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The SOCIETY for POPULAR ASTRONOMY
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Electronic News Bulletin No. 215 2007 February 18
====================================================


Here is the latest round-up of news from the Society for Popular
Astronomy. The SPA is Britain's liveliest astronomical society, with
members all over the world. We accept subscription payments online
using our secure site and can take credit and debit cards. You can
join or renew via a secure server or just see how much we have to
offer by visiting http://popastro.c.topica.com/maafSrPabwDtXcixLLVb/


SPECIAL MEMBERSHIP OFFER

If you are not already a member of the SPA now is a great time to
join! We are offering to those who receive ENBs and are not already
members a SPECIAL JOINING PRICE of only £12.00 saving £3.00 on the
usual UK annual rate. (Overseas rates vary but discount still applies
-- see our website for details.) You will also receive FREE, THREE
CD-Roms -- 'Window on the Universe' Parts 1 and 2 and 'Window on
the UK'.

Join NOW by going to our secure website at www.popastro.com and click
the 'join now' button. To claim the your discount enter ENB017 when
asked for your voucher reference. To join by post send your details
with payment (£12.00 -- 1 year or £24.00 -- 2 years -- UK rate) to SPA
Membership, 36 Fairway, Keyworth, Nottingham, NG12 5DU, quoting
reference ENB017. Please note -- this offer is valid only until 2007
February 28 and does not apply to renewals of membership.



FIREBALLS UPDATE
By Alastair McBeath, SPA Meteor Section Director

Back in ENB 207, I gave the most recent update on the impressive,
magnitude -5/-9 fireball from 22:54 UT on 2006 July 18-19, that
was widely seen from SE England, Belgium and the Netherlands. A
more detailed review of the event's trajectory has now been published
in the first issue of the Arbeitskreis Meteore's (AKM's) journal
"Meteoros" this year (Vol. 10, No. 1, pp. 25-28; in German) by
AKM fireball and meteorite analyst Dieter Heinlein, and Pavel Spurny
of the Astronomical Institute at Ondrejov in the Czech Republic, the
coordinating centre for the European Fireball Network (EFN) of
all-sky cameras, which Network has operated continuously since the
1960s.

The fireball's trail was imaged by five EFN stations, four in western
Germany, and Klaas Jobse at Oostkapelle in the Netherlands. Data
from these showed the fireball had an atmospheric trajectory that
started at 99.5 km altitude above western Belgium, at 50.667 deg
N, 3.669 deg E, and ended at 45.2 km altitude above the North
Sea between Holland and England at 51.924 deg N, 2.327 deg E, all
of which parameters were pleasingly close to those preliminary values
calculated earlier by SPA analyst David Entwistle, based on details
prepared by Belgian VVS Meteor Director Chris Steyaert, given in
ENB 207. The object's atmospheric entry velocity was 37.7 km/sec,
decelerating markedly to 20 +/- 1 km/sec by the end. Its orbit, which
was unusually elongated - aphelion was found to be ~ 7 Astronomical
Units from the Sun - could not be correlated with any known
meteoroid stream, though as a whole, its character was similar to those
of the short-period comets.

Moving closer to the present, early February has been trying to outdo
early January for the number of fireballs reported to the Section. The
February 3-4, ~20:31 UT event, featured in the Special ENB of
February 10 now has sightings from 22 separate places recorded in the
Section files, most concentrated in Wales and SW England, but among
the more recent outlying reports has come one from Co Down in
Northern Ireland. Unfortunately, the actual path of the object continues
to prove elusive, with just a generally east to west trending track high
above the Celtic Sea and/or St George's Channel still the only close
match that can be suggested.

A much better probable trajectory has been established for a
fragmenting, magnitude -7/-10 event on February 6-7, at 19:56 UT,
because on this occasion, part of the trail was imaged too, by Section
contact Klaas Jobse in Holland again. Details from Klaas' image can
be found on his website:
http://popastro.c.topica.com/maafSrPabwDtYcixLLVb/ Six UK visual
reports have come in too, from South London, Buckinghamshire,
Hampshire, Berkshire and Cheshire, plus there were other visual
sightings made in the Netherlands. More details on the likely
trajectory, including a sketch map of the projected surface path, are
available on the "Recent Fireball Sightings" page of the SPA website,
http://popastro.c.topica.com/maafSrPabwDtZcixLLVb/ , but the essentials
are that the trail probably
began at around 80 km altitude above the southern North Sea at
approximately 51.5 degrees N, 2 degrees E. From there, the object
flew northwestwards on a very shallow trajectory, only about 8
degrees from the horizontal, to end at ~ 61 km altitude, ~ 10 km east
of Thetford, Norfolk, close to 52.4 degrees N, 0.8 degrees E, as a
best-estimate. The mean atmospheric velocity for the photographed
part of the trail, not allowing for atmospheric deceleration, was
~ 24 km/sec. There is little likelihood any meteorites would have
survived from this unfortunately, and the very shallow approach angle
means the potential fall zone is enormous - possibly anywhere from the
northern Pennines to the southern Grampians, assuming this trajectory
estimate is correct.

