Number 105, December 4, 1992 by Phillip F. Schewe and Ben Stein THE SHARPEST PICTURE YET OF AN ACTIVE GALACTIC NUCLEUS has been recorded by the Hubble Space Telescope. The picture shows a doughnut-shaped disk of dust 300 light-years across in NGC 4261, a galaxy in the Virgo cluster. The presence of huge radio jets emanating from the galaxy's core suggests to some astronomers that a black hole lurks there. If so, the disk may be providing the fuel for the black hole. (Science News, 28 Nov. 1992.) LUNAR TIDAL EFFECTS are the main cause of error in determining the mass of the Z boson. Scientists at the CERN laboratory in Geneva, Switzerland, with help from workers at SLAC and the University of Lausanne, have found that the Moon's gravitational pull warps the LEP (Large Electron Positron) collider by as much as a millimeter (out of a total circumference of 27 km). This blurs the Z mass estimates by about 10 MeV. Hereafter, calibrations of the beam energy will take into account the phase of the Moon. (CERN press release, 23 Nov. 1992.) RUMORS OF TOP-QUARK EVENTS in the data sample of high-energy proton-antiproton collisions accumulated by the CDF (Collider Detector at Fermilab) experiment surfaced again, this time at a divisional (particle physics) meeting of The American Physical Society in Chicago two weeks ago. But the discussion of intriguing hints of the top's existence is not the same thing as an announcement of a discovery, as CDF co-leader Melvin Shochet was quick to point out. (Science, 20 Nov. 1992.) Controversy earlier in the year arose because of an unauthorized analysis and discussion of CDF data. (Science, 24 July 1992.) NUCLEAR AGE BEGAN 50 YEARS AGO on December 2, 1942 when Enrico Fermi's team of scientists initiated the first sustained nuclear chain reaction in a pile constructed under the University of Chicago's football stadium. The pile used 385 tons of graphite and 50 tons of uranium. Subsequent nuclear energy milestones include the first nuclear bomb blast (July 16, 1945), hydrogen bomb (Nov. 1, 1952), nuclear-powered submarine (U.S.S. Nautilus, Jan. 1955), and reactor-powered generation of commercial electricity (Arco, Idaho, July 1955). (The New York Times, 1 Dec. 1992.) SPACECRAFT UPDATE: The Mars Observer craft was launched on 25 September 1992 and will reach Mars in August 1993. It will assume a polar orbit and begin mapping the planet's surface. The Pioneer Venus craft ran out of fuel and fell into Venus's clouds on 8 October 1992 after having served for 14 years, far longer than the originally-planned 243 days. Meanwhile, Voyager 1 is now 50 astronomical units (4.6 billion miles) away from the Sun and traveling at an angle of 35 degrees to the ecliptic plane. Astronomers hope that both Voyager 1 and Voyager 2 will reach the heliopause (the start of interstellar space) at an estimated distance of 60-90 AU. Finally, the Magellan spacecraft is now performing its fourth cycle of mappings of the Venusian surface. Project scientists would like to extend the mission and send the craft into a lower orbit, the better to probe the planet's interior by searching for gravity anomalies. (Astronomy, Jan. 1993.)