Space travel represents a daunting challenge for human beings. Space is devoid of any life-support elements for Earth-born humans. Remember that one of those life-support elements is gravity. So human space travelers must carry all of their life-support systems along with them and find a way to create a sustained artificial gravity vector of yet-to-be determined minimum or maximum value. For short Earth orbit missions, carried consumables and repair parts that can be re-supplied from Earth provide a near-term, acceptable solution. For future long missions the supply of consumables (oxygen, water, food, and power) must be self-sustainable onboard the spacecraft. Spare parts must be in sufficient supply to assure operation of critical hardware. However, as humans attempt to explore further and further from Earth, the system that enables increasingly distance travels is still propulsion. In fact, food and other life-sustaining matter increase linearly with travel time and crew size, while Tsiolkowski's law shows that accelerating a spacecraft by expelling mass (i.e., using Newton's third principle) needs a propellant mass that increases exponentially with increasing speed and initial mass. Thus long travel times are a balance between the mass controlled by propulsion performance and the mass contributed by human support systems. No matter what support systems are available for humans, without appropriate propulsion the necessary time and distance cannot be traversed. So whether human travelers or an automatic robotic system occupies the spacecraft, the propulsion system is the single key element. Remember, in space whatever velocity is imparted remains essentially unchanged. In order to orbit a distant object, the spacecraft must slow down to the initial speed of launch, and equal propellant mass ratio must be expended to decelerate the vehicle as was spent accelerating the vehicle. As we shall see, this propellant mass is not trivial.
The former USSR orbited the first artificial satellite, Sputnik, in 1957. Eleven years later six Apollo missions to the moon enabled 12 astronauts to stand on the moon, explore its surface, and return samples [Stafford, 1970]. There was one short-lived attempt at building an orbital station using an empty Saturn V upper stage tank: an empty Saturn V, S-IV upper stage tank was outfitted to be inhabitable as the Skylab [Skylab, Aviation Week 1985]. After Skylab was permitted to enter the atmosphere and be destroyed, all United States human exploration ended. Not until the next century would the United States, using Russian hardware, place a habitable orbital station into orbit. In that almost 30-year gap, the nations of the former Soviet Union (USSR) launched a series of Salyut orbital stations, culminating with MIR, the seventh Russian orbital station. MIR had served successfully for 15 years, which was about three times its design life. Then in 2001, after suffering the ravages of solar radiation and the space environment, it was deorbited into the Pacific Ocean [Aviation Week, MIR Deorbit, 2001]. This ended a long Russian history of humans living in space on an orbital station. In fact Salyut 6 had to be shut down because of a leak in the hypergolic propellant lines for the station-keeping rocket engines. A former student at Moscow Aviation Institute that had the Salyut orbital propulsion system as a design project was now a cosmonaut. After being launched to Salyut 6 on a Soyuz rocket, he repaired the leak with equipment he helped design and re-established the orbital station operation [Cosmonaut, Private Communication, Los Angeles, 1984]. In 2000 the International Space Station (ISS) was established in the Russian orbital plane of 55° and was constructed with a large fraction of Russian hardware. Its re-supply is primarily a responsibility of Russia with its Progress/Soyuz launch system, and many of the more massive components can be lifted with the Russian Proton launcher if the Space Shuttle is not available for the mission. As with MIR, the key to successful utilization of an orbital station is the frequent and reliable transportation system that can regularly maintain supplies and rotate crewmembers. In effect what is required is a "train" to and from space that operates with the scheduled frequency and reliability of a real train. The principal difference between a rocket-to-space and a train-to-space is that trains are two-way transportation for people and materials. When one of the authors visited Baikonur in 1990, the Soyuz launch complex had launched 92 Soyuz rockets in the previous 12 months, which is a very good record, but other than allowing the return of astronauts, Soyuz it is a one-way transportation system.
The Russian experience is the only database about humans and long-term exposure to the near-Earth space and the microgravity-micromagnetic environment. In fact, discussions colleagues have had with Russian researchers indicate the human physiology might become irreversibly adapted to microgravity after periods in orbit that exceed one year [Hansson, 1987, 1991]. With other experiments that compared animal physiology response in low Earth orbit (LEO) to geostationary Earth orbit (GEO) using Rhesus monkeys [Hansson, 1987, 1991, 1993] there were differences in adrenal cortex manufactured hormone effectiveness that were initially attributed to the absence of the Earth's magnetic field in configuring hormone receptor sites. This experience showed how much remains to be learned about the adaptability of the human physiology and chemistry to space. In fact one conclusion that might be drawn from the Russian data is that the human physiology is too adaptable. That is, the human physiology attempts to convert a gravity physiology into a micro-gravity physiology. There is a debate as to whether the gravity of the Moon is sufficient to induce a gravity physiology. Former astronaut Thomas Stafford thinks that it might be, but only time spent on the Moon will tell [Stafford, 1990]. If the Russian data on the essential presence of a low-level magnetic field is confirmed, then that will be an additional environmental requirement for long-term human space travel. Now the Unites States is just beginning to gather data on long-term orbital exposure with the International Space Station (ISS) in the Russian orbital plane of 55°.
As distances of missions from Earth increase, the propulsion challenge increases because the mission time increases. Missions need to be made within the possible lifetime of the project team, that is approximately 20 earth years. Earth years are specified because as the fraction of light speed increases, the time dilatation for the crew increases. That is a 20-Earth-year mission for the Earth-bound project team will not have the same time duration as 20 years for the space-based crew.
There are two classes of mission possible. The first is a one-way mission that explores a distant object and electronically communicates the information to Earth. Remember that if that is to a celestial object one light-year away, then communication will take a two-Earth-year round trip! The second is a two-way mission in which something is returned to Earth after exploring a distant object. This can deliver a greater trove of information than the one-way mission. However, a return mission is far more challenging. If the returning spacecraft travels at the speed of light, then the returning spacecraft will appear at Earth at the same time the light traveling from their destination shows them leaving!
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