Topic: Late November Astronomy Bulletin

[Clive]

COMET ISON
NASA

Comet ISON is now within the orbit of the Earth as it plunges headlong
towards the Sun for a close encounter on November 28.  Although it
is not as bright as many forecasts predicted, it is putting on a fair
show for observatories around the Solar System.  Spacecraft and
amateur astronomers alike are taking pictures of the comet's green
coma and filamentary double tail.  Because ISON has never passed
through the inner Solar System before (it is a first-time visitor from
the distant Oort cloud), experts do not know what will happen next.
Astronomers say that there are three possible outcomes:

1)  Disintegration before November 28.
    Some comets have disintegrated.  Recent examples include Comet
LINEAR (C/1999 S4) in 2000 and Comet Elenin (C/2010 X1) in 2011.
ISON is now reaching the region near the Sun where such comets have
disintegrated.  ISON is being observed by so many telescopes on Earth
and beyond that, if it does disintegrate, it will be the best-observed
case of cometary disruption in history.

2)  Disintegration and/or evaporation around November 28.
    If ISON survives the next few days intact, it faces a very close
approach to the Sun.  At perihelion, its equilibrium temperature
will be about 2700°C, hot enough to vaporize much of the dust and rock
on its surface.  While it may seem incredible that anything could
survive such a high temperature, the rate at which ISON might
evaporate is small in comparison with the size of the comet's nucleus.
ISON needs to be about 200 m across to survive; current estimates
of its actual size are in the range 500-2000 m.  It helps that the
comet is moving very fast, so it will not be exposed for long to such
a high temperature.  But even if it survives the rapid vaporization of
its exterior, it will get so close to the Sun that the Sun's gravity
might cause it to disintegrate.  Destroyed comets can still be
spectacular, though.  Sun-grazing Comet Lovejoy, for instance, passed
within 100,000 miles of the Sun's surface in 2011 December.  It
disintegrated, forming a spectacular tail of dust.

3)  Survival
    The final case is straightforward: ISON survives its brush with
the Sun and emerges with enough nuclear material to continue as an
active comet.  If it survives intact, it will probably lose enough
dust near the Sun to produce a fine tail, which at best might stretch
for tens of degrees and grace the early-morning sky somewhat as Comet
McNaught (C/2006 P1) did in 2007.


ASTEROID WITH SIX COMET-LIKE TAILS
NASA

Astronomers using the Hubble telescope have seen an asteroid,
designated P/2013 P5, with six comet-like tails of dust.  The tail
structures have changed dramatically in just 13 days as the comet has
ejected dust, as it has been doing from time to time for at least
some months.  Astronomers believe that the asteroid's rotation rate
may have increased to the point where its surface has started to fly
away.  They do not believe that the tails are the result of an impact
with another asteroid, because they have not seen a large quantity of
dust produced all at once.  Scientists using the Pan-STARRS survey
telescope in Hawaii announced their discovery of the asteroid on
August 27.  P/2013 P5 appeared as an unusually fuzzy-looking object.
The multiple tails were discovered when Hubble was used to take a more
detailed image on September 10.  When Hubble looked at the asteroid
again on September 23, its appearance had totally changed; it looked
as if the entire structure had swung round.

Modelling by the Max Planck Institute for Solar-System Research in
Lindau suggested that the tails could have been formed by a series of
impulsive dust-ejection events, on about April 15, July 18 and 24,
August 8 and 26, and September 4.  Radiation pressure from the Sun
stretched the dust into streamers.  Radiation pressure could also have
spun the asteroid up to such an extent that its weak gravity no longer
could hold it together.  If that happened, dust etc could slide to its
equator, shatter and fall off, and drift into space to make a tail.
So far, probably only about 100 to 1,000 tons of dust has been lost.
The asteroid's nucleus, about 300 m across, is thousands of times more
massive than the estimated amount of ejected dust.  Astronomers will
continue observing P/2013 P5 to see whether the dust leaves the
asteroid in the equatorial plane.  If it does, that would be strong
evidence for a rotational breakup.  It appears that P/2013 P5 is a
fragment of a larger asteroid that broke apart in a collision roughly
200 million years ago; there are many other fragments in similar
orbits.  Meteorites thought to have come from those bodies show
evidence of having been heated to as much as 800°C, making it likely
that the asteroid is composed of metamorphic rocks and does not hold
any ice as a comet does.


