Astronomy Picture of the Day . ... MESSENGER s First Day . ... One solar day on a planet is the length of time from noon to noon. A solar day lasts 24 hours on planet Earth. On Mercury a solar day is about 176 Earth days long. The Comet Hartley 2 Cruise . ... Early last November, small but active Comet Hartley 2 (103/P Hartley) became the fifth comet imaged close-up by a spacecraft from planet Earth. ... Comet and CME on the Sun . ... Did a sun-diving comet just cause a solar explosion? ...