The image is taken from the film 2001: A Space Odyssey and might be seen as a 1968 prediction of future space exploration based on the screenplay written by Stanley Kubrick and Arthur C. Clarke. Given that the first man-made object reached the surface of the Moon in 1959, which was then followed by the first manned mission to land on the Moon in 1969, the film was not necessarily an unreasonable prediction of space exploration set some 33 years into the future. However, in 2018, we possibly need to table a retrospective question:
Why has this future not already been realised?
Again, initial references will be made to earlier discussions, i.e. Space: An Overview, which might provide some general indication of the scale of our local solar system and its position within the Milky Way galaxy, along with an array of possible and speculative propulsion systems. However, in the expanding scope of the human ecosystem now under discussion, we might want to consider the inclusion of all space in the universe and, in so doing, all galaxies and planetary systems beyond our local solar system. Of course, while science fiction has little problem with travelling the vast distances between the stars, the science of reality has found this to be exceedingly difficult, if not impossible, especially if we have to accept the speed of light as an absolute upper limit. For the two nearest stars are Alpha Centauri A and B, which form a binary pair system approximately 4.3 lightyears from Earth, where one lightyear equates to 9,461 billion kilometres.
Note: In terms of a velocity relative to the Sun, NASA’s unmanned
If we discount faster-than-light technology for the moment and restrict sustained acceleration to 9.81 m/s2 [1g], it would take 353.7 days to approach lightspeed, ignoring all relativistic effects on the crew for simplicity. In this time, it would have travelled 4,580 billion kilometres or approximately 0.5 lightyears of the 4.3 lightyear journey to Alpha Centauri. As such, we might estimate the fastest one-way travel time to be 5.3 years, i.e. 1-year acceleration to lightspeed plus 3.3 years to Alpha Centauri plus 1-year deceleration, again, ignoring time dilation effects on the crew. However, this optimistic estimate ignores all the known problems associated with a relatively large spacecraft colliding with interstellar particles at relativistic velocities - see Limits and Signatures of Relativistic Spaceflight for details. Therefore, should 10% of lightspeed be a more practical limit to manned spacecraft in the foreseeable future, the journey time to Alpha Centauri might be closer to 40 years with little assistance from time dilation to stop the aging of its human crew.
OK, but surely space exploration within the local solar system is a much more realistic goal?
While this is undoubtedly true, there are still significant technology barriers to large-scale human space exploration, which need to be taken into account along with an understanding of the needs and limitation of the human physiology. In addition, there is a need to assess cost versus benefits of space exploration, which might help explain why the last manned mission to the Moon was in 1972, i.e. over 45 years ago. At a relatively close distance of 384,400 kilometres, the journey time to the Moon took 3 days, where the astronauts only spent 22 hours on the surface before making the return journey back to Earth, i.e. a one week round-trip.
Note: The journey time from Earth to Mars might be in the order of 250 days based on the assumption that Mars is at its closest point of 78 million kilometres from the Earth, which occurs every 2 years. However, the travel time depends not only on the positions of the planets, but the trajectory taken and how much fuel is burnt on route – see Path to Mars for details. If we assume the mission remains in orbit or on the surface for 2 months, we might estimate the mission time to be about 2.5 years, i.e. 910 days rather than just 7 days.
While all the technical details are not central to this discussion, or specific to Mars, it is an example that might help characterise the problems of human exploration of space in 3 distinct areas,
- lift-off payload systems
- space propulsion systems
- onboard life support systems.
While aspects of these problem areas will be expanded in subsequent discussions, we might initially outline the scope of each. Clearly, any mission to another planet, even a relatively close one, will need to lift a lot of heavy equipment into space, which will require multiple launch vehicles. By way of a comparison, the International Space Station (ISS) has a mass of about 400,000kg, which might be similar in size to that required for any human Mars mission. In the case of the ISS, it took over 16 launches to lift all the equipment into space, but possibly over a hundred launches for all the subsequent assembly and logistics flights to complete the ISS over the course of 15 years. The ISS was initially estimated to cost $10 billion over 10 years but ended up costing 10 times that amount, i.e. $100 billion, spread over nearly three decades.
