Topic: Mid October Astronomy Bulletin

[Clive]

APPROACHING COMET
Spaceweather.com

Comet 103P/Hartley 2 is approaching the Earth for a close encounter on
October 20, when it will be only 11 million miles (0.12 AU) away and
may be dimly visible to the naked eye from dark-sky sites.  Images can
be found at http://spaceweather.com  .  The comet has a green coma
almost 0.5 degrees in diameter and is currently about 7th magnitude.
It can be seen throughout the night in the constellation Cassiopeia.
A chart by Sky and Telescope can be found at
LINK .  On November 4 the EPOXI spacecraft
(formerly called Deep Impact) will pass only 435 miles from the
comet's active icy nucleus.


'POTENTIALLY HABITABLE' PLANET
University of California

Astronomers have identified what they describe as the most promising
candidate yet for a 'habitable' planet outside our solar system.  It
orbits the red-dwarf star Gliese 581, 20 light-years away in the
constellation Libra.  Gliese 581 has had a somewhat chequered history
of 'habitable-planet' claims.  Two previously detected planets in the
system lie at the edges of the 'habitable' zone.  One, designated
planet c, is on the hot side of that zone, that is to say, if there
were any water there it would be liable to be in the form of steam;
another, d, is on the cold side, where any water would be liable to be
ice.  While some astronomers would still like to think that d might be
'habitable' if it has a thick atmosphere that helps warm it, others
are sceptical.  The newly discovered body is designated g.  To a
certain optimistic class of astronomers, a 'potentially habitable'
planet is one where life might be sustainable, but it certainly would
not necessarily be one that even they would consider to be a nice
place to live.  'Habitability' really depends on many factors, but the
only one that is suggested to be potentially acceptable about the
planet referred to here (and only over a small area, at that) is that
there is probably an area that is sometimes at a temperature at
which water would be liquid as opposed to being either ice or steam.
The use of such an emotive term as 'habitable', merely to describe a
region (which is bound to exist within a certain range of distances
from any hot object) where the temperature is such that water (if
there were any there) might exist in liquid form, may be supposed to
represent a deliberate effort on the part of the people who use it to
encourage sources of funding believe that they are on the point of
discovering Life on Other Worlds.  In actual fact there is of course a
lot of difference between being at the sort of temperatures that we
are used to on the Earth and being the abode of Little Green Men, but
the difference tends to be swept under the carpet.

The new findings also include a report of a second new planet that
brings to six the total number of planets thought to surround Gliese
581.  All are calculated to have nearly circular orbits, which are
claimed to promote stable climates, although even _that_ is not so
obvious from the one example that we know and have at hand.  Gliese
581g, the newfound 'potentially habitable' one, has a mass equivalent
of three or four Earths and orbits its star in 37 days, so the seasons
(if any) would come round very rapidly.  The planet is tidally locked
to the star, meaning that one side is always facing the star and
baking in perpetual daylight, while the side facing away from the star
is in permanent darkness.  The only 'habitable' zone on the planet^s
surface might be the boundary between shadow and light, known as the
terminator, with surface temperatures freezing on the dark side and
boiling on the light side -- rather a drawback that might suggest that
'habitability' is a bit qualified even in this prize case.


MAGELLANIC STREAM
Harvard-Smithsonian Center

The Magellanic Stream is an arc of hydrogen gas spanning more than
100° of the sky; it trails behind the Milky Way's neighbouring
galaxies, the Large and Small Magellanic Clouds.  Our galaxy, the
Milky Way, has been thought to be the dominant gravitational force in
forming the Stream from the Magellanic Clouds, but a new computer
simulation indicates that the Magellanic Stream resulted from a past
close encounter between the two dwarf galaxies rather than effects of
the Milky Way.

Previous models required the Magellanic Clouds to complete an orbit
about the Milky Way in less than 2 billion years in order for the
Stream to form.  Measurements from the Hubble telescope seem to
rule out such an orbit, and suggest that the Magellanic Clouds are new
arrivals and not long-time satellites of the Milky Way.  To try to see
how the Stream could form without relying on a close encounter with
the Milky Way, theoreticians set up a simulation assuming that the
Magellanic Clouds were a stable binary system on their first passage
about the Milky Way.  They postulated that the Magellanic Stream and
the 'Bridge' between the two Clouds are similar to the tail and bridge
structures seen in other interacting galaxies and formed before the
Milky Way captured the Magellanic Clouds.  According to the
simulation, while the Clouds did not actually collide, they came close
enough together for the Large Cloud to pull large amounts of hydrogen
away from the Small Cloud.  That tidal interaction gave rise to the
Bridge that we see between the Clouds, as well as the Stream.

The model may be held to indicate that tidal interactions between
dwarf galaxies may be a powerful mechanism to change the shapes of
those galaxies without the need for repeated interactions with a
massive galaxy like the Milky Way.  While the Milky Way may not have
drawn the Stream material out of the Magellanic Clouds, the Milky
Way's gravity does now shape the orbits of the Clouds and thereby
controls the appearance of the tail.


CHINA LAUNCHES MOON MISSION
BBC News

China has launched a rocket carrying the Chang'e-2 probe destined for
the Moon.  Chang'e-2 will be used to test key technologies and collect
data for future landings.  The probe is to circle just 15 km above the
surface in order to take photographs of possible landing sites.  It is
China's second lunar probe; the first was launched in 2007 and stayed
in space for 16 months before being intentionally crashed on to the
Moon's surface.  China launched its first manned flight into low
Earth-orbit in 2003; two more followed, with the most recent one
in 2008.  China says it will send a rover on its next lunar mission,
and it has ambitions to send astronauts later.


LOST FOOTAGE OF APOLLO 11
NASA

Long-lost footage of Neil Armstrong descending the ladder of the
Apollo 11 lunar module has been screened in public for the first time
in Sydney this week.  The film runs for a few minutes and is
considered to be some of the best recording of the historic 1969
Moon-walk, but the film was lost in archives for many years and was
badly damaged when found.  It depicts the first few minutes of
Armstrong's descent, which was recorded in Australia while NASA was
still scrambling for a signal, and shows a far clearer image than was
initially screened worldwide.

Instrumentation in Australia played a key role in the Apollo 11
mission, including provision of the television signal, after Armstrong
decided to attempt the moonwalk early, putting the United States just
beyond the horizon.  John Sarkissian -- historian and astronomer in
charge of the Australian side of the recordings restoration project --
said the unseen minutes were the "best quality of Armstrong descending
the ladder.  NASA were using the Goldstone (California) station
signal, which had its settings wrong, but in the signals being
received by the Australian stations you can actually see Armstrong",
he said.  "In what people have seen before you can barely see
Armstrong at all, you can see something black -- that was his leg."

There was a "long detective story" involved in the search for the
film, and Sarkissian said that it took painstaking frame-by-frame work
to shift the material from the deteriorating black-and-white film to
digital format.  Digitising the recording was "significant in the
space-flight history context", allowing it to be preserved and copied
for future generations.