Gravity Probe B and Navy Yard–Ballpark station: Difference between pages

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{{WMATA infobox

'''Gravity Probe B''' ('''GP-B''') is a [[satellite]]-based mission to measure the [[stress-energy tensor]] (the distribution, and especially the motion, of matter) in and near [[Earth]], and thus to test related models; in application of [[Einstein]]'s [[general relativity|general theory of relativity]]. Mission scientists view it as the second gravity experiment in space, following the successful launch of [[Gravity Probe A]] (GP-A) in [[1976]].

|name = Navy Yard

|picture = Navy_yard.jpg

|opened = [[December 28]], [[1991]]

|platform = Center platform

|RTU = -

|line info = {{WMATA line|previous=[[Anacostia (Washington Metro)|Anacostia]]|line=Green|next=[[Waterfront-SEU (Washington Metro)|Waterfront-SEU]]}}

|}}'''Navy Yard''' is a [[Washington Metro]] station in [[Washington, DC]] on the [[Green Line (Washington Metro)|Green Line]]. The station is located in [[Washington DC (southeast)|Southeast Washington]], with entrances on M Street at Half Street and New Jersey Avenue.

The station is named for the nearby [[Washington Navy Yard]]. The industrial neighborhood is a focus for redevelopment; the Arthur Capper and Carrollsburg Dwellings, nearby [[public housing]] projects, are scheduled for demolition. The Southeast Federal Center, a U.S. government office complex built on the former Navy Yard Annex, opened in [[2000]]; the station serves many commuters to the new headquarters of the [[United States Department of Transportation]] and other agencies. The main tourist attraction is the [[Navy Museum]], on the grounds of the Navy Yard.

==Overview==

[[Image:Gravity Probe B.jpg|thumb|300px|right|Gravity Probe B with solar panels folded]]

''Gravity Probe B'' is a relativity [[gyroscope]] experiment funded by [[NASA]]. Efforts are being headed up by the [[Physics]] department at [[Stanford University]] with [[Lockheed Martin]] as the primary subcontractor. According to mission plans, it will test two unverified predictions of that theory:

Plans for a [[Washington Nationals#The_Ballpark_Controversy|new baseball stadium]] for the [[Washington Nationals]] also call for expansion of the station to serve game-day crowds.

The experiment plans to check, very precisely, tiny changes in the direction of spin of four gyroscopes contained in an [[Earth]] satellite orbiting at 650 [[km]] (400 [[statute mile]]s, 350 [[nautical mile]]s) altitude and crossing directly over the poles. So free are the gyroscopes from disturbance that they should provide an almost perfect [[space-time]] reference system. They are intended to measure how space and time are "warped" by the presence of the Earth, and, more profoundly, if and how much the Earth's rotation "drags" space-time around with it; the so-called [[frame-dragging]] effect. Previously, only two analyses of the [[laser-ranging]] data obtained by the two [[LAGEOS]] satellites, published in [[1997]] and [[2004]], claimed to have found the frame-dragging effect with an accuracy of about 20 percent and 10 percent respectively ([http://xxx.lanl.gov/abs/gr-qc/9704065], [http://www.nature.com/news/2004/041018/full/041018-11.html]).

The next prediction is so-called [[gravitomagnetism]] or [[geodetic effect]] - a field generated by the rotation of Earth and similar to the [[magnetism]] in electrodynamics.

When the mission is successfully completed, GP-B will be the most precise attempt at verification of any effect predicted by general relativity.

The launch was planned for [[April 19]], [[2004]] at [[Vandenberg Air Force Base]] but was scrubbed within 5 minutes of the scheduled launch window due to changing winds in the upper atmosphere. An unusual feature of the mission is that it only had a one-second launch window due to the precise orbit required by the experiment. On [[April 20]] at 9:57:23 AM [[Time zone|PDT]] (16:57:23 [[UTC]]) the spacecraft was launched successfully. The satellite was placed in orbit at 11:12:33 AM (18:12:33 UTC) after a cruise period over the south pole and a short second burn. The mission is planned to last 16 [[month]]s.

