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Gravity Probe B

Testing Einstein's Universe

WEEKLY UPDATE FOR 3 JUNE 2005:

GRAVITY PROBE B MISSION STATUS AT A GLANCE

Item Current Status
Mission Elapsed Time 409 days (58 weeks/13.40 months)
Science Data Collection 280 days (40 weeks/9.18 months)
Current Orbit # 6,036 as of 5:00 PM PST
Spacecraft General Health Good
Roll Rate Normal at 0.7742 rpm (77.5 seconds per revolution)
Gyro Suspension System (GSS) All 4 gyros digitally suspended in science mode
Dewar Temperature 1.82 kelvin, holding steady
Global Positioning System (GPS) lock Greater than 96.0%
Attitude & Translation Control (ATC)

X-axis attitude error: 245.9 marcs rms
Y-axis attitude error: 230.0 marcs rms

Command & Data Handling (CDH) B-side (backup) computer in control
Multi-bit errors (MBE): 2 (in SRE computer on 5/30)
Single-bit errors (SBE): 7 (daily average)
Telescope Readout (TRE) Nominal
SQUID Readouts (SRE) Nominal
Gyro #1 rotor potential -0.2 mV (As of 5/26)
Gyro #2 rotor potential -1.0 mV (As of 5/26)
Gyro #4 rotor potential -0.7 mV (As of 5/26)
Gyro #3 Drag-free Status Backup Drag-free mode (normal)

MISSION DIRECTOR'S SUMMARY

As of Mission Day 409, the Gravity Probe B vehicle and payload are in good health. All four gyros are digitally suspended in science mode. The spacecraft is flying drag-free around Gyro #3.

Late last Thursday, Gyro #2 transitioned from its normal digital suspension to analog backup suspension mode. This gyro was re-suspended in digital mode last Saturday, 28 May 2005, and it has remained digitally suspended since that time. The root cause of this transition is still under investigation.

Last Sunday, 20 May 2005, the SQUID Readout Electronics (SRE) computer experienced two multi-bit errors (MBEs). One of these errors was located in a data buffer location that cleared itself, thus removing the error. The second error was located in a section of program code that is not planned for use. In response, the Mission Operations team has since removed this code module from the computer's database, so that it cannot be accessed inadvertently.

Various planned preliminary calibration tests are continuing, including some tests of Gyro #3 and the drag-free system. Such tests are standard operating procedure on experimental missions. During these tests, we purposely vary certain parameters that would not be expected to change greatly during the experimental portion of the mission for the purpose of characterizing instrument component performance. In order to maximize the length of the science data collection phase of the GP-B mission, we are currently running some preliminary calibrations that can safely be performed while we are still collecting science data. Our final calibration tests, which must be completed before the helium in the Dewar is depleted, involve placing torques (forces) on the gyros, so we can only perform these tests after data collection is finished—early in August.

MISSION NEWS—THE GP-B PROBE'S REALLY COOL WINDOWS: Part I

Note: This week's GP-B Mission News story is too long to cover in one update, so we've divided it into two parts. We will post Part II next week.

Consider the following: GP-B's four gyroscopes are mounted in a quartz block, which is suspended near the bottom of a 9-foot long vacuum chamber, shaped like a cigar tube and called “the probe.” The probe is mounted along the central axis of the Dewar, surrounded by a bath of superfluid helium, maintained at a temperature of 1.8 Kelvin (-456 degrees Fahrenheit). Sitting on top of the quartz block and monolithically bonded to it is the telescope. Above the telescope, the cylindrical probe wall rises about three more feet into the neck of the Dewar, ending at the "Top Hat," which caps off the Dewar and the probe. The Top Hat provides an interface for over 400 cables and tubes that connect various parts of the science instrument (gyros and telescope) inside the probe with the many electronics boxes, computers, and pure helium gas for spinning up the gyros that are all mounted in the bays of the spacecraft frame and the Forward Equipment Enclosure (FEE) surrounding the Top Hat.

