The xenon ions are what is being expelled at 40 kilometers per second.
Brandon Weigel
11

The mass of 1.15E27 xenon atoms is 250Kg.

Xenon is no magical substance. To push 500 Kg at 20Km/s velocity you need to expel 250 Kg of something at 40Km/s. The mass of single xenon ion doesn’t matter here.

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