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Electronic News Bulletin No. 232 2007 November 25

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

By Alastair McBeath, SPA Meteor Section Director

Initial comments reaching the Section after last weekend's predicted
Leonid maxima suggest few people saw anything at all, other than the
underside of solid cloud-sheets. This situation affected much of the UK
and the eastern USA judging by the correspondence so far. The sole
positive British report as yet was from Martin McKenna in Co. Derry,
Northern Ireland, who watched in between other observations on
November 17-18, but who was quite disappointed with the Leonid rates
he saw. There is a link to Martin's web report from the UK Weather
World's Space Weather Forum topic at: .
However, the preliminary International Meteor Organization "live"
results page, available from the homepage,
suggested Leonid Zenithal Hourly Rates (ZHRs) that night were around
20-30, rising to between 30-40 on November 18-19. These values are
only first estimates and are liable to change as fresh data comes through,
but they do suggest Leonid rates were at least somewhat better than the
normal ~15 on both nights. Nothing till now indicates a specific peak
around 23h-00h UT on November 18-19, as mentioned in ENB 230
(archived at: ), but elevated Leonid counts do
seem to have been present about as anticipated generally.

Grateful thanks go to all those who have sent in reports on how
(frequently badly!) their Leonid observing efforts went. All further
positive results from the shower would be very welcome. Details of
what to send and where to can be found on the Section's webpages off
the Meteor homepage, .

By Alastair McBeath, SPA Meteor Section Director

The month's main meteoric event is expected to be the Geminid
maximum, due on December 14, within about 2h20m of 16:45 UT. The
waxing crescent Moon will set between 20h-21h UT across the UK
then, while the peak's timing is still during astronomical twilight here,
and although the radiant will have risen by then from most places, it will
be unhelpfully low till roughly moonset. The shower's strength means
something of the peak may be visible despite this, but maybe only a few
meteors per hour. Watching is likely to be more rewarding from
moonset onwards on December 14-15, particularly as Geminid rates
often stay near maximum levels for 6-10 hours to either side of the peak.
After mid-evening, the radiant remains well on-view for the rest of the
night, culminating around 02h UT. Highest ZHRs should be circa 120
judging by recent returns, so with luck and clear, dark skies, observed
Geminid rates might be approaching a meteor every minute or two by
late evening then. Meteor activity from the shower normally remains
good, if lower, for a night or two before, and sometimes a night after,
the maximum in an average year, with effectively no moonlight for most
of the night on all these dates in 2007. Geminids are medium speed and
often bright, though few leave persistent trains. Much lower Geminid
rates may be seen away from the peak in any dark skies from
December 7 to 17. The shower's parent object, Apollo asteroid
3200 Phaethon (possibly an extinct comet) passes closest to Earth since
its 1983 discovery on December 10, 0.12 astronomical units away
(about 18 million kilometres). It will be around magnitude +13 in Virgo,
but it is not expected this relative proximity will have any effect on
Geminid rates.

After the Geminids have faded away, this year's Ursid peak is scheduled
to fall between ~01:00-03:30 UT on December 21-22, when ZHRs
may be 10-50 or so (they were ~30 for a time in both 2000 and 2006
recently - see ENBs 211 and 212 for the 2006 event, archived at and respectively).
However, only at rare returns does the activity exceed ~10-15. While
this peak timing is very favourable for the UK, the bright Moon, visible
all night (full on December 24), is not, particularly as typical Ursid
meteors are faint, and of slow to medium speed. Ursid activity usually
runs from December 17-26 overall, but is typically low to undetectable
early and late in this spell. The shower's radiant at maximum is near
Kochab, the brighter of the two "Guard" stars in Ursa Minor, so is
circumpolar from the UK, though highest towards dawn. Like the
Geminids, the Ursid parent body is nearby this December, Comet
8P/Tuttle, at perihelion on 2008 January 26, but is brightest and closest
to Earth on New Year's Day, perhaps reaching magnitude +6 near then,
in Pisces. It will be close to Gamma Cassiopeiae on December 21-22.

More information on these and other showers active in December,
including radiant charts, can be found among the SPA's Meteor
webpages, at: . Good luck, and clear skies!

New Scientist

A system intended to spot impending asteroid impacts mistook ESA's
Rosetta spacecraft for an incoming rock and issued an alert that a
near miss was imminent. The Rosetta spacecraft is intended to visit
comet Churyumov-Gerasimenko in 2014. On its way, it must pass by the
Earth three times to obtain the momentum needed to reach the comet.
It is due to pass only 5300 kilometres above the Pacific Ocean at
2100 UT on Tuesday December 13. During the close approach, Rosetta
will look for shooting stars in the upper atmosphere and observe the
magnetosphere. It will also image urban regions in Asia, Africa and
Europe, and measure the light reflected from the Moon.

A fortnight ago three observatories in the US noticed an unidentified
object, heading our way. That prompted the Minor Planet Center in
Cambridge, Massachusetts, which coordinates the search of near-Earth
objects, to issue a potential near-miss warning. The warning was
later retracted after Moscow-based astronomer Denis Denisenko noticed
that the object's trajectory closely matched Rosetta's. The mix-up
caused a few red faces, but the Minor Planet Center complains that the
incident "highlights the deplorable state of availability of
positional information on distant artificial objects". The lack of a
centralised database makes checking incoming objects against known
space probes difficult; it is not clear, however, who should compile,
maintain, and pay for such a database.

New Scientist

Supernova 2006gy was seen in 2006 September in a galaxy 240 million
light-years away. The blast was 100 times more powerful than a normal
supernova, suggesting the exploding star was more than a hundred times
the mass of the Sun. But a puzzling aspect of the supernova was that
the debris contained large amounts of hydrogen, whereas such a massive
star should have shed its outer hydrogen layers at an earlier stage.

