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Gravity Probe B

Testing Einstein's Universe



Currently, the target date for the Gravity Probe B space vehicle to launch from Vandenberg Air Force Base in Southern California is Saturday, April 17, 2004. We are continuing to explore the feasibility of pushing the date up to Thursday, April 15th, which is the earliest possible launch date. The exact time of launch depends on the date. On April 17th, the launch would be around 10:09 AM Pacific Daylight Time, and about 4 minutes later for each day prior, or 4 minutes earlier for each day thereafter.

We update these highlights every Friday, and we will post any new information about the launch on this Web site and on the GPB Update email list as soon as it becomes available.


The following activities are currently underway at VAFB:
  • Reconditioning of the dewar back to a pre-launch temperature of 1.65 K has begun.
    Conditioning is a long, iterative "topping off" process of evaporating helium gas from the dewar and adding more liquid helium until the dewar is 95% full. The helium gas “boiled off” during the evaporation phase removes heat from the dewar, converting some of the remaining liquid helium to a superfluid state—a fluid state with special properties that only helium exhibits when cooled to almost absolute zero. The liquid helium that is lost during the evaporative cooling process must then be replaced. After several of these conditioning cycles, 95% of the helium in the dewar ends up in the superfluid state, which will enable the helium to last throughout the duration of the mission.
  • As of Thursday, the temperature of the dewar's main tank was 1.8525K, and the main tank was 96.2% full. The Guard Tank level was 69.1%.
  • The Gas Management Assembly (GMA) is currently undergoing rate of rise testing, and so far, all calculations are within expected limits.
  • Regression testing on the reworked Experiment Control Unit (ECU) is continuing as planned, and thus far, the ECU has been performing nominally.
  • General functional testing of the spacecraft has proceeding on schedule, and the spacecraft has been performing nominally.
  • The launch vehicle is surrounded by the Mobile Service Tower (MST) on the launch pad. Boeing technicians are preparing to power up the launch vehicle on March 8, 2004.

Photo: The GP-B Cryo group performing a sub-atmospheric transfer of liquid helium into the dewar's main tank. Click on the thumbnail to view an enlargement of the photo.


The Los Angeles Times newspaper has tentatively scheduled a front page, column one story on Gravity Probe B for next Monday or Tuesday, February 23 or 24, 2004. If you can't find the L. A. Times at your local newsstand, you can read the story on the newspaper's Web site:

On Sunday evening, December 28, 2003, the Cover story on the CBS Evening News was on the subject of time—examined from both human and cosmic perspectives. The story featured interviews with a manager from Torneau Time Machines (the largest watch store in New York City), physicist Brian Greene (a proponent of String Theory), and our own Gravity Probe B Principal Investigator, Francis Everitt. Typical of television news, the entire story lasted a little over three minutes, but it was heart warming to see Francis and our spacecraft on the national news. Perhaps the best sound byte from Francis was a quote from St. Augustine that CBS chose not to use: "What was God doing before he made heaven and earth? ... He was preparing hell for those that would pry into such profound mysteries."

Click here to view a video clip of the CBS News story about time.


If you are interested in automatically receiving these weekly highlights and other important GP-B mission information by email, you can subscribe to our Gravity Probe B Update email list by sending an email message to "" with the command "subscribe gpb-update" in the body of the message (not in the Subject line). You can unsubscribe from this mailing list at any time by sending an email message to the same address with the command, "unsubscribe gpb-update" in the body of the message.

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