Paula Cleggett-Haleim
Headquarters, Washington, D.C       April 19, 1991
(Phone:  202/453-1547)

Frank O'Donnell
Jet Propulsion Laboratory, Pasadena, Calif.
(Phone:  818/354-5011)


RELEASE:  91-59

GALILEO ANTENNA DEPLOYMENT STUDIED BY NASA

	Intensive analysis of the problem that prevented 
deployment of the Galileo spacecraft's high-gain antenna is 
continuing at NASA's Jet Propulsion Laboratory (JPL), 
Pasadena, Calif.  A "tiger team" of specialists from a 
variety of engineering disciplines -- including industry 
representatives -- has been assembled to study the problem 
and how to correct it.
 
	Galileo Project officials say they expect to carry out 
considerably more analysis and ground tests before 
determining a date to make another deployment effort.  The 
deployment difficulty poses no immediate problems for the 
spacecraft, which otherwise is functioning properly.
 
	The problem arose Thursday, April 11, 1991, when 
commands to unfurl the umbrella-like antenna were issued by 
Galileo's computers.  The deployment action -- very similar 
to opening a conventional umbrella -- was expected to be 
concluded in less than 3 minutes.
 
	Data from Galileo, however, indicate that the antenna 
unfurled partially but did not completely unfold.  One side 
of the antenna appears to be deployed more fully than the 
other side, suggesting that some restriction may be 
affecting a portion of the antenna's movement.

                       -more -


 	Data that the JPL team has been studying include 
readings from the spacecraft's sun sensor and from its spin 
detectors, which offer engineers information on the current 
state of the antenna.  In addition, data from Galileo's 
power system provide details on how the deployment attempt 
proceeded and possible clues on the nature of the 
restriction.  Engineers say that continued analysis of the 
data -- and tests of identical antenna equipment on the 
ground -- are important to avoid any action that could 
damage onboard equipment.
 
	The 16-foot-diameter, high-gain antenna -- a modified 
version of the design used in NASA's Earth-orbiting 
Tracking & Data Relay Satellites -- has a surface made of 
gold-plated molybdenum wire woven into a mesh.  The mesh is 
stretched across 18 graphite-epoxy ribs and connected with 
quartz cords.
 
	The antenna has been stowed behind a sun shield since 
Galileo's launch in October 1989 to avoid heat damage while 
the spacecraft flew closer to the sun than the orbit of 
Earth.
 
	The antenna deployment mechanism is driven by a set of 
redundant motors which turn a worm gear.  This gear pushes 
a nut connected to levers which spread the antenna's ribs, 
much as an umbrella is opened.
 
	Unfurling of the antenna will enable Galileo to send 
scientific data to Earth at much higher rates over greater 
distances than it can with the two low-gain antennas it has 
used since launch.  
 
	Project officials say Galileo will still conduct its 
planned flyby of the asteroid Gaspra on October 29 even if 
the antenna is partially deployed.  In that event, pictures 
and other data would be stored on the spacecraft's onboard 
tape recorder and relayed to the ground when Galileo 
approaches for its flyby of Earth in December 1992.
 
	JPL manages the Galileo Project for NASA's Office of 
Space Science and Applications.

                              - end -


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