CometISONcouldwellbethebrightestandmostspectacularcometforageneration.ItshouldappearabovetheeasternhorizonfromDecember2013asagloriousstreakacrossthesky.ISONhasbeentravellingtowardsthesunfortenthousandyearsandwillmakeonlyoneorbitthroughitscoronabeforedisap