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Gravity Probe B

Testing Einstein's Universe



Gravity Probe B's many successes in its first week on orbit will ensure a smooth transition into the science phase of the mission and the best possible experimental accuracy.

  • The spacecraft has already achieved a science mission orbit, within the plane of the Guide Star, IM Pegasi, and its inclination error is one sixth of that expected.
  • In the quiet environment of space, the gyro readout system is performing significantly better than it did during any ground testing. All four SQUIDs (Super-conducting Quantum Interference Devices) are fully functional and have detected calibration signals with high precision. Noise levels are below the allowable mission requirements.
  • The electrical power system is fully functional and is providing adequate power for all operating conditions. Eclipse operation (when the spacecraft is in the shadow of the Earth) meets all requirements. There is no evidence of solar array motion that might disturb the experiment.
  • All other spacecraft subsystems are fully functional. All four gyros have been checked out and are performing well. They have all been electrically suspended in analog mode; digital suspension activities will commence shortly. The team is in the process of updating data tables for the Gyro Suspension System (GSS) to aid in achieving digital suspension of the gyros.
  • The spacecraft’s Attitude Control System (ATC) is maintaining a stable attitude (relative position in orbit—pitch, yaw and roll). We expect to lock onto the Guide Star, IM Pegasi, within a few days, after completing on-orbit re-calibration of the spacecraft’s 16 micro thrusters. During thruster re-calibration, it was observed that one of the redundant micro thrusters was stuck partially open. It was isolated and removed from the system, and the flow of helium to the remaining micro thrusters has been adjusted to compensate for its removal.

Overall, one week after launch, it appears that all of the spacecraft’s subsystems are functioning well—meeting or exceeding 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 performing as planned, and the Stanford-NASA-Lockheed Martin Mission Operations Team has been performing superbly.

Throughout the 40-60 day Initialization & Orbit Checkout (IOC) period, we will update this Web site and our GP-B email list weekly, or more often as necessary, to report any changes in status or noteworthy GP-B 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.)


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.
  • 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.

Photos: The first photo, showing the Delta II rocket and payload lifting off of the launch pad was taken by a photographer from the Boeing Corporation. The second photo, showing the rocket in the air, at the end of its vapor trail, was taken by Jim Burns, a member of the Stanford GP-B team at Vandenberg AFB. The shot of the rocket's vapor trail high above the coastline was sent to us by John Dickson of Thank you John! The photos of the rocket engine glow and the two photos showing the spacecraft separating from the second stage rocket were extracted from NASA KSC's launch video. Click on the thumbnails to view enlarged copies of these photos.


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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