DAWN MISSION TO CERES EXTENDED
NASA
NASA has authorized a second extension of the Dawn mission at Ceres, the
largest object in the asteroid belt between Mars and Jupiter. During
this extension, the spacecraft, which has been orbiting Ceres since 2015
March, will descend to lower altitudes than ever before. The spacecraft
will continue at Ceres for the remainder of its investigation and will
remain in a stable orbit indefinitely after its hydrazine fuel runs out.
The Dawn flight team is studying ways to manoeuvre Dawn into a new
elliptical orbit, which may take the spacecraft to less than 200 km from
the surface of Ceres at closest approach. Previously, Dawn's lowest
altitude was 385 km. A priority of the second Ceres mission extension
is collecting data with Dawn's gamma-ray and neutron spectrometer,
which measures the number and energy of gamma rays and neutrons. That
information is important for understanding the composition of Ceres'
uppermost layer and how much ice it contains. The spacecraft will also
take visible-light images of Ceres' surface with its camera, as well as
measurements of Ceres' mineralogy with its visible and infrared mapping
spectrometer.
The extended mission additionally allows Dawn to be in orbit while Ceres
goes through perihelion, its closest approach to the Sun, which will
occur in 2018 April. At closer proximity to the Sun, more ice on Ceres'
surface may turn to water vapour, which may in turn contribute to the
weak transient atmosphere detected by ESA's Herschel space observatory
before Dawn's arrival. Building on Dawn's findings, the team has
hypothesized that water vapour may be produced in part from energetic
particles from the Sun interacting with ice at shallow depths in Ceres'
surface. Scientists will combine data from ground-based observatories
with Dawn's observations to study these phenomena further as Ceres
approaches perihelion.
ASTEROID VISITS FROM BEYOND SOLAR SYSTEM
NASA
A small, recently discovered asteroid -- or perhaps a comet -- appears
to have originated from outside the Solar System, coming from somewhere
else in our Galaxy. If so, it would be the first 'interstellar object'
to be observed and confirmed by astronomers. The unusual object -- for
now designated A/2017 U1 -- is less than 400 metres in diameter and is
moving remarkably fast. Astronomers are using telescopes around the
world and in space to observe this notable object, in an effort to learn
about the origin and possibly composition of the object. A/2017 U1 was
discovered on October 19 by the University of Hawaii's Pan-STARRS 1
telescope during the course of its nightly search for near-Earth objects.
Astronomers immediately realized that it was an unusual object. Its
motion could not be explained as either a normal Solar-System asteroid
orbit or a comet orbit. Combined data from follow-up images taken at
the ESA telescope on Tenerife proved that the object came from outside
the Solar System. It is the most extreme orbit that NASA scientists
have ever seen. The object is moving extremely fast, and on such a
trajectory that we can say with confidence that it is on its way out of
the Solar System and will not come back. The team plotted the object's
current trajectory and even looked into its future. A/2017 U1 came from
the direction of the constellation Lyra, cruising through interstellar
space at a brisk 25.5 km/s.
The object approached the Solar System from almost directly 'above' the
ecliptic, the approximate plane in space where the planets and most
asteroids orbit the Sun, so it did not have close encounters with any
of the major planets during its plunge toward the Sun. On Sept. 2, the
small body crossed under the ecliptic plane just inside Mercury's orbit
and then made its closest approach to the Sun on Sept. 9; answering to
the Sun's gravity, it made a hairpin turn under our Solar System,
passing under the Earth's orbit on Oct. 14 at a distance of about 24
million kilometres -- about 60 times the distance to the Moon. It has
now shot back up above the plane of the planets and, travelling at
44 km/s with respect to the Sun, is speeding toward the constellation
Pegasus. Astronomers have long suspected that such objects should
exist, because during the process of planet formation a lot of material
should be ejected from planetary systems. What is surprising is that we
have never seen interstellar objects pass through before. Since this is
the first object of its type ever discovered, rules for naming such
objects will need to be established by the International Astronomical
Union.
