I suggest that the estimated cost of $1.5 trillion in the commentary by O. Glenn Smith and Paul D. Spudis for a nine-mission Mars program, while providing a useful reference point, is unrealistic.

The space shuttle program is ending after completion of the international space station. The United States will surely miss the capabilities of these unique vehicles and our superb astronauts to transport high value payloads to low Earth orbit (LEO) and deploy, service, and repair them. The shuttle has provided America with assured crew access to LEO for over 30 years. After it is gone, there will be no defined U.S. human spaceflight program for the first time since the announcement of project Mercury in 1958.