Shields for Icarus Part1: The Impact Hazard
by Adam Crowl
Space is really, really BIG and immense speeds are necessary for space vehicles to cross the gulfs between the stars within a reasonable time-frame. At such high speeds (in the case of “Icarus” somewhere between 30,000 to 60,000 km/s) a collision with a sand-grain massing just a milligram would release 450 megajoules of kinetic energy, which is equivalent to about 100 kilograms of TNT. Such a collision hazard seems insuperable if we focus on the kinetic energy of sand grains, but just how many are out there? More specifically just how many collisions does a space-vehicle need protection from during a typical mission? Fortunately space dust can be observed and measured in bulk because it gets in the way of starlight and we’re learning more about its actual nature via various probes encountering it as the stuff enters the solar system. From studies of how the Interstellar Medium (ISM) absorbs starlight we know that, on average, 0.03 solar masses per cubic parsec of gas and dust fills the void between the stars. The ISM is composed of 99% Hydrogen/Helium and 1% other elements, and of the latter ~50%-10% has formed into dust specks with a size range typically between 100 to 10 nanometres across. The relative numbers increase with the decreasing size of the dust specks. For the purposes of a conservative estimate I am assuming the numbers scale with the mass of a dust particle. Thus dust specks 10 times bigger, and thus 1,000 times heavier, are 1,000 times rarer than other smaller specks. The actual observed size/number distribution results in a somewhat lower frequency of smaller dust specks, so I am over-estimating their numbers. Let’s put some figures to those dust speck numbers. A cubic parsec is about 28E+48 cubic metres and thus about 6E+43 to 6E+46 typical dust specks occupy that volume. Each individual speck thus sits in a volume of between ~450 to 450,000 cubic metres of empty space. Such typical specks mass between ~1E-18 to 1E-21 kilograms meaning when encountered by a probe doing 0.1c (30,000 km/s) the kinetic energy released in collision is ~0.00045 to 4.5E-7 Joules. Though a tiny amount of energy it’s also concentrated into a tiny area (1E-14 to 1E-16 square metres respectively) and thus represent a huge intensity, forming an intense pinpoint burst of hot plasma, but with a small total energy. Robert Freitas has proposed star-probes need to be needle-shaped, perhaps 1 cm wide, to avoid the collision hazard. We can compute that with a circular cross-section 1 cm across a needle probe encounters a speck of dust roughly once every ~0.19 to 190 seconds at 0.1c (remember numbers depend on size, thus the range in times.) Based on the ISM mass density, those dreaded 1 milligram space sand-grains are – at the very worst – encountered roughly once every ~600 kilo-lightyears by a 1 cm wide space-probe. And that’s assuming ALL the dust is in the form of 1 milligram space sand-grains, which we know it isn’t, because such dust would reflect significant amounts of starlight, something unobserved in interstellar space. Sand-grains are seen in great numbers around newly formed stars, but not between them. Of course an interstellar probe with a cross section of 2,000 square metres (e.g.“Daedalus”) is a MUCH bigger target. It would encounter a space sand-grain, in the worst case scenario sketched here, once every 1,500 AU. Fortunately that gives it plenty of distance to deploy counter-measures in. Just what sort of counter-measures is the subject of Part 2.