FRESH FINDINGS FROM CASSINI
NASA
The Cassini spacecraft ended its journey on Sept. 15 with an intentional
plunge into the atmosphere of Saturn, but analysis continues on the
mountain of data the spacecraft sent during its long 'life'. The
spacecraft's Ion and Neutral Mass Spectrometer (INMS) returned a lot
of direct measurements of the components in Saturn's upper atmosphere,
which stretches almost to the rings. From those observations, the team
sees evidence that molecules from the rings are raining down onto the
atmosphere. That influx of material from the rings was expected, but
INMS data show hints of ingredients more complex than just water, which
makes up the bulk of the rings' composition. In particular, the
instrument detected methane, a volatile molecule that scientists would
not expect to be abundant in the rings or found so high in Saturn's
atmosphere.
Chief among the questions that scientists hope to answer by using data
from Cassini is the age and origin of the rings. Theoretical modelling
has shown that, without forces to confine them, the rings would spread
out over hundreds of millions of years -- much younger than Saturn
itself. Such spreading happens because faster-moving particles that
orbit closer to Saturn occasionally collide with slower particles on
slightly farther-out orbits. When that happens, some momentum from the
faster particles is transferred to the slower particles, speeding the
latter up in their orbit and causing them to move farther out. The
inverse happens to the faster, inner particles. Previous research had
shown that gravitational tugs from the moon Mimas are solely responsible
for halting the outward spread of Saturn's B ring -- that ring's outer
edge is defined by the dark region known as the Cassini Division. Ring
scientists had thought that the small moon Janus was responsible for
confining the outer edge of the A ring, but a new modelling study shows
that the A ring's outward creep is kept in check by a confederation of
moons, including Pan, Atlas, Prometheus, Pandora, Janus, Epimetheus and
Mimas.
AMATEUR DETECTS COMET ORBITING DISTANT STAR
RAS
A 'citizen scientist' was the first to detect tell-tale signs that a
comet was orbiting a distant star monitored by the Kepler space
observatory. The discovery marks the first time that the presence of an
object as small as a comet has been inferred by observing dips in the
intensity of light from a star. Such dips usually signal crossings of
planets or other objects in front of the star, which briefly block a
small fraction of its light. In this case things were different: the
researchers were able to pick out the comet's tail, a trail of gas and
dust, which blocked about 0.1 per cent of the star's light as the comet
streaked by. The data came from the Kepler space telescope, a stellar
observatory that was launched in 2009. For four years, the spacecraft
monitored about 200,000 stars for dips in brightness caused by transit-
ing exoplanets. To date, the mission has identified and confirmed more
than 2,400 exoplanets, mostly orbiting stars in the constellation
Cygnus, with the help of automated algorithms that quickly sift through
the data, looking for the characteristic dips. The smallest exoplanets
detected thus far measure about one-third the diameter of the Earth.
Comets, in comparison, are only the size of a small city at their
largest, making them much more difficult to detect. But on March 18
this year Thomas Jacobs, an amateur astronomer who makes it his hobby to
comb through Kepler's data, was able to pick out several curious light
patterns amid the noise. Jacobs is part of the 'Planet Hunters'
citizen-scientist project established by Yale University, which enlists
amateur astronomers in the search for exoplanets. The idea was that the
human eye might be able to notice things that a computer would miss.
Astronomers could name 10 types of objects that those people have found
in the Kepler data but that algorithms could not find, because of the
pattern-recognition capability of the human eye. During the search, the
amateur observed three unusual dips in the light coming from KIC
3542116, a faint star located 800 light-years away - he flagged the
events and alerted a professional astronomer with whom he had collab-
orated in the past to interpret his findings. A further three transits
were subsequently found. The asymmetry in the light curves resembled
disintegrating planets, with long trails of debris that would continue
to block a bit of light as the planet moves away from the star.
However, such disintegrating planets orbit their star, transiting
repeatedly. In contrast, no such periodic pattern had been observed in
the transits identified. The only kind of body that could do the same
thing and not repeat is one that probably gets destroyed in the end. In
other words, instead of repeatedly orbiting the star, the objects must
have transited, then ultimately flown too close to the star, and
vaporised. The only thing that fits the bill, and has a small enough
mass to be destroyed, is a comet.
SPOTS ON SUPERGIANT STAR
RAS
Astronomers recently discovered that spots on the surface of a super-
giant star are driving huge spiral structures in its stellar wind.
Massive stars are responsible for producing the heavy elements that are
included in life on Earth. At the end of their lives they scatter the
material into interstellar space in catastrophic explosions called
supernovae - without those dramatic events, our Solar System would never
have formed. Zeta Puppis is an evolved massive supergiant star. It is
about sixty times the mass of the Sun, and seven times hotter at the
surface. Massive stars are rare, and usually found in binary or
multiple systems. Zeta Puppis is special however, because it is a
single massive star, moving through space alone, at a velocity of about
60 km/s. One theory is that Zeta Puppis has interacted with a binary or
a multiple system in the past, and been thrown out into space at an
incredible velocity. Using a network of nano-satellites from the
BRIght Target Explorer (BRITE) space mission, astronomers monitored
the brightness of the surface of Zeta Puppis over a six-month period,
and simultaneously monitored the behaviour of its stellar wind from
several ground-based professional and amateur observatories.
The observations revealed a repeated pattern every 1.78 days, both at
the surface of the star and in the stellar wind. The periodic signal
turns out to reflect the rotation of the star through bright spots tied
to its surface, which are driving large-scale spiral-like structures in
the wind, dubbed 'co-rotating interaction regions' or CIRs. By studying
a spectral line of ionized helium in the star's wind, astronomers
clearly saw some 'S' patterns caused by areas of CIRs induced in the wind
by the bright surface spots. In addition to the 1.78-day periodicity,
the research team also detected random changes on time-scales of hours
at the surface of Zeta Puppis, strongly correlated with the behaviour of
small regions of higher density in the wind (known as clumps) that
travel outwards from the star. The results are exciting because there
is evidence, for the first time, of a direct link between surface varia-
tions and wind clumping, both random in nature. After several decades
of puzzling over the potential link between the surface variability of
very hot massive stars and their wind variability, these results are a
significant breakthrough in massive-star research, essentially owing to
the BRITE nanosats and the large contribution by amateur astronomers.
The physical origins of the bright surface spots and the random
brightness variations discovered in Zeta Puppis remain unknown at this
point, and will be the subject of further investigations, probably
requiring many more observations by space observatories, large ground-
based facilities, and small telescopes alike.
SOURCE OF GRAVITATIONAL WAVES OBSERVED
ESA/Hubble Information Centre
The Hubble space telescope has observed for the first time the source
of a gravitational wave, created by the merger of two neutron stars.
The merger created a kilonova -- an object predicted by theory decades
ago -- that ejects heavy elements such as gold and platinum into space.
The event also provides the strongest evidence yet that short-duration
gamma-ray bursts are caused by mergers of neutron stars. This discovery
is the first glimpse of multi-messenger astronomy, bringing together
gravitational waves and electromagnetic radiation. On 2017 August 17
the Laser Interferometer Gravitational-Wave Observatory (LIGO) and
the Virgo Interferometer both alerted astronomical observers all round
the world about the detection of a gravitational-wave event named
GW170817. About two seconds after the detection of the gravitational
wave, ESA's INTEGRAL telescope and NASA's Fermi gamma-ray space
telescope observed a short gamma-ray burst from the same direction.
In the night following the initial discovery, a fleet of telescopes
started a hunt to locate the source of the event. Astronomers found
it in the lenticular galaxy NGC 4993, about 130 million light-years
away. A point of light was shining where nothing was visible before,
and that set off one of the largest multi-telescope observing campaigns
ever -- among them the Hubble space telescope. Several different teams
of scientists used Hubble over the fortnight following the gravitational
-wave event alert to observe NGC 4993. Using Hubble's high-resolution
imaging capabilities they managed to get the first observational proof
for a kilonova, the visible counterpart of the merging of two extremely
dense objects -- most likely two neutron stars. Such mergers were first
suggested more than 30 years ago, but this marks the first reasonably
firm observation of such an event. The distance to the merger makes the
source both the closest gravitational-wave event detected so far and
also one of the closest gamma-ray burst sources ever seen.