Astronomers at the University of Amsterdam now suggest that 2006gy may
have been the result of a multiple star collision in a dense stellar
cluster. They say dozens of stars, some of them hydrogen-rich, could
have collided to form a giant of over 100 Suns, which blew up in an
explosion that outshone its home galaxy. Computer simulations suggest
that multiple collisions are not improbable in very dense star
clusters. Our own galaxy contains two such super-dense clusters (the
Arches cluster and the Quintuplet cluster), close to its centre. It
may be significant that 2006gy occurred close to the core of its
galaxy. If the astronomers are right, the dense cluster of stars
should become visible once the supernova has faded sufficiently. as
should happen within a few years.

Astronomers at the University of California, however, have put forward
an alternative proposal. They reckon to have shown how multiple
explosions in a single, very massive star could account for 2006gy's
behaviour. In their model, each explosion produces an expanding shell
of material, and when new ejecta catch up and collide with an older
shell, so much energy is released that the result will look like an
over-luminous supernova.


Gemini consists of two 8-metre telescopes -- one in Hawaii and one in
the Chilean Andes -- which together can be used to observe the entire
sky. The UK invested about 35 million pounds in the capital phase of
the Gemini telescopes, in which we have a 23% stake. In 2002,
Britain also joined the European Southern Observatory, which runs
telescopes in Chile but none in the north. Now, British astronomers
are objecting to a plan being considered by the government research
council that funds British astronomy for the UK to withdraw from the
Gemini partnership, a 15-year agreement whereby Britain shares the
telescopes with America, Canada, Australia and three South-American
countries. The astronomers say that they will no longer be able to
make important observations in the northern hemisphere.

BBC News

The European Space Agency (ESA) has drawn up a shortlist of what it
thinks are the best places to look for life on Mars, potential landing
sites for a mission called ExoMars, to be launched in 2013. A rover
having a mass of just over 200kg will search for evidence of past or
present life. It will have a drill that can go 2m below the surface.
The shortlisted sites have some of Mars' oldest rocks, which were in
contact with water just after the planet's formation. Of particular
interest is a class of hydrated minerals known as phyllosilicates, or
clay minerals, which contain water in their crystalline structure.
The sites are: Mawrth Vallis -- an ancient valley covered with
light-coloured clay-rich minerals; Nili Fossae -- large eroded surface
fracture partially filled with clay-rich debris from a space impact;
Meridiani Planum -- where the rover Opportunity found possible
evidence for an ancient sea; Holden Crater -- an ancient lake bed with
layered sedimentary deposits; and Gale Crater -- an impact crater with
exposed layered deposits. NASA has an analogous shortlist of six
landing sites for its Mars Science Laboratory mission, due to fly in
late 2009. However, there is no chance of the US and Europe choosing
the same landing site.

University of Arizona

Eight white-dwarf stars detected between 1,000 and 2,000 light-years
away represent a previously unknown category of stars. White dwarfs
result from the collapse of star cores in dying stars whose nuclear
fusion has ceased. They usually have a mass about that of the Sun,
but are only a bit larger than the Earth because they have blown off
their outer layers; what is left behind is a small, dim and extremely
dense core made largely of carbon and oxygen surrounded by an
atmosphere of hydrogen or helium. The eight newly described ones have
atmospheres primarily of carbon, with little or no trace of hydrogen
or helium. They may have formed from stars much more massive than the
Sun but not quite massive enough to explode as supernovae. There are
at least a couple of other white dwarfs that might fit into the new
category; all of them are among about 10,000 new white dwarfs
recently identified in the Sloan Digital Sky Survey.

BBC News

A study by astronomers at the University of Florida suggests that
moons like the Earth's, which may have formed in catastrophic
collisions, are rare. The currently favoured theory holds that the
Moon was created when an object the size of Mars collided with the
Earth billions of years ago. The impact hurled into orbit a lot of
debris, some of which eventually consolidated to form the Moon.
When a moon forms from a violent collision, dust should be blasted
everywhere, so if there were lots of moons forming, dust should be
observed around lots of stars.

Using the Spitzer Space Telescope, the astronomers searched for the
dusty signs of similar collisions around 400 stars that are all about
30 million years old. That is roughly the age that our Sun is
supposed to have been when the Moon was formed. They found only
1 out of the 400 stars to be immersed in the telltale dust. Taking into
consideration the amount of time that the dust should endure, and the
age range at which moon-forming collisions might occur, the scientists
estimated the probability of a solar system making a satellite like
the Earth's to be at most 5 to 10 per cent.

The Register

The Messenger probe is nearing the halfway point of its 7.9-billion-
kilometre journey, which when completed will make it the first man-
made object to orbit the planet Mercury. Since its launch in 2004
August, Messenger has flown by the Earth once and Venus twice. Now
it is nearing its goal: on 2008 January 14 it will fly within 200
kilometres of the surface of Mercury, making it the first spacecraft
to pass the planet since Mariner 10 did so in 1974. Messenger will
then make two additional passes by Mercury and three deep-space
manoeuvres, which together will slow the spacecraft down enough to
enter an orbit round Mercury on 2011 March 18. Mariner 10 also flew
past the planet three times, but was able only to photograph 45 per
cent of the surface and carried out no other scientific investigation.
Among the goals of the present mission are mapping the elemental and
mineralogical composition of Mercury's surface, global imaging of the
surface at a resolution of hundreds of metres or better, and
determining the structures of the planet's gravitational and magnetic
fields. In less than two months from now it is expected to begin to
transmit close-up views of Mercury.

Bulletin compiled by Clive Down

(c) 2007 the Society for Popular Astronomy

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