Editing Gravity Probe B (section)

==Overview==

Gravity Probe B was a relativity [[gyroscope]] experiment funded by NASA. Efforts were led by the [[Stanford University]] physics department with [[Lockheed Martin]] as the primary subcontractor. Mission scientists viewed it as the second relativity experiment in space, following the successful launch of [[Gravity Probe A]] (GP-A) in <time>1976</time>.

The mission plans were to test two 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 satellite orbiting at {{cvt|650|km}} altitude, crossing directly over the poles. The gyroscopes were intended to be so free from disturbance that they would provide a near-perfect [[spacetime]] reference system. This would allow them to reveal how space and time are "warped" by the presence of the Earth, and by how much the Earth's rotation "drags" space-time around with it.

The geodetic effect is an effect caused by space-time being "curved" by the mass of the Earth. A gyroscope's axis when [[parallel transport]]ed around the Earth in one complete revolution does not end up pointing in exactly the same direction as before. The angle "missing" may be thought of as the amount the gyroscope "leans over" into the slope of the space-time curvature. A more precise explanation for the space curvature part of the geodetic precession is obtained by using a nearly flat cone to model the space curvature of the Earth's gravitational field. Such a cone is made by cutting out a thin "pie-slice" from a circle and gluing the cut edges together. The spatial geodetic precession is a measure of the missing "pie-slice" angle. Gravity Probe B was expected to measure this effect to an accuracy of one part in {{val|10,000}}, the most stringent check on general relativistic predictions to date.

The much smaller frame-dragging effect is an example of [[gravitomagnetism]]. It is an analog of [[magnetism]] in [[classical electrodynamics]], but caused by rotating masses rather than rotating electric charges. Previously, only two analyses of the [[laser-ranging]] data obtained by the two [[LAGEOS]] satellites, published in <time>1997</time> and <time>2004</time>, claimed to have found the frame-dragging effect with an accuracy of about 20% and 10% respectively,<ref>
{{cite arXiv
 |last1=Ciufolini | first1=I.
 |last2=Lucchesi | first2=D.
 |last3=Vespe | first3=F.
 |last4=Chieppa | first4=F.
 |title=Detection of Lense–Thirring Effect Due to Earth's Spin
 |eprint=gr-qc/9704065
 |date=1997
}}</ref><ref>
{{cite news
 |title=Einstein's warp effect measured
 |url=http://news.bbc.co.uk/2/hi/science/nature/3762852.stm
 |work=BBC News
 |date=21 October 2004
 |access-date=14 May 2009
}}</ref><ref>
{{cite journal
 |last=Peplow |first=M.
 |title=Spinning Earth twists space
 |journal=[[Nature (journal)|Nature News]]
 |date=2004
 |doi=10.1038/news041018-11
}}</ref> whereas Gravity Probe B aimed to measure the frame dragging effect to a precision of 1%.<ref>{{cite web |title=Overview of the GP-B Mission |url=http://einstein.stanford.edu/MISSION/mission1.html |website=Gravity Probe B |publisher=Stanford University |year=2011 |access-date=18 January 2015}}</ref> A recent analysis of [[Mars Global Surveyor]] data has claimed to have confirmed the frame dragging effect to a precision of 0.5%, although the accuracy of this claim is disputed.<ref>
{{cite journal
 |last=Krogh |first=K.
 |title=Comment on 'Evidence of the gravitomagnetic field of Mars'
 |journal=[[Classical and Quantum Gravity]]
 |volume=24
 |issue = 22
 |pages=5709–5715
 |date=November 2007
 |doi=10.1088/0264-9381/24/22/N01
 |bibcode = 2007CQGra..24.5709K |arxiv=astro-ph/0701653
 |s2cid=12238950
}}</ref>

The launch was planned for <time datetime="2004-04-19">19 April 2004</time> 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 <time datetime="2004-04-20 09:57:23-07:00">20 April, at 9:57:23 AM</time> [[Time zone|PDT]] (<time datetime="2004-04-20 16:57:23Z">16:57:23</time> [[Coordinated Universal Time|UTC]]) the spacecraft was launched successfully. The satellite was placed in orbit at <time datetime="2004-04-20 11:12:33-07:00">11:12:33</time> AM (<time datetime="2004-04-20 18:12:33Z">18:12:33</time> UTC) after a cruise period over the south pole and a short second burn. The mission lasted 16 months.
 	
Some preliminary results were presented at a special session during the [[American Physical Society]] meeting in <time datetime="2007-04">April 2007</time>. NASA initially requested a proposal for extending the GP-B data analysis phase through <time datetime="2007-12">December 2007</time>. The data analysis phase was further extended to <time datetime="2008-09">September 2008</time> using funding from [[Richard Fairbank]], Stanford and NASA, and beyond that point using non-NASA funding only.<ref name = "Gugliotta2009"/> Final science results were reported in <time>2011</time>.