1962	0.035g	1g	-	4 rpm	6 m/s
Gilruth "optimum"	1969	0.3g	0.9g	8%	6 rpm 2 rpm	-
Gordon & Gervais	1969	0.2g	1g	8%	6 rpm	7 m/s
Stone	1973	0.1g	1g	25%	6 rpm	10 m/s
Cramer	1985	0.1g	1g	0.03g	3 rpm	7 m/s

A key issue in developing space tourism is the question, which people are not only willing but are also able to participate. It was shown that people who are in a generally good health condition would be able to participate in space tourism. Micro gravity is desired as a unique feature of space but can impose health problems at long stay time, which especially applies to the personnel of a space hotel. Therefor and for comfort reasons of the guests, artificial gravity is absolutely required for a space hotel.

Radiation is the major health issue in space tourism. In case of normal radiation background, the protection, which is provided by a typical space hotel structure would not cause health problems for the guests. The in orbit stay time of hotel personnel must be limited six month or less.

Special emphasis must be put on the questions how to deal with high radiation events like solar flares. Either "storm shelters" with heavy shielding must be provided or the whole space hotel must be evacuated to Earth.

In conclusion in can be stated that the addressed health issues are no showstoppers for the implementation of space tourism.

1. Ernst Messerschmid, Reinhold Bertrand, Frank Pohlemann, 'Raumstationen - Systeme und Nutzung', Springer-Verlag, Berlin, 1997
2. Hans Kiefer, Winfried Koelzer, 'Strahlen und Strahlenschutz' , Springer-Verlag, Berlin, 1986
3. Bundesminister des Innern, 'Berechnungsgrundlage fŸr die Ermittlung von Kšrperdosen bei Šu§erer Strahlenexposition durch Photonen und Berechnungs-Grundlage fŸr die Ermittlung von Kšrperdosen bei Šu§erer Strahlenexposition durch Elektronen, insbesondere durch b - Strahlung', Gustav Fischer Verlag, Stuttgart, 1986
4. Hans-Michael Veith (Ed.), 'Strahlenschutzverordnung 1989', Bundesanzeiger Verlags-GmbH,Kšln,1989
5. MARSEMSI-Study, Technical Note SD-TN-AI-275, ESA, 1992/3/
6. Theodore W. Hall, "Artificial Gravity and the Architecture of Orbital Habitats", International Symposium on Space Tourism, Bremen, March 20-22, 1997