 Flexibility will continue to be the key, not only to retention within the workforce, but really bringing that culture that everybody's striving for. Warren says offering a choice keeps employees engaged. Retention is key. Stephan Kaufman, CBS News. Four volunteers who spent more than a year inside NASA's first simulated Mars environment have emerged. They spent 378 days inside a 1,700-square-foot space at Johnson Space Center in Houston to simulate a mission to the Red Planet.