==Experimental setup==

The ''Gravity Probe B'' experiment comprises four gyroscopes and a reference [[telescope]] sighted on HR8703 (also known as [[IM Pegasi]] [http://simbad.u-strasbg.fr/sim-id.pl?protocol=html&Ident=HR+8703]), a [[binary star]] in the constellation [[Pegasus (constellation)|Pegasus]]. In [[polar orbit]], with the gyro spin directions also pointing toward HR8703, the frame-dragging and geodetic effects come out at right angles, each gyroscope measuring both.

[[Image:Einstein gyro gravity probe b.jpg|thumb|350px|right|One of the most perfect [[sphere]]s ever created by humans. A [[fused quartz]] [[gyroscope]] for the [[Gravity Probe B]] experiment which differs from a perfect sphere by no more than a mere 40 atoms of thickness as it [[refraction|refracts]] the image of Einstein in background. It is thought that only [[neutron star]]s are smoother.]]

The gyroscopes are the most spherical objects ever made. Approximately the size of ping pong balls, they are perfectly round to within forty atoms. They are composed of [[fused quartz]] and coated with [[niobium]]. The gyros' spin axes are sensed with [[SQUID]]s.

The gyroscopes are housed in a [[Dewar flask|dewar]] of [[superfluid]] [[helium]], maintaining a temperature of under 2 [[kelvin]]s (−271 degrees [[Celsius]], −456 degrees [[Fahrenheit]]). Near [[absolute zero]] temperatures are required in order to minimize molecular interference.

This particular star was chosen for multiple reasons. First, it needed to be bright enough to be usable for sightings. Then it was close to the ideal positions at the [[equator]] of the [[Equatorial coordinate system|sky coordinates]]. Also important was a well understood motion in the sky, which was helped by the fact that this star emits relatively strong [[Radio astronomy|radio signals]]. As a preparation for the setup of this mission, astronomers analyzed the radio based position measurements taken over the last few years to understand its motion as precisely as needed.

==Mission progress==

*[[April 20]], [[2004]]

**Launch of ''GP-B'' from Vandenberg AFB and successful insertion into polar orbit.

*[[April 28]], [[2004]]

**Mission controllers started the "Initialization and Orbit Checkout" phase (IOC), which was expected to last 40-60 days. At this point all gyros were spun up and the SQUID detectors were being checked. All other spacecraft subsystems performed well, including solar power and the attitude control system.

*[[May 1]], [[2004]]

**During the IOC the primary computer of the spacecraft received too much radiation to cope with the built-in error correction mechanism. ''GP-B'' switched over to the backup computer as designed. Since the spacecraft crosses over the polar areas of the Earth with their high radiation, this was anticipated by the designers. The primary computer was repaired and put back into service. All science instruments on board were working perfectly throughout this incident.

*[[May 14]], [[2004]]

**The spacecraft went into "safemode" for a short period when some of the helium micro-thrusters behaved in an unstable way. This problem was addressed quickly and ''GP-B'' went back into IOC mode. The cause of this incident was a high-pressure condition in the [[dewar]], which was reached due to warm (10 K) helium being used to remove [[magnetic flux]] from the gyroscopes. Mission members believe that the IOC phase will still be completed on time after a total 60 mission days.

*[[July 13]], [[2004]]

**The preparations for the science phase of the mission reached a major milestone: One of the gyros (No. 4) reached the science ready speed of 6,348 rpm (105.8 Hz) during a short test.

*[[July 16]], [[2004]]

**An unexpectedly large slowdown of gyro 4 was detected during the full-speed spin-up of gyro 2. Although some "leakage" effect was expected, the amount seen led mission planners to search for ways to diminish the effect for this final step towards the science phase. This investigation took close to a week and delayed the planned spin-up of gyro 1 and 3.

**Ground tests had indicated that a good signal-to-noise ratio for science data is reached, once the gyro spin rate exceeds 80 Hz. However, mission managers stress that a slightly lower number will also be sufficient for entering the science phase of ''GP-B''.

