Stellar encounters with the Oort cloud based on Hipparcos data

We have combined Hipparcos proper-motion and parallax data for nearby stars with ground-based radial velocity measurements to find stars that may have passed (or will pass) close enough to the Sun to perturb the Oort cloud. C lose stellar encounters could deflect large numbers of comets into the inne r solar system, which would increase the impact hazard at Earth. We find th at the rate of close approaches by star systems (single or multiple stars) within a distance D (in parsecs) from the Sun is given by N = 3.5D(2.12) MY r(-1), less than the number predicted by a simple stellar dynamics model. H owever, this value is clearly a lower limit because of observational incomp leteness in the Hipparcos data set. One star, Gliese 710, is estimated to h ave a closest approach of less than 0.4 pc 1.4 Myr in the future, and sever al stars come within 1 pc during a +/- 10 Myr interval. We have performed d ynamical simulations that show that none of the passing stars perturb the O ort cloud sufficiently to create a substantial increase in the long-period comet flux at Earth's orbit.