S&T News Bulletin - Apr 21



The weather forecast on Neptune calls for changeable skies, according to
astronomer Heidi Hammel.  She has been using the Hubble Space Telescope to
monitor the distant planet, and its appearance has changed radically since
Voyager 2 flew by in 1989.  The Great Dark Spot in Neptune's southern
hemisphere is no longer, but a new dark spot has appeared in the north.
And huge, bright clouds appear, evolve, and disappear from week to week.
Sunlight is about 900 times weaker on Neptune than here on Earth, so it
probably has little effect on the planet's weather.  Instead, the cloud
systems may be spawned by heat rising from the planet's interior, which
radiates twice as much energy to space as Neptune receives from the Sun.


A team of astronomers led by Anita Cochran report that the Hubble
Telescope may have detected extremely faint objects beyond Neptune in what
is called the Kuiper Belt.  The objects are roughly magnitude 28, which
would make them only 20 km or so across.  The astronomers painstakingly
combined 34 HST images, then looked for things moving the way you'd expect
for objects in distant, low-inclination orbits.  This process turned up 59
candidate objects in a field only 4 arcminutes on a side.  If these really
are members of the Kuiper Belt, there must be about 60,000 such objects
per square degree of sky.  That would translate to a total of some 100
million comets traveling in low-inclination orbits and shining brighter
than the HST's magnitude-28 limit.


The annual Lyrid meteor shower is predicted to reach maximum on Saturday
morning, April 22nd.  But light from the last-quarter Moon will interfere
somewhat, and the Lyrids are considered a weak shower anyway. Ordinarily,
even ideal conditions would yield only about 15 Lyrids per hour. But
surprises happen, and in 1982 the hourly rate briefly jumped to a 100 or
more. Some meteor mavens suspect that the Lyrids have a 12-year
periodicity, in which case this could be another surprising year.


There's good news coming out of the Bat Cave, which is what everyone calls
the center used to control the Clementine mission.  According to Trevor
Sorenson, ground controllers regained full control of the spacecraft on
April 10th.  All the systems and sensors seem functional despite having a
dead battery and being in deep freeze for 8 months.  On the 15th the solar
panels were pointed toward the Sun and some test images taken.  "So far,"
Sorenson says, "we have not found anything broken."  The mission team
estimates that the spacecraft has enough thruster gas on board to change
velocity by about 200 meters per second, and various options for an
extended mission are being explored.


The Sun undergoes an annular eclipse on April 29th that will be visible
>from  South America.  In the U.S. partial phases will be visible only from
southern Florida.  See page 73 of April's SKY & TELESCOPE for more

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   | Stuart Goldman         Internet: sgoldman@cfa.harvard.edu  |
   * Associate Editor                 mrastro@aol.com           *
   | Sky & Telescope                                            |
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