RECORD-SETTING ASTEROID FLY-BY
NASA
On Feb. 15 an asteroid about 50 metres across will pass the Earth only
17,200 miles above the surface. There is no danger of a collision,
but the object, designated 2012 DA14, is making the closest approach
of an object of such a size since regular surveillance began in the
1990s. Our neighbourhood is littered with fragmentary material,
ranging from dust up to objects kilometres across. Much of it comes
from the asteroid belt, while some may be the remains of long-dead,
burnt-out comets. NASA's Near-Earth Object Program helps to find and
keep track of it all, especially pieces that come close to us. 2012
DA14 is a fairly typical near-Earth mini-asteroid, and is probably
made of stone (rather than metal or ice). Astronomers estimate that
an asteroid like 2012 DA14 flies past the Earth, on average, every 40
years, yet one actually strikes the planet only every 1200 years or
so. The impact of a 50-metre asteroid is not cataclysmic. It was
probably a similar-sized object that formed the mile-wide Meteor
Crater in Arizona when it struck about 50,000 years ago. Also, in
1908, something about the size of 2012 DA14 exploded in the atmosphere
above Siberia, levelling hundreds of square miles of forest.
Researchers are still studying the 'Tunguska Event' for clues to the
impacting object.
2012 DA14 will come closer than the belt of geosynchronous satellites
which provide weather data and telecommunications. The odds of an
impact with a satellite are extremely remote, since almost nothing
orbits where DA14 will pass the Earth. It will be watched by radar;
the echoes will not only refine the orbit of the asteroid, allowing
better predictions of future encounters, but also reveal its physical
characteristics such as size, spin, and reflectivity. During the
hours around closest approach, the asteroid will brighten until it
resembles a star of the 8th magnitude. Theoretically, that is an easy
target for amateur telescopes, but the asteroid will be racing across
the sky, moving almost a degree a minute. That will be hard to keep
up with with a telescope, but it should be tolerably easy to watch
with binoculars [if only one knew where to look! - it is an
extraordinary weakness of this item that it gives no information on
that most important point! -- Ed.] For more information about 2012
DA14 and other asteroids of interest, visit
http://neo.jpl.nasa.govDID AN 8th-CENTURY GAMMA-RAY BURST IRRADIATE THE EARTH?
RAS
It was announced in 2012 that high levels of the isotopes carbon-14
and beryllium-10 found in tree rings formed in 775 AD suggested that a
burst of radiation struck the Earth in 774 or 775. Carbon-14 and
beryllium-10 form when radiation from space collides with nitrogen
atoms, which then decay to those heavy forms of carbon and beryllium.
Earlier research ruled out a nearby supernova explosion, as nothing
was recorded in observations at the time and no remnant has been
found. It was also considered whether a solar flare could have been
responsible, but that is not powerful enough to cause the observed
excess of carbon-14. Large flares are likely to be accompanied by
ejections of material from the solar corona, leading to vivid displays
of aurorae, but again there are no historical records of them.
Scientists offer another explanation, consistent with both the
carbon-14 measurements and the absence of any recorded events in the
sky. They suggest that two compact stellar remnants, i.e. black
holes, neutron stars or white dwarfs, collided and merged together.
When that happens, a lot of energy is radiated in the form of gamma
rays. The burst of gamma rays is intense but short, typically lasting
less than two seconds. If that is really the explanation for the
ancient radiation burst, then the merging stars could not be closer
than about a kiloparsec (1 kpc is about 3000 light-years), or it would
have led to the extinction of some terrestrial life. On the basis of
the carbon-14 measurements, scientists believe that the gamma-ray
burst originated in a system between 1 and 4 kpc away. If they are
right, then that would explain why no records exist of a supernova or
auroral display. Other work suggests that some visible light is
emitted during short gamma-ray bursts, but might only be seen for a
few days and be easily missed, but nonetheless it may be worthwhile
for historians to look again through contemporary texts. Astronomers
could also look for the merged object, a 1200-year-old black hole or
neutron star the right distance away but without the characteristic
gas and dust of a supernova remnant. If the gamma-ray burst had been
much closer to the Earth it would have caused significant harm to the
biosphere. But even so far away, away, a similar event today could
cause havoc with the sensitive electronic systems that advanced
societies have come to depend on. The challenge now is to establish
how rare such carbon-14 spikes are i.e. how often such radiation
bursts hit the Earth. In the last 3000 years, the maximum age of
trees alive today, only one such event appears to have taken place.
COMET OF THE CENTURY?
NASA
Astronomers say that a comet currently near the orbit of Jupiter will
pass very close to the Sun, and later this year could blossom into a
striking naked-eye object visible even in broad daylight. Russian
astronomers Vitali Nevski and Artyom Novichonok found the comet in
2012 September. It bears the name of their night-sky survey
programme, the International Scientific Optical Network (ISON). For a
comet at its present distance, it is actually very bright. Its
appearance suggests that is losing gas and dust from a fairly large
nucleus in the 1--10-km range. On 2013 Nov. 28, it will fly through
the Sun's atmosphere little more than a million km from the surface.
