NEWLY DISCOVERED ASTEROID IS A COMPANION TO THE EARTH
RAS
Astronomers from Armagh have found that a recently discovered
asteroid, designated 2010 SO16, has been following the Earth in its
motion around the Sun for at least the past 250,000 years, and may be
related to the origin of our planet. The asteroid was found by the
WISE infrared survey satellite, launched in 2009. Its average
distance from the Sun is identical to that of the Earth, and its orbit
is very Earth-like. Most near-Earth Asteroids (NEAs) have very
eccentric orbits, but the orbit of the new object is almost circular,
so it does not come close to any other planet -- only to the Earth.
The researchers investigated how stable the orbit is and how long the
asteroid has occupied it. To take account of the current uncertainty
in the asteroid's orbit they performed their calculations for a number
of orbits, all cloned from the presently accepted one but slightly
changed by amounts representing the present uncertainties. They
computed the evolution of the orbits under the gravity of the Sun
and the planets for two million years into the past and the future.
They found that all the clones remained in a 'horseshoe' state with
respect to the Earth. In that configuration, an object mimics closely
the orbital motion of our planet round the Sun, but as seen from the
Earth it appears slowly to trace out a horseshoe shape in space.
Asteroid 2010 SO16 takes 175 years to move from one end of the
horseshoe to the other. Although in one sense its orbit is remarkably
similar to the Earth's, the asteroid keeps well away from the Earth.
It has probably been in the horseshoe-shaped relative orbit for at
least several hundred thousand years, never coming closer than 50
times the distance to the Moon. That is where it is now, near the end
of the horseshoe trailing the Earth.
Currently, three other 'horseshoe' companions of the Earth are known,
but, unlike 2010 SO16, they will linger for a few thousand years at
most before moving on to different orbits. The new object, with an
estimated diameter of 200--400 metres, is by far the largest of our
horseshoe asteroids.
VESTA -- IS IT REALLY AN ASTEROID?
NASA
On 1807 March 29, German astronomer Heinrich Wilhelm Olbers discovered
Vesta. Now, some 200 years later, as the 'Dawn' spacecraft prepares
to begin orbiting Vesta, scientists are aware of the unusual nature of
the object. Vesta is called an asteroid because it lies in the main
asteroid belt between Mars and Jupiter, but it is not a typical one.
The vast majority of objects in the main belt have dimensions of 100
km or less, whereas Vesta is 530 km in diameter. Its layered structure
(core, mantle and crust) is the key trait that makes Vesta more like
planets such as the Earth, Venus and Mars than the other asteroids.
Like the planets, Vesta had sufficient radioactive material in it when
it coalesced to release enough heat to melt rock and enable lighter
layers to float to the outside, a process called differentiation.
Officially, Vesta is a 'minor planet' -- a body that orbits the Sun
but is not a proper planet or comet. But there are more than half a
million minor planets in our Solar System, so the label doesn't give
Vesta much distinction. 'Dwarf planets' -- which include Dawn's
second destination, Ceres -- are another category, but Vesta isn't
quite large enough to qualify as one. Dawn scientists prefer to think
of Vesta as a 'protoplanet' because it is a dense, layered body that
orbits the Sun and began in the same fashion as Mercury, Venus, the
Earth and Mars, but never fully developed. In the early history of
the Solar System, objects became planets by merging with other
Vesta-sized objects. Other bodies have collided with Vesta and
knocked bits off it. Those bits became debris in the asteroid belt
known as Vestoids, and even hundreds of meteorites that have ended up
on the Earth. But Vesta never collided with anything big enough to
disrupt it, and it remained intact; it could be regarded as a time
capsule from that early era.
Vesta, then, has survived bombardment in the asteroid belt for over
4.5 billion years, so it has possibly the oldest planetary surface
in the Solar System. When Dawn arrives at Vesta in July, the
south pole will be in full sunlight, giving a clear view of a huge
crater at the south pole. That crater may reveal the layer cake of
materials inside Vesta. The orbit design allows Dawn to map fresh
areas as the seasons progress over its 12-month visit. The spacecraft
will make many measurements, including high-resolution data on
surface composition, topography and texture. The spacecraft will also
measure Vesta's gravitational field to learn more about its internal
structure.
RING 'RIPPLES' LINKED TO COMETS
BBC News
Scientists say that ripples observed in the ring systems of Saturn and
Jupiter were caused by comets. The ripples, which the researchers
say resemble the undulations of corrugated iron, have been detected both
in Saturn's rings and in Jupiter's lesser-known rings. The ripples in
Jupiter's rings are believed to have been caused by the comet
Shoemaker-Levy 9, which struck the planet in 1994. The researchers
analysed images of Jupiter's rings taken by the Galileo spacecraft in
1996 and 2000 and by the New Horizons probe in 2007. They also
looked at images of Saturn's rings taken by the Cassini spacecraft
during 2009. What they found were undulations that they liken to a
corrugated-iron roof, which when lit from a low angle appear
as alternating dark and light bands.
