accelerators or particle-beam based weaponry, microwaves and plasma-based weaponry. Particle beams involve the acceleration of charged or neutral particles in a stream towards a target at extremely high velocities, the impact of which creates a reaction causing immense damage. Most of these weapons are theoretical or impractical to implement currently, aside from lasers which are starting to be used in terrestrial warfare. That said, directed-energy weapons are more practical and more effective in a vacuum (i.e. space) than in the Earth's atmosphere, as in the atmosphere the particles of air interfere with and disperse the directed energy. Nazi Germany had a project for such a weapon, considered a wunderwaffe, the sun gun, which would have been an orbital concave mirror able to concentrate the sun's energy on a ground target. Practical considerations Space warfare is likely to be done at far bigger distances and speeds than combat on Earth. The vast distances pose big challenges for targeting and tracking, as even light requires a few seconds to cover hundreds of thousands of kilometers. For example, if trying to fire on a target at the distance of the Moon from the Earth, one sees the position of the target slightly more than a second earlier. Thus even a laser would need ~1.28 seconds, meaning a laser-based weapon system would need to lead a target's apparent position by 1.28×2 = 2.56 seconds. A projectile from a railgun recently tested by the US Navy would take over 18 hours to cross that distance, if it travels in a straight line at a constant velocity of 5.8 km/s along its entire trajectory. Three factors make engaging targets in space very difficult. First, the vast distances mean that an error of even a fraction of a degree in the firing solution can mean a miss by thousands of kilometers. Second, spaceflight involves tremendous speeds by terrestrial standards—a geostationary satellite moves at 3.07 km/s, and objects in low Earth orbit move at ~8 km/s. Third, though distances are huge, targets remain relatively small. The International Space Station, currently the largest artificial object in Earth orbit, measures slightly over 100m at its largest span. Other satellites can be vastly smaller, e.g. Quickbird measures only 3.04m. External ballistics for stationary terrestrial targets is enormously complicated—some of the earliest analog computers were used to calculate firing solutions for naval artillery, as the problems were already beyond manual solutions in