WASHINGTON — Lingering doubts about the flightworthiness of the Delta 2 rocket’s Italian-built oxidizer tanks have prompted NASA to postpone the launch of a pair of sun-observing satellites at least several more weeks.

An upcoming Global Positioning System satellite launch for the U.S. Air Force, however, is in the clear because the Delta 2 built for that mission does not include one of the suspect tanks.

NASA’s two Solar Terrestrial Relations Observatory (STEREO) spacecraft were slated for an Aug. 31 liftoff from Cape Canaveral Air Station, Fla. NASA officials, however, have delayed the launch to allow Boeing Launch Services more time to determine if the Delta 2 assigned to the STEREO mission has the same second-stage tank defect that has delayed the launch of NASA’s five-probe Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission from October into December.

In early July, while building the Delta 2 rocket NASA ordered for the THEMIS mission, Boeing engineers at the firm’s Decatur, Ala., rocket-manufacturing facility discovered during routine testing that the rocket’s second stage leaked. The problem was traced back to an oxidizer tank Boeing buys from Alcatel Alenia Space in Turin, Italy.

A portion of that tank has since been cut out for material thickness testing to determine why it leaked, according to Doug Shores, a Boeing spokesman in Decatur. Shores said Boeing was still working with Alcatel Alenia Space and NASA to identify the root cause of the THEMIS tank’s failure and determine any corrective actions that need to be taken.

Shores said Boeing has a plan in place to replace the THEMIS tank and launch the mission later this year. But the company’s “first priority,” he said, “is to ensure that the STEREO tank is flight worthy.”

NASA and Boeing have been working for more than a month to do just that.

In late July, NASA announced that it was slipping STEREO’s launch 11 days to Aug. 31 in order to de-stack the Delta 2’s second stage and take it to a nearby facility for leak testing. When no leak was detected, the second stage was returned to the launch pad and added back to the stack.

For most of August, preparations continued for an end of the month launch. At that point the biggest threat to the schedule was a potential range conflict with the Space Shuttle Atlantis’ planned Aug. 27 launch. However, 10 days before STEREO’s was slated to lift off, NASA announced that it was postponing the launch again — this time until no earlier than Sept. 18 — to permit additional evaluation of the tank issue.

Jim Adams, the STEREO deputy project manager at NASA’s Goddard Space Flight Center in Greenbelt, Md., said Aug. 28 that it was decided that the null result from the leak test conducted earlier in the month was no guarantee that that tank “does not have the same problem” as the THEMIS tank.

Adams said the failure investigation into the THEMIS tank leak uncovered “process problems in the way the tanks are made” and that Boeing and Alcatel Alenia Space have so far been unable to “totally exonerate the STEREO tank.”

Shores would not say whether manufacturing mistakes were to blame for the THEMIS tank failure. He said the investigation has not concluded and that it would be “too early to speculate” on exactly why the tank leaked and what corrective actions might be necessary.

Nicolas Brun, director of communications at Alcatel Alenia Space, said the Delta 2 tanks delivered to Boeing are a Boeing design that met all Boeing specifications. Brun said in an Aug. 31 interview that Boeing and Alcatel Alenia continue to work together to determine the cause of the problem. Brun said Alcatel Alenia is unaware of any manufacturing defect that could have caused a leak, but that the investigation into the issue is continuing.

Boeing has been buying second-stage tanks for the Delta 2 from Alcatel Alenia Space since 2002 and Shores said the tanks have performed well, flying on five successful missions to date, most recently the April 28 launch of NASA’s CloudSat and the French-U.S. Calipso satellite.

Prior to transferring the second-stage tank production to Alca tel Alenia Space, Boeing built the tanks itself in Huntington Beach, Calif.

Shores said the Delta 2 rockets ordered by the U.S. Air Force for GPS satellites launches were built before second-stage tank production shifted to Italy and so are not being scrutinized for the problem found on the THEMIS rocket.

The next GPS launch is slated for Sept. 21, just three days after STEREO’s next 14-day launch window opens.

If STEREO cannot be cleared in time to make that window, Adams said the next two-week opportunity arrives in mid-October. He said the delays pose no threat to the mission’s science objectives.

“We can do the mission just about any time we get off the ground,” Adams said in an Aug. 28 interview. The only restriction, he said, is launching STEREO in phase with the Moon in order to take advantage of the lunar swingby each spacecraft needs to perform to get into it proper heliocentric orbit. Each month offers a 14-day window with two daily launch opportunities: a brief two-minute window followed an hour later by a 15-minute window.

With aid of the lunar swingbys, one STEREO probe will be placed ahead of the Earth in its orbit and the other will travel behind. The twin observatories will work together to capture three-dimensional images of coronal mass ejections and other solar events.