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We made VLBI observations at 8.4 GHz between 1997 and 2005 to estimate thecoordinates of the "core" component of the superluminal quasar, 3C 454.3, theultimate reference point in the distant universe for the NASA/StanfordGyroscope Relativity Mission, Gravity Probe B. These coordinates are determinedrelative to those of the brightness peaks of two other compact extragalacticsources, B2250+194 and B2252+172, nearby on the sky, and within a celestialreference frame (CRF), defined by a large suite of compact extragalactic radiosources, and nearly identical to the International Celestial Reference Frame 2(ICRF2). We find that B2250+194 and B2252+172 are stationary relative to eachother, and also in the CRF, to within 1-sigma upper limits of 15 and 30micro-arcsec/yr in RA and decl., respectively. The core of 3C 454.3 appears tojitter in its position along the jet direction over ~0.2 mas, likely due toactivity close to the putative supermassive black hole nearby, but on averageis stationary in the CRF within 1-sigma upper limits on its proper motion of 39micro-arcsec/yr (1.0c) and 30 micro-arcsec/yr (0.8c) in RA and decl.,respectively, for the period 2002 - 2005. Our corresponding limit over thelonger interval, 1998 - 2005, of more importance to GP-B, is 46 and 56micro-arcsec/yr in RA and decl., respectively. Some of 3C 454.3's jetcomponents show significantly superluminal motion with speeds of up to ~200micro-arcsec/yr or 5c in the CRF. The core of 3C 454.3 thus provides forGravity Probe B a sufficiently stable reference in the distant universe.

VLBI for Gravity Probe B. III. A Limit on the Proper Motion of the "Core" of the Quasar 3C 454.3