Topic: Early December Astronomy Bulletin

[Clive]

ORBITER OBSERVES MOVEMENT OF MARS SAND DUNES
NASA

Images from the Mars Reconnaissance Orbiter show sand dunes and
ripples moving across the surface of Mars at dozens of locations and
shifting up to several metres.  The sandy surface is more dynamic than
was previously realised.  The air on Mars is very thin, so it takes
strong winds to move a grain of sand.  Wind-tunnel experiments have
shown that it would take winds of 80 mph to move sand on Mars,
compared with only 10 mph on Earth.  Measurements by Viking landers in
the 1970s and early 1980s, as well as climate models, indicated that
such winds should be rare on Mars.  The first hints that Martian dunes
move came from the Mars Global Surveyor about ten years ago, but its
cameras were not good enough to be sure.  The rovers also detected
hints of shifting sand; the mission team was surprised to see grains
of sand dotting the rovers' solar panels, and also saw the rovers'
track marks filling in with sand.


NEW EVIDENCE FOR LIQUID WATER ON EUROPA
NASA

The surface of Jupiter's satellite Europa is completely covered with a
shell of ice.  Pictures returned by the Galileo probe that was in
orbit around Jupiter for some years led some scientists to suggest
that under the icy surface there exists a global salt-water ocean that
contains more liquid water than all of the Earth's oceans combined.
Owing to the chill so far from the Sun, of course the ocean surface is
completely frozen; most scientists have thought that the ice crust
is tens of miles thick.  Now, scientists associated with the Galileo
probe have discovered what appears to be a body of liquid water the
volume of the North-American Great Lakes locked inside the icy shell.
They were looking at two roughly circular, bumpy features that they
called 'chaotic terrain' on Europa's surface.  (Not a good expression,
since 'terrain' refers uniquely to the Earth.)  On the basis of
similar processes seen on Earth -- on ice shelves and under glaciers
overlying volcanoes -- they developed a model to try to explain how
the features form.  The model suggests that the features on Europa's
surface are formed by mechanisms that involve significant exchange
between the icy shell and the underlying lake, and that many more such
lakes might exist under Europa's shell.


THE DISTANCE AND NATURE OF CYGNUS X-1
Astropublishing

Since its discovery almost half a century ago, the powerful source of
X-rays known as Cygnus X-1 has been one of the most-studied objects in
the sky.  It appears to be a double system, consisting of a black hole
and a blue supergiant star.  Its distance has been put at between 5800
and 7800 light-years.  Astronomers at the Harvard-Smithsonian Center
for Astrophysics have now used the intercontinental network of radio
telescopes known as the VLBA (Very-Long-Baseline Array) and refined
the distance somewhat, finding it to be 6070 light-years with an
uncertainty of about 400.  Using that value, they have calculated
that the mass of the black hole is 15 times that of the Sun, and that
it rotates at the exceptionally high rate of 48000 rpm.  The VLBA
measured the motion of Cygnus X-1 within the Galaxy in addition to
its distance, and it appears that the motion is actually slower than
would be expected if the black hole had been born in a supernova
explosion.  Other evidence also points to the possibility that the
black hole was produced without a supernova explosion, the progenitor
star collapsing by itself.


MARS SCIENCE LABORATORY LAUNCHED
NASA

NASA has launched the Mars Science Laboratory, which carries a
car-sized rover named Curiosity, and hopes to demonstrate a precise
landing by 'sky-crane' near the foot of a mountain inside Gale Crater
on 2012 August 6.  For nearly two years after landing, the rover is to
investigate whether the region has ever offered conditions favourable
for microbial life, including the chemical ingredients for life.
Curiosity has a drill and scoop at the end of a robotic arm to gather
soil and powdered samples of rock interiors, then sieve and parcel out
such samples into instruments inside the rover.  It carries 10
instruments with a total mass 15 times those on the Mars rovers Spirit
and Opportunity.  It has a laser-firing instrument for checking the
elemental composition of rocks from a distance, and an X-ray-
diffraction instrument to help to identify minerals in powdered
samples.

Curiosity is twice as long and five times as heavy as Spirit or
Opportunity.  Because of its one-ton mass (though it weighs only
400 kg on Mars owing to the lower gravity there), Curiosity is too
heavy to employ airbags to cushion its landing as previous Mars rovers
have done.  Instead, it has a rocket-powered descent stage that will
lower the rover on tethers as the rocket engines control the speed of
descent.  The landing site should allow Curiosity to drive to layers
of the mountain within Gale Crater.  Observations from orbit have
identified clay and sulphate minerals in the lower layers, indicating
a wet history.


NANOSAIL-D 'SAILS' HOME
Science Daily

After spending more than 240 days 'sailing' around Earth, NanoSail-D
-- a 'nanosatellite' that deployed NASA's first solar sail in low
Earth-orbit -- has successfully completed its mission.  Launched in
2010 November as a payload on a small satellite named FASTSAT,
NanoSail-D unfurled its sail on Jan. 20.  A main objective was to test
the capability of a such a sail, having a low mass and large surface
area, to bring down de-commissioned satellites and space debris by
causing them to re-enter and burn up in the atmosphere.