Note: While we might extend the payload lift capacity for present-day heavy-lift launch vehicles to 150,000kg, it would still require multiple launches at a possible cost of $150 million per launch plus some additional number to complete the assembly of the Mars mission craft in space. NASA have estimated the overall cost of a manned mission to Mars at about $100 billion over a possible 30-year timeframe, although past history might suggest that costs would only escalate over time.
While there are many different types of rocket engines, i.e. those needed to get into space and then move around in space, they are all essentially chemical engines that need to carry the fuel onboard. However, there is a possible distinction between lift-off payload engines and in-space propulsion thrusters, where the former need to burn up a huge amount of fuel to get the required payload into space. However, while present-day lift-off payload engines are very expensive and generally non-reusable, a long in-space journey to Mars presents another problem as chemical engines are ‘mass-limited’ in terms of how much fuel can be carried onboard. While a new generation of in-space ion thrusters work differently by using a much smaller amount of gas, which is then accelerated to very high speeds, they have very low acceleration and can be ‘energy-limited’ rather than ‘mass-limited’ in that the ion engine needs a lot of electrical energy to power the ion engine.
Note: Although ion engines may deliver about ten times as much thrust per kilogram of fuel as a chemical engine, they are very low-thrust devices, such that it cannot be used as a launch engine. Equally, while ion engines can sustain thrust for much longer periods of time, the low-thrust implies a slow rate of acceleration. The current assessment of ion engines suggests that they are probably an impractical form of propulsion for heavy payloads, e.g. manned missions to Mars, for the foreseeable future.
However, any manned mission to another planet would also involve the complexity of the life support systems required to sustain the crew for 2-3 years, which even in the case of the initial Mars missions may still involve a crew of 6. For these systems will, at minimum, need to provide air, food, water and waste disposal, which may amount to 100,000 kilograms of mass and, as indicated, add considerably to the complexity and cost of the mission. The complexity of maintaining this life support system might be characterised in terms of the Biosphere projects, which were originally intended to demonstrate the viability of maintaining a closed ecological system, on Earth, which might then be partially replicated to support and maintain human life in outer space and possibly help support longer-term colonies on other planets, e.g. Mars. While recognising that the scope of the Biosphere projects was wider than the requirements for a 2-3 year Mars mission, the scale and nature of the various failures associated with the biosphere project must be an issue of concern. For even a minimal 2-year Mars mission would not have the option to simply open the door and step back into the life support system of planet Earth, which was one eventually taken by the occupants of the biosphere project when things went wrong. Of course, the inhabitants of the biosphere also had one other distinct advantage, i.e. they were still living in Earth’s gravity.
Note: To-date, the longest amount of time spent in space is 437 days, as such we do not really know how long humans can live, healthily, in space. However, the crews of the ISS are providing important and necessary data, which might suggest that zero-gravity space may not necessarily be a healthy long-term environment for humans. For humanity did not evolve to live in space and many of our biological systems only function properly under Earth’s gravity. For example, muscles are constantly exercised by gravity and begin to atrophy in zero-gravity, which includes the muscles of the heart and in the neck that help support the weight of the head. This health issue is but one of many that may question the immediate viability of long-term manned missions in space, but not necessarily its longer-term probability.
In some respects, there may be an argument that humanity should initially restrict its space exploration to the Moon, where the required technology can be further developed and tested. For unlike Mars, any missions to the Moon still retains an ‘escape plan’ in the form of a return to planet Earth in 3-days, rather than 1-2 years, should anything go seriously wrong with the life support systems or the health of the human crew. However, this position does not imply that wider exploration of the solar system has to be halted, simply that it might be carried out more effectively by increasingly AI autonomous systems , which do not require expensive and complex life support systems. Therefore, in the remainder of this section of discussions concerning space developments, some attempt will be made to expand on the issues outlined in a little more detail, because space exploration will probably come to play an important part in the plurality of ‘brave new worlds’ of the future.