The math may not add up to make AST SpaceMobile stock a long-term success.Space investors are in love with AST SpaceMobile (ASTS -1.60%) stock -- and for good reason.Three years ago, AST broke on the scene as a rather sketchy-looking special purpose acquisition company (SPAC) initial public offering (IPO). It promised to build "the first and only space-based cellular broadband network accessible directly by standard mobile phones," enticing investors with dreams of "five billion mobile subscribers" paying f ...