If it survives, it could emerge shining as brightly as the Moon,
briefly visible near the Sun in broad daylight. Some reporters have
already started calling ISON the 'Comet of the Century', but
astronomers see that as premature, especially since we are not an
eighth of the way through the century yet. In 1973, a distant comet
named Kohoutek promised to put on a great show, much like ISON, but
the actual apparition was a big let-down and the comet could barely be
seen. Comets are notoriously unpredictable. Tidal forces and solar
radiation have been known to destroy comets. A recent example is
Comet Elenin, which broke apart and dissipated in 2011 as it
approached the Sun. Elenin, however, was a much smaller comet.
A better comparison, perhaps, is Comet Lovejoy, which flew through the
Sun's atmosphere in 2011. Lovejoy emerged seemingly all right and
wowed southern-hemisphere observers with a garish tail for weeks,
though it transpired that its nucleus had been pretty well destroyed.
Comet ISON is probably at least twice as big as Comet Lovejoy and will
pass a bit farther from the Sun's surface. That would seem to favour
Comet ISON surviving and ultimately putting on a good show. One of
the most exciting possibilities would be a partial break-up. If Comet
ISON splits, it might appear as a 'string of pearls' when viewed
through a telescope and might even resemble the famous Comet
Shoemaker-Levy 9 that hit Jupiter in 1994. If it breaks up, the
fragments would continue along the same trajectory as the original
comet. Whatever happens, northern sky-watchers will get a good view.
For months after it passes by the Sun, Comet ISON will be well placed
for such observers. It will pass almost directly over the North Pole
and be visible all night.
ARE SUPER-EARTHS ACTUALLY MINI-NEPTUNES?
RAS
In the last two decades astronomers have found hundreds of 'exo-
planets' in orbit around other stars. One type is the 'super-Earths'
that are thought to have a high proportion of rock but at the same
time are significantly bigger than our own Earth. Now a new study
suggests that such planets are actually surrounded by extended
hydrogen-rich envelopes. Rather than being super-Earths, they are
more like mini-Neptunes. 'Super-Earths' follow a different
evolutionary track to the planets found in our Solar System, but an
open question is whether they can evolve to become rocky bodies like
the terrestrial planets. To try to answer that, astronomers looked at
the impact of radiation on the upper atmospheres of super-Earths
orbiting the stars Kepler-11, Gliese 1214 and 55 Cancri. Those
planets are all a few times more massive and slightly larger than the
Earth and orbit very close to their respective stars. The way in
which the masses of planets scale with their sizes suggests that they
have solid cores surrounded by hydrogen or hydrogen-rich atmospheres,
probably captured from the clouds of gas and dust from which they
formed.
The new model suggests that the short-wavelength extreme-ultraviolet
light of the host stars heats up the gaseous envelopes of such
planets, so that they expand up to several times their normal radius
and gas escapes from them fairly quickly. Nonetheless most of the
atmosphere remains in place over the whole lifetime of the stars they
orbit. If the scientists' results are right, then super-Earths
further out from their stars, in the 'habitable zone', where the
temperature would allow liquid water to exist, would hold on to their
atmospheres even more effectively. If that happens, they would be
much less likely to be habitable.
RED EXPLOSIONS: SECRET LIFE OF BINARY STARS REVEALED
University of Alberta
Researchers have long debated about what happens when binary stars,
two stars that orbit one another, come together in a 'common
envelope'. When this dramatic cannibalizing event ends there are two
possible outcomes: the two stars merge into a single star or an
initial binary transforms into an exotic short-period one. The event
is believed to take anywhere from a dozen days to a few hundred years
to complete -- an extremely fast time frame in terms of celestial
events. More than half of all stars in the Universe are binary stars,
but it was not known what a common-envelope event would look like
until now. After analyzing the physics of what happens in the outer
layers of a common envelope, researchers found that hot and ionized
material in the common envelope cools and expands, then releases
energy in the form of a bright outburst of red light. The researchers
linked such theoretically anticipated common-envelope outbursts with
recently discovered luminous red novae, mysterious transients that are
brighter than novae and just a bit less luminous than supernovae. The
research both provides a way to identify common-envelope events and
explains the luminosity generated during such an event.
WHERE ARE ALL THE DWARF GALAXIES?
Science Daily
Some people have claimed that the Universe consists of about 75% 'Dark
Energy' (whatever they may mean by that), 20% 'Dark Matter' and 5%
ordinary matter. Galaxies and matter in the Universe have been said
to clump in an intricate network of filaments and voids, known as the
Cosmic Web. Computer experiments have suggested that in such a
Universe a huge number of 'small' 'dwarf galaxies' with masses 'only'
about a thousandth of that of the Milky Way should have formed in our
cosmic neighbourhood. Only a handful of such galaxies, however, are
actually observed orbiting around the Milky Way. The observed
scarcity of dwarf galaxies is a major challenge to the understanding
of galaxy formation.
