Gagarin's flight of thirty two years ago was a surprise for people. Later, with progress in manned space flight, various attitudes toward "human in space" evolved. A typical question asked by scientists who were satisfied by unmanned space missions concerned the usefulness of human beings in space in comparison with automated systems. However the question, man or robotics, was not constructive for discussing the future of space activities. Thus, I thought the idea of productivity in space would be a better context to discuss the engineering of space systems requiring human beings and robotics.

More recently, I consider the context of the "space frontier" is the most important in my thoughts on "humans in space". The usefulness of human beings and the importance of productivity in space are only one aspect of "humans in space". In history it has been the frontier that motivates people to take action for new opportunities. And it will be so in the future. My view on Man in Space is that human beings will go to space because they want to be there, rather than in order to work under weightless conditions.

Generally speaking, new frontiers should be the new territory for people to visit and live in. To make it possible in outer space, high technology is essential in location of the living places, assurance of living environment in space and transportation to and from there. Fundamentals of the essential technology were efficiently and successfully developed by governments in the past space programs. These can be considered to be mature enough to be applied for development of the space frontier in various ways in future.

To develop the space frontier, we made unmanned survey and then exploration with robotics, followed by brief manned stays. At present, the first step has extended as far as the entire space of the solar system. The second step has been taken in near earth space including the moon. And now we are at the threshold of permanently manned space existence. What actually happens will be more complicated, since there are many target places and routes to which this evolution of steps can be applied. A space station is considered the next step toward the space frontier among various options. The reason is that it should be a key element of space infrastructure for permanently manned lunar activities and a manned Mars expedition. We have arrived at the crossroads to the next steps: lunar bases, manned Mars expedition, as well as the international space station.

All of these are proposed as government-funded programs. In this respect, every scenario is an extension of past space programs. However, political and economical crises in the world seem to endanger these programs which were justified as national programs by old standards. Furthermore, evidence of the stagnation of existing space activities can be observed in the trend of population growth in space, as follows.

I would like to look at the trend of the human population in the near-earth space which is the sum of every astronaut's flight time in each year, and can be defined as the average number of human beings in orbit. (Fig. 1)

The population increased rapidly in the seventies to reach an average level of one person in space, and kept increasing in the early eighties until it reached two people. Then, the growth rate should through the present. Obviously outer space is large enough to accommodate many more people. The reason can be explained by the delay of the international space station. If the original version of the space station was constructed in the early nineties, the population would have been growing continuously.

To see the trend in more detail, we will examine the flight time of individual astronauts. In this case, a population of one person equals various combinations of number of astronauts and their flight time. Recently, Russian astronauts flew mainly in their space station for longer periods than American astronauts flew in the Space Shuttle, which means that fewer Russians than Americans experienced space flight if the average numbers are equal. This will also apply in the case of the international space station, where several crew members with stay for periods longer than a month.

As a result, it is predicted that even if the international space station is built, a relatively small number of astronauts will be employed, and they will be more specialized to aim at higher productivity. Eventually, manned space flights remain a special territory supported by governments for a few elite groups of "superman" astronauts. Nevertheless, if the station had been built as planned, the space population would have kept growing in the nineties high enough to be counted by two digit numbers.

However, the project implementation has been delayed and the population growth has stooped. Among various reasons to explain the delay, financial difficulty is the largest one. This implies that the proposed budget is considered to exceed the amount that tax payers are willing to pay. In other words, people may be questioning whether to continue to pay their own money to support the elite groups. Taxpayers generally think that a certain portion of their money can be allotted to maintain the national prestige, but more money should be used for their benefits.

To get much stronger support for space programs, we have to carry not only a few influential politicians but many more public visitors to space. The only possible way to realize this is commercialization of manned space flights, that is, space travel for the general public. However, the currently planned space systems will not work effectively for this purpose, because they are based on the traditional space technology and engineering which supported scientific and military missions, ignoring economy and passenger service.

Although space tourism was a dream of spaceflight for people, actual manned spaceflights were far from space travel as it had been expected. What is urgently required is to give a definite concept of space tourism to the general public, which will be used as a guidebook to the space frontier. To challenge this route to the space frontier, the Japanese Rocket Society has just started a study on space tourism, which consists of four study areas; space medicine, finance, transportation and passenger services. This first study aims to identify the most practical approach to this new business, and to come up with a definite concept of engineering design, and hopefully a business framework.

