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S&T News Bulletin - Dec 9



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         SKY & TELESCOPE'S NEWS BULLETIN -- DECEMBER 9, 1994
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CLOSE-CALL ASTEROID

On December 9th, shortly after 19:00 Universal Time, a barn-size asteroid
hurtled through the Earth-Moon system and missed our planet by a scant
100,000 km.  The first warning of its existence had come just 14 hours
earlier when James V. Scotti, using the Spacewatch telescope in Arizona,
picked up a 17th-magnitude speck of light streaking northwestward through
Cetus and Aries. At the time of its closest approach this object, now
designed 1994 XM1, was moving incredibly fast, crossing 1 deg of sky every
3 minutes.

CLOSE-IN ASTEROID

Just three days earlier, Robert H. McNaught found a 16th-magnitude
asteroid near Orion's Belt on Schmidt plates taken at Siding Spring,
Australia.  Orbital measurements later later showed it to be a rare Aten
asteroid, the type that circles the Sun in less time than the Earth does. 
Fewer than 20 Atens are known, and the new find, designated 1994 XL1,
appears to have the shortest revolution period of all -- only 6.5 months. 
Its mean distance from the Sun is 0.67 astronomical unit, slightly less
than the mean orbital radius of Venus! Traveling a route inclined about 30
deg to the ecliptic, this 1994 XL1 is perhaps 300 km across.  It probably
would not have been discovered but for its highly eccentric orbit; on the
night of discovery it was 1.02 a.u. from the Sun, about as far as it ever
gets.

COMET BORRELLY

If you're still tracking Periodic Comet Borrelly, these days you'll find
it skirting northward between Ursa Major and Lynx.  It doesn't get high in
the northeast sky until around midnight, which is just as well considering
the waxing Moon now present in the evening sky.  Its total brightness is
estimated near 8th magnitude at present, according to Charles Morris, who
spotted it through binoculars on December 8th.  A detailed chart for
finder chart appears in December's S&T on page 76, but here are positions
for the coming week, given for 0h Universal time.

          R.A. (2000) Dec.
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Dec 11   9h 18m   +44.7 deg
    13   9  22    +46.3
    15   9  26    +47.9

THICK-SKINNED MOON

This past week the American Geophysical Union held its semiannual meeting
in San Francisco.  There planetary geologist Maria Zuber described lunar
gravity data gathered earlier this year by the Clementine spacecraft.  She
notes that the Moon's crust averages 68 km thick and varies from
essentially 0 km under Mare Crisium to 107 km north of the crater Korolev
on the lunar farside.  Zuber also concludes that the lunar exterior became
rigid early in its existence and that the Moon's interior is probably more
complex than previously thought. But there's no definitive word yet on
whether it has a solid-metal core.

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   *------------------------------------------------------------*
   | Stuart Goldman         Internet: sgoldman@cfa.harvard.edu  |
   * Associate Editor                 mrastro@aol.com           *
   | Sky & Telescope                                            |
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   | Belmont, MA  02178           Magazine of Astronomy         |
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