A letter from the White House to the Senate Appropriations Committee has estimated that a single Space Launch System would cost over two billion dollars. Throughout the almost ten-year long development process, a cost estimate had yet to be released. The first launch could happen in 2021, with NASA not refuting the cost valuation. This estimate includes both the fixed and marginal cost of the rocket, including maintenance and employees. A second rocket would cost significantly less; however, the two billion dollar estimate did not include other costs, such as the development and addition of the Exploration Upper Stage. This means the ballpark estimate is five billion per flight.