JOHNSON SPACE CENTER, Houston (AP) -- Shuttle astronauts on board the international space station are preparing for another excursion outside today.

This time they'll be testing a caulking gun and goo on some extra thermal protection tiles that have been deliberately damaged. NASA wants to see how well the patching material works in case it's ever needed for a real repair on the tiles, which are vital to the shuttle's fiery re-entry.

The potential fix was developed after the 2003 Columbia disaster, when the shuttle was destroyed during re-entry because of a hole in the wing. NASA wants the test results back before the Atlantis repair mission to the Hubble Telescope in August. If the shuttle were to be damaged on takeoff, it would not be able to use the space station as a repair refuge because the two craft won't be in the same orbit.

This fourth spacewalk comes just past the midway mark in Endeavour's 16-day mission.

(Copyright 2008 by The Associated Press. All Rights Reserved.)