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Gravity Probe B

Testing Einstein's Universe



As of Mission Day #17, the Gravity Probe B spacecraft continues to perform well, and we are expecting a smooth and successful transition into the science phase of the mission.

  • The spacecraft remains in a science mission orbit, within the plane of the Guide Star, IM Pegasi. The gyro readout system performance continues to exceed expectations, and all four SQUIDs (Super-conducting Quantum Interference Devices) are functional and calibrated, with very low noise levels. Power and thermal systems meet all of our mission requirements. All spacecraft subsystems continue to perform nominally.
  • All four gyros have been electrically suspended in analog mode, and gyros #1, #2 and #4 are now digitally suspended; we expect gyro #3 to transition from analog to digital suspension shortly.
  • Last weekend, the spacecraft was hit by radiation while passing over the Earth’s south magnetic pole. This radiation caused data errors in the spacecraft’s primary (A-side) computer, which exceeded its capacity for self-correction. Thus, by design, the spacecraft automatically switched over to the backup (B-side) computer, placed the spacecraft in a “safe” mode, and put the planned timeline of events on hold.
  • The automatic switch over from primary to backup computer worked flawlessly. The GP-B mission operations team has since re-booted the primary computer, restored its data parameters, and then commanded the spacecraft to switch back to the primary computer, which is once again in control. During this incident, the GP-B science instrument continued to function perfectly--as expected--with all four gyros remaining suspended in their assigned modes.
  • The spacecraft’s Attitude Control System (ATC) is continuing to maintain a stable attitude (relative position in orbit—pitch, yaw and roll). However, the process of locking onto the Guide Star, IM Pegasi, has been delayed a few days by the South Pole radiation incident.

Overall, at two and a half weeks after launch, it appears that all of the spacecraft’s subsystems are continuing to meet or exceed mission requirements, in preparation for beginning the science experiment.

The spacecraft is being controlled from the Gravity Probe B Mission Operations Center, located here at Stanford University. Mission operations have demonstrated that the hardware developed for the GP-B mission is functioning as planned, and the Stanford-NASA/MSFC-Lockheed Martin operations team is continuing to perform superbly.

Please Note: During the Initialization & Oribit Checkout (IOC) Phase of the GP-B mission, we will update this Web site and send out an email update once a week (usually on Thursday or Friday) to keep you appraised of our progress. From time to time, we may post and email extra updates, as warranted by mission events.


Find the Gravity Probe B satellite in the sky at NASA's satellite tracking web site. See where GP-B is with respect to the terminator (the day-night boundary on the Earth's surface), or just enter your zip code to see if GP-B might be over your neighborhood. The best time to look for it is usually at dusk.


If you are interested in following the IOC procedures more closely, you'll find a schedule and description of them on pages 12-14 of the Gravity Probe B Launch Companion. This document also provides an overview and explanation of the Gravity Probe B experiment and information about the spacecraft and GP-B's amazing technologies. Click here to download the GP-B Launch Companion in Adobe Acrobat PDF format. (Please note: this file is 1.6 MB, so it may take awhile to download if you have a slow Internet connection.)

Photos: The first photo, showing the Delta II rocket and payload lifting off of the launch pad was taken by photographer Bill Hartenstein ( . The second photo—actually three photos electronically "stitched together"— was taken by Marian Chuang, a systems enginee at Lockheed Martin Missles & Space Corporation. Tthe photo showing the spacecraft separating from the second stage rocket was extracted from NASA KSC's launch video. The final "photo" is actually a drawing of the GP-B spacecraft on orbit, as depicted in an animation created by NASA's Marshall Space Flight Center. Click on the thumbnails to view enlarged copies of these photos.


Click here to view a 3 1/2 minute QuickTime video clip of the GP-B launch, produced by the Stanford News Service. Please note that the video requires Quick Time to play. Click here to link to download Quick Time.


In addition to this Web site, here are some other Web sites that have information, photos, and video of the GP-B launch and mission.

  • The ELV Missions Virtual Launch Center Web page on the John F. Kennedy Space Center Web site has information and several streaming video clips covering the GP-B mission. (You can view these video clips free of charge, but you will need to have either the Real Media Player or Windows Media Player installed on your computer to view them.)
  • NASA's Marshall Space Flight Center Gravity Probe Web page has a number of great photos from the GP-B launch, including photos of the spacecraft separation, as well as other information about Gravity Probe B.
  • The Science @ NASA Web site, hosted by NASA's Marshall Space Flight Center, posts several stories each month about scientific research projects in which NASA is involved. This site currently features two general interest stories about Gravity Probe B: In search of Gravitomagnetism and A Pocket of Near Perfection. (In addition to the Web versions, these stories are also available in both plain text and streaming audio formats.)
  • Photographer William G. Hartenstein's Web site has an extraordinary set of photos that he took on launch day.
  • Another very comprehensive source of information about the GP-B launch is the Spaceflight Now Web site. This site contains an excellent photo gallery, as well as a number of Quicktime video clips of the launch. However, you have to become a subscriber to this site ($$$) in order to view the video clips.


Sunday evening, April 18th, a feature story about Gravity Probe B and principal investigator, Francis Everitt, aired on ABC World News Tonight. Click here to read a text version of the ABC News story. Also on April 18th, NPR's David Kestenbaum talked with GP-B's principal investigator, Francis Everitt on the program All Things Considered. On Friday, April 16th, Gravity Probe B Co-Principal Investigator, John Turneaure, was interviewed by Ira Flatow on NPR Talk of the Nation—Science Friday.

On Tuesday, April 13,2004, Gravity Probe B was the lead story in the Science section of the New York Times, and it was one of the front page stories in the San Jose Mercury News. (You'll need to register on the Web sites of these newspapers to view these stories online.) In addition, a story about GP-B appeared on the New Scientist Web on April 13, 2004.

The official pre-launch Gravity Probe B mission and science briefing was held on Friday, April 2, 2004 at 1:00 PM Eastern Daylight Time at NASA Headquarters in Washington, D.C. The participants in the briefing (pictured from left to right in the photo) were:

  • Anne Kinney, Director of Astronomy/Physics Division, NASA Headquarters
  • Rex Geveden, Program Manager, GP-B and Deputy Director, NASA's Marshall Space Flight Center in Huntsville, Alabama.
  • Francis Everitt, GP-B Principal Investigator at Stanford University, Stanford, California
  • Bradford Parkinson, GP-B Co-Principal Investigator at Stanford University, Stanford California
  • Kip Thorne, Feynman Professor of Theoretical Physics, California Institute of Technology, Pasadena, California

You can view a Real Player streaming video of this briefing on the Kennedy Space Center GP-B Web site.

Following this press briefing, stories about Gravity Probe B appeared in a number of newspapers around the country. For example:

  • The Boston Globe carried a front page story, which you can read online.
  • The San Francisco Chronicle carried a front page story, which you can read online.
  • On the Internet, a story about GP-B, written by Associated Press science writer Andrew Bridges, became one of April 3rd's most popular stories on Yahoo News.
  • CNN also ran a story about GP-B, which you can view online.


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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