The launch of the second NASA Mars Exploration Rover (MER-B), Opportunity, aboard a Boeing Delta II Heavy booster from Cape Canaveral has been pushed back one day to 26 June. This will enable engineers to complete pre-flight preparation delayed as a result of the two day delay to the launch of MER-A, Spirit, on 10 June.
The hybrid Delta II Heavy uses the larger solid rocket boosters which powered the fated Delta III booster, to provide the additional power required after the more optimal launch window of the Spirit. The mission has until 15 July to fly or it will miss the window.
Source: Flight Daily News