Light from the guide star, IM Pegasi, must be able to shine down through the Top Hat, into the probe, and ultimately into the telescope. Thus, the probe needed to have some kind of window system that would allow starlight from IM Pegasi to pass into the telescope, while keeping infrared and microwave radiation (heat energy) out and maintaining an ultra-high vacuum and ultra cold environment inside the probe. There also had to be some way of blocking extraneous light from the sun that could burn out the sensitive photon detectors in the telescope.

Furthermore, in order to be tested prior to launch, this window system had to work here on earth, where the differential between the pressure and temperature inside and outside the probe is far greater than in space. In addition, this probe window system needed to:

  • Remain virtually free of condensation and contamination.
  • Avoid creating extraneous reflections between windows.
  • Maintain the vacuum seal inside the probe.
  • Withstand the pressure and temperature on the outermost window, which could cause significant lensing or bulging that would distort the starlight from the guide star.
Needless to say, designing and building such a system of windows inside the probe turned out to be another of GP-B's formidable engineering challenges.

Over the years, three probes, labeled A, B, and C, were constructed, each improving on the previous one. Probe C is the flight probe on-board the spacecraft. Probe B currently sits on a stand outside the conference room where we hold our daily status briefings, and Probe A is in an off-site storage facility.

The window system that evolved consists of four circular windows--three inside the probe and one hermetically sealed on the end of the probe. Above the end window, a conical sunshade extends several feet to the top of the spacecraft. The inside of the sunshade is lined with baffles that absorb and redirect stray light from the sun. A shutter at the bottom of the sunshade can be closed to prevent any light from entering the probe or telescope. The sunshade and shutter address the issue of sunlight entering the telescope and burning out its photon detectors.

The three windows inside the probe are numbered from #1, closest to the telescope, to #3 further up the tube. Window #4 forms the vacuum seal at the end of the probe, above the Top Hat. Window #4 has a number of special requirements and characteristics that will be described in next week's update. The three inner windows are about 6 1/2" in diameter, with 6" clear aperture. They are made of fused quartz--the same material used to make the quartz block, gyros and housings, and the telescope, and they have anti-reflective coatings, not unlike the coatings used on eyeglasses, that minimize reflections off the window surfaces and block infrared light, while allowing visible light to pass through. In fact, these coatings provide increased visible light transmission over uncoated glass. In addition to enabling the light from the guide star to reach the telescope, the other main purpose of these windows is to successively dissipate all of the heat entering the probe through three copper heat-exchange stations that are thermally connected to four successive layers of heat exchangers that line the Dewar's outer shell.

The lower portion of the probe, which resembles a cigar tube, is suspended from the neck tube of the Dewar. Its cylindrical shell is made of aluminum, which conducts the cryogenic temperature of the liquid helium surrounding it to the science instrument assembly (telescope and gyros) inside. Above the telescope, the material comprising the cylindrical shell of the probe's neck tube changes from aluminum to a gamma alumina ceramic composite. The upper composite probe shell is joined to the lower aluminum shell with an O-ring seal of epoxy and Indium, a very soft metallic element that remains soft at low temperatures. The composite material comprising the upper shell of the probe, where the windows are located, is a very low thermal conductor, so that it does not pass heat down into the lower portion of the probe, which must remain at the cryogenic temperature of 1.8 K. 

Moving up the cylindrical probe wall from the telescope, the three inner windows are mounted in thermal conducting copper brackets that are attached to the first three of four thermal platforms. These platforms are strategically spaced along the upper portion of the probe, so that they connect to four successive heat exchanger layers of the Dewar shell that terminate in the Dewar neck. Each thermal platform consists of a pair of copper bands, one inside the probe wall, with a matching band at the same location on the outside of the probe wall. The inner bands were first frozen with liquid nitrogen, inserted into their correct positions inside the probe, heated so that they expanded to fit tightly against the inner probe wall, and then glued into place with epoxy. The outer bands were positioned outside their inner probe counter parts and shrunk for a tight fit against the outer probe wall and likewise glued into place with epoxy. Thus, each heat platform is comprised of a sandwich of copper bands, conducting heat from the inner band through the thin probe wall to the outer band. In turn, the outer bands of the probe's thermal platforms each touch, and thus conduct the heat to a copper ring in the Dewar neck, corresponding to one of the Dewar's four successive heat exchanger layers.

