Hi,
Here is the latest report from the Gravity Probe B folks:
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GRAVITY PROBE B MISSION UPDATE FOR 16 SEPTEMBER 2005
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GP-B STATUS AT A GLANCE
===================
Mission Elapsed Time: 514 days (73 weeks/ 16.85 months)
--IOC Phase: 129 days (4.2 months)
--Science Phase: 352 days (11.6 months)
--Final Calibration Phase: 33 days
Current Orbit #: 7,586 as of 5:30PM PST
Spacecraft General Health: Good
Roll Rate: Normal at 0.4898 rpm (2.04 minutes per revolution)
Gyro Suspension System (GSS): All 4 gyros digitally suspended
Dewar Temperature: Not Available
Global Positioning System (GPS) lock: Greater than 99.8%
Attitude & Translation Control (ATC): 137.8 marcs rms (as of 9/15)
Y-axis error: 176.4 marcs rms (as of 9/15)
Command & Data Handling (CDH): B-side (backup) computer in control
Multi-bit errors (MBE): 0
Single-bit errors (SBE): Not Available
Telescope Readout (TRE): Nominal
SQUID Readouts (SRE): Nominal
Gyro #1 rotor potential: +2.7 mV (as of 9/13)
Gyro #2 rotor potential: +2.5 mV (as of 9/13)
Gyro #3 rotor potential: +1.2 mV (as of 9/13)
Gyro #4 rotor potential: +3.4 mV (as of 9/13)
Gyro #1 Drag-free Status: Backup Drag-free mode
(OFF during some calibration maneuvers)
MISSION DIRECTOR'S SUMMARY
=======================
As of Mission Day 514, the Gravity Probe B
vehicle and payload are in good health, with all
subsystems performing nominally. We still have
helium remaining in the Dewar, and the spacecraft
is currently flying drag-free around Gyro #1.
The helium in the Dewar has now surpassed its
estimated lifetime by more than two weeks. We
have thus continued working our way through a
prioritized list of calibration tests. Most of
these tests have involved slewing the telescope
(and spacecraft) to "visit" stars (both real and
virtual) in the neighborhood around the guide
star, IM Pegasi. In each case, we visit a
neighboring location for a period of time and
then lock back onto IM Pegasi for a number of
hours. The purpose of these excursions to
neighboring stars is to precisely calibrate the
torques imparted onto the science gyroscopes by
purposely misaligning the telescope axis (and
spacecraft roll axis) by varying amounts from the
direction of the gyros' spin axes.
Last Friday, we visited a virtual star (place
where no stars visible to the telescope exist)
located 0.1 degrees west of our guide star, IM
Pegasi, in the direction of neighboring star HR
Pegasi. We remained fixed on that virtual star
location for 48 hours and then returned to IM
Pegasi, where we remained for 24 hours.
While locked onto IM Pegasi, we switched
drag-free control from gyro #1 to gyro #3 in
order to perform some calibration tests on gyro
#1. During this calibration test, the pressure in
the Dewar dropped, and the spacecraft experienced
thruster instability in the z-axis direction
(along the spacecraft/telescope roll axis). This
behavior was similar to instabilities that we had
previously seen in June 2004, during the
Initialization and Orbit Checkout (IOC) phase of
the mission. The root cause is under
investigation, but there is some speculation that
this may be the result of a micrometeorite
striking the spacecraft. To rectify this
situation, we switched the operating mode of our
micro thrusters to "open loop"--that is, turning
off the thruster pressure sensors and relying
instead on pre-defined thruster calibration
curves to control thruster flow. We will leave
the micro thrusters in open loop mode until the
helium is depleted.
Last Tuesday, 13 Septmber, we visited the very
bright (magnitude 2.45) neighboring star Alpha
Pegasi (also known as Markab or HD 218045),
located about 3.25 degrees south of IM Pegasi. We
remained locked on Alpha Pegasi for 12 hours.
Upon returning to returning to IM Pegasi. We
switched drag-free control back to gyro #1.
Then, last Wednesday, we sent commands to the
spacecraft to decrease its roll rate from 0.7742
rpm (77.5 seconds per revolution) to 0.4898 rpm
(122.5 seconds per revolution). This resulted in
some interference with a SQUID calibration
signal, and the frequency of the calibration
signal was subsequently changed, eliminating this
issue. The spacecraft has since been performing
nominally at the slower roll rate.
Finally, today, we initiated procedures to
de-fluxing the SQUIDS---that is, removing
electromagnetic flux from the SQUIDs by heating
them up a few kelvins, and then allowing them to
cool back down to their normal cryogenic
operating temperature of 1.8 kelvin. Assuming
that we still have helium remaining this weekend
and next week, this process is a prelude to
performing several gyro housing exploration
tests, followed by a switch from backup to
primary drag-free mode.
GP-B MISSION NEWS--RUNNING ON EMPTY, AND VISITING
THE BRIGHTEST STAR IN THE CONSTELLATION PEGASUS
==========================================================
If you drive a car, you have likely
experienced--at least once--the situation where
the needle on your car's gas gage moves into the
red zone, and the light comes on indicating that
the gas tank is nearly empty. But, unless you've
been in this situation a few times with any
particular car, you don't know for sure how much
farther you can drive before the engine finally
sputters and stops running. This is essentially
the situation with our GP-B spacecraft right now.
We know that we're in the "red zone" with our
helium very nearly depleted, but how many more
orbits will actually be sustained is basically
anyone's guess.
