In 2014, comet C/2013 A1, known as sliding spring, came within 140,000 Km of the planet Mars.  This is a bit more than a third of the distance from the Earth to the Moon.  Comets are small, so gravitationally this interaction was insignificant, but from an electromagnetic point of view, things were shaken up big time! Comets are small, relatively speaking.  A typical comet is a few kilometers across, about the size of a big city.  But with sunlight melting ices and liberating gases and dust from the comet’s interior, the part of the comet we see in the sky,...