O meaning in Space Science ?

Omicron (uppercase Ο, lowercase ο, literally 'small o': όμικρον < ὂ μικρόν - ò mikrón, micron meaning 'small' in contrast to omega) is the 15th letter of the Greek alphabet. In the system of Greek numerals it has a value of 70. This letter is derived from the Phoenician letter ayin . In classical Greek, omicron represented the sound [o] in contrast to omega [ɔː] and ου [oː]. In modern Greek, omicron represents the mid back rounded vowel /o̞/ , the same sound as omega. Letters that arose from omicron include Roman O and Cyrillic O.