Ed Campion Headquarters, Washington, D.C.                             April 23, 1993 (Phone:  202/358-1780)  Kyle Herring Johnson Space Center, Houston (Phone:  713/483-5111)  RELEASE:  93-76  HUBBLE TELESCOPE SERVICING MISSION SCHEDULED FOR ELEVEN DAYS  	The December flight of Endeavour on Space Shuttle mission STS-61 to service the Hubble Space Telescope (HST) has been scheduled as an 11 day mission designed to accommodate a record five spacewalks with the capability for an additional two, if needed.  	The decision to schedule five extravehicular activities, or EVAs, was reached following extensive evaluations of underwater training, maneuver times required using the Shuttle's robot arm based on software simulations and actual EVA tasks on previous missions.  	"Basically what we've done by going to five EVAs rather than three is to repackage our margin so that we have the capability to respond to the dynamics, or unknowns, of spacewalks," Mission Director Randy Brinkley said. "It improves the probabilities for mission success while providing added flexibility and adaptability for reacting to real-time situations."  	In laying out the specific tasks to be completed on each of the spacewalks, officials have determined that changing out the gyros, solar arrays and the Wide Field/Planetary Camera (WF/PC) and installing the Corrective Optics Space Telescope Axial Replacement (COSTAR) are priority objectives during the mission.  	"When we looked at accomplishing all of the tasks, highest through lowest priority, and recognizing that the major tasks -- gyros, solar arrays, WF/PC and COSTAR -- would consume most of the time set aside for each spacewalk, five EVAs were deemed appropriate," said Milt Heflin, Lead Flight Director for the mission.  	While the five spacewalks will be unprecedented, the use of two alternating spacewalk teams will alleviate placing more stress on the crew than previous missions requiring two, three or four EVAs.  	"We have paid close attention to lessons learned during previous spacewalks and factored these into our timeline estimates for five EVAs," Heflin said.  "In planning for all Space Shuttle missions, it is necessary to formulate a work schedule that represents as realistic a timeline as possible to accomplish the mission objectives."  	Planning currently calls for at least five water tank training sessions that include support from the Mission Control Center, called joint integrated simulations, lasting between 10 and 36 hours.  In addition, many stand alone underwater training "runs" will practice individual tasks in each spacewalk.  	Various refinements to the specific tasks on each spacewalk will be made based on actual training experience during the months prior to the mission.  Also, lessons learned from other spacewalks leading up to the flight will be valuable in assisting the STS-61 crew in its training techniques.  	Endeavour's June flight and Discovery's July mission both will include spacewalks to evaluate some of the unique tools to be used on the HST mission. The evaluations will help in better understanding the differences between the actual weightlessness of space and the ground training in the water tanks at the Johnson Space Center, Houston, and the Marshall Space Flight Center, Huntsville, Ala.  	Also, the inflight spacewalking experiences will assist in gaining further insight into the time required for the various tasks and expand the experience levels among the astronaut corps, the flight controllers and trainers.  	Designed to be serviced by a Space Shuttle crew, Hubble was built with grapple fixtures and handholds to assist in the capture and repair procedures.  	The telescope was launched aboard Discovery in April 1990.  At that time the NASA mixed fleet manifest showed the first revisit mission to HST in 1993 to change out science instruments and make any repairs that may have become necessary.