As an interesting historical note, the thermal conduction from the inner copper band through the probe wall to the outer copper band worked quite well in Probe A and Probe B. However, for some reason, when these thermal platform band assemblies were completed in the final Probe C, the thermal conduction through the probe wall was significantly diminished. At this stage in the development of Probe C, it would have taken years to redesign and fabricate a new probe with the proper heat exchanger for the windows. Fortunately, Lockheed Martin Lead Designer, Gary Reynolds, collaborating with GP-B Co-Principal Investigator, John Turneaure, now an Emeritus Professor of Physics at Stanford, came up with a work-around solution that saved the probe: they decided to drill small holes through the outside copper bands, through the probe wall, and into the inner band…but not all the way through the inner band, which would destroy the vacuum inside the probe. Then, they inserted copper dowels into these holes and glued them in place with a special epoxy. The epoxied copper dowels established a strong thermal connection from the inner bands to the outer bands, as well as ensuring that the vacuum seal inside the probe remained intact.

Next week, in Part II of this story, we will talk about Window #4, which has its own special set of requirements. We will also describe how the whole system works, including the temperature differentials at each successive thermal platform and the successive loss of light from the guide star, as it passes through some 24 optical surfaces on its way to the telescope's photon detectors.

In last week's update/highlights, we described the GP-B probe's structure, including the difference between the aluminum shell in the lower part of the probe and the composite shell in the probe neck, the four thermal platforms in the probe neck, and the three 6” round windows that are thermally attached to the first three of the four thermal platforms.

The fourth (topmost) thermal platform has no window connected to it. Rather, the topmost window (Window #4), is embedded inside the top end of a large metal collar, called the "Cross Flange." Window #4, which is about 8” in diameter (2" larger in diameter than the three internal windows), is sometimes called the "warm window," as opposed to the three "cold windows" inside the probe neck. The Cross Flange in which Window #4 is embedded forms the top end of the probe and is bolted with a vacuum seal to the upper end of the Top Hat. It contains two large exhaust valves that were used to evacuate all gases from the probe shortly after launch. The tall, conical Sunshade is bolted to the outside top end of the Cross Flange.

Window #4, along with its unique hermetic vacuum seal, was an engineering tour de force--a synergy of design constraints, ingenuity, and engineering prowess. Because this window forms the vacuum seal at end of the probe, it had to be made of a material that was strong enough to withstand the pressure differential between the high vacuum inside the probe and normal atmospheric pressure and temperature outside the probe without lensing or bulging. In addition, this window had to be a very good thermal conductor, so that it would conduct heat uniformly in all directions from the center outward, with very little temperature rise in the center. Of course, this window also had to be optically transparent, allowing the light from the guide star, IM Pegasi, to enter the probe undistorted, but at the same time, it also had to block electromagnetic radiation. Finally, this window had to perform these functions both in orbit and in extensive testing here on Earth, where the pressure and temperature differential inside and outside the probe is much greater than in space.

What kind of window material meets all of these requirements? The answer is sapphire. Sapphire is extremely strong, it has very high thermal conductivity, it is an electrical insulator, and it is optically transparent. Sapphire has a crystalline structure, that could cause a phenomenon called birefringence or double refractions of a single light beam. However, due to the sapphire's strength, Window #4 could be made relatively thin, and birefringence was not a problem. In addition to an anti-reflective coating, which was also applied to the three fused quartz windows, Window #4 was treated with an electromagnetic interference (EMI) coating to attenuate the ambient low frequency and microwave electromagnetic radiation leaking into the probe through the wide open optical aperture, both in space and on the ground. Because EMI coatings use metal, which is opaque, they reduce the optical transmission of the window. Thus, there was a tradeoff between the effectiveness of the EMI shield and optical transmission requirement. Finally, Window #4 also included a thermal emissivity or “low e” coating that reduced the amount of heat it transmits into the probe.

