On: NASA says Webb’s excess fuel likely to extend its lifetime expectations

[slackfan]

Headline question - do they believe, or do they know? If the former - I worry for any space agency that can't do basic math.

[jffry → slackfan]

Article answer to your headline question:

  "It's around 20 years of propellant, roughly speaking,” though he added that it’s a preliminary estimate.

There are likely many factors that influence propellant usage which are unknowable - for example, photon pressure from the sun, which can vary over time in an unpredictable way.