Hubble captured images of the galaxy in visible and infrared light,
witnessing a new bright object within NGC 4993 that was brighter than a
nova but fainter than a supernova. The images showed that the object
faded noticeably over the six days of the Hubble observations. Using
Hubble's spectroscopic capabilities the teams also found indications of
material being ejected by the kilonova as fast as one-fifth of the speed
of light. Astronomers were surprised at just how closely the behaviour
of the kilonova matched predictions. It looked nothing like known
supernovae, and so confidence was soon very high that this really was a
kilonova. Connecting kilonovae and short gamma-ray bursts to neutron-
star mergers has so far been difficult, but with the multitude of
detailed observations following the detection of the gravitational-wave
event GW170817 has now finally verified the connection. The spectrum
of the kilonova looked exactly the way that theoretical physicists had
predicted the outcome of the merger of two neutron stars would appear.
It ties this object to the gravitational-wave source beyond all
reasonable doubt. The infrared spectra taken by Hubble also showed
several broad features that could be identified with some of the
heaviest elements in nature. The observations may accordingly help to
solve another long-standing question in astronomy -- the origin of heavy
chemical elements, like gold and platinum. In the merger of two neutron
stars, the conditions appear to be just right for their production.
Now, astronomers won't just look at the light from an object, as we've
done for hundreds of years, but also listen to it. Gravitational waves
provide us with complementary information about objects which are very
hard to study by observations only of electromagnetic waves. So pairing
gravitational waves with electromagnetic radiation will help astronomers
to understand some of the most extreme events in the Universe.
OBSERVATIONS MAY HINT AT NATURE OF DARK MATTER
ESA/Hubble Information Centre
Using the Hubble space telescope, astronomers have discovered that the
brightest galaxies within galaxy clusters 'wobble' relatively to the
clusters' centres of mass. That unexpected result is inconsistent with
predictions made by the current standard model of dark matter. With
further analysis it may provide insights into the nature of dark matter,
perhaps even indicating that new physics is at work. Dark matter
constitutes just over 25 per cent of all matter in the Universe but
cannot be directly observed, making it one of the biggest mysteries in
modern astronomy. Invisible haloes of such elusive dark matter enclose
galaxies and galaxy clusters alike. The latter are massive groupings of
up to a thousand galaxies immersed in hot intergalactic gas. Such
clusters have very dense cores, each containing a massive galaxy called
the 'brightest cluster galaxy' (BCG). The standard model of dark
matter (cold dark matter model) predicts that once a galaxy cluster has
returned to a 'relaxed' state after experiencing the turbulence of a
merging event, the BCG does not move from the cluster's centre. It is
held in place by the enormous gravitational influence of dark matter.
But now, a team of Swiss, French, and British astronomers has analysed
ten galaxy clusters observed with the Hubble telescope, and found that
their BCGs are not fixed at the centre as expected. The Hubble data
indicate that they are 'wobbling' around the centre of mass of each
cluster long after the galaxy cluster has returned to a relaxed state
following a merger. In other words, the centre of the visible parts of
each galaxy cluster and the centre of the total mass of the cluster --
including its dark matter halo -- are offset, by as much as 40,000
light-years.
That indicates that, rather than a dense region in the centre of the
galaxy cluster, as predicted by the cold dark matter model, there is a
much shallower central density. That is a striking signal of exotic
forms of dark matter right at the heart of galaxy clusters. The
wobbling of the BCGs could be analysed only because the galaxy clusters
studied also act as gravitational lenses. They are so massive that they
warp space-time enough to distort light from more distant objects behind
them. That effect, called strong gravitational lensing, can be used to
make a map of the dark matter associated with the cluster, enabling
astronomers to work out the exact position of the centre of mass and
then measure the offset of the BCG from that centre. If the 'wobbling'
is not an unknown astrophysical phenomenon and is in fact the result of
the behaviour of dark matter, then it is inconsistent with the standard
model of dark matter and can only be explained if dark-matter particles
can interact with each other -- a strong contradiction to the current
understanding of dark matter. That may indicate that new fundamental
physics is required to solve the mystery of dark matter.
Topic: Early November Astronomy Bulletin (Read 2195 times)