Editing Gravity Probe B (section)

== Experimental setup ==
[[Image:Einstein gyro gravity probe b.jpg|thumb|At the time, the fused quartz [[gyroscopes]] created for Gravity Probe B were the most nearly perfect [[sphere]]s ever created by humans.<ref>
{{cite web
 |last        = Barry
 |first       = P.L.
 |title       = A Pocket of Near-Perfection
 |url         = https://science.nasa.gov/headlines/y2004/26apr_gpbtech.htm
 |publisher   = [[NASA|Science@NASA]]
 |date        = 26 April 2004
 |access-date  = 20 May 2009
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20090512163541/http://science.nasa.gov/headlines/y2004/26apr_gpbtech.htm
 |archive-date = 12 May 2009
}}</ref> The gyroscopes differ from a perfect sphere by no more than 40 [[atom]]s of thickness. One is pictured here [[refraction|refracting]] the image of [[Albert Einstein]] in background.]]
[[Image:Gravity Probe B Confirms the Existence of Gravitomagnetism-en.jpg|thumb]]

The ''Gravity Probe B'' experiment comprised four [[Gyroscope#London moment|London moment gyroscopes]] and a reference [[telescope]] sighted on [[IM Pegasi]], a [[binary star]] in the constellation [[Pegasus (constellation)|Pegasus]]. In [[polar orbit]], with the gyro spin directions also pointing toward IM Pegasi, the frame-dragging and geodetic effects came out at right angles, each gyroscope measuring both.

The gyroscopes were housed in a [[Dewar flask|dewar]] of [[Superfluid helium-4|superfluid helium]], maintaining a temperature of under {{convert|2|K|C F|sigfig=3|abbr=out|lk=on}}. Near-[[absolute zero]] temperatures were required to minimize molecular interference, and enable the [[lead]] and [[niobium]] components of the gyroscope mechanisms to become [[superconductivity|superconductive]].

At the time of their manufacture, the gyroscopes were the most nearly spherical objects ever made (two gyroscopes still hold that record, but third place has been taken by the silicon spheres made by the [[Alternative approaches to redefining the kilogram#Avogadro project|Avogadro project]]).  Approximately the size of [[ping pong]] balls, they were perfectly round to within forty atoms (less than {{val|10|u=nm}}). If one of these spheres were scaled to the size of the Earth, the tallest mountains and deepest ocean trench would measure only {{convert|2.4|m|ft|sigfig=1|abbr=on}} high.<ref>
{{cite web
 |last=Hardwood |first=W.
 |title=Spacecraft launched to test Albert Einstein's theories
 |url=http://www.spaceflightnow.com/delta/d304/
 |work=[[Spaceflight Now]]
 |date=20 April 2004
 |access-date=14 May 2009
}}</ref> The spheres were made of [[fused quartz]] and coated with an extremely thin layer of [[niobium]].  A primary concern was minimizing any influence on their spin, so the gyroscopes could never touch their containing compartment.  They were held suspended with electric fields, spun up using a flow of helium gas, and their spin axes were sensed by monitoring the magnetic field of the superconductive niobium layer with [[SQUID]]s. (A spinning superconductor generates a magnetic field precisely aligned with the rotation axis; see [[London moment]].)

IM Pegasi was chosen as the guide star for multiple reasons. First, it needed to be bright enough to be usable for sightings. Then it was close to the ideal positions near the [[celestial equator]]. Also important was its well-understood motion in the sky, which was helped by the fact that this star emits relatively strong [[Radio astronomy|radio signals]]. In preparation for the setup of this mission, astronomers analyzed the radio-based position measurements with respect to far distant quasars taken over several years to understand its motion as precisely as needed.