S&T News Bulletin - Feb 25

I sent this Friday, but I never saw it appear, so here it is (again):



Astronomers using the Hubble Space Telescope have found that Europa, one
of Jupiter's four large satellites, is enveloped by an extremely tenuous
atmosphere of molecular oxygen.  The discovery is not altogether
unexpected, since Europa is thought to be covered with water ice.  When
sunlight, cosmic rays, and micrometeorites strike the moon's icy surface,
water molecules are blasted into space and quickly dissociated into
hydrogen and oxygen.  The hydrogen escapes readily, leaving the heavier
oxygen behind.  But it's really a stretch to even call this an atmosphere:
according to team leader Doyle Hall, the surface pressure on Europa is
barely a hundred billionth that on Earth.  The discovery by HST
complements the finding of oxygen on the neighboring moon Ganymede, which
was detected last year by ground- based telescopes.  Ganymede probably
doesn't have an atmosphere, however, as its surface is colder than
Europa's.  Instead, Ganymede's oxygen is most likely a contaminant within
water ice lying on the surface.  For comparison, Europa is a bit smaller
than Earth's Moon, while Ganymede has a diameter about half again larger.


The Clementine spacecraft, which fell silent last year soon after leaving
lunar orbit, appears to be alive after all.  On February 20th tracking
engineers used one of the 70-meter antennas in NASA's Deep Space Network
to reestablish contact for about 50 minutes.  Details are still sketchy,
but apparently enough telemetry was received to confirm that the
spacecraft continues to function.  This was a surprise to everyone.  The
spacecraft left lunar orbit last May 3rd after a phenomenally successful
mapping effort, but two weeks later a computer malfunction caused the
depletion virtually all of its thruster gas. Spinning rapidly and out of
control, Clementine appeared doomed. Frantic attempts to correct the
problems failed. On May 27th the derelict spacecraft flew within 400 km of
the Moon and was flung into heliocentric orbit, its planned flyby of the
asteroid Geographos no longer possible.


If all goes according to plan, the Space Shuttle *Endeavour* will be
launched at 1:37 a.m. EST on March 2nd.  That will mark the beginning of a
record-breaking 16-day mission devoted largely to astronomy.  The
principal payload is Astro 2, a suite of three telescopes optimized for
studies at ultraviolet wavelengths.  These flew on the shuttle once
before, in 1990, but the mission was marred by a number of equipment
glitches that reduced the scientific yield.  Many lessons were learned,
and this time the crew and their ground support will be better prepared. 
Observations are planned of objects as near as the Moon and as distant as
a quasar 10 billion light-years away.

The News Bulletin is provided as a service to the amateur-astronomer
community by Sky & Telescope magazine.  Electronic distribution is
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   | Stuart Goldman         Internet: sgoldman@cfa.harvard.edu  |
   * Associate Editor                 mrastro@aol.com           *
   | Sky & Telescope                                            |
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