Corrugations were found across Saturn's entire C ring, stretching for
thousands of kilometres. A similar pattern had been observed
previously in the fainter D ring. The researchers believe that they
were caused by debris, most likely from a comet, striking the rings,
and tilting them. Over time, the spiral becomes more tightly wound,
and it may be decades before the rings flatten out again. The team
was then able to rewind the process using mathematical models to give
an estimated date of the impact event. For Saturn, they arrived at a
point in 1983, but have not yet identified a possible candidate comet.
With Jupiter, they detected at least two spirals and so possibly two
impact events. When they wound back the process for one of the
spirals, they reached a point in 1994, the year of the Shoemaker-
Levy 9 impact. The second spiral led them to an estimated impact
event in 1990, which the scientists hypothesise may have been caused
by debris from a previous close encounter with Shoemaker-Levy 9.
COOL PAIR OF BROWN DWARFS
ESO
Observations with Very Large Telescope (VLT), along with two other
telescopes, have shown that there is a new candidate for the coldest
known star -- a brown dwarf with a temperature of about 100° Celsius
-- extraordinarily cold for the surface of a star. The object is cool
enough to begin crossing the blurred line dividing small cold stars
from big hot planets. Brown dwarfs are 'failed stars' which lack
enough mass for gravity to trigger the nuclear reactions that make
stars shine. The newly discovered brown dwarf, identified as CFBDSIR
1458+10B, is the dimmer member of a binary brown dwarf system 75
light-years away. The object was first found to be a binary by the
adaptive-optics system on the Keck II telescope in Hawaii.
Astronomers then used the adjacent Canada-France-Hawaii telescope to
determine the distance. Finally the ESO VLT was used to observe the
object's infrared spectrum and measure its temperature.
The hunt for cool objects is an active astronomical topic. The Spitzer
space telescope has recently identified two other faint objects as
possible contenders for the coolest known brown dwarfs, although
their temperatures have not been measured precisely. The team is
planning to observe CFBDSIR 1458+10B again to determine its properties
more accurately and to begin mapping the binary's orbit, which, after
perhaps a decade of monitoring, may allow astronomers to determine
the binary's mass.
SUPER-LUMINOUS SUPERNOVA DISCOVERED
McDonald Observatory
University of Texas astronomers have found another extremely bright
'super-luminous supernova' to add to that new class of exploding stars
that they identified a few years ago. Supernova 2008am, which is more
than a billion parsecs away, is one of the most intrinsically bright
exploding stars ever observed. At its peak, it was over 100 billion
times brighter than the Sun. It emitted enough energy in one second
to satisfy the power needs of the United States for a million times
longer than the Universe has existed -- even the Texans admit to being
outclassed!
Analysis of the light from SN 2008am indicated that its
extraordinary luminosity most likely came from interaction of the
debris from the explosion with a surrounding envelope of gas
that the star had previously ejected. The researchers think that
the progenitor star might have been of the type known as 'luminous
blue variables' -- massive stars that puff off layers of material
episodicallly, the most famous example being Eta Carinae.
'Super-luminous supernovae' (SLSNe) are about 100 times brighter than
standard core-collapse supernovae, but extremely rare. Normal
supernovae go off at a rate of about one per century in a galaxy;
SLSNe may be more than a thousand times more rare. The usual ideas
about how supernovae are powered, and why they are so bright, do not
seem to apply to them. Not all SLSNe are the same; there seems to be
a variety of progenitor stars that can give different outcomes, the
common factor being their luminosity. The fates of different stars
depend on their masses. The team defines three categories of high-
mass stars that explode as supernovae:
In the least massive case, around 10 to 20 solar masses, a star
collapses in on itself because its iron core cannot hold out against
the crushing gravity of the star's weight. This is the classic
'core-collapse supernova' with normal brightness. The second
progenitor category consists of more-massive stars, perhaps up to 100
solar masses, which puff off layers of material before they explode,
and it is the interaction between the supernova ejecta and the
previously puffed-off material that can cause the supernova to
brighten to the super-luminous range. The final category includes the
most massive progenitor stars, those of more than 100 solar masses.
In their case, the current thought is that they make electron-positron
pairs, because they are so hot; that process destabilizes the whole
star, and it contracts, ignites the thermonuclear fuel, and then
explodes, blowing the whole star up. Those are called 'pair-
instability' supernovae. Of the three types of explosions, the first
two would leave behind a stellar remnant in the form of a neutron star
or black hole, but in the third type the most massive stars would
explode completely, leaving no remnant.
HUBBLE 'RULES OUT' ALTERNATIVE TO DARK ENERGY
NASA
Astronomers using the Hubble telescope claim to have ruled out an
alternate theory on the nature of 'dark energy' after recalculating
the expansion rate of the Universe with greater accuracy. The
Universe appears to be expanding at an increasing rate. Some believe
that that is because the Universe is filled with 'dark energy' that
works in the opposite way to gravity. One alternative to that
hypothesis is that an bubble of relatively empty space 8 billion
light-years across surrounds our galactic neighbourhood. If we lived
near the centre of such a void, observations of galaxies being pushed
away from each other at accelerating speeds would be an illusion.
