Number 112, January 28, 1993 by Phillip F. Schewe and Ben Stein
THE FLUX OF COSMIC RAYS FROM THE SMALL MAGELLANIC CLOUD is smaller than
one would expect if cosmic rays were extragalactic in origin. Measurements
by the Gamma Ray Observatory of gamma rays (with energies greater than
100 MeV) resulting from the collision of cosmic-ray nucleons and electrons
with interstellar matter recorded a flux of 0.5 x 10**-7 photons/sec-cm**2,
one fifth the expected flux (P. Sreekumar et al., 11 Jan. 1993 Physical
Review Letters). The GRO scientists conclude that cosmic rays are almost
certainly galactic in origin; determining the nature of their source is
another matter. (Nature, 21 Jan. 1993.)
THE RACE IS ON TO ACHIEVE BOSE CONDENSATION , a state of matter in which
an ensemble of bosons collapses to a single quantum state at very low temperature.
According to Carl Wieman of the University of Colorado (303-492-6963),
Bose condensation represents the lowest possible energy state for matter.
Daniel Kleppner of MIT (617-253-4876) is trying to achieve Bose condensation
in a cloud of atomic hydrogen supercooled by liquid helium. He further
cools and condenses the atoms through the process of "evaporative
cooling," in which the atoms are held in a magnetic field where only
the hottest atoms have enough energy to escape. Only the coldest atoms
remain, clumping together in the trap as a result of their lethargy. Wieman
is also trying to apply evaporative cooling to his setup of laser-cooled
cesium atoms, but has not achieved a density of atoms sufficiently high
to trigger the process. He plans to design a "magnetic lens"
that would increase the density to desired levels. Kleppner is adjusting
his experiment to better monitor the trapped hydrogen atoms and thereby
pinpoint the conditions that would optimize their density and temperature.
Both groups and those at other labs are optimistic about achieving Bose
condensation in the near future. (Discover, February 1993.)
THE HUBBLE SPACE TELESCOPE SERVICING MISSION , planned by NASA for Dec.
1993, will address the problems of the 2.4-m primary mirror's spherical
aberration and the jitter in the solar array, and will carry our various
upgrades such as the replacement of the Wide Field/Planetary Camera. The
aberration, which causes light falling on the outer edge of the primary
mirror to be focused 38 mm farther along the central axis than light from
the center of the mirror, will be undone by the Corrective Optics Space
Telescope Axial Replacement (COSTAR), a module that brings the light into
a much tighter focus. (Astronomy, Feb. 1993.)
APPLICATIONS OF HIGH-TEMPERATURE SUPERCONDUCTING (HTS) materials are
coming along, slowly. Several companies, such as the American Superconducting
Corp. and the Intermagnetics General Corp., now make 100-m bismuth-based
HTS wires able to carry currents of 10,000 A/cm**2. Unfortunately the wires
can't operate yet in high magnetic fields, which precludes their use in
magnets or motors. A temporary solution might be to lower the operating
temperature from 77 K down to 30 K, where magnetic flux lines stay put
and the superconducting state is preserved. Meanwhile, HTS films have been
used in a commercial product, namely SQUID detectors marketed last year
by Conductus. (Science, 15 Jan. 1993.)
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