From: LARRY KLAES (ljk4_at_msn.com)
Date: Mon Jul 05 2004 - 07:08:47 PDT
----- Original Message -----
From: Bob Kahn<mailto:kahn_at_relgyro.stanford.edu>
To: gpb-update_at_lists.Stanford.EDU<mailto:gpb-update_at_lists.Stanford.EDU>
Sent: Saturday, July 03, 2004 4:22 PM
Subject: Gravity Probe B Update -- July 2, 2004
=========================================
GRAVITY PROBE B MISSION UPDATE -- July 2, 2004
=========================================
Please Note: During the Initialization & Orbit Checkout (IOC) Phase
of the GP-B mission, we update our Web site and send out this email
update once a week (usually on Thursday or Friday) to keep you
apprised of our progress. From time to time, we may send out extra
updates, as warranted by mission events.
At a little over ten weeks into the mission, the spacecraft is in
excellent health, with all subsystems performing well. The
spacecraft's orbit remains stable, ready for the transition into the
Science Phase. All four gyros are digitally suspended and have
completed calibration testing at approximately 0.3 Hz (20 rpm) spin
rates. Two problematic micro thrusters, which were preventing the
spacecraft from sustaining a drag-free orbit, have been isolated, and
the thruster-control software was modified to optimize Attitude and
Translation Control system (ATC) functionality without them. Over the
past two weeks, the spacecraft's roll rate was increased from 0.3 rpm
to 0.9 rpm as part of the process of uniformly distributing and
balancing the mass of the spacecraft.
During this past week, we completed the mass trim procedure at 0.9
rpm, using movable weights on long screw shafts to alter the
spacecraft's center of mass from front to back and from side to side.
The mass trim operation is necessary to precisely align the
spacecraft's roll axis so that it passes through the centers of the
gyros and the telescope's line of sight. The mass trim procedure also
balances the mass of the spacecraft so that it rolls smoothly around
this axis.
Also, this past week, we decreased the spacecraft's roll rate
incrementally from 0.9 rpm back to 0.5 rpm. During the first
roll-down decrement to 0.7 rpm, we discovered that the distribution
of the liquid helium in the Dewar is less predictable during
roll-down than it is during roll-up. When the spacecraft's roll rate
is slowed too quickly, the liquid helium begins to slosh around. The
resulting displacement of the center of mass from the sloshing helium
affects the micro thrusters, resulting in a significant increase in
the time required to complete the roll-down.
Last weekend, in preparation for optimizing ATC performance with 14
instead of 16 micro thrusters, we uploaded revised drag-free
thruster-control software to the on-board computer. After completing
the mass trim maneuver and stabilizing the spacecraft at the 0.5 rpm
roll rate, we re-booted the on-board computer with the revised
software. The re-boot went very smoothly, and a checkout of the new
software confirms that the two problematic thrusters are isolated and
receiving no helium, while the remaining 14 thrusters are responding
to commands as expected. With the new software up and running, we
have begun successfully testing both primary and backup drag-free
modes around gyro #1.
Meanwhile, gyro #1 and gyro #3 are currently in the process of being
spun up to 3 Hz (300 rpm), and likewise, gyros #2 and #4 will be spun
up to 3 Hz next week. Over this weekend, the team will also re-lock
the science telescope on the guide star, IM Pegasi, with the
spacecraft rolling at 0.5 rpm and balanced along the telescope's axis
of sight. Before we begin calibration testing of the gyros at a spin
rate of 3 Hz next week, our goal is to have the spacecraft rolling
smoothly at 0.5 rpm, locked onto the guide star, and in a drag-free
orbit around one of the gyros.
This week, we are pleased to announce two welcome additions to our
The second addition to our Web site is a 1/20 scale, paper model of
The spacecraft is being controlled from the Gravity Probe B Mission
Clarification of Centrifugal vs Centripetal Force from Last Week's Update
In the first procedure, called "bubble wrap," the spacecraft's roll
The words centripetal and centrifugal are in fact antonyms defined as follows:
LEARN MORE ABOUT OUR GUIDE STAR, IM PEGASI
Also, the ETH Institute of Astronomy in Zurich, Switzerland, is
Some readers have asked for alternative names for IM Pegasi as might
LEARN MORE ABOUT IOC & GRAVITY PROBE B
Our GP-B Web site, http://einstein.stanford.edu Our Web site also includes a 3 1/2 minute Quicktime streaming video
TRACK THE GP-B SATELLITE ON THE WEB OR WITH YOUR PDA
Also, Big Fat Tail Productions (http://www.bigfattail.com FOLLOW THE GP-B MISSION ELSEWHERE ON THE INTERNET
NASA's Marshall Space Flight Center Gravity Probe B.com Web page
ABOUT THE GPB-UPDATE EMAIL LIST
If this email update was forwarded to you by someone else and you
You can unsubscribe at any time by sending an email message to
This archive was generated by hypermail 2.1.6
: Mon Jul 05 2004 - 07:19:36 PDT
GP-B Web site. First, we have added a Visual Tour of our spacecraft
and payload. You can access this tour by using the navigation menu
along the left edge of our Web pages. Choose the second item, The
Engineering Story, and from that sub-menu, choose Visual Tour, or
simply enter the following URL into your Web browser:
http://einstein.stanford.edu/content/vehicle_tour/index.html
the GP-B spacecraft that you can download as a PDF file, print out,
and assemble. There are two versions of the PDF file-a 12 MB
high-quality version and a 2 MB low-quality version. The only visible
difference between the versions is that the colors are not as bright
and saturated in the low-quality version, but it will download in
much less time for people with low-speed Internet connections. The
URL for these PDF files is: http://einstein.stanford.edu/p_model
Both versions include two pages of instructions and six pages of
images to cut out and assemble. You'll need scissors, an Exacto
knife, a straight edge, glue (glue sticks and hot glue guns work
well), Scotch tape, two 9.5" long, 1/8" diameter wooden dowels
(shish-kabob skewers work well) about 3-5 hours, and patience to
assemble the model. Kate Stephenson, a Stanford graduate in
Mechanical Engineering, created both the spacecraft visual tour and
the paper model. Over the past few years, Kate has worked on a number
of graphic design projects at GP-B, including our Web site.
Operations Center, located here at Stanford University. The
Stanford-NASA/MSFC-Lockheed Martin operations team is continuing to
perform superbly.
====================================================
Several subscribers emailed me to say that I should have used the
term centripetal force, rather than centrifugal force to describe the
bubble wrap process in last week's update--and they were correct. The
statement about bubble wrap should have read:
rate was increased in incremental steps, from 0.3 rpm to 0.9 rpm. The
increased roll rate begins to rotate the liquid helium, effectively
pushing it outwards as it tries to move in a straight line with its
inertia. The Dewar walls hold it in with a centripetal force. This
wraps the helium uniformly around the outer shell. Distributing the
liquid helium uniformly along the spacecraft's roll axis helps to
ensure that the science telescope can remain locked on the guide star
while the spacecraft is rolling.
--centrifugal : tending to move away from a center.
--centripetal : tending to move toward a center.
=====================================
For an overview about why we chose IM Pegasi as our guide star, and
its importance to the GP-B mission, see the Guide Star FAQ on our Web
site: http://einstein.stanford.edu/content/faqs/faqs.html#guidestar
In addition, see pages 18-20 of the Gravity Probe B Launch Companion:
http://einstein.stanford.edu/highlights/GP-B_Launch_Companion.pdf
working with the Harvard-Smithsonian Center for Astrophysics to
provide detailed optical information about the GP-B guide star, IM
Pegasi. You can find out about the ETH Institute's work in monitoring
magnetic activity on IM Pegasi and the Doppler Imaging Technique used
for this purpose at:
http://www.astro.phys.ethz.ch/staff/berdyugina/private/GP/gravity_probe.html
be found in different stellar catalogs and charts. Those include: IM
Peg, HR 8703, HD 216489, SAO 108231, BD +16 4831, and FK5: 3829
==================================
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." You can download a copy of this
document, in Adobe Acrobat PDF format, from
http://einstein.stanford.edu/highlights/GP-B_Launch_Companion.pdf
information about the Gravity Probe B experiment, general relativity,
and the amazing technologies that were developed to carry out this
experiment.
clip of the launch, edited by Mark Shwartz and Delana Lindsey of the
Stanford News Service. We will be posting more video clips on the Web
site soon.
=============================================
You can track the GP-B satellite on the Web using NASA's Java-based
J-Pass satellite tracking application at:
http://science.nasa.gov/realtime/JPass/
created shareware satellite tracking software that runs on Personal
Digital Assistants (PDAs) using either the Palm OS or Pocket PC
operating systems. Like all PDA shareware, you can try out either
version for free. If you like the software and decide to use it, Big
Fat Tail asks that you pay a nominal shareware fee.
==============================================
In addition to our Stanford GP-B Web site, 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. The URL is: http://www.ksc.nasa.gov/elvnew/gpb/vlcc.htm
(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.)
(http://www.gravityprobeb.com
GP-B launch, including photos of the spacecraft separation, as well
as other information about Gravity Probe B.
============================
The email distribution list for this GP-B Weekly Highlights update is
maintained on the Stanford University email lists server. These email
updates are automatically sent to GP-B program internal email
distribution lists at Stanford, Lockheed Martin, and NASA. If your
email address is not included in one of these distribution lists, you
have subscribed--or been subscribed by someone else--to this email
list.
wish to subscribe, simply send an email message to
"majordomo_at_lists.Stanford.edu<mailto:majordomo_at_lists.Stanford.edu>" with the command "subscribe
gpb-update" in the body of the message (not in the Subject line).
"majordomo_at_lists.Stanford.edu<mailto:majordomo_at_lists.Stanford.edu>" with the command, "unsubscribe
gpb-update" in the body of the message.
--
**********************************
NASA - Stanford - Lockheed Martin
Gravity Probe B Program
"Testing Einstein's Universe"
http://einstein.stanford.edu