Stockholders’ Equity and Stock-based Compensation - Common Stock Outstanding (Details) - shares	Aug. 13, 2021	Sep. 30, 2021	Dec. 31, 2020
Class of Stock [Line Items]
Common stock outstanding (in shares)	79,772,262	80,580,232	62,510,690
Percentage of common stock outstanding	100.00%
Momentus Space, LLC Unit Holders
Class of Stock [Line Items]
Common stock outstanding (in shares)	50,419,505
Percentage of common stock outstanding	63.00%
Public Stockholders
Class of Stock [Line Items]
Common stock outstanding (in shares)	13,695,257
Percentage of common stock outstanding	17.00%
Stable Road Acquisition Corporation And Affiliates
Class of Stock [Line Items]
Common stock outstanding (in shares)	4,657,500
Percentage of common stock outstanding	6.00%
Private Investment in Public Equity Investors
Class of Stock [Line Items]
Common stock outstanding (in shares)	11,000,000
Percentage of common stock outstanding	14.00%