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The Kessler syndrome , proposed by NASA scientist Donald J. Kessler in 1978 , is a theoretical scenario in which the density of objects in low Earth orbit ( LEO ) is high enough that collisions between objects could cause a cascade effect where each collision generates space debris that increases the likelihood of further collisions .
 Kessler, NASA, Donald J. Kessler
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Kessler, NASA, Donald J. Kessler