Galileo High Gain Antenna (HGA) Failure (1991)


In April 1991, the Galileo spacecraft executed a deployment sequence which was to open the High Gain Antenna (HGA) like an umbrella, but it never reached the fully deployed position. A formal failure investigation attributed the failure to the design of the rib retention mechanism. According to this scenario, the most likely failure mechanism is friction in the pin/socket interface on the antenna rib midpoint restraint. Preloading of the ribs when the antenna was stowed at the factory damaged the ceramic coating on the pin engaged by the V-groove socket; the coating served to retain the molybdenum disulfide dry lubricant. Accumulated stresses from vibration testing, rib preloading, four cross-country trips, and the post-launch ignition of the upper stage further dispersed the lubricant film. The resulting friction caused asymmetrical deployment, resulting in restraining forces which further reduced the torque available from the deployment drive system.

The HGA was largely inherited from an antenna developed for the Tracking Data Relay Satellite (TDRS) system. JPL design changes included substitution of two conical Inconel pin sockets with one conical and one V-groove Inconel socket. Selection of an Earth orbital antenna design, even though proven in that application, was not fully consistent with the Galileo mission. Inheritance and other design reviews failed to reveal the existence of high surface stresses. In addition, a lessons learned on Voyager II to use spring assisted mechanical deployments was not followed. The deep space mission subjected the redesigned antenna to environmental conditions not encountered by TDRS in Earth orbit, and the VEEGA mission profile instituted after Challenger extended both the duration of those conditions and the time to deployment.

Galileo illustrates the difficulty of reproducing the spaceflight environment in the ground test of large and complex mechanisms, even when full design review and environmental testing are undertaken. Flight antenna deployment test failed to disclose the problem because (1) vacuum test was performed without the vibration-induced relative motion between the pins and sockets and (2) oxides and contaminants present during ground test on the bare titanium pins lubricated the mechanism. Similarly, ambient ground tests did not reveal the failure mode due to the lower coefficient of friction of the titanium pin/socket interface in air. Additional testing of the deployment mechanism would only have worn out the deployment drive system.

Work-arounds using the Low Gain Antenna, new data compression techniques, and the spacecraft's recorder are expected to meet 70 percent of the mission objectives.

Reference(s): "Galileo HGA Deployment Pin Walkout Analysis Final Report," JPL D-9932, July 1992

Lesson(s) Learned

Design changes intended to improve the reliability of inherited hardware may introduce new failure mechanisms. The mission impact of such design changes may best be understood through a "physics of failure" approach to reliability analysis. Failure physics issues relevant to antenna support bearings, for example, may include oxidation, cold welding, galling, static and sliding friction, lubrication transfer, Hertzian contact stresses, and plastic deformation, as well as operational issues such as long-term storage, ground handling, the mission environment, and mission duration.


  1. In the design of preloaded mechanisms, consider the potential for high contact stresses on pin/socket interfaces to destroy the integrity of the lubricant film. Take into account the potential for lubricant effectiveness to decrease over time.
  2. Due to its high wear rate in air, carefully evaluate the use of molybdenum disulfide drylube on a mechanism that will be tested or operated under non-vacuum conditions.
  3. Hardware should be inheritently robust or redesigned to accommodate major changes in spacecraft system design, changes in spacecraft handling or the mission profile. Inheritance, design and peer reviews should fully consider the effect of such changes on known failure mechanisms.
  4. One-shot, non-redundant, mechanisms should be designed for simplicity and fault tolerance-particularly where the mechanisms are preloaded prior to long-term storage, or where they endure extended periods under atmospheric and vacuum conditions prior to actuation.


JPL has referenced this lesson learned as additional rationale and guidance supporting Paragraph 6.6 ("Engineering Practices: Inheritance") in the Jet Propulsion Laboratory standard "Flight Project Practices, Rev. 7," JPL DocID 58032, September 30, 2008.
In addition, JPL has referenced the lesson as supporting Paragraph ("Mechanical Configuration/Systems Design: Mechanisms - Use of Kick-Off Springs for Assured First Motion") in the JPL standard "Design, Verification/Validation and Operations Principles for Flight Systems (Design Principles)," JPL Document D-17868, Rev. 3, December 11, 2006.

Mission Directorate(s)

Additional Key Phrase(s)

  • Parts Materials & Processes
  • Hardware
  • Flight Equipment
  • Environment
  • Spacecraft

Approval Info

  • Approval Date: 1997-03-7
  • Approval Name: Carol Dumain
  • Approval Organization: 125-204
  • Approval Phone Number: