I find the clarification on the "centrifugal" force quite interesting. As explained in physics books, there is no such thing as what is called the centrifugal force.

The natural tendency of any object is to continue moving in a straight line (inertia), so no "force" is needed to pull an object to the outside; in a circular movement, there is an unbalanced force pulling the object to the inside.

>Hi,
>
>Here is another update on Gravity Probe B. Gravity Proble B has been a little overshadow this week by the Cassini mission to Saturn.
>
>#--------------------------------------------
>
>=========================================
>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
>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 second addition to our Web site is a 1/20 scale, paper model of
>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.
>
>The spacecraft is being controlled from the Gravity Probe B Mission
>Operations Center, located here at Stanford University. The
>Stanford-NASA/MSFC-Lockheed Martin operations team is continuing to
>perform superbly.
>
>Clarification of Centrifugal vs Centripetal Force from Last Week's Update
>====================================================
>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:
>
>In the first procedure, called "bubble wrap," the spacecraft's roll
>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.
>
>The words centripetal and centrifugal are in fact antonyms defined as follows:
>--centrifugal : tending to move away from a center.
>--centripetal : tending to move toward a center.
>
>#----------------------------------------------------
>
>Regards,
>
>LelandJ