Topic: Mid June Astronomy Buletin

[Clive]

WIND-DRIVEN MARS ROVER?
NASA

New research shows that a wind-driven 'tumbleweed' Mars rover would
be capable of moving across rocky Martian 'terrain'*.  There is quite
--------------------
*Terrain: landform that pertains to the Earth, 'Terra' (cf. terra
firma, terrestrial).  Abuse of language to apply it to Mars!  -  ED.
--------------------
a lot of interest within NASA to pursue the tumbleweed rover design,
but one of the questions regarding the concept is how it might perform
on the rocky surface of Mars.  Scientists developed a computer model
to determine how varying the diameter and mass of a tumbleweed rover
would affect its speed and ability to avoid getting stuck in Martian
rock fields.  Rock fields are common on the surface of Mars, which
averages one rock per square metre.  They found that, in general, the
larger the diameter, and the lower the overall weight, the better the
rover performs.  In addition, the study found that a tumbleweed rover
would need to have a diameter of at least six metres in order to
achieve an acceptable level of performance -- meaning that the rover
could hope to move through rock fields without getting stuck.

The model also suggested that tumbleweed rovers are more likely to
bounce than roll across the surface, owing to the spacing of the rocks
and the size of the rovers.  Earth-based testing alone cannot
establish with certainty whether a particular design will work on
Mars.  Mars has approximately three-eighths of the Earth's gravity,
and the atmospheric density at the surface of Mars is only duplicated
around 100,000 feet altitude here.  Tumbleweed rovers are attractive
because they can cover much larger distances, and on rougher
'terrain', than the rovers that have already been sent to Mars.  While
tumbleweed rovers would not be controllable like wheeled rovers, they
would also not need a power supply for mobility -- they would
literally be blown across the Martian landscape by the wind.


MORE ATOMIC HYDROGEN
Astrophysical Journal

By taking a new look at archival data, Australian astronomers have
discovered that galaxies around us contain about a third more atomic
hydrogen gas than was previously supposed.  The study also shows that
the gas is distributed very differently from how it was in the past,
with much less in the galaxies' outer parts.  That implies that it is
much harder for galaxies to pull the gas in and form new stars, which
is probably why stars are forming 20 times more slowly now than in the
past.  The new finding does not help to solve the problem of 'dark
matter' -- lots of mass, detectable by its gravity, that has not been
properly identified.  Even though there is more atomic hydrogen than
was thought, it is not enough to solve the dark-matter problem.


GAMMA-RAY BEAMS FROM MILKY WAY CENTRE
Astrophysical Journal

As galaxies go, our Milky Way is pretty quiet.  Active galaxies have
cores that glow brightly, powered by supermassive black holes
swallowing material, and often eject twin jets in opposite directions.
In contrast, the Milky Way's centre presently shows little activity,
but two beams of gamma-rays, or jets, have been revealed by the Fermi
space telescope.  They extend from the Galactic Centre to a distance
of 27,000 light-years above and below the Galactic plane.  They are
the first such gamma-ray jets ever found.  The faint jets are a ghost
or after-image of what existed a million years ago, and suggest that
the Milky Way had an active galactic nucleus in the relatively recent
past.

The jets may be related to the gamma-ray bubbles that Fermi detected
in 2010.  Those bubbles, too, stretch 27,000 light-years from the
centre of the Milky Way.  However, whereas the bubbles are
perpendicular to the Galactic plane, the gamma-ray jets are tilted at
an angle of 15 degrees.  That may reflect a tilt of the accretion disc
surrounding the black hole.  The magnetic field embedded in the disc
accelerates the jet material along the spin axis of the black hole,
which may not be aligned with the Milky Way.  The jets and bubbles
also formed differently.  The jets were produced when plasma squirted
out from the Galactic Centre, following a corkscrew-like magnetic
field that kept it tightly focused, whereas the gamma-ray bubbles were
probably created by a 'wind' of hot matter blowing outward from the
black hole's accretion disc.  As a result, they are much broader than
the narrow jets.

Both the jets and bubbles are powered by 'inverse Compton scattering'.
In that process, electrons moving near the speed of light collide with
low-energy light, such as radio or infrared photons.  The collisions
increase the energy of the photons into the gamma-ray part of the
electromagnetic spectrum.  The discovery leaves open the question of
when the Milky Way was last active.  A minimum duration can be
calculated by dividing the jet's 27,000-light-year length by its
approximate speed -- so it must have lasted at least 27,000 years.
However, it may have persisted for much longer; the jets probably
flickered on and off in response to large variations in the rate at
which material fell into the hole.


STFC ENDS SUPPORT FOR MAJOR TELESCOPES
RAS

The Science and Technology Facilities Council (STFC) is to end support
for two major astronomical telescopes.  The decision, a consequence of
ongoing real-terms cuts to the UK science budget by the Government,
will almost certainly see the UK Infrared Telescope (UKIRT) in Hawaii
cease operations in the autumn of 2013, and the James Clerk Maxwell
Telescope (JCMT) do the same a year later, with the loss of around 40
jobs.  UK involvement in the William Herschel Telescope (WHT), sited
on La Palma in the Canaries, which observes the sky in visible light,
will continue.  Without the WHT, UK astronomers would be unable to
observe the northern hemisphere of the sky at optical wavelengths.
Such access is also critical for instrument development and for
observations that complement new radio observatories like the pan-
European LOFAR array.


NEW X-RAY TELESCOPE ABOUT TO BE LAUNCHED
NASA

NuSTAR, the Nuclear Spectroscopic Telescope Array, is being prepared
for launch later this month.  NuSTAR will be the first space telescope
to create focused images of cosmic X-rays with the highest energies.
Those are the same types of X-rays that doctors use to see bones and
airports use to scan bags. The telescope will have more than 10 times
the resolution and more than 100 times the sensitivity of its
predecessors that operated in a similar energy range.  High-energy
light is difficult to focus because it is reflected only if it strikes
the mirror almost tangentially.  NuSTAR operates with nested shells of
mirrors, of which it has 133 in each of two optical units.  The
mirrors were moulded from ultra-thin glass similar to that found in
laptop screens and glazed with even thinner layers of reflective
coating.  NuSTAR will observe the hottest, densest and most energetic
objects.  It will work with other telescopes in space now, including
Chandra, which observes lower-energy X-rays.  Together, they will
provide a more complete picture of the objects, and will also
investigate the production of heavy elements by exploding stars.