Topic: Mid March Astronomy Bulletin

[Clive]

ASTEROID FLYBY
Spaceweather.com

A newly-discovered asteroid 2009 DD45 came within 72,000 km
(0.00048 AU) of the Earth on March 2.  That's only twice the height of
a geostationary communications satellite.  The asteroid is thought to
be 30 to 40 metres across, perhaps similar in size to the Tunguska
impactor of 1908.  Although the object passed very close by
astronomical standards, it was still more than 11 Earth radii away.
Statistically, therefore, only one in 11-squared passes -- less than 1%
-- within that distance would result in an actual impact.


NEWLY FOUND MOON LINKED TO SATURN'S G RING
NASA

The Cassini spacecraft has found within Saturn's G ring a faint
embedded moonlet.  Scientists believe that it is a main source of the
G ring and its single ring arc.  The G ring is one of the outer
diffuse rings.  (Saturn's rings have been named in the order in which
they were they were discovered, and the nomenclature now appears to be
less than systematic; working outwards they are D, C, B, A, F, G and
E.)  Within the faint G ring there is a relatively bright and narrow,
250-kilometre-wide arc of ring material, which extends one-sixth of
the way around the ring's circumference.  The moonlet moves within
that arc.  Previous Cassini plasma and dust measurements already
indicated that the partial ring may be produced from relatively large,
icy particles embedded within the arc, such as the recently recognised
moonlet.

The scientists first saw the moonlet on 2008 August 15, and were able
to confirm it by finding it in two earlier images.  They have since
seen it on several occasions.  It is too small to be resolved by
Cassini's cameras, so its size cannot be measured directly, but has
been estimated by comparison of its brightness with that of another
small Saturnian moon, Pallene.  They have also found that the
moonlet's orbit is being disturbed by the larger, nearby satellite
Mimas, which keeps the ring arc together.  The new discovery brings
to three the number of Saturnian ring arcs with embedded moonlets
found by Cassini.  The new moonlet may not be alone in the G-ring arc;
previous measurements with other Cassini instruments implied the
existence of a population of particles, possibly ranging in size from
1 to 100 metres.  Early next year, Cassini's will be in a position to
take a closer look at the arc and the moonlet.


PLUTO'S LOWER ATMOSPHERE INVESTIGATED
ESO

Using the Very Large Telescope, astronomers have made observations
of the atmosphere of Pluto.  They found unexpectedly large amounts
of methane in the atmosphere, and also discovered that the atmosphere
is hotter than the surface by about 40 degrees, although it still
reaches only a frigid minus 180 degrees Celsius.  Those properties of
Pluto's atmosphere may be due to the presence either of patches of
pure methane or else of a methane-rich layer covering the surface.
Pluto, which is about a fifth the size of the Earth, is composed
primarily of rock and ice.  As it is about 40 times further from the
Sun than the Earth on average, it is very cold, with a surface
temperature of about minus 220 degrees Celsius.

It has been known since the 1980s that Pluto has a tenuous atmosphere,
which consists of a thin envelope of mostly nitrogen, with traces of
methane and probably carbon monoxide.  As Pluto moves away from the
Sun during its 248-year orbit, its atmosphere gradually freezes and
falls to the ground.  In periods when it is closest to the Sun -- as
it is now -- the temperature of the solid surface rises, causing the
ice to sublimate into gas.  Until recently, only the upper parts of
the atmosphere of Pluto could be studied.  By observing stellar
occultations, when the planet as it moved across the sky passed in
front of a background star, astronomers were able to demonstrate that
Pluto's upper atmosphere was some 50 degrees warmer than the surface,
or minus 170 degrees Celsius.  Those observations could not shed any
light on the atmospheric temperature and pressure near Pluto's
surface, but the new observations made with an infrared spectrograph
have indicated that the atmosphere as a whole, not just the upper
layers, has a mean temperature of minus 180 degrees Celsius, and so it
is indeed 'much hotter' than the surface.  The infrared observations
also indicate that methane is the second-most-common gas in Pluto's
atmosphere, representing 0.5% of the molecules.


QUASAR COLOURS REVEAL DUSTY UNIVERSE
Science Daily

Intergalactic space appears to be filled with a haze of tiny, smoke-
like 'dust' particles that dim the light from distant objects and
subtly change their colours, according to a team of astronomers from
the Sloan Digital Sky Survey.  Galaxies contain lots of dust, most of
it formed in the outer regions of dying stars.  The surprise is that
dust is observed hundreds of thousands of light-years outside the
galaxies, in intergalactic space.

The Sloan team considered the colours of distant quasars whose light
passes in the vicinity of foreground galaxies on its way to the Earth.
Dust grains block blue light more effectively than red light; it
appears that quasars are reddened by intergalactic dust, and the
reddening extends up to ten times the radii of the foreground galaxies
themselves.  Supernova explosions and outflows from massive stars
drive gas out of some galaxies and the gas may carry dust with it, but
it is also possible that the dust may be pushed directly by the
radiation pressure of starlight.