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Sky & Telescope News Bulletin - 06/06/97

SKY & TELESCOPE NEWS BULLETIN
JUNE 7, 1997

A DRY MOON AFTER ALL?

A new report, published in SCIENCE for June 6th, suggests that there
might not be any water at the lunar poles after all. Radar echoes
collected during the Clementine mission in April 1994 hinted that
patches of ice might lie near the lunar south pole, mixed with dust in
the frigid floors of permanently shadowed craters. But additional
radar work with the 1,000-foot Arecibo radio dish has dimmed that
prospect. Exploiting favorable Earth-Moon geometry in 1992, Donald
Campbell and Nicholas J. S. Stacy used Arecibo's radar beam to peer
into the shadowed craters. The presence of water ice would have
changed the returning beam's polarization in a telltale way, but that
was not seen. "We don't see anything that suggests ice," says
Campbell. However, Clementine scientists are standing by their
results. The truth may be learned later this year, when a spacecraft
called Lunar Prospector returns to the Moon to conduct a sensitive
search for ice in the polar regions. If the Moon proves to be bone
dry, as the Arecibo data suggest, it would place stringent constraints
on the amount of water that could be delivered to the lunar surface by
infalling comets -- miniature or otherwise.

JUPITER: WET & DRY

Results from the Galileo spacecraft have restored scientists' belief
that the clouds of Jupiter contain water. This view had been shaken
when Galileo's probe found virtually no water during its December 1995
descent into the clouds. However, trhe orbiter's near-infrared mapper
has found some areas so humid that, according to investigator Robert
Carlson, "it's either going to rain or it's raining right now." 
Apparently the probe chanced to enter the atmosphere in one of a few
very dry parcels, which together cover only about 2% to 5% of the
planet. These dry spots appear to be areas of downwelling gas. "Winds
rise from the deep atmosphere and lose water and ammonia," explains
Glenn Orton (JPL). "At the top, when they converge and drop back down,
nothing is left to condense into clouds, and a dry clearing is created."

CLOSE-CALL STAR

Using newly released data from the Hipparcos satellite, a team of
astronomers led by Joan Garcia-Sanchez (JPL) has found a small handful
stars that did or will pass within 10 light-years of us. The closest
future approacher will be Gliese 710, now a 10th-magnitude red dwarf
63 light-years away in Ophiuchus. About a million years from now this
star will be only about a light-year from the Sun and will shine at a
brilliant magnitude 0.6. At that distance it may actually pass through
the outer Oort Cloud, sending a deadly shower of comets our way.
Statistically, over time other stars should have come as close or
closer, and the infall of distant comets may have triggered some of
the mass extinctions that have plagued life on our planet.

KING OF THE KUIPER BELT

A sizable body discovered last October in the Kuiper Belt beyond
Neptune has a unique orbit with links to the distant Oort Cloud of
comets. Designated 1996 TL66, this object is currently some 5.2 billion
kilometers (35 astronomical units) from the Sun. Assuming it has a
dark surface, the new find is rather large and could be up to 500 km
across. Discoverers Jane Luu (Harvard University) and her colleagues
calculate that 1996 TL66 is now close to the perihelion of a much more
eccentric orbit, one with a semimajor axis of 84 a.u. and a period of
nearly 800 years. No other known object bridges the void between the
Kuiper Belt and the far more distant Oort Cloud. Whatever its origin,
1996 TL66 undoubtedly represents the first of many such discoveries.
Luu's group estimates that thousands more bodies of comparable size
and distance await discovery within 30 degrees of the ecliptic plane.

HALE-BOPP UPDATE

Observers in the Southern Hemisphere report that Comet Hale-Bopp has
waned to about magnitude 2.5, though it is still obvious to the
unaided eye in deep evening twilight. The comet's once-glorious dust
tail is now just a degree-long stub visible in binoculars.



THIS WEEK'S "SKY AT A GLANCE"

  Some daily events in the changing sky, from the editors of SKY & TELESCOPE

JUNE 8 -- SUNDAY

  * The red long-period variable star R Sagittarii should be at maximum light 
(7th magnitude) around this date.

