Topic: Late October Astronomy News

[Clive]

NO ICE ON MOON AFTER ALL?
New Scientist

A decade ago, the Lunar Prospector spacecraft suggested that the
Moon's poles have large concentrations of hydrogen near the surface,
which could be in the form of frozen water deposited by comets.  That
would be very useful for a possible future base on the Moon, providing
water for astronauts and hydrogen fuel for their vehicles.  The
Shackleton Crater at the south pole has been a prominent candidate for
a future base station, since it contains a ledge on its rim that would
be a good landing spot.  If the crater were also to hold frozen water,
it would be an excellent location.  But that possibility seemed to
evaporate when radar signals formerly attributed to water-ice were
also found to reflect off sunlit areas where ice could not survive.
So it had been hoped that the Japanese spacecraft Kaguya, which was
launched in 2007 September, could shed light on the question by
observations from lunar orbit.

The spacecraft contains a camera that can obtain images of the Moon's
surface even in the near-total darkness of its south pole.  The inside
of the crater receives no direct sunlight, but for a short period
during summer time in the Moon's southern hemisphere (November and
December in the Earth's calendar), a small part of its rim catches
some sunlight, which is then scattered to the crater floor.  Jaoanese
scientists analysed pictures of the crater taken at such a time.  The
images resolve objects as small as 10 metres across.  They provided a
full profile of the crater, including details of tiny craters on its
floor and two landslides from the inner wall.  The most striking
feature was what was missing.  If there had been ice, there would have
been brighter reflections from its surface, but none was visible.
Instead, the images showed just dull lunar soil.  That does not
completely rule out the possibility of frozen water within the crater
-- it could be buried, or the ice crystals could be dirty and mixed
with particles of soil.  But there may be no water at all, and the
hydrogen could be combined in another compound like methane.  Ice
particles trapped within the lunar soil could still be useful for a
human base station, but that would depend on the difficulty of mining
the mixture and extracting the water.


PHOBOS PROBABLY MADE OF RUBBLE
ESA

Mars has two small moons.  The larger one, Phobos, is an irregular
lump of rock measuring 27 by 22 by 19 kilometres.  It has been
observed in a series of close encounters by the Mars Express
spacecraft, and is now considered almost certain to be a rubble pile
rather than a single solid object.  The pictures taken by the
spacecraft enable an accurate 3-dimensional model of Phobos to be
made, so that its volume can be determined with some precision.
During one of the nearest ecounters, the spacecraft's radio signals
were monitored from the Earth to record the changes in frequency
brought about by Phobos' gravitational acceleration of Mars Express,
to enable the calculation of the precise mass of the moon.  When the
team has finished doing the sums putting the mass and volume data
together, it will know the density, which will be an important clue
as to how the moon was formed.  Previously, radio tracking from the
Soviet Phobos 88 mission and from other spacecraft orbiting Mars had
provided the most accurate mass.  The team's current mass estimate
for Phobos is about one-billionth the mass of the Earth.  Preliminary
density calculations suggest that it is 1.85 grams per cubic
centimetre, which is very similar to that of some asteroids.
Asteroids that share Phobos' density are known as D-class.  They are
believed to be highly fractured bodies containing caverns and voids
because they are not solid but are just collections of pieces weakly
held together by gravity, 'rubble piles'.  Spectroscopic data from
Mars Express and previous spacecraft show that Phobos has a
composition similar to that of such asteroids.  That suggests that
Phobos, and probably Deimos too, are captured asteroids.  However,
one circumstance remains difficult to explain in that picture:
usually, asteroids are captured into just random orbits around the
planet concerned, but Phobos orbits above Mars' equator -- a very
specific case.


OUTER SOLAR SYSTEM NOT SO CROWDED
Harvard-Smithsonian Center for Astrophysics

The Taiwanese-American Occultation Survey (TAOS) spent two years
periodically photographing portions of the sky to look for small
bodies orbiting beyond Neptune, in a region of the Solar System called
the Kuiper Belt.  The survey looked for Kuiper Belt objects (KBOs)
with sizes between 3 and 28 kilometres.  Since such objects are too
small to see directly, the survey watched for stars to dim as KBOs
passed in front of and occulted them.  After accumulating more than
200 hours of data (only 100 hours' work a year, a duty cycle of little
over 1%) watching for stellar flickers lasting a second or less, TAOS
had not observed any occultations.

