Number 155, December 13, 1993 by Phillip F. Schewe and Ben Stein
THE HUBBLE SPACE TELESCOPE REPAIR MISSION has been successful. Space
Shuttle astronauts installed a module (COSTAR) to correct the optical aberration
in Hubble's main mirror. Hubble also received several new gyroscopes, new
solar panels, and an upgraded wide- field planetary camera. (The New York
Times, 10 Dec.)
PRINCETON'S TFTR TOKAMAK HAS PRODUCED 5.6 MEGAWATTS OF POWER from fusion
reactions. The previous record power output for a fusion reactor was 1.7
megawatts at the Joint European Torus (JET) in 1991. The high power at
Princeton (as with JET) was made possible by using tritium. Deuterium-tritium
reactions result in the emission of a surplus energy of 17.6 MeV, whereas
deuterium-deuterium reactions---the ones studied at Princeton until last
week---release only 3.2 MeV of surplus energy. The trouble with tritium
is that it is radioactive and has to be produced artificially. The short
burst of fusion at TFTR lasted only about one second; furthermore, despite
the high power output more energy was put into initiating the fusion than
was gotten out of it. (The Washington Post, 11 Dec.)
SOLAR WIND SPEEDS are twice as great (800 km/sec) at a latitude of 45
degrees south as in the plane of the ecliptic, new Ulysses measurements
show. The Ulysses spacecraft, on its way toward a point beneath the sun's
southern pole, does not look at the 5000-K photosphere, the sun we see
with our eyes, but rather at the much hotter (millions of K) corona; it
studies the magnetic fields and the particles cast out by the sun in a
part of the solar system where no probe has ever been before. Ulysses scientists
(Ed Smith of JPL and others), who spoke at last week's meeting of the American
Geophysical Union in San Francisco, have discovered that shock waves, set
up when certain fast gusts of solar wind overtake slower gusts, can accelerate
ionized atoms that have entered the solar system from the interstellar
medium. It was previously thought that these "anomalous cosmic rays"
originated only in the outer precincts of the solar system.
MANTLE PLUMES CONTAIN RECYCLED OCEAN CRUST AND SEDIMENT, William M.
White of Cornell University reported at the AGU meeting. Mantle plumes
are the upwellings of hot rock that originate either at the boundary between
the mantle and the core 2900 km below the surface, or at a convective boundary
(if one exists) between the upper and lower mantle, which seismic measurements
suggest may be located 660 km below the surface. Measurements of the lead-to-cerium
ratio in volcanic rock originating from plumes in the Society Islands,
which include Tahiti, led White's team to conclude that some of the material
must have been part of the ocean floor in the past. These findings suggest
a picture whereby ocean slabs sink very deeply into the mantle, only to
re-emerge on the surface as material in mantle plumes. Earlier this year,
Jon D. Woodhead of Australian National University and his colleagues made
a similar conclusion through analysis of oxygen isotope ratios from igneous
rocks studied in a volcanic region near Pitcairn Island.
THE FIRST OBSERVED TRANSIT OF MERCURY across the sun's corona was recorded
by the x-ray satellite Yokhoh. A series of pictures show the tiny planet
below the limb of the sun and silhouetted against the bright x-ray glow
of the corona. (Science, 19 Nov.)
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