An example of an industrial perspective of space tourism is shown by Table 1. The industry involves space vehicle manufacturers which might be interested in production of at least one hundred vehicles which carry fifty passengers on each flight. Flight frequency should be two times per day, and in average four hundred flights in a year. If passengers enjoy spaceflight briefly for as short a time as three hours, the total number of passengers will be two million, which corresponds to more than six hundred population in space. Thus, the contribution of space stations to the space population will be negligible.

The effects of space tourism will be enormous in two respects. One is that faster progress may be expected for the advanced space activities to be conducted by the government elite groups. I expect these future programs would get much stronger and wider support from those space visitors who experience the new environment for themselves.

The other is that the demand for mass transportation will change the situation of high cost space transportation, which is now the obstacle blocking the way to the space frontier. If the price of earth to space transportation is reduced by two orders of magnitude as indicated by Hunter (ref.1), not only more and longer space travel will become feasible, but also the most basic infrastructure for industrialization of space to develop extraterrestrial energy and mineral resources will have been completed.

Considering these possibilities for people in space in the coming century, space medicine seems to have focused its main effort on the specific purpose of assuring the safety of astronauts flying on early space vehicles. From my view point as an engineer, there are two features of the achievements of space medicine in past space programs.

One is that space medicine was life science in the space environment, and especially in weightless condition. It inevitably included the environment of space vehicles to carry astronauts to and from space. Consequently the study was closely related to the design engineering of space systems and vehicles. The study results were often compromises between medical allowances and design criteria. In early projects, technical difficulties of vehicle design seemed to be overcome by tough medical standards for the crew.

The other feature of space medicine was concerned with labor science, perhaps together with human engineering. To justify manned space missions, it was preferable to demonstrate the superiority of human beings to automatic systems. In addition to the important human factors of creativity and adaptability, astronauts are expected to work cooperatively with machines. Thus, taking various lessons and training, astronauts become extremely able workers in space. Space medicine is very important in the selection of tough "superman" and "superwoman" astronauts.

An appropriate medical standard to assure passengers' health and safety is essential for space tourism to open the space frontier to the general public. The medical standard should be much looser than the present one, since ordinary people have to be accepted as passengers. This will reduce rocket performance, due to, for example, additional mass and lower acceleration. In addition to this requirement, rocket vehicles for this use will be required to fly repeatedly like aircraft. Both of these new requirements would impose more difficult design problems on designers of such passenger rocket vehicles. Fortunately, new materials and technology are available to improve vehicle performance. Fig. 2 is a picture of one of the Phenix series vehicles, which was designed for the first attempt of sightseeing space flight.

It will be very important for engineers and medical specialists to work together to find the best compromise between safety assurance and practical vehicle design. In this respect, space medicine is expected to provide appropriate new directions to engineers. To achieve the most positive aspect of tourist services, space medicine will indicate what is the best design for passengers to watch the earth or to enjoy weightlessness from the medical viewpoint. Health management of the space crew will necessarily involve space medicine, while public health in space flight will be an important field of research as well as of practice. It should be stressed that medicine will be more influential on the design and engineering of space systems than before.

Fig. 2 . 1985 Phoenix design, mixed mode, single-fuel version (2)

In this discussion, space tourism is considered to be a breakthrough to open the space frontier to the general public, and consequently to strengthen support for ongoing advanced space programs.

To realize space tourism, space medicine is expected to indicate clear directions for designing new space systems and vehicles to be used for tourists from the general public. The medical standard for tourists and onboard crew is expected to be similar to that for the current airline business.

It will be essential for engineers to closely cooperate with medical specialists to develop new space systems, new medical and engineering standards, and finally a new culture in the space frontier.

1. Maxwell W Hunter, Fall 1989, " The SSX - True Spaceship", Journal of Practical Applications in Space, Vol. 1, No.1
2. Gary C Hudson, November 1991, "History of the Phoenix VTOL SSTO and Recent Developments in Single-Stage Launch Systems", AAS 91-643, included in Proc. of International Space Year in the Pacific Basin, AAS advances in the astronautical sciences, Vol. 77