DAWN SPACECRAFT PROMPTS REVISION OF VESTA'S HISTORY
NASA/Jet Propulsion Laboratory

Data from the Dawn mission suggest that the history of the asteroid
Vesta is more complicated than was previously thought.  If Vesta's
formation had followed the script for the formation of rocky planets
like our own, heat from the interior would have created distinct,
separated layers of rock (generally, a core, mantle and crust).  In
that case, the mineral olivine should be concentrated in the mantle.
But Dawn's observations of the huge southern-hemisphere craters that
exposed the lower crust and should have excavated the mantle did not
find evidence of olivine there.  Instead, there were clear signatures
of olivine in the surface material in the northern hemisphere.  Such
blatant discrepancies from expectation must indicate, at the very
least, that Vesta has had a more complex evolutionary history than
had been supposed.


BLACK HOLES FOUND IN GLOBULAR STAR CLUSTERS
Texas Tech University

Globular star clusters are large groupings of stars thought to contain
some of the oldest stars in the Universe.  In the same distance as
from the Sun to Proxima Centauri, globular star clusters could have a
million to tens of millions of stars.  The stars are close enough
together to collide with one another occasionally.  It was believed
that the interaction of stars would be liable to eject any black holes
that formed.  While that theory may still be wrong, astronomers have
said it might still be somewhat true.  Black holes might still get
ejected from globular clusters, but not as readily as initially
believed.

In 2007, researchers made the first discovery of a black hole in a
globular star cluster, in the external galaxy NGC 4472.  They found it
by seeing X-ray emission from the gas falling into the black hole and
heating up to a few million degrees.  Now researchers from Texas have
discovered the first examples of black holes in a globular cluster in
our own Galaxy.  They used the Very Large Array of radio telescopes in
New Mexico, and detected a certain type of emission made by holes as
they assimilate stars.  When a hole captures a star, most of the
material falls into the hole, but some is thrown outwards in a jet,
with the emission of a characteristic type of radio signal.


HUBBLE OBSERVES SUPERNOVAE
NASA/Goddard Space Flight Center

Supernovae occur when stars reach the ends of their 'lives' with
dramatic explosions, expelling most of their material into space.
Last year, a supernova called SN 2012im occurred in the spiral galaxy
NGC 6984.  Now another star in the same galaxy has exploded, forming
supernova SN 2013ek.  SN 2012im is called a Type Ic supernova, while
2013ek is a Type Ib.  Both types are caused by the core collapse of
massive stars that have shed their outer layers of hydrogen.  Type Ic
supernovae are thought to have lost more of their outer envelope than
Type Ib, including a layer of helium.  Hubble observations taken on
August 19 were intended to locate the new explosion more precisely.
It is so close to last year's that the two events seem to be linked --
the chance of two completely independent supernovae so close together
exploding within a year of one another is very small.  It was
initially suggested that the new one might be the old one somehow
flaring up again, but further observations indicate that they are
separate supernovae -- although they may be related in some as-yet-
unknown way.


MAVEN MARS ORBITER
NASA

Billions of years ago when the Solar System was young, Mars was a very
different world from the one it is today.  Liquid water flowed in long
rivers that emptied into lakes and shallow seas and a thick atmosphere
blanketed the planet and kept it warm.  In that environment, living
microbes might have found a home, starting Mars on a path to becoming
a second life-filled planet next to our own.  Today, Mars is bitterly
cold and desiccated.  Its thin atmosphere provides scant cover for a
surface marked by dry riverbeds and empty lakes.  In an effort to find
out what happened, NASA has sent to Mars a new orbiter called MAVEN
['Mars Atmosphere and Volatile EvolutioN'!], which is due to arrive in
September next year.

Mars could have been wet and warm 4 billion years ago only if it then
had a thick atmosphere.  A thick blanket of CO2 and other greenhouse
gases would have provided the warmer temperatures and the greater
atmospheric pressure required to keep liquid water from freezing solid
or boiling away.  Something caused Mars to lose that blanket.  One
possibility is the solar wind.  Unlike the Earth, Mars is not
protected by a global magnetic field.  Instead, there are 'magnetic
umbrellas' scattered around the planet, that shelter only part of the
atmosphere.  Erosion of exposed areas by the solar wind might have
slowly stripped the atmosphere away over billions of years.  Recent
isotopic measurements of the Martian atmosphere by the rover Curiosity
support that idea: light isotopes of hydrogen and argon are depleted
with respect to their heavier counterparts, suggesting that they have
been preferentially lost into space.

Scientists have also speculated that the planet's surface might have
absorbed the CO2 and locked it up in minerals such as carbonates, but
in recent years Mars rovers and orbiters have failed to find enough
carbonate to account for the missing gas.  MAVEN's instruments are
intended to document the flow of CO2 and other molecules into space.
Once scientists know how quickly Mars is losing CO2 now, they may be
able to extrapolate backwards in time to estimate the total amount
lost, and decide whether loss to space was the most important driver
of Martian climate change.