Spaceship Neptune will spend about two hours getting up to a maximum altitude of about 100,000 feet (30,000 meters), pulled gently upward by a 650-foot-tall (200 m) balloon filled with buoyant hydrogen."Helium has become quite difficult to obtain," Space Perspective founder and co-CEO Taber MacCallum said during a news conference today, explaining the choice of lift gas. "It's used for a lot of medical practices and for launching rockets."