METEORITE--ASTEROID LINK SOLVED
MIT
For the last few years, it has seemed puzzling that the vast majority
of asteroids that come near the Earth are of a type that matches only
a tiny fraction of meteorites. Since meteorites are supposed mostly
to be pieces of asteroids, that discrepancy was hard to explain, but a
solution has now been suggested. The smaller rocks that most often
fall to Earth, it seems, come straight in from the main asteroid belt
between Mars and Jupiter, rather than from the 'near-Earth asteroid'
population.
In the main belt, the population is much more varied, and approximates
to the mix of types that is found among meteorites. But why would the
things that most frequently hit us match that distant population
better than they match the objects that are right in our neighbourhood?
That's where the idea emerged of a fast track all the way from the
main belt to landing on the Earth's surface. The fast track, it turns
out, is caused by a process called the Yarkovsky effect, which was
discovered long ago but whose significance has only recently been
recognized.
The Yarkovsky effect causes asteroids to change their orbits as a
result of absorbing the Sun's heat on one side and radiating it in a
different direction owing to their rotation. That produces a small
thrust that operates continuously in one direction and can in time
alter the object's path. Because the surface area of an object is
proportional to the square of its size whereas its mass goes as the
cube, the effect acts most strongly on the smallest objects. Thus,
for rocks of boulder size and smaller, typical of meteorites, the
Yarkovsky effect can play a major role, moving them from anywhere in
the asteroid belt on to paths that can intersect the Earth. For
larger asteroids a kilometre or so across, about which some people
have taken to worrying as potential threats to the Earth, the effect
is too weak to alter their orbits at all quickly.
'INNER OORT CLOUD' OBJECT?
Sloan Digital Sky Survey
A minor planet with the prosaic name 2006 SQ372 was discovered by the
Sloan Digital Sky Survey (SDSS) through the application of a searching
algorithm to data that were actually taken to look for supernovae --
an area of sky of about 200 square degrees was observed every clear
night in the autumns of 2005, 2006, and 2007. 2006 SQ372 has an
unusual orbit, an ellipse that is four times longer than it is wide;
the object is presently slightly closer to us than Neptune but is
beginning the outward leg of a 22,500-year journey that will take it
to a distance of something like 1600 times the distance from the Earth
to the Sun. The only known object with a comparable orbit is Sedna --
a distant, Pluto-like dwarf planet discovered in 2003 -- but 2006
SQ372's orbit takes it more than half as far again from the Sun, and
its orbital period is nearly twice as long.
The new object is much smaller than Sedna, probably 30-60 miles across
instead of nearly 1,000. It may be cometary in nature, but it never
gets close enough to the Sun to develop a long, bright tail of
evaporated gas and dust. Even at its most distant turning point, 2006
SQ372 will be ten times closer to the Sun than the supposed main body
of the supposed Oort Cloud. The existence of an 'inner' Oort cloud
has been suggested for many years, but SQ372 and perhaps Sedna are the
first objects found that could be deemed to have originated there.
2006 SQ372 was bright enough to find with the SDSS only because it is
near its closest approach to the Sun. Since the SDSS survey observed
less than 1% of the sky, there could well be many similar objects to
be discovered.
XMM-NEWTON FINDS MASSIVE CLUSTER OF GALAXIES
ESA
The orbiting X-ray observatory XMM-Newton has discovered a very
massive cluster of galaxies in the distant Universe. The newly
discovered cluster, called 2XMM J083026+524133, is estimated to
contain as much mass as a thousand large galaxies; much of it is in
the form of 100-million-degree gas. It was first observed by chance
as XMM-Newton was studying another celestial object, and was placed
in a catalogue for a future follow-up. Astronomers discovered the
cluster as they were analysing the catalogue, which is based on 3,500
X-ray images which together cover about 1% of the sky, and contains
more than 190,000 individual X-ray sources. The team was looking for
extended patches of X-rays that could either be nearby galaxies or
distant clusters of galaxies. J083026+524133 stood out because it was
so bright. Images made in visible light by the Sloan Digital Sky
Survey did not show any obvious nearby galaxy in that location, but
when the team obtained a deep exposure with the Large Binocular
Telescope in Arizona they found a cluster of galaxies calculated to be
at a distance of 7.7 billion light-years.
DO GALAXIES HAVE A MINIMUM MASS?
New Scientist
The Milky Way's galactic companions all seem to have much the same
amount of mass in their cores. A collection of at least 22 dwarf
galaxies orbits the Milky Way; the brightnesses range over a factor of
10,000. Astronomers analysed the motions of stars in the innermost
1000 light-years, where they might expect common properties to emerge
if any existed, in 18 of the dwarf galaxies. They measured the
velocities of hundreds of stars in orbit around the galaxies' centres,
which allowed them to calculate the masses of the galaxies' cores.
Surprisingly, the masses all came out about the same -- roughly 10
million solar masses. What the 'New Scientist' is claiming is that
most of the mass is dark matter, and that the dimmest galaxies had
10,000 times more dark matter than visible matter. It says that that
is an unusual ratio, and by way of an example says that the Milky Way,
for example, contains only roughly 10 times as much dark matter as
ordinary matter. The similarity of the masses hints that the dwarf
galaxies must have at least that much mass in order to form.
The 'New Scientist' article goes on to speculate about the nature of
dark matter and to draw various conclusions that do not necessarily
follow from the observational facts. There is a misunderstanding
whereby differences in the ratio of the mass to the luminosity between
different objects are assigned (as they are in the paragraph above) to
ratios of 'dark matter' to 'visible matter'. A (normally unspoken)
assumption is that the proper ratio of mass to light is that exhibited
by the Sun. If the Milky Way has ten times as much total mass in
relation to its total brightness it is said to have ten times as much
dark matter as visible matter. But that is not a sensible way of
describing the situation. A star of early-M spectral type is about
five magnitudes (a factor of 100) fainter than the Sun but has about
half as much mass, so it has a mass-to-light ratio of 50 in solar
units. It does not seem helpful to conclude, as by analogy with the
assertions above one would be obliged to do, that an early-M star has
fifty times as much dark matter as visible matter, or that there is
anything mysterious about the former.
FIRST LIGHT FOR THE FERMI SPACE TELESCOPE
NASA
NASA has announced that its new gamma-ray telescope, formerly known as
GLAST, has passed its orbital checks and has been formally renamed the
'Fermi Gamma-ray Space Telescope' in honour of Prof. Enrico Fermi
(1901-1954), a pioneer in high-energy physics. Scientists expect
Fermi, by observing gamma-rays, to make many discoveries involving
highly energetic sources such as pulsars and black holes. Since the
spacecraft's launch on June 11, scientists have tested and calibrated
its two instruments, the Large-Area Telescope (LAT) and the GLAST
Burst Monitor (GBM).
The large-area telescope has obtained in four days an all-sky image
similar to the one that the now-defunct Compton gamma-ray observatory
took years of observations to produce. It scans the entire sky every
three hours when operating in survey mode, which will occupy most of
the telescope's observing time during the first year of operations, to
allow scientists to monitor rapid changes. The telescope is sensitive
to photons with energies ranging from 20 MeV (million electron volts)
to over 300,000 MeV. The high end of the range, which corresponds to
energies more than 5 million times greater than dental X-rays, is
little explored. The spacecraft's secondary instrument, the GBM,
detected 31 explosions of the sort known as gamma-ray bursts in its
first month of operations. The GBM is sensitive to less-energetic
gamma-rays than the LAT, giving it a complementary view of the broad
gamma-ray spectrum.
Topic: Late August Astronomy News (Read 1785 times)