Engineers are trying to determine whether critical bolts holding the shuttle Atlantis' KU-band antenna box in place are securely threaded, a potentially serious issue that could require tricky repairs before the ship's Aug. 27 launch, sources said Sunday.

File image of KU-band antenna stowed in Atlantis payload bay. Credit: NASA-KSC

A two-day flight readiness review to assess Atlantis' ground processing, to resolve open issues and to set an official launch date begins Tuesday at the Kennedy Space Center. The launch window for the 116th shuttle mission opens Aug. 27 at 4:30 p.m. and closes Sept. 7.

The KU-band antenna bolt issue will be discussed Monday, but it's not yet clear whether the issue can be resolved before the flight readiness review begins or whether additional work will be needed to determine what, if anything, needs to be done.

The issue involves four bolts that hold the antenna support box to the forward right side of Atlantis' cargo bay. The KU-band antenna is used to relay voice, video and data between the shuttle and NASA's fleet of communications satellites.

During an earlier launch campaign, engineers discovered problems with a certain type of bolt that in some cases were too short and not sufficiently threaded, or screwed in. Engineers began an assessment of similar bolts used elsewhere in the shuttle.

As it turns out, the same type bolts are used to secure the shuttle's KU-band antenna box to the payload bay wall. Engineers replaced the bolts in the shuttles Discovery and Endeavour but not in Atlantis.

File image of KU-band antenna deployed in space. Credit: NASA-JSC

The bolts have been in place since the antenna was installed aboard Atlantis and there has never been a problem. But it is known that some of the bolts could be engaged just a few threads worth because of a "tolerance stackup" in the way the bolts and washers are installed, according to engineers familiar with the matter.

Should the box break free during ascent, it would fall the length of the shuttle's 60-foot-long cargo bay and could cause catastrophic damage.

The bolts in question cannot be easily inspected at the launch pad. Kennedy Space Center engineers are studying paperwork and earlier analyses to determine whether the bolts are, in fact, sufficiently threaded. At the same time, sources said, they also are studying ways to replace the bolts at the pad, if necessary, a procedure that has never been done before.

Atlantis' launch window is defined by the international space station's orbit and by a post-Columbia requirement to launch two missions in daylight for photo documentation of the shuttle's heat shield and external tank foam insulation.

Because of unexpected foam shedding during the first post-Columbia mission last year, NASA extended the daylight requirement to the third flight in the sequence.

The shuttle Discovery successfully flew in July and a repeat performance by Atlantis would clear the way for the resumption of night launchings, greatly expanding available launch windows.

But if Atlantis isn't off the ground by the end of the September window, NASA would be faced with the prospect of just two launch days in October, none in November and just one in December. As such, agency engineers want to resolve the bolt issue as soon as possible.

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