Starflight Review

In a recent article, Ian Crawford briefly reviews the technical feasibility of starflight.  The main limit is economic: an ability to collect 50,000 tons of (deuterium/helium-3) nuclear fuel (the same weight as today’s total annual uranium mined, but vastly harder to collect) and launch that into space seems nearly sufficient. But of course with advanced robotics, nanotech, etc., that much may not be necessary:

The most technically mature concepts for achieving rapid interstellar travel are those based on nuclear fusion propulsion, of which the Daedalus study (Bond et al., 1978) is still the most detailed engineering assessment available in the literature. Daedalus was designed to accelerate a 450-tonne scientific payload to 12% of the speed of light. … This would permit a travel time of 36 years to the nearest star, although the resulting vehicle would be very massive (requiring approximately 50,000 tonnes of nuclear fuel) and far beyond present capabilities to construct. …

In the decades following the original Daedalus study, technical advances in a number of fields have occurred which may make fusion-powered vehicles of the Daedalus type more practical. … Developments in miniaturization … would ensure that a much less massive payload would be required … The National Ignition Facility … is, albeit unintentionally, building up technical competencies directly relevant to the development of fusion-based space propulsion systems. …

Impacts with interstellar grains will be potentially damaging for space vehicles. … However, the problem is not as severe as [many fear]. … The size of typical interstellar grains … in the solar neighborhood are expected mostly to be submicron in size. … The mass of a 1 um [= 10^-6 m] radius grain of silicate composition is 10^-14 kg, and its kinetic energy at 0.1c is 4.5 J. …

Until recently, it would have been expected that 1 um would be an absolute upper limit for the size of interstellar grains in the warm (T * 6000 K), low-density (n * 0.1–0.2 hydrogen nuclei cm^-3) interstellar cloud [LIC] which surrounds the Sun. However, recent … measurements … have identified a high-mass tail to the local interstellar grain population extending to perhaps as high as 10^-12 kg (i.e., 4.5 um radius). …

Martin (1978) … adopted beryllium as a potential shielding material. … Adopting an interstellar dust density of 6.2 x10^-24 kg m^-3 (i.e., that determined by Landgraf et al., 2000), we find that erosion by interstellar dust at a velocity of 0.1c would be expected to erode of the order of 5 kg m^-2 of shielding material over a 6-light-year flight. The need to provide such shielding will certainly add to the mass of an interstellar probe, but it hardly seems to be the showstopper. …

Interstellar particles even larger than the largest inferred from the spacecraft dust detectors (of the order of 40 um in size) may have been detected by meteor radar observations, and the possibility of colliding with even larger particles over the course of a voyage of several light-years cannot be discounted. Indeed … crudely extrapolating to higher masses, we would infer the spatial density of 100 um grains to be about 4 x10^-17 m^-3. Thus, over the 6 light-year flight … we might expect of the order of two impacts per square meter with such large particles. …

It is … certainly possible to envisage appropriate counter-measures. … By far the simplest solution, developed by Bond (1978) for the Daedalus study, would be for the spacecraft to be preceded by a fine cloud of small dust particles (ejected from the vehicle and thus traveling at the same velocity but a small distance ahead), such that any incoming large grains would be destroyed by collisions within this artificial dust cloud before they have a chance to reach the main vehicle. …

Journeys to the nearer stars with travel times of decades (necessitating velocities of the order of ten percent of the speed of light) will be a considerable technological (as well as economic and political) undertaking. The magnitude of the difficulties should not be underestimated, but neither should they be exaggerated. There is a large technical literature which demonstrates that rapid interstellar space travel is not physically impossible and is a legitimate technological goal for the centuries ahead. (more)

Of course if you want to slow down your ship at the other end instead of flying by at 0.1c, you need a much larger ship, with roughly 10 million tons of fuel.

Added 11a: Apparently, many of you need to review the rocket equation.

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