69508
In the shorter term , solar thermal propulsion has been proposed both for longer-life , lower-cost , more efficient use of the sun and more-flexible cryogenic upper stage launch vehicles and for on-orbit propellant depots .
 sun
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: media, plant, vehicle, astronomical object, time, disease, animal, location, person, organization, mythical figure, food, biology, geographical phenomenon, instrument, event 
Text: {0} 
Answer:
 sun