*[[August 27]], [[2004]]

**Mission manangers announced that ''GP-B'' entered its science phase, today. On mission day 129 all systems were configured to be ready for data collection, with the only exception being gyro 4, which needs further spin axis alignment.

**After weeks of testing it was decided to use the "backup-up drag-free" mode, which balances the spacecraft around gyro 3. Also, the rotation period of ''GP-B'' was adjusted to 0.7742 rpm in order to avoid harmonic interferences with the sample rate during data taking.

**They also report that it was planned to continue tuning the drag-free performance of the Attitude and Translation Control (ATC) system in the early portion of the Science Phase to correct for an unknown force, which is causing excess helium flow from the Dewar through the micro thrusters.

*[[September 7]], [[2004]]

**The main computer suffered a "double-bit" error in its memory. The ___location of this error was non-critical to the mission and the function of the spacecraft. A correction that fixed the problem was successfully uploaded. All other subsystems are reported to continue to perform well.

*[[September 16]], [[2004]]

**''GP-B'' officially enters its science phase with all four gyros aligned.

*[[September 23]], [[2004]]

**Due to problems with gyro 3, ''GP-B'' went into "safe mode". The mission team was able to ensure minimal impact to the science, safe the spacecraft, and switch the control system setup. It is now maintaining the drag-free orbit around gyro 1.

*[[September 24]], [[2004]]

**The mission went back into science mode.

*[[October 19]], [[2004]]

**Gyro 1 showed the same behavior as gyro 3 earlier, which prompted mission members to switch back to a drag-free orbit around gyro 3. Adjustments were made to both gyro suspension systems (GSS) to avoid future problems. All this was done in a span of three hours, and science data collection was interrupted only briefly.

*[[November 10]], [[2004]]

**When passing over the [[South Atlantic Anomaly]] during a strong [[solar storm]], a memory error in a critical region put ''GP-B'' into safe mode. This incident caused a computer to reboot and put the gyros into "analog mode." After about two days all memory problems were fixed and science data became available again. At first, it was assumed a [[proton]] hit from the storm was the cause, but later analysis showed that this was not the case. Instead, an earlier error at a presumed non-critical memory position was causing the "safe mode", when the memory was accessed during routine maintenance.

*[[January]], [[2005]]

**[[Image:Solar activity 012005.JPG|thumb|Proton flux due to Solar flares, January 2005]] A series of strong [[solar flare]]s disrupted data taking for several days. On [[January 17]] a very powerful radiation storm created multi-bit errors in the onboard computer memory, and saturated the telescope detectors so that ''GP-B'' lost track of the guide star. The science team, however, is confident that the temporary loss of science data will have no significant effect on the results. On [[January 20]] the high level of proton flux is still generating "single bit errors" in ''GP-B'' memory, but the telescope is locked on the guide star again, and the gyroscope electronics seem to perform nominally.

*[[March 14]], [[2005]]

**The onboard backup computer (B-side) rebooted after a safemode event, which came two weeks after the switch-over from the nominal computer (A-side). Both events were triggered by the occurrence of Multi-Bit Errors (MBEs) in the memory of each computer. It took mission members about 29 hours to recover and transfer back to the nominal state, with the guide star locked in.

*[[May 6]], [[2005]]

**Mission members deduce from a "heat pulse test" that there is enough liquid helium on board the space craft to cool the experiment until sometime between late August and early September of [[2005]]. They are preparing to start the calibration procedures, and thus end the science phase, in early August.

*[[August 15]], [[2005]]

**The science phase of the mission ended and the spacecraft instruments transitioned to the final calibration mode.

*[[September 26]], [[2005]]

**The calibration phase ended with liquid helium still in the dewar. The spacecraft was returned to science mode pending the depletion of the last of the liquid helium.