That hypothesis is said to have been invalidated by a refinement of
the Hubble constant to a precision claimed to be just 3.3%, that
allows for a better characterization of 'dark energy's' behaviour.
The team had to try to determine accurate distances to galaxies near
and far and to compare those distances with the speeds at which the
galaxies concerned appear to be receding because of the expansion of
space. The Hubble constant relates the speed at which a galaxy appears
to recede to its distance away. Because they could not physically
measure the distances to galaxies, the researchers had to rely on
stars or other objects to serve as cosmic yardsticks. They need to be
objects whose intrinsic brightness -- brightness that hasn't been
dimmed by distance, an atmosphere, or stellar dust -- that is known
and constant. Their distances, therefore, can be inferred by
comparing their true brightness with their apparent brightness as seen
from here.
To calculate longer distances, the astronomers had to fall back on the
class of exploding stars called type Ia supernovae -- stellar
explosions that supposedly all have similar luminosities and are
brilliant enough to be visible far across the Universe. By comparing
the apparent brightness of type Ia supernovae and pulsating Cepheid
stars in comparatively nearby galaxies, the astronomers could
accurately measure their intrinsic brightness and therefore calculate
distances to type Ia supernovae in far-flung galaxies. Of course they
were implicitly obliged to assume that supernovae had exactly the same
brightness at early times as they do today, despite the progressive
enrichment of stellar and interstellar material with heavy elements as
time has gone on.
PIONEER ANOMALY SOLVED
JPL
During the last decade or so, the 'Pioneer Anomaly' has presented a
puzzle. The Pioneer 10 and 11 spacecraft were launched towards
Jupiter and Saturn in the early 1970s. After their respective flybys,
they continued on escape trajectories out of the Solar System, both
decelerating under the force of the Sun's gravity. But they have
seemed to be slowing faster than they ought to, as if being pulled by
an extra unseen force towards the Sun. This deceleration is tiny:
just (8.74 ± 1.33) times 10 to the power (-10) metres per second per
second. The question is how that extra deceleration arises.
Spacecraft engineers' first thought was that heat emitted by the
spacecraft could cause exactly that kind of deceleration, but when
they examined the way heat was produced on the craft (by on-board
plutonium) and how it must be emitted, they were unable to make the
numbers chime. At most, thermal effects could account for only 67% of
the deceleration. Now scientists at the Instituto de Plasmas e Fusao
Nuclear in Lisbon say that they have identified where the thermal
calculations went wrong. They have re-done the calculations using a
model of not only how the heat is emitted but also how it is reflected
off the various parts of the spacecraft. The reflections turn out to
be crucial: previous calculations have only estimated the effect of
reflections, but this time the team modelled exactly how the emitted
heat is reflected and in which direction it ends up travelling. It
turns out that heat from the back wall of a part of the spacecraft
called the main equipment compartment is reflected from the back of
the spacecraft's antenna. Since the antenna points Sunward, towards
the Earth, reflections off its back would tend to decelerate the
spacecraft. The radiation from this wall will, in a first iteration,
reflect off the antenna and thereby add a radiation-pressure contri-
bution to the force in the direction of the Sun. That extra component
of force makes all the difference, and it looks now as if the puzzle
of the anomalous acceleration of the Pioneer probes can finally be put
to rest.
JODRELL BANK TO BE IN CHARGE OF THE LARGEST RADIO TELESCOPE
BBC News
Jodrell Bank Observatory in Cheshire has been selected as the
headquarters for a £1.3bn project to build the world's biggest radio
telescope. An agreement to run the Square Kilometre Array (SKA) from
Jodrell Bank was signed in Rome by Australia, China, the Netherlands,
New Zealand, South Africa, France, Germany, Italy, and the UK. The
location of the SKA itself is presently undecided; it could be built
either in Australia or Southern Africa. The new headquarters at
Jodrell Bank will open next January, superseding the existing project
office at the University of Manchester. Jodrell Bank has been
responsible for some very important astronomical discoveries since it
was established after the Second World War, but owing to constant
funding worries in recent years there have been doubts over its
future. SKA is hoped to be one of the great scientific endeavours of
this century, and the immense cost means it has to be undertaken as an
international collaboration. It should be able to see the hydrogen in
the first stars and galaxies to form after the Big Bang, and to map
the positions of the nearest 100 million galaxies. The Square
Kilometre Array takes its name from the size of its collecting area,
but instead of a single radio dish a kilometre across, it will be made
up of thousands of smaller ones. The total collecting area will,
however, be approximately one square kilometre, giving 50 times the
sensitivity, and 10,000 times the survey speed, of the best present-
day telescopes.
Topic: Eary April Astronomy Bulletin (Read 716 times)