Perhaps the greatest challenge with Window #4 was its hermetic vacuum seal, which took many design iterations to arrive at a working solution. At the time Window #4 was constructed, rubber O-rings were typically used to create vacuum seals. O-rings would probably have worked in space, but they did not provide a good enough seal here on the ground, where the pressure was much greater. Another option was to braze (very high temperature soldering) a metallic seal onto the outer edge of the sapphire window. However, brazing was deemed too risky, given thermal expansion issues and the very high cost of sapphire. Ultimately, the Stanford-Lockheed Martin engineering collaboration came up with a unique seal that had never before been tried: Window #4 was surrounded by an Indium-coated "C" seal, lined with a thin layer of gold against the sapphire, and held in place by a ring of springy nickel-chromium-iron alloy called Inconel® that is more typically used in formula race car exhaust systems. As of this update, the probe has maintained it ultra low vacuum throughout months of ground testing, plus 13 months in orbit, so this hermetic seal must be working.

After Window #4 was sealed in place, a vacuum was established inside the probe. This vacuum was more than 100 times greater than the vacuum of space in GP-B's orbital region, 641 km (400 miles) above the Earth. Light and some heat enter the probe through Window #4, and some heat from the electronics boxes and heaters located on the spacecraft's frame also enter the probe through the top hat. The visible light passes down the probe, through all four windows, and into the telescope. Much of the heat is initially dissipated to the Dewar's outermost heat exchanger through the topmost thermal platform #4 (which has no window attached to it). Any remaining heat then continues down the probe, eventually striking Window #3, where a large percentage of it is dissipated out through Thermal Platform #3 to the Dewar's second heat exchanger. Likewise, whatever heat still remains travels further down the probe, striking Window #2, where much of it is dissipated out through Thermal Platform #2 and the next heat exchanger. Finally, whatever tiny amount of heat still remains, travels down the probe to Window #1, where most of it is dissipated out through Thermal Platform #1 to the Dewar's innermost heat exchanger.

Thus, moving down through the probe towards the telescope, the average temperature drops significantly from window to window. The actual temperature ranges recorded at each window thus far in the mission are as follows:

  • Window #4: 210 K to 255 K
  • Window #3: 125 K to 155 K
  • Window #2: 72 K to 92 K
  • Window #1: 30 K to 38 K
  • Window #0: 3.3 K to 3.5 K (This is not actually a stand-alone window, but rather the forward plate of the telescope, where the Image Divider Assembly sits.)

The low numbers in each range are from the end of September, when the least Sunlight strikes the Dewar. The high numbers in each range are the current values, when the spacecraft is in full-Sun throughout each orbit. Note that for the inner three windows, the temperatures in the table shown above are measured at the copper mounts, where the window frames connect to the thermal platforms. The actual temperatures of the windows themselves cannot be measured, but they differ slightly from the values shown above. The copper mounts have weak thermal links to both the probe and to the window glass. This way the glass in the three inner windows never experiences any thermal shock from the probe. However, the copper mounts themselves are very good thermal conductors, ensuring that the window glass remains in a very symmetric thermal environment along the axis of the probe. Any axial asymmetry in the windows' thermal environment would cause a distortion in star image.

