Editing Gravity Probe B (section)

{{short description|Orbital gravitational observatory}}
{{Use dmy dates|date=December 2020}}
{{Infobox spaceflight
| name                  = Gravity Probe B
| image                 = Artist concept of Gravity Probe B.jpg
| image_caption         =

| mission_type          = Astrophysics
| operator              = [[NASA]]/[[Stanford University]]
| website               = {{url|https://einstein.stanford.edu}}
| COSPAR_ID             = 2004-014A 
| SATCAT                = 28230
| mission_duration      = {{time interval|20 April 2004 16:57:24|8 December 2010}} <ref name=GPB/>

| spacecraft_bus        = 
| manufacturer          = [[Lockheed Martin]]
| dry_mass              = 
| launch_mass           = {{convert|3100|kg|lb|abbr=on}}<ref name=GPB>{{cite web |url=http://einstein.stanford.edu/content/fact_sheet/GPB_FactSheet-0405.pdf |title=Gravity Probe B |work=NASA Facts |date=February 2005 |publisher=[[NASA]] and [[Stanford University]] |access-date=17 March 2011 }}</ref>
| dimensions            = {{convert|6.4|×|2.6|m|ft|abbr=on}}<ref name=GPB/>
| power                 = 606&nbsp;W<br/><small>Spacecraft: 293&nbsp;W<br/>Payload: 313&nbsp;W<ref name=GPB/></small>

| launch_date           = {{start-date|20 April 2004, 16:57:24|timezone=yes}}&nbsp;UTC
| launch_rocket         = [[Delta II|Delta II 7920-10C]]  
| launch_site           = [[Vandenberg Air Force Base|Vandenberg]] [[Vandenberg AFB Space Launch Complex 2|SLC-2W]]
| launch_contractor     =

| disposal_type         = Decommissioned
| deactivated           = {{end-date|8 December 2010}}
| decay_date            =

| orbit_epoch           = <time>2004-04-20 15:57:00</time> UTC<ref name=NSSDC>{{cite web|url=https://nssdc.gsfc.nasa.gov/nmc/spacecraft/display.action?id=2004-014A|title=Spacecraft orbit: Gravity Probe B|publisher=National Space Science Data Center|year=2004|access-date=18 January 2015}}</ref>
| orbit_reference       = [[geocentric orbit|Geocentric]]
| orbit_regime          = [[Low Earth orbit|Low Earth]]
| orbit_periapsis       = {{convert|641|km|mi|abbr=on|sp=us}}<ref name=NSSDC/>
| orbit_apoapsis        = {{convert|645|km|mi|abbr=on|sp=us}}<ref name=NSSDC/> 
| orbit_inclination     = 90.007º<ref name=GPB/>
| orbit_semimajor       = {{convert|7027.4|km|mi|abbr=on|sp=us}}
| orbit_eccentricity    = 0.0014<ref name=GPB/>
| orbit_period          = 97.65 minutes<ref>{{cite web |author1=G. Hanuschak |author2=H. Small |author3=D. DeBra |author4=K. Galal |author5=A. Ndili |author6=P. Shestople |url=http://einstein.stanford.edu/content/sci_papers/papers/Hanuschak_ION_Paper.pdf |title=Gravity Probe B GPS Orbit Determination with Verification by Satellite Laser Ranging |access-date=17 March 2011 }}</ref>
| apsis                 = gee

| insignia              = Gravity Probe B logo (KSC-03PD3038).jpg
}}

'''Gravity Probe B''' ('''GP-B''') was a [[satellite]]-based experiment whose objective was to test two previously-unverified predictions of general relativity: the [[geodetic effect]] and [[frame-dragging]]. This was to be accomplished by measuring, very precisely, tiny changes in the direction of spin of four gyroscopes contained in an Earth-orbiting satellite at {{cvt|650|km}} of altitude, crossing directly over the poles.

The satellite was launched on 20 April 2004 on a [[Delta II]] rocket.<ref>
{{cite web
 |url=http://einstein.stanford.edu/content/faqs/faqs.html#launch
 |title=Frequently asked Questions |website=Gravity Probe B |publisher=Stanford University
 |at=Answers to Spacecraft and Mission Operations Questions: 1.&nbsp;When and where was GP-B launched, and where can I find photos, video or news clips of the launch?
 |access-date=14 May 2009
}}</ref> The spaceflight phase lasted until <time>2005</time>;<ref>
{{cite web
 |url=http://einstein.stanford.edu/content/faqs/faqs.html#operations
 |title=Frequently asked Questions |website=Gravity Probe B |publisher=Stanford University
 |at=Answers to Spacecraft and Mission Operations Questions: 4.&nbsp;Where is the GP-B Mission Operations Center (MOC) for controlling the spacecraft in orbit?
 |access-date=14 May 2009
}}</ref> Its aim was to measure [[spacetime|spacetime curvature]] near Earth, and thereby the [[stress–energy tensor]] (which is related to the distribution and the motion of matter in space) in and near Earth. This provided a test of [[general relativity]], [[gravitomagnetism]] and related models. The principal investigator was [[Francis Everitt]].

Initial results confirmed the expected geodetic effect to an accuracy of about 1%. The expected frame-dragging effect was similar in magnitude to the current noise level (the noise being dominated by initially unmodeled effects due to nonuniform coatings on the gyroscopes). Work continued to model and account for these sources of error, thus permitting extraction of the frame-dragging signal. By <time datetime="2008-08">August 2008</time>, the frame-dragging effect had been confirmed to within 15% of the expected result,<ref name = "Gugliotta2009">
{{cite news
 | last = Gugliotta | first = G.
 | title = Perseverance Is Paying Off for a Test of Relativity in Space
 | work = [[The New York Times]]
 | date = 16 February 2009
 | url = https://www.nytimes.com/2009/02/17/science/17gravity.html?_r=1
 | access-date =18 February 2009
}}</ref> and the <time datetime="2008-12">December 2008</time> [[NASA]] report indicated that the geodetic effect was confirmed to be better than 0.5%.<ref>
{{cite web
 |author1=Everitt, C.W.F. |author2=Parkinson, B.W. |url=http://einstein.stanford.edu/content/final_report/GPB_Final_NASA_Report-020509-web.pdf
 |title=Gravity Probe B Science Results—NASA Final Report
 |date=2009
 |access-date=2 May 2009
}}</ref>

In an article published in the journal ''[[Physical Review Letters]]'' in <time>2011</time>, the authors reported analysis of the data from all four gyroscopes results in a geodetic drift rate of {{val|−6601.8|18.3|ul=mas|upl=yr}} and a frame-dragging drift rate of {{val|−37.2|7.2|u=mas|up=yr}}, in good agreement with the general relativity predictions of {{val|−6606.1|0.28|errend=%|u=mas/yr}} and {{val|−39.2|0.19|errend=%|u=mas/yr}}, respectively.<ref name=PRL/>