Number 115 (Story #3), February 25, 1993 by Phillip F. Schewe and Ben Stein THE CLOSEST KNOWN PULSAR, PSR J0437-4715 (with a rotation period of 5.75 msec), is only about 400 light years from Earth, several times closer than any other known millisecond pulsar. It was discovered by astronomers in Australia (with the Parkes 64-m radio telescope) making a southern-sky search for msec pulsars. They estimated (very roughly) the distance to the pulsar by measuring the dispersion of its radio pulses caused by the waves' passage through the intervening ionized interstellar medium; the dispersion was the smallest for any known pulsar. PSR J0437-4715 is close enough, however, that its distance might be computable independently by parallax techniques, in which case the local interstellar electron density could be measured. Rosat and the Gamma Ray Observatory will be searching for the object at x-ray and gamma wavelengths, respectively. (Simon Johnson et al., Nature, 18 Feb. 1993.)