Topic: Mid - February Astronomy Bulletin

[Clive]

ANATOMY OF AN ASTEROID
ESO

Astronomers using measurements taken with the New Technology Telescope
(NTT) at La Silla in Chile have found evidence that an asteroid has a
varied internal structure: different parts of the asteroid (25143)
Itokawa have different densities.  The team has measured the speed at
which that small near-Earth asteroid spins and how that spin rate is
changing over time.

Itokawa is an intriguing object, as it has a strange peanut shape,
revealed by the Japanese spacecraft Hayabusa in 2005.  To probe its
internal structure astronomers used images gathered from 2001 to 2013,
by the NTT and other telescopes, to measure its brightness variation
as it rotates.  The data were used to derive the asteroid's spin
period and how it is changing over time.  In combination with
knowledge of the asteroid's shape, that brought to light the
complexity within its core.  The spin of asteroids and other small
bodies in space can be affected by sunlight.  That phenomenon, known
as the YORP effect (an acronym for its discoverers), occurs when
absorbed sunlight is re-emitted from the surface of the object in the
form of heat.  When the shape of the asteroid is very irregular the
heat is not radiated evenly, and that creates a tiny, but continuous,
torque on the body and changes its spin rate.  The astronomers found
that the YORP effect was slowly accelerating the rate at which Itokawa
spins.  The change in rotation period is tiny -- a mere 0.045 seconds
per year.  But it was very different from what was expected and can
only be explained if the two parts of the asteroid's peanut shape have
different densities. Until now, the properties of asteroid interiors
could be inferred only from rough overall density measurements.  The
glimpse into the diverse innards of Itokawa has led to speculation
regarding its formation.  One possibility is that it formed from the
two components of a double asteroid after they bumped together and
merged.  Finding that asteroids can have inhomogeneous interiors has
far-reaching implications, particularly for models of binary-asteroid
formation.


'ROGUE ASTEROIDS' MAY BE THE NORM
Massachusetts Institute of Technology

To get an idea of how the early Solar System may have formed,
scientists often look to asteroids.  Those relics of rock and dust
represent what today's planets may have been like before they
differentiated into bodies of core, mantle, and crust.  In the 1980s,
scientists' view of the Solar System's asteroids was essentially
static: asteroids that formed near the Sun remained near the Sun,
while those that formed farther out stayed on the outskirts.  But in
the last decade, astronomers have detected asteroids with compositions
unexpected for their locations in space.  Scientists considered them
to be anomalous 'rogue' asteroids.  But a new map developed by
researchers from MIT and the Paris Observatory charts the size,
composition and location of more than 100,000 asteroids throughout the
Solar System, and shows that rogue asteroids are actually more common
than previously thought.  Particularly in the main asteroid belt --
between Mars and Jupiter -- the researchers found a compositionally
diverse mix of asteroids.  It has been suggested, unfortunately
without any actual mechanism being proposed, that Jupiter may have
'drifted' closer to the Sun, dragging with it a host of asteroids that
originally formed in the colder edges of the Solar System, before
deciding to move back to its current position.  Such a migration may
(so it is said) have simultaneously knocked around more close-in
asteroids, scattering them outward.

To create a comprehensive asteroid map, the researchers first analyzed
data from the Sloan Digital Sky Survey, which uses a large telescope
in New Mexico to observe a lot of galaxies, but included in the survey
are data for more than 100,000 asteroids.  The team grouped the
asteroids by size, location, and composition.  That last category was
defined by asteroids' origins -- whether in a warmer or colder
environment -- a characteristic that can supposedly be determined by
whether an asteroid's surface is more reflective at redder or bluer
wavelengths.  The team then tried to account for observational biases.
While the survey includes more than 100,000 asteroids, they are the
brightest ones.  Asteroids that are smaller and/or less reflective
may be overlooked, so an asteroid map based on observations may
unintentionally leave out an entire population.  The researchers
determined that the survey was probably complete down to a diameter of
5 km, so they confined their analysis to asteroids above that size.
They grouped them by size and composition, and mapped them into
distinct regions of the Solar System.  They observed that for large
asteroids, the traditional pattern holds true: the further from the
Sun, the colder the asteroids appear.  But for smaller asteroids, that
trend seems to break down: those that look to have formed in warmer
environments can be found not just close to the Sun, but throughout
the Solar System, and asteroids that resemble colder bodies beyond
Jupiter can also be found in the inner asteroid belt, closer to Mars.