*[[September 29]], [[2005]]

**The liquid helium in the dewar finally ran out, and the experiment began to warm up.

**Drag-free mode turned off.

==History==

A station serving the Navy Yard area existed in original plans for Metro; however, the routing of the Green Line below proved controversial. In [[1976]] the original routing was rejected as too costly and disruptive. A new study proposed a more westerly path which would move the Anacostia station west, replace the Good Hope Road station with one at [[Congress Heights (Washington Metro)|Congress Heights]], and terminate at Brinkley instead of [[Branch Ave (Washington Metro)|Branch Ave]]. In December [[1977]] public hearings this route was criticized as disserving poorer landowners in the area, but WMATA approved the western route in [[1980]], scheduled to open in [[1986]]. Supporters of the Branch Avenue route then took the case to the [[U.S. District Court]].

The court ruled in February [[1981]] that the 1977 hearings were invalid, as insufficient public notice had been given, and issued an injunction halting construction below the [[Waterfront-SEU (Washington Metro)|Waterfront station]]. New hearings were held in June [[1982]], but the court again ruled against WMATA in October [[1983]]. A third set of hearings in July [[1984]] finally selected the present route, allowing constructon to commence.[http://www.roadstothefuture.com/Metro_Branch_Ave_Opening.html]

The conceptual design for this mission was first proposed by Leonard Schiff ([[Stanford]]) and Edward E. Pugh ([[U.S. Department of Defense]]) (independently of each other) in [[1959]]. It was proposed to NASA in [[1961]], and it supported the project with funds in [[1964]]. This grant ended in [[1977]] after a long phase of engineering research into the basic requirements and tools for the satellite.

Navy Yard is the last station prior to crossing the [[Anacostia River]]; the tunnel below the Anacostia was the first drilled with a [[tunnel boring machine]] in the United States. Service to the station finally began on [[December 28]], [[1991]] with the extension of the Green Line to [[Anacostia (Washington Metro)|Anacostia]].

In [[1986]] NASA changed plans for the [[Space Shuttle|shuttle]], which forced the mission team to switch from a shuttle-based launch design to one that is based on the [[Delta 2]], and in [[1995]] tests planned of a prototype on a shuttle flight were cancelled as well.

==References==

Gravity Probe B marks the first time in history that a university has been in control of the development and operations of a space satellite funded by NASA.

* RoadstotheFuture.com: "[http://www.roadstothefuture.com/Metro_Branch_Ave_Opening.html Metrorail Branch Avenue Route Completion]"

==See also==

*[[Timeline of gravitational physics and relativity|Timeline of Gravitational Physics]]

*[[Gravity]]

*[[Gravitomagnetism]]

*[[Frame-dragging]]

*[[Tests of general relativity]]

==External links==

* WMATA: [http://www.wmata.com/metrorail/Stations/station.cfm?station=84 Navy Yard Station]

*[http://news.bbc.co.uk/2/hi/science/nature/3639193.stm#graphic Graphic explaination of how Gravity Probe B works]

* Stationmasters.com: [http://www.stationmasters.com/System_Map/NAVYYARD/navyyard.html Navy Yard Station]

*[http://einstein.stanford.edu Gravity Probe B Web site at Stanford]

* world.nycsubway.org: [http://world.nycsubway.org/us/washdc/green/wmata-green-navy.html Navy Yard Station]

*[http://www.ksc.nasa.gov/elvnew/gpb NASA GP-B launch site]

* The Schumin Web Transit Center: [http://transit.schuminweb.com/rail/washington/stations/green/navy-yard.asp Navy Yard Station]

*[http://mocc.vandenberg.af.mil/launchsched.asp VAFB launch schedule]

*[http://www.rdrop.com/users/green/school/framdrag.htm Frame Dragging]

*[http://www.phy.duke.edu/~kolena/framedrag.html General Relativistic Frame Dragging]

*[http://www.washingtonpost.com/ac2/wp-dyn/A43049-2004Jul11?language=printer Layman's article on the project progress]

*[http://news.bbc.co.uk/2/hi/science/nature/3762852.stm Latest LAGEOS results reported at BBC news]

[[Category:Tests of general relativity]]

[[Category:Satellites]]

[[Category:Washington Metro stations]]

[[de:Gravity Probe]]