An international team of researchers has studied that issue within the
Constrained Local UniversE Simulations project (CLUES). The CLUES
simulations use the observed positions and peculiar velocities of
galaxies within tens of millions of light-years of us, to simulate the
local environment of the Milky Way. The main goal of the project is
to simulate the evolution of the Local Group -- the Andromeda and
Milky Way galaxies and their low-mass neighbours -- within their
observed large-scale environment. Analysing the CLUES simulations,
the astronomers have now found that some of the far-out dwarf galaxies
in the Local Group move with such high velocities with respect to the
Cosmic Web that most of their gas can be stripped and effectively
removed. They call this mechanism 'Cosmic Web Stripping', since it is
the pancake and filamentary structure of the cosmos that is
responsible for depleting the dwarfs' gas supply. The dwarfs move so
fast that even the weakest membranes of the Cosmic Web can rip off
their gas. Without a large gas reservoir out of which to form stars,
dwarf galaxies might be so small and dim as hardly to be visible
today, so the supposedly missing dwarfs might actually exist but
simply be too faint to be seen.
HOW THE UNIVERSE HAS COOLED SINCE THE BIG BANG
CSIRO Australia.
Astronomers using the CSIRO Australia Telescope Compact Array near
Narrabri, New South Wiles, have measured the temperature that the
Universe had when it was half its current age. Because light takes
time to travel, when we look far into space we see the Universe as it
was in the past -- as it was when light left the galaxies we are
looking at. So to look back half-way into the Universe's history, we
need to look half-way across the Universe. To measure a temperature
at such a great distance, the astronomers studied gas in an unnamed
galaxy 7.2 billion light-years away (a redshift of 0.89). The only
thing keeping the gas 'warm' is the cosmic background radiation --
the glow left over from the Big Bang. By chance, there is another
powerful galaxy, a quasar (PKS 1830-211), lying behind the unnamed
galaxy. Radio waves from that quasar pass through the gas of the
foreground galaxy. As they do so, the gas molecules absorb some of
the energy of the radio waves, leaving on the radio waves a
distinctive 'fingerprint' from which the astronomers could calculate
the gas's temperature. They found it to be 5.08 Kelvin: extremely
cold, but not as cold as today's Universe, which is at 2.73 Kelvin.
According to the Big Bang theory, the temperature of the cosmic
background radiation drops smoothly as the Universe expands. That is
just what is seen in the measurements. In round figures, at half its
present age the Universe was twice as 'hot' as it is now, exactly as
the theory would wish.
NASA JOINS ESA's 'DARK UNIVERSE' MISSION
NASA/Jet Propulsion Laboratory.
NASA has joined the European Space Agency's (ESA's) Euclid mission, a
space telescope designed to investigate the cosmologists' brain-
children called dark matter and dark energy. Euclid is intended for
launch in 2020 and is to spend six years mapping the locations and
measuring the shapes of as many as 2 billion galaxies spread over more
than one-third of the sky. It will study the evolution of the
Universe, and possibly throw light on the dark matter and dark energy
that are said to influence its evolution in ways that are still
admitted not to be understood. The telescope will be stationed at the
Lagrange point L2. At that location the gravitational pulls of two
large masses, the Sun and Earth in this case, keep a small object,
such as the Euclid spacecraft, at a relatively stationary position, in
this case behind the Earth as seen from the Sun. Euclid will try to
map the dark matter in the Universe.
Reproducing the more questionable content of the published item that
is summarized here, we report that it says that matter as we know it
-- the atoms that make up the human body, for example -- is only a
fraction of the total matter in the Universe. The rest, about 85%, is
dark matter consisting of particles of an unknown type. [Can't help
it if this item does not chime with the one next but one above -- it
might suggest that we are not dealing with scientific facts! -- Ed.]
Dark matter was first postulated in 1932, but surely history is trying
to tell us something when, after some 80 years, it still has not been
detected directly. It is called dark matter because it does not
interact with light. It seems somehow to by-pass the 'periodic table
of the elements', which has no spaces left for mystery elements that
could be the principal content of the Universe. It flies in the face
of every rational consideration. But dark matter is said to interact
with ordinary matter through gravity and to bind galaxies together
like an invisible glue. While dark matter pulls matter together, dark
energy pushes the Universe apart at ever-increasing speeds. In terms
of the total mass-energy content of the Universe, dark energy
dominates. Even less, if possible, is known about dark energy than
dark matter! Euclid will use two techniques to study the Universe,
both involving precise measurements of galaxies billions of light-
years away. The observations will yield (so it is claimed) the best
measurements yet of how the acceleration of the Universe has changed
over time, providing new clues about the evolution and fate of the
cosmos.
Topic: Mid February Astronomy Bulletin (Read 2168 times)