And so, continuing our prioritized calibration
tests, last week we visited the star Alpha Pegasi
(aka Markab/HD 218045/ HR 8781), located about
3.25 degrees southward of IM Pegasi. Alpha Pegasi
is one of the defining stars in the constellation
Pegasus, the flying horse. It is approximately
110 light years away, and its luminosity is about
95 times greater than our Sun. In fact, the
designation of "Alpha" Pegasi means that it is
the brightest star in the constellation Pegasus.
A set of composite photos from the Palomar
Observatory Sky Surveys (POSSI and POSSII)
accompanying this week's Mission News story on
our Web site clearly show the comparative
luminosity or brightness of Alpha Pegasi, Zeta
Pegasi, HR Pegasi, IM Pegasi, and HD 216635--all
the stars in the neighborhood of IM Pegasi that
we have visited recently.
Following are some excerpts from Burnham's
Celestial Handbook (a three-volume compendium of
astronomical information), about Alpha Pegasi:
The name Markab or Marchab, is from the Arabian
word for saddle, though the term might also refer
to a ship. Other Arabic names were Matn al Faras,
the Horse's Shoulder, and Yed Alpheras, the
Horse's Forearm or Hand.
ŠPegasus is, of course, the famed Flying Horse of
Greek Mythology, one of the most curious, but
also one of the loveliest concepts created by the
ancient myth-makers of the Greek world. In the
legend, he was born from the blood of the Medusa,
when that monster had been slain by Perseus, and
his name, it is thought, comes from the Greek
'Pegae,' the Springs of the Ocean' at the place
of his birth.
ŠPegasus was tamed by Athena or Minerva according
to Greek legend, and given to the Muses, in whose
service he became the symbol of poetic
inspiration; in another tradition, he carried the
thunder and lightning for Zeus.
In the sky, Pegasus appears turned over on his
back with his body outlined by the Great Square
(Alpha, Beta, and Gamma Pegasi, along with the
Alpha of Andromeda.) His front legs are marked by
Eta and Iota Pegasi, and his head by Epsilon
Pegasi; the great wings are not clearly
indicated, but would like more or less at the
position of the 'Circlet of Pisces,' some 10
degrees below the southern edge of the Great
Square.
At this time of year in North America, the
constellation Pegasus is clearly visible above
the Eastern horizon during the evening hours, as
shown in the sky charts accompanying this week's
Mission News on our GP-B Web site.
As we noted last week, as long as we still have
helium in the Dewar, we will continue working our
way through our prioritized list of calibration
tests. When the helium actually does run out, we
will post a notice on our Web site and send out a
message to the subscribers of our GP-B Update
email list. NASA will also issue a news release,
and we will then post the content of that release
on our Web site and send it to our email
subscribers.
===================
PREVIOUS GP-B UPDATES
===================
If you wish to read any of our previous updates,
our GP-B Web site includes a chronological
archive of all the updates/highlights (with
photos and drawings) that we have posted over the
past 8 years:
http://einstein.stanford.edu/highlights/hlindexmain.html=============================
OTHER LINKS THAT MAY INTEREST YOU
=============================
Our GP-B Web site,
http://einstein.stanford.edu
contains lots of information about the Gravity
Probe B experiment, general relativity, and the
amazing technologies that were developed to carry
out this experiment.
Visual tour of the GP-B spacecraft and payload
from our GP-B Web site:
http://einstein.stanford.edu/content/vehicle_tour/index.html
PDF file containing a 1/20 scale, paper model of
the GP-B spacecraft that you can download print
out, and assemble:
http://einstein.stanford.edu/content/paper_model.
NASA's Marshall Space Flight Center also has a
series of Web pages devoted to GP-B:
http://www.gravityprobeb.com
Photo, taken through a telescope by Swiss physics
teacher and amateur astronomer Stefano Sposetti,
of GP-B spacecraft in orbit, passing near IM
Pegasi:
http://aida.astronomie.info/sposetti
The Harvard-Smithsonian Center for Astrophysics
(Cambridge) and York University (Toronto), with
contributions from the Observatoire de Paris,
have been studying the motions of the guide star,
IM Pegasi for over a decade. To find out more,
visit:
http://www.yorku.ca/bartel/guidestar/
In addition, you'll find information in the Guide
Star FAQ on our Web site:
http://einstein.stanford.edu/content/faqs/faqs.html#guidestar
and on pages 18-20 of the Gravity Probe B Launch
Companion:
http://einstein.stanford.edu/highlights/GP-B_Launch_Companion.pdf
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/
Also, you can track the GP-B satellite on
Personal Digital Assistants (PDAs) using either
the Palm OS or Pocket PC operating systems with
software from Big Fat Tail Productions:
http://www.bigfattail.com
The Einstein Exhibition at the Skirball Cultural
Center in Los Angeles has closed.However, you can
visit the American Museum of Natural History's
virtual Einstein exhibit on the Web at:
http://www.skirball.org/exhibit/amnh_frame.html
==========================
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==========================
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--
**********************************
NASA - Stanford - Lockheed Martin
Gravity Probe B Program
"Testing Einstein's Universe"
http://einstein.stanford.edu
Bob Kahn
Public Affairs Coordinator
Phone: 650-723-2540
Fax: 650-723-3494
Email: kahn@relgyro.stanford.edu
**********************************
#-------------------------------------
Regards,
LelandJ
