WASHINGTON — Launch of NASA’s two-satellite Solar Terrestrial Relations Observatory (Stereo) mission aboard a Delta 2 vehicle has been delayed another month to allow more time to verify the flight worthiness of the rocket’s second stage.

As it now stands, Stereo will not lift off from Florida’s Cape Canaveral Air Force Station before Oct. 18. The launch has been delayed several times since July, when concerns surfaced about an oxidizer tank inside the Delta 2 second stage. An identical tank, supplied by the Turin, Italy-based division of Alcatel Alenia Space, leaked during routine testing of a Delta 2 second stage being prepared for another NASA mission slated to launch late this year: the five-spacecraft Time History of Events and Macroscale Interactions during Substorms .

In early August, NASA removed the second stage from Stereo’s Delta 2 and took it to a nearby facility for leak testing. When no leak was detected, the hardware was returned to the launch pad and added back to the stack. Launch preparations resumed, but lingering concerns prompted NASA to postpone the launch again to no earlier than Sept. 18.

The latest Stereo delay, announced Sept. 1, will allow technicians to once again remove the Delta 2 second stage and inspect the oxidizer tank from the inside to verify its fitness for flight. NASA also plans to use the extra time to make sure that none of the rocket’s electrical systems were harmed by lightning strikes during Tropical Storm Ernesto, which passed through the area Aug. 30. Stereo , which rode out the storm at the Astrotech Space Operations Facility in Titusville, Fla., was not harmed.

Once launched, the twin Stereo spacecraft will spend five years capturing 3-D images of coronal mass ejections and other solar disturbances.