To minimize ghost images in the field of view of the telescope, formed by light reflecting between windows, each of the four windows was canted (tilted) 2 degrees and clocked perpendicular to the next window. In other words, if Window #4 was canted 2 degrees towards 12 o'clock, Window #3 was canted 2 degrees towards 3 o'clock, Window #2 was canted 2 degrees towards 6 o'clock and Window #1 was canted 2 degrees towards 9 o'clock. The window canting directs reflections between the windows away from the telescope's field of view. Without the window canting, the telescope would see clusters of dimmer stars surrounding a brighter star, instead of just a single star. 

Even with all the special coatings on the windows, the amount of starlight that reaches the telescope's photon detectors is somewhat diminished by each optical surface that it passes through. All told, the starlight encounters some 24 optical surfaces, including the front and back of each probe window, the main telescope plates, the telescope mirrors, the beam splitter, prisms, mirrors, and lenses in the image divider, and beam splitters, mirrors, and lenses in the photon detector packages. Thus, by the time the starlight from IM Pegasi reaches the telescope's photon detectors, only about 8-12% of the light makes it to all 8 photo detectors combined! That is, 2-3% of the initial starlight beam actually falls on each of the four detector pairs (one primary detector pair, plus one redundant backup detector pair in both the X- and Y-axes.) Considering that IM Pegasi is only a 5.6 magnitude star at it brightest, only a very small amount of light actually hits these detectors, but it has been adequate to keep the telescope centered on the guide star throughout the mission.

The table below provides an example of the decrease in light moving down through the probe, surface-by-surface, from Window #4 to the telescope's photon detectors. Note: This table is a simplified model of the probe's optical performance. Actual values differ somewhat and are published in various scientific journals.

Surface

Typical Efficiency

Remaining Light

Number of Paths

Light in Each Path

Notes

raw starlight

n/a

100.0%

1

100.0%

5.8 average magnitude star - pretty dim to start with

window 4 top

88%

88.0%

1

88.0%

Window 4 also acts as an EMI shield and vacuum seal

window 4 bottom

88%

77.4%

1

77.4%

 

window 3 top

96%

74.3%

1

74.3%

Each window reduces the thermal load into the probe.  Four was chosen as the right balance between heat rejected versus light lost.

window 3 bottom

96%

71.4%

1

71.4%

 

window 2 top

96%

68.5%

1

68.5%

 

window 2 bottom

96%

65.8%

1

65.8%

 

window 1 top

96%

63.1%

1

63.1%

 

window 1 bottom

96%

60.6%

1

60.6%

 

telescope forward plate top

96%

58.2%

1

58.2%

The telescope forward plate supports the IDA, DPA's, and secondary mirror

telescope forward plate bottom

96%

55.9%

1

55.9%

 

telescope primary mirror

90%

50.3%

1

50.3%

The telescope mirrors focus the starlight from a 6" wide beam to a 0.001" point for the IDA to divide it

telescope secondary mirror

90%

45.3%

1

45.3%

 

telescope tertiary mirror

90%

40.7%

1

40.7%

 

IDA baseplate bottom

96%

39.1%

1

39.1%

IDA = Image Divider Assembly

IDA baseplate top

96%

37.5%

1

37.5%

 

IDA beamsplitter

68%

25.5%

2

12.8%

The IDA beamsplitter is a "half silvered" plate that makes copies of the beam (1 reflected, 1 transmitted) to be split in the X and Y axes

IDA roof prism

90%

23.0%

4

5.7%

The roof prisms slice the beam into left and right halves; when they're equal, you're pointed at the center of the guide star

IDA fold mirror

90%

20.7%

4

5.2%

Fold mirrors steer the beams to the detectors

IDA lens entrance

96%

19.8%

4

5.0%

after division at the roof prism, the beams are expanding (nearly BB sized at this point); these lenses control the expansion

IDA lens exit

96%

19.1%

4

4.8%

 

DPA fold mirror

90%

17.1%

4

4.3%

DPA = Detector Package Assembly

DPA lens entrance

96%

16.5%

4

4.1%

These lenses refocus the now slightly expanding beams into tight spots on the tiny photodiodes

DPA lens exit

96%

15.8%

4

4.0%

 

