NASA Photo ID: STS056-21-001      File Name: 10073219.gif 
 
Film Type: 35mm CN                Date Taken: 04/17/93 
 
Title: STS-56 Earth observation of a sunset in the southern hemisphere 
 
Description: 
 
STS-56 Earth observation shows a sunset in the southern hemisphere, as seen  
from east to west by the astronaut crewmembers aboard the Earth-orbiting  
Discovery, Orbiter Vehicle (OV) 103. Classic color changes from reds to 
blues  are caused by white (composite) light passing through density and 
atmospheric  particulate boundaries.  Colors are refracted preferentially 
according to the  diameter of the particles in the visible spectrum with 
longer wavelengths  (reds) refracted the most.  The phenomena can be seen 
here as the reds (0.7  micron) are bent the most while blues (0.4 micron) 
are bent the least.  The  dark substance suspended in the upper 
stratosphere in the left portion of  this scene is of unknown origin.  NASA 
scientists studying the STS-56  photography feel that the most likely 
sources for these substances are  volcanic ash or sulfur dioxide (SO2) 
ejected into the stratosphere by  volcanoes, such as the recent eruptions 
of Mt. Pinatubo (Philippines), Mt  Spurr (Alaska), and Mt. Hudson (Chile).