CURIOSITY MARS ROVER FACES MINI SAND DUNE
BBC NEWS

The 'Curiosity' Mars rover is to try to drive over a one-metre-high
dune.  The sand bank is currently blocking the robot's path into a
small valley and a route with fewer of the sharp rocks that lately
have been making big dents in the vehicle's aluminium wheels.  NASA
engineers will take no risks, however.  The rover will be commanded
initially to climb only part way up the dune to see how it behaves.
The team is mindful that the 'Spirit' rover was lost in a sand trap in
2009, and the 'Opportunity' rover, which has just celebrated 10
working years on the planet, very nearly went the same way in 2005
when it became stuck for several weeks in a deep dirt pile.  Curiosity
has already had one scuff at the base of the barrier, using a wheel to
test the sand's consistency.  The robot would have no problem managing
the incline but mission planners will be concerned about the potential
for any rocks hidden inside the dune to damage or snare Curiosity.
Engineers believe the path ahead between two scarps will be kinder on
the rover's 50cm-diameter wheels, which have been taking a hammering
during the one-ton (only 8 cwt on Mars) vehicle's traverse across the
floor of Mars' equatorial Gale Crater.  Recent close-up pictures
reveal multiple punctures, rips and dimples in Curiosity's metal
'tyres'.  Getting over the small dune would offer the rover smoother
ground in the future.  The next objective is a place where scientists
hope to drill into freshly exposed bedrock and look for traces of any
complex carbon chemistry that might be present.  Curiosity's ultimate
goal is to get to the foothills of the big central mountain that
dominates the crater floor, which is still several kilometres away to
the SW.  Since landing in 2012 August, Curiosity has travelled almost
5 km.


MARS ORBITER EXAMINES NEW CRATER
NASA

Space rocks hitting Mars excavate fresh craters at a rate of more than
200 per year.  A new image from the Mars Reconnaissance Orbiter shows
a crater about 30 metres in diameter at the centre of a radial burst
painting the surface with a pattern of bright and dark tones.  The
scar appeared some time between pictures taken of that place in 2010
July and in 2012 May.  The new image, which is of higher resolution,
was taken on 2013 Nov. 19.


RED SKIES DISCOVERED ON EXTREME BROWN DWARF
RAS

A peculiar example of a brown dwarf with unusually red skies has been
discovered by a team of astronomers from the University of Hertford-
shire's Centre for Astrophysics Research.

Brown dwarfs straddle the line between stars and planets.  They are
too massive to be considered as planets but not massive enough to fuse
hydrogen in their cores and develop into stars.  As they have not got
an internal source of energy, they are cold and very faint, and keep
on cooling over time.  One called ULAS J222711-004547 caught the
researchers' attention for its extremely red appearance.  Observations
with the VLT in Chile have shown that the reason for its peculiarity
is the presence of a very thick layer of dust in its upper atmosphere.
The size of the dust grains influences the colour of the sky.  The
atmosphere of that particular brown dwarf appears to contain water
vapour, methane and probably some ammonia but, unusually, it is
dominated by clay-sized particles that are mostly made of mineral
dust, of materials like enstatite and corundum.


OLDEST KNOWN STAR
The Australian National University.

A team led by astronomers at the ANU has observed one of the oldest
known stars in the Universe, which formed shortly after the Big Bang
13.7 billion years ago.  It was discovered at Siding Spring
Observatory with the SkyMapper telescope, which is searching for
ancient stars as it conducts a five-year project to produce the first
digital map the southern sky.  It is around 6,000 light-years away,
and its composition suggests that it formed in the wake of a
primordial star which had a mass 60 times that of our Sun.  It has
been thought that primordial stars died in extremely violent
explosions which polluted huge volumes of space with iron.  But the
ancient star shows signs of lighter elements such as carbon and
magnesium, and no sign of iron.  That is thought to indicate that the
primordial star's supernova explosion was of surprisingly low energy.