Conclusions Can We Reach Stars

The focus of this chapter has been on giving a technology answer to a question going back to the first men gazing at the stars: What is there? Are there beings like us? Can we go there? To answer the last question, we enrolled the ultimate known power source, fusion. The calculations and analyses presented, however, leave the question still without a clear answer: within the constraints posed by the physics we know, even fusion propulsion is rather limited if stars are our destination. Stripping fusion rocket concepts of their exotic mystique leads to a rather disappointing future scenario: thrust may even be in the 105 N range and Isp in the 107 s, but at the price of strikingly large engine mass (hundreds or thousands of tons). These Isp are infinitely better than those of chemical propulsion, but still way lower than needed to carry humans on interstellar travel and exploration within reasonable human timescales. The first fundamental limitation in traveling over quasi-interstellar and stellar distances is that mass conversion into energy by fusion is about a factor 5 larger than in fission, but still limited to fractions of a percent. Fusion propulsion will make traveling beyond our Solar System practicable, but only to destinations much closer than the nearest star: even the Oort Cloud is too far away to be explored by a manned vehicle. The mass of a ship bound for Proxima Centauri and still meeting the constraints posed by our physics would be so large, and the time to cross the gulf in between would be so long, as to effectively make manned trips in practice unfeasible, although not physically impossible.

Only matter annihilation can lower mass consumption significantly, and enable practical travel of robotic spacecraft and (perhaps) some crewed ships. Matterantimatter "fusion" is still at the conceptual level, and was not analyzed in this chapter (its energy is released essentially in the form of radiation not easily made into thrust). Harnessing antimatter is the last hope for practical interstellar travel: the scientific and engineering challenges are formidable, but the performance could also be so outstanding to enable travel speed close to light speed.

At these speeds there is a second fundamental limitation. Physics itself rules out, for the time being, any process in which matter could be accelerated beyond the speed of light. It is very difficult, except in science fiction novels, to envisage a ship where the crew lives and works without external support for many years or even a decade, knowing that any form of communication would take years to send and receive, and that (if everything turns out well, and if the "twin paradox" holds) when they go back they would find a different Earth and all their friends, family and colleagues already dead.

Robotic interstellar trips are easier to conceive: either by fusion, or by building matter-antimatter propelled robotic spacecraft, radiation and shielding would be less critical problems, and acceleration could be much higher than the 1 g human beings can sustain. If the time paradoxes due to relativity still apply, their impact on the will and resources to invest in such trips would be critical. Short of breakthroughs in physics enabling the control of inertia, interstellar missions, whether manned or unmanned, will be realized only when trip times of the order of many decades become not only feasible, but also accepted by the public. Nevertheless, there are indeed space exploration visions based on robots capable of independent operation, from orbit capturing around a planet to descent and to roaming on the planet surface. For instance, Dr. W. Fink at CalTech [Hsu, 2008] is developing robotics incorporating decision-making software based on sensor integration. The same CalTech research group has in fact proposed testing such technology on a future Europa or Titan mission.

In the same skeptical spirit, it is doubtful that efficient unmanned exploration can be carried on as done so far for Mars: telecommunication times will be too long to respond to specific situations. Any robotic "crew" that can be designed to carry on stellar or quasi-stellar exploration will have to be endowed with such sophisticated artificial intelligence the likes of which we cannot even imagine at present.

However, these rather sobering or pessimistic conclusions may be the ultimate key to stellar travel. Perhaps, if no breakthroughs in physics ever occur, at a certain point in its history humankind will accept that stars cannot be ''visited'' but only reached, that is, once in a lifetime. That means that, as happened on Earth in the past, some humans will choose to leave Earth for good. When this happens, fusion will then be the means of propulsion.

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