JUNE 9 -- MONDAY

  * The eclipsing variable star SZ Herculis has an eclipse tonight while 
visible from much of North America. See the June Sky & Telescope, page 76, 
for how to time these events to track the star's behavior. "This was the most 
fun I've had (astronomically) since last year's Perseids," writes David Beard 
of Newmanstown, Pennsylvania, after timing an eclipse of SZ Herculis on the 
night of May 27th. "Observing gets stale without some sort of project to 
direct the attention and lend purpose. This was nearly ideal. It could be 
completed in one evening, served some nominally useful purpose, was 
slow-paced enough to allow plenty of time to see the rest of the sky, and 
involved both field and desk work." All you'll need is the June issue, a 
watch, and a 4-inch telescope.

  * Also tonight, a 6.2-magnitude star in Capricornus, SAO 164249, will be 
occulted by the faint asteroid 170 Maria early Tuesday morning. You can try 
timing the star's disappearance with the smallest of telescopes or even 
tripod-mounted binoculars. The occultation path is predicted to run from Texas 
or Louisiana through Saskatchewan and Alberta from about 9:23 to 9:29 
Universal Time. However, errors in the catalog position of the star or 
asteroid could move the occultation track hundreds of kilometers east or west.

Maria is very small, only about 46 kilometers in diameter, but it is near the 
eastern end of its retrograde loop, so it will be moving slowly against the 
backdrop of stars. The occultation could thus last for up to 12 seconds. A 
finder chart is in the June Sky & Telescope, page 79. For more information see 
the map and table in the February issue page 73. For late prediction updates 
call the voice recording at 301-474-4945 a day or so before the event.

JUNE 10 -- TUESDAY

  * The Moon shines below Regulus this evening.

JUNE 11 -- WEDNESDAY

  * Mars is within 1/3 degree of the 3.8-magnitude star Beta Virginis tonight 
and tomorrow night.

  * Jupiter's moon Io casts its shadow onto Europa from 1:39 to 1:42 Eastern 
Daylight Time Thursday morning. See the June Sky & Telescope, page 78.

JUNE 12 -- THURSDAY

  * First-quarter Moon (exact at 12:51 a.m. Eastern Daylight Time Friday 
morning). Mars is to the Moon's left.

JUNE 13 -- FRIDAY

  * Mars is to the Moon's right as seen from North America. The Moon occults 
Mars for central Africa.

JUNE 14 -- SATURDAY

  * Spica is well to the lower left of the Moon as darkness falls. Mars is 
roughly twice as far to the Moon's right.


 ============================
  THIS WEEK'S PLANET ROUNDUP
 ============================

MERCURY is becoming lost in the glow of sunrise.

VENUS is emerging into evening view. Look for it just above the 
west-northwest horizon about 40 minutes after sunset. It's far to the 
lower left of Capella.

MARS, in the head of Virgo, shines pale orange in the southwest during early 
evening. Mars is fading as it drops far behind Earth in our race around the 
Sun. In a telescope it now appears only about 8.5 arcseconds across.

JUPITER, in Capricornus, rises around midnight daylight saving time. It's 
high in the south at dawn.

URANUS and NEPTUNE are west of Jupiter. They're detectable in binoculars in 
early-morning hours if you have a detailed enough finder chart. Use the one 
in the May Sky & Telescope, page 84.

PLUTO, at the Ophiuchus-Scorpius border, is well up in the south-southeast 
after dark. It's only 14th magnitude.

(All descriptions that relate to your horizon or zenith are written for the 
world's midnorthern latitudes. Descriptions that also depend on your 
longitude are for North America.)


Copyright 1997 Sky Publishing Corporation. S&T's Weekly News Bulletin
and "Sky at a Glance" stargazing calendar are provided as a service to
the astronomical community by the editors of SKY & TELESCOPE magazine.
Widespread electronic distribution is encouraged as long as this
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