DPA beamsplitter

68%

10.7%

8

1.3%

These duplicate the beams onto redundant sets of photodiodes

One final mission note about light and heat from Earth entering the probe: The GP-B spacecraft design includes a shutter at the base of the Sunshade that can be closed to prevent Sunlight from entering the telescope and overpowering its sensitive photon detectors and the telescope readout electronics (TRE) in the event that the spacecraft loses attitude control. The original spacecraft design called for the shutter to remain open throughout each orbit. However, it occurred to members of the design team that the temperature in the cryogenic part of the probe would remain more constant if the shutter were closed during "Guide Star Invalid" periods--anytime the spacecraft moves behind the Earth--thus preventing Earth light (and thus heat) from entering the probe. Earth light includes bright Sunlight reflected off the Earth at mid-day, glancing Sunlight when the spacecraft is pointing at the edge of the Earth, and light originating from Earth, such as city lights and forest fires. The intensity of Earth light is always much greater than light from the guide star, and thus it tends to saturate the TRE detector amplifiers and also results in a very slight warming of the lower, cryogenic section of the probe. Thus, at launch last year, the mission plan called for closing the shutter during the Guide Star Invalid period of each orbit.

The opening and closing movement of the shutter was tested on the ground before launch and found to have no significant effect on the Attitude and Translation Control (ATC) system. However, during the Initialization and Orbit Checkout (IOC) following launch, the GP-B team discovered that shutter movement was significantly detectable by the science gyros, and that its movements could adversely affect their performance. (This issue would have been very difficult, time-consuming, and expensive to test for prior to launch.) Thus, during the IOC phase, we decided to revert to the original design plan of leaving the shutter open during the Guide Star Invalid portion of each orbit and allowing the light and heat radiated from Earth to enter the probe and very slightly increase the temperature in the telescope detector platforms and the quartz block housing the SQUIDs and gyros. This small amount of heat is not a problem. Most of it is drawn off and conducted into the Dewar by the Cryopump, located between Window #1 and the front plate of the telescope. (The Cryopump, itself, is a very interesting technology and will be the subject of a future update.) The temperature of the remaining cryogenic components quickly normalizes when the spacecraft emerges from behind the Earth and re-enters the Guide Star Valid state.

The telescope's photon detectors continue to generate data during each Guide Star Invalid period, but generally, we do not record this data in our ground telemetry passes. However, for a few months of the year, when the Sun is at it closest approach to IM Pegasi and when the alignment of Sun, Earth, Moon, and spacecraft is such that the spacecraft is only receiving light generated by the Earth itself or when the portion of Earth under the spacecraft is bathed in Moonlight, we have collected this data in order to calibrate the sensitivity of the photon detectors. Interestingly, Moonlight is just bright enough to enable the photon detectors to create a nocturnal brightness map of the Earth's surface features--clouds, oceans, forests, deserts, and so on--as seen through the telescope's very narrow field of view. When the Moon and Sun are both on the opposite side of the Earth from the spacecraft, the detectors pick up city lights, forest fires, and any other light-dark patterns generated here on Earth.


UPDATED NASA/GP-B FACT SHEET AVAILABLE FOR DOWNLOADING

We recently updated our NASA Factsheet on the GP-B mission and experiment. You'll now find this 6-page document (Adobe Acrobat PDF format) listed as the last navigation link under "What is GP-B" in the upper left corner of this Web page. You can also click here to download a copy.


Drawings & Photos: Tthe composite photo showing the GP-B spacecraft in orbit above the Earth was created with Adobe Photoshop by GP-B Public Affairs Coordinator, Bob Kahn.The schematic diagrams of the GP-B Dewar and probe, and the photos of the gyro housing, SRE box and all of the probe photos are from the GP-B Image Archive here at Stanford University. The photos of the flight backup inner probe windows were taken by Bob Kahn and Ken Bower. Click on the thumbnails to view these images at full size.

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