Shields for Icarus: Part 2 – Navigational Deflectors for Real
posted by Adam Crowl on October 25, 2010
One famous science-fictional nod to the hazards of high speed interstellar travel are the navigational deflectors used on the most famous line of SF starships of all, the “Enterprise” starships. All of them feature a glowing forward facing disk which, as made explicit in “The Next Generation” series, is a deflector shield against hazardous impacts when travelling through space-time at high sub-light speeds. Curiously the 0.2c quoted as the maximum impulse speed for the NCC1701-D model “Enterprise” is about right for “Icarus” too. Can “Icarus” be defended by something straight of “Star Trek”? A proposed means of decelerating from interstellar speeds is the magnetic-sail, which is a large loop of superconducting wire producing an artificial magnetosphere around the moving spacecraft. By deflecting interstellar ions, the magnetic field forms a semi-spherical zone forward of the vehicle where the magnetic pressure of the field and the pressure of colliding ions are evenly balanced. A magnetopause forms, in which ions are reversed in direction and their change in momentum produces an equal, but opposed, change in momentum in the magnetic-sail, and thus the spacecraft to which it is attached. Interestingly the Sun’s magnetosphere already acts like a deflector shield, forcing the ions and small charged particles of dust to flow around the Sun as it moves against the average flow of the Galaxy. Exposed to energetic photons (ultraviolet and x-ray) and high-energy ions (cosmic rays) the interstellar dust is charged. The very smallest dust particles, up to a certain diameter, are completely excluded from the inner Solar System by the Sun’s magnetosphere, while particles a bit larger are significantly deflected. Only the high-end of the dust size range is able to penetrate. In the case of a moving magnetic-sail, the atoms of the Interstellar Medium (about 90%-50% of the ISM) are actually ionized by its rapidly changing magnetic-field strength, in a process akin to that used to ionize gas in a Pulsed Inductive Thruster. If you imagine an atom drifting through space at typically 15 km/s, to then encounter a magnetic field approaching at 60,000 km/s is to experience a change in field sufficiently quick enough to ionize the atom. In effect the ship is creating a shock-wave in the ISM which is producing a lot of extra charge as atoms are ionized. All those suddenly energetic electrons could be sufficient to increase the charge on the ISM dust, thus increasing the deflector effect. The question needing investigation is whether this is sufficient to provide protection against all the ISM dust, or whether some additional defences will be needed. Cosmic sand-grains, with the kinetic energy of 100 pound bombs, while rare, will perhaps still need some means of interception by “Icarus”. The original “Daedalus” study proposed an artificial dust cloud moving 200 kilometres ahead of the main vehicle and this might prove sufficiently effective. Alternatively newer materials have become available which might provide multilayer protection – carbon allotropes, the most exciting of which is graphene. Graphene is basically a single sheet of graphite – a hexagonal grid form of carbon in the form of immensely strong sheets of covalently bonded carbon atoms, but held together between the sheets via via weak hydrogen bonds to make graphite. Peeling away single layers of graphene has now become possible and it has all sorts of surprising properties. What I’m interested in for shielding is making a large, low-mass “bumper” which cosmic sand-grains run into before hitting the craft. After passing through several layers of graphene the offending mass is totally ionized and forms a high-energy spray of particles, but particles that can now be deflected by the vehicle’s cosmic-ray defences (akin to the mag-sail, but smaller with a higher current) and safely diverted away from sensitive parts. To put the bumper in place, perhaps 100 kilometres ahead, it can be deployed via a small sub-vehicle – sheets made from carbon fibre are surprisingly springy and can self-unfold from a small volume. Once in place it might be kept in place by firing lasers at super-reflective patches on the bumper. Via reflecting ~2,000 times the laser achieves far more push than a single pulse of energy can achieve. Circuitry is being made from graphene in laboratories around the world, thus the bumper isn’t a passive mass. Multiple layers could work together to track any grains that pass through without being totally ionized. This causes a signal to be sent back to the vehicle which then activates its final layer of defence, high-powered lasers. In microseconds the lasers either utterly ionize the target or give it a sideways nudge via ablation – blowing it violently to the side via a blast of plasma. Such an active tracking bumper would need to be further away than 100 km to give the laser defence time to react, though 1/600th of a second can be a lot of computer cycles for a fast artificial intelligence. The lasers might use advanced metamaterials to focus the beam onto a speck at ~100 km, without needing to physically turn the laser itself in such a split-second. Highly directional, high-powered microwave phased arrays exist which already do so purely electronically and an optical phased-array isn’t a stretch beyond current technology. In conclusion, contrary to some sceptical voices, an interstellar probe doesn’t have to passively plow its way through a potentially deadly interstellar medium. A variety of magnetic, physical and optical defences are possible, even desirable, to protect a fast interstellar probe like “Icarus”. On our eventual ‘star treks’ we will have navigational deflectors to defend us.