Crawford reported in 2010 on those properties of the interstellar medium that are within 15 light-years of the Sun, which will be relevant for the planning of future rapid (v ≥ 0.1C) interstellar-space missions to the nearest stars. As the detailed properties of the local interstellar medium (LISM) may only be come apparent after interstellar probes have been able to make in situ measurements, the first such probes will have to be designed conservatively with respect to what can be learned about the LISM from the immediate environment of the Solar System.
It follows that studies of interstellar vehicles should assume the lowest plausible density when considering braking devices,which rely on transferring momentum from the vehicle to the surrounding medium, but the highest plausible densities when considering possible damage caused by the impact of the vehicle with interstellar material. Some suggestions for working values of these parameters are provided.This paper is a submission of the Project Icarus Study Group.
This review of the properties of the local interstellar medium has revealed several conclusions relevant to the design of quasi-relativistic (v≥0.1C) interstellar space vehicles, such as that considered in the Icarus study:
1. Many important properties of the LISM are still poorly defined, including the distribution of nearby clouds,their physical properties (especially density and ionization state), the size distribution of interstellar dust grains, and the nature of the ‘inter-cloud’ material, which will occupy a significant fraction of the path length towards many possible target stars.
2. Given these uncertainties, it is important that the first generation of interstellar space vehicles be designed conservatively with respect to key interstellar medium properties. That is, such studies should assume the lowest likely density if considering braking devices which rely on transferring momentum from the vehicle to the surrounding medium (such as magsails), but the highest plausible densities when considering possible damage caused by impact of the vehicle with interstellar material.
3. In order to maintain flexibility with regard to the choice of target star, it would be a mistake to assume that the target will necessarily lie within one of the relatively high density LISM clouds.
4. In situ spacecraft detections of interstellar dust entering the Solar System imply a local interstellar dust mass density of ~6.2 x 10-24 kg m-3. This is somewhat higher than previous estimates and will have to be taken into account by the dust protection system of any quasi-relativistic (v ≥0.1C) interstellar space vehicle. The requirements of the dust protection system become more demanding for longer journeys and, especially, for higher velocities.
5. Observations over the last decade (2005-2010) have revealed an unexpected high-mass tail to the local interstellar grain-size distribution. Individual particles with masses as high as 10-12 kg (4.5 um radius) are almost certainly present. Moreover, radar detections of interstellar dust particles entering the Earth’s atmosphere imply a population of interstellar grains with masses of >3 x 10-10 kg (corresponding to radii 430 mm).
While there are difficulties reconciling the presence of such large grains with other astronomical observations, a conservative planning assumption would be that particles as large as 100 um radius (10-8 kg), and perhaps larger, might be encountered in the course of a several light-year journey through the LISM. The kinetic energy of such large particles striking an interstellar space vehicle with a relative velocity of >0.1C are considerable (4.5 x 106 J), and some kind of active dust detection and mitigation system may need to be considered.
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