Noctilucent Clouds
For those who fear that there may be nothing left in the world to discover (hardly a chance!), consider this item from Space Weather News for April 25, 2007:
NIGHT-SHINING CLOUDS: NASA's AIM spacecraft left Earth Wednesday on a two-year mission to study mysterious noctilucent (night-shining) clouds. Hovering at the edge of space, these clouds were first noticed in the 19th century; they are remarkable for their electric-blue color and sharp, wavy ripples. In recent years noctilucent clouds have been growing brighter and spreading. What causes them? Theories range from space dust to global warming. For the next two years, AIM will scrutinize the clouds from Earth orbit to learn what they may be telling us about our planet. Visit http://spaceweather.com for more information about the AIM mission, pictures of noctilucent clouds and observing tips.