PHYSICS NEWS UPDATE A digest of physics news items by Phillip F. Schewe, American Institut

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PHYSICS NEWS UPDATE A digest of physics news items by Phillip F. Schewe, American Institute of Physics Number 180 May 27, 1994 THE EXISTENCE OF BLACK HOLES is now established observationally about as well as is possible with the release of new Hubble Space Telescope (HST) pictures of fast gas near the core of galaxy M87. Black holes have been adduced to explain a variety of high-energy phenomena, from quasars to the creation of positrons at the center of the Milky Way. Images of the cores of some galaxies had shown a pileup of stellar light, suggestive of the presence of a massive object, but many skeptical astronomers held out for stronger evidence; they insisted on tracking the movement of matter---in the case of M87 a disk of gas---orbiting the presumed black hole. The gas, which at a distance of only 60 light years out from the center of M87 could only have been resolved by HST, moves at a velocity of more than a million mph. This in turn suggests the presence of 1-2 billion-solar-mass black hole. (The New York Times, The Washington Post, and other papers, 26 May.) WHEN THE FRAGMENTS OF COMET SHOEMAKER-LEVY strike Jupiter one after the other in mid-July, what will we see? A panel of scientists, addressing this subject at the Spring Meeting of the American Geophysical Society in Baltimore this week, agreed that a lot depended on the size of the bang. Hubble Space Telescope scientist Harold Weaver said that his best estimate of the size of the larger comet chunks was 1-2 km. Mordecai-Mark Mac Low of the University of Chicago has performed computer simulations which show that the fireball of hot gas (not unlike that of a nuclear bomb) resulting from the breakup of comet fragments in Jupiter's atmosphere will rise above the cloud tops. A 1-km fragment may well trigger an explosion equivalent to a million nuclear bombs; the fireball for such an event, Mac Low believes, might be visible above the limb of Jupiter even though the actual impact site will not yet have rotated into view. Drake Deming of NASA Goddard discussed the sound waves (accounting for as much as 30% of the impact energy) that will move through Jupiter's atmosphere following each impact. Such waves would eventually refract upwards into Jupiter's stratosphere where they might be imaged by infrared detectors on Earth. The Galileo spacecraft, on its way to Jupiter, is actually in a position to directly observe the impact sites, although the nature of its detectors and the speed at which data can be downloaded precludes full images. According to Torrance Johnson of JPL, careful information from other telescopes about the exact timing of the impacts may help the retrieval of selected Galileo measurements---such as an overall brightening of Jupiter due to the impacts --- in the days following the event. THE CLEMENTINE SPACECRAFT has produced the best map of the moon yet. This map is global, including the little-studied polar regions. It is multi-spectral: shot at 11 different wavelengths, the pictures provide information about the mineral compositions and the ages of moon rocks. And it has good spatial resolution: the vertical topography of the moon, for example, was measured to within 100 m, an improvement by a factor of 10 over previous maps. This accuracy permitted an extensive study of the South Pole-Aitken basin, the deepest (12 km) and largest (2500 km across) impact basin in the solar system. Speaking at the AGU meeting, Eugene Shoemaker of the Lowell Observatory said that the lunar phase of the mission had been a complete success. Clementine's secondary scientific task, a rendezvous with the asteroid Geographos, failed because of the uncontrolled spinning of the spacecraft after its departure from lunar orbit. - END OF FILE - ==========


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