scope:

Description of Driving Event:

At approximately 10:30 a.m. on August 26, 2001, gaseous nitrogen (GN2) was erroneously connected to the Gravity Probe-B (GP-B) guard tank vent line as the GP-B Science Payload (Figure 1) was readied for transport from Stanford University (SU) to Lockheed Martin (LM) in Palo Alto, California. At approximately 2:30 p.m. (after the GP-B Science Payload arrived at LM), the error was recognized and the GN2 was disconnected. Gaseous helium (GHe) was the intended gas to be connected instead of GN2. Since the guard tank is at a much lower temperature than the freezing point of GN2, concerns arose over the possible detrimental effects of frozen nitrogen (N2) in the vent line and/or the guard tank itself.

The purpose of the move was to transfer the GP-B Science Payload from SU to LM for later integration with the spacecraft. The move was uneventful except for the discovery of the connection of the incorrect gas to the guard tank. The immediate cause of the incident was the inadvertent connection of the GN2 to the guard tank instead of utilizing the required GHe.

A procedure to check the guard tank vent line impedance was executed on September 15, 2001. The data showed significant but not total blockage of the vent line with frozen N2. The procedure added heat to the guard tank and flowed gas through the vent line until there was no further indication of N2 in the tank or line. There is no reason to believe that damage to the hardware had occurred since the vent line was only partially blocked. There was an impact on the schedule due to the recovery time, and there will be an associated cost based on the schedule impact.