The Kuiper Belt contains objects in a range of sizes -- a few large
ones (the 'dwarf planets' Pluto, Eris, Makemake and Haumea) and many
more smaller ones.  The commonness of a given size might offer a clue
as to the history of planet formation and dynamics.  In particular,
the size distribution of KBOs may reflect a history of agglomeration,
in which colliding objects tended to stick together, followed by
destructive collisions, where collisional velocities were high enough
to shatter the rocks involved.  Astronomers wondered whether they
would find more and more objects at smaller and smaller sizes, or
whether the distribution levelled out.  Unless there has been some
miscalculation over the sensitivity of the TAOS observational
procedures, the fact that no occultations were seen sets an upper
limit to the number density of KBOs in the relevant size range.
The outer Solar System now appears not to be as crowded as some
theories suggest, perhaps because small KBOs have already stuck
together to form larger bodies or frequent collisions have ground them
down into even smaller bits below the threshold of the survey.


PLANETS IN COLLISION?
UCLA

Astronomers have been studying a star known as BD +20 307, which is in
the constellation Aries and is surrounded by about a million times
more dust than exists around our Sun.  They expected BD +20 307 to
prove to be a young star, with the massive dust ring signalling the
final stages in the formation of a planetary system.  A major revision
of ideas was called for last May, when Carnegie astronomers showed
that BD +20 307 is actually a close binary system, composed of two
stars, both very similar in mass, temperature and size to our own Sun,
orbiting about their common centre of mass every 3.42 days.  The
patterns of element abundances in the stars indicate that they are
several billion years old, like our Solar System.  The origin
suggested for the extraordinary quantity of dust, orbiting the binary
pair at about the same distance as Earth and Venus are from our Sun,
is a collision between two terrestrial planets.  That would have had
to have happened quite recently by astronomical standards, no more
than a few hundred thousand years ago, because at such distances from
a star small dust particles get pushed away by stellar radiation while
larger pieces get reduced to dust in collisions within the disc and
are then pushed away.  A serious objection to the idea of a major
collision is, however, that it seems most unlikely that bodies that
have orbited stably for billions of years would suddenly decide to
collide vigorously just now, but no more plausible idea has been put
forward so far.


MASSIVE DOUBLE STAR SYSTEM
ESO

A new image of the star-forming region Gum 29 shows that a small
cluster of stars, only 1 to 2 million years old, called Westerlund 2
includes one of the most massive double-star systems known.  Gum 29
is a region of hydrogen gas that has been stripped of its electrons
(ionised) by the intense radiation of the hot young stars at its
centre.  Astronomers call that an HII {'aitch-two') region, and this
particularly fine example is over 200 light-years across.  The latest
measurements indicate its distance as about 26,000 light-years,
placing it towards the outside edge of the Carina spiral arm of the
Milky Way.

Previous observations have shown that a pair stars on the south-
preceding side of the cluster are particularly massive.  They have
masses of 82 and 83 times that of our Sun and orbit one another in
approximately 3.7 days.  They are both Wolf-Rayet stars -- massive
stars nearing the end of their lives, expelling vast quantities of
material as their swansong.  X-ray observations show that streams
of material from each star continually collide, creating a blaze of
X-ray radiation.


FIRST GAMMA-RAY-ONLY PULSAR
NASA

A pulsar is a rapidly spinning neutron star, the crushed core left
behind when a massive star explodes.  Astronomers have catalogued
nearly 1800 of them.  Although most of them have been found through
their pulses at radio wavelengths, some also beam energy in other
forms, including visible light and X-rays.  Now the orbiting Fermi
gamma-ray telescope has discovered a pulsar that seems to pulse only
in gamma-rays.  The object lies within a supernova remnant known as
CTA 1, about 4600 light-years away in the constellation Cepheus.  It
emits 1000 times the energy of our Sun. and its lighthouse-like beam
sweeps across the Earth every 316.86 milliseconds.  Fermi scans the
entire sky every three hours and detects photons with energies ranging
from 20 million to more than 300 billion times the energy of visible
light.  The instrument sees only about one gamma-ray per minute from
CTA 1, but that is enough for scientists to piece together the neutron
star's pulsing behaviour, its rotation period, and the rate at which
it is slowing down.

A pulsar's beams arise because neutron stars possess intense magnetic
fields and rotate rapidly.  Charged particles stream outwards from the
star's magnetic poles at nearly the speed of light to create the
gamma-ray beams Fermi sees.  Because the beams are powered by energy
drawn from the neutron star's rotation, the pulsation period gradually
increases as the neutron star spins down.  In the case of CTA 1, the
rotation period is increasing by about 12 microseconds a year.