Gravitational Constant: Experimental Bound on Its Time Variation

Analysis of radar-echo time delays, primarily between Earth and Mercury, yields an upper limit on the fractional variation of the gravitational constant of 4 parts in ${10}^{10}$ per year. Continuation of these radar measurements for five more years-even at the present level of accuracy-would allow the uncertainty to be reduced reliably to 3 parts in ${10}^{11}$ per year, a limit close to the effect predicted by some theorists.