February 7-8 brought two more fireballs, the first at about 19:40 UT
(seen from Cheshire and Cumbria, magnitude -3 or brighter), the
second around 21:00 UT (very bright, from Falkirk), then February
12-13 produced a third sometime between 22:31-22:34 UT (brighter
than magnitude -3; Wiltshire).

Fireball reports (meteors of at least magnitude -3) from the British
Isles
and nearby are always welcomed by the Meteor Section. Notes of
what data is most essential and where to submit it can be found on the
Section's Fireball Observing webpage at:
http://popastro.c.topica.com/maafSrPabwDt0cixLLVb/


LUNAR OCCULTATION OF STARS WITHIN THE PLEIADES OPEN CLUSTER (M45)
Friday, 2007 February 23
By Jon Harper, Occultation Section

This is a good opportunity for astro-photographers to take some
interesting pictures of the very broad crescent Moon (9 hours before
First Quarter), in juxtaposition with the bright members of one of the
most famous open clusters. Faint Earthshine on the Moon's dark
hemisphere will enhance the spectacle. If you have a telescope you
will have many chances to practice your timing skills, or simply to
watch the stars disappear one by one as the Moon travels eastwards
through the north-western part of the cluster. The UT times of the
expected disappearances of the brighter members follow. The first
time given is that at Greenwich, the second is for Edinburgh:

Celaeno (16 Tauri) m(v) 5.4 22h 53m, 22h 41m
Taygeta (19 Tauri) m(v) 4.3 22h 56m, 22h 49m
Asterope (21 Tauri) m(v) 5.8 23h 15m, 23h 15m
Maia (20 Tauri) m(v) 3.9 23h 15m, 23h 06m

If you need a more detailed set of predictions for your locality, and
of fainter stars due to be occulted, then please get in touch with me
and I can send these as a MS Excel document.

Please send any timings, observations and graphics you may obtain to
me also if you can. I would be very pleased to receive them. There
is a downloadable report form at:
http://popastro.c.topica.com/maafSrPabwDt1cixLLVb/

My e-mail address is jonvran@aol.com.


SATELLITES OF NEPTUNE
IAU

The International Astronomical Union has named recently discovered
satellites of Neptune as follows:

Neptune IX Halimede = S/2002 N 1
Neptune X Psamathe = S/2003 N 1
Neptune XI Sao = S/2002 N 2
Neptune XII Laomedeia = S/2002 N 3
Neptune XIII Neso = S/2002 N 4


WEBB TELESCOPE MIRRORS READY FOR POLISHING
NASA

The James Webb Space Telescope's 18 hexagonal mirror blanks are ready
to be polished. They will form a mirror 6.6 m in diameter, more than
two and a half times the diameter of the Hubble primary mirror, but it
will weigh only about half as much. It cannot fit into a rocket when
assembled, so the 18 segments are designed to fold, much as the leaves
of a drop-leaf table, and will be unfolded in space.


SATURN'S MOON ENCELADUS SANDBLASTS NEIGHBOURS
New Scientist

Enceladus, a Saturnian satellite 513 km in diameter, has been found to
be the source of ice particles that form Saturn's E ring, which is
hundreds of thousands of kilometres across. In 2005, observations
made with the Hubble telescope indicated that Enceladus and some of
its neighbouring moons are extraordinarily reflective. Now, a new
study suggests that high-speed ice particles from Enceladus are
'sandblasting' its neighbours to make their surfaces so white.

The researchers found that the moons in the densest part of the E ring
-- Enceladus and Tethys -- are also the most reflective. Mimas, Dione
and Rhea, which are in more tenuous parts of the E ring, are less
reflective, but still brighter than moons such as Epimetheus and
Janus, which orbit outside the ring altogether. The team argues that
the level of reflectivity depends on how many ring particles slam into
the moons. When the icy, micron-sized particles strike the ice-rich
surfaces of the moons at speeds of several kilometres per second, they
stir up fresh ice that constantly re-coats the moons' surfaces and
prevents their being darkened by the constant bombardment by charged
particles from the Sun.


A LITTLE LIGHT IS SHED ON DARKEST GALAXIES
BBC News

Scientists have been worrying over the large amounts of seemingly dark
matter in many galaxies. There are sometimes serious discrepancies
between different estimates of galactic masses, in the sense that the
mass judged from a galaxy's internal velocity dispersion may be
substantially greater than that attributable to its observable stars.
Dwarf spheroidal galaxies, of which faint examples have been
discovered orbiting the Milky Way and Andromeda galaxies, exhibit such
discrepancies. The scientists tried to simulate on a computer what
may happen when an initially gas-rich dwarf galaxy interacts with a
larger, Milky-Way-sized one. It has been recognised for a long time
that the drag force, or 'ram pressure', felt by the small galaxy as it
moves through the more massive one must strip away the dwarf galaxy's
interstellar gas. The recent model also indicated that the gravity of
the larger system would commandeer many of the dwarf system's luminous
stars. The result, said the team, would be a galaxy where most of the
visible matter was absent, leaving mainly dark matter behind; but that
still did not identify the actual nature of that matter.



Bulletin compiled by Clive Down

(c) 2007 the Society for Popular Astronomy


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