PHAETHON CONFIRMED AS ROCK COMET
RAS
Most meteor showers occur when the Earth ploughs through streams of
debris released from comets in the inner Solar System. The Geminids,
which grace the night sky annually in December, are one of the best
known and most spectacular of the dozens of meteor showers. However,
astronomers have known for 30 years that the Geminids are not caused
by a comet but by a Sun-grazing 5-km-diameter asteroid called (3200)
Phaethon. Phaethon has now been found to show a comet-like tail of
dust particles blown backwards by radiation pressure from the Sun.
The tail, however, does not arise like a comet's, through the
vaporization of an icy nucleus. Researchers believe that, during its
close approach to the Sun, Phaethon becomes so hot that rocks on the
surface crack and crumble to dust under the extreme heat.
Until recently astronomers had never caught Phaethon in the actual act
of throwing out particles. In 2010. however, astronomers Jewitt and
Jing Li found Phaethon to be anomalously bright when closest to the
Sun. The key to their success was their use of the STEREO Sun-
observing spacecraft. Phaethon at perihelion appears only 8° from the
Sun, making observations with normal telescopes impossible. Now, in
further STEREO observations from 2009 and 2012, astronomers have
observed a comet-like tail extending from Phaethon. The tail gives
incontrovertible evidence that Phaethon ejects dust but that still
does not tell us why. Comets do it because they contain ice that
vaporizes in the heat of the Sun, creating a wind that blows embedded
dust particles from the nucleus. Phaethon's closest approach to the
Sun is just 14 per cent of the average Earth-Sun distance, i.e. 0.14
Astronomical Unit (AU). That is the closest perhelion distance of
any named asteroid, and implies that Phaethon must reach temperatures
over 700°C -- far too hot for ice to survive.
The astronomers believe that thermal fractures and desiccation
fractures (like mud cracks in a dry lake bed) may be launching small
dust particles that are then picked up by sunlight and pushed into the
tail. While this is the first time that thermal disintegration has
been found to play an important role in the Solar System, astronomers
have already detected around some 'nearby' stars unexpected amounts of
hot dust that might have been similarly-produced. So, is Phaethon an
asteroid or a comet? Asteroids and comets tend to occupy entirely
different regions of the Solar System; asteroids between Mars and
Jupiter (roughly 2 to 3.5 AU) and comets om the frigid trans-Neptunian
realms (30 AU and beyond). According to the nature of its orbit,
Phaethon is definitely an asteroid. But by ejecting dust it behaves
like a 'rock comet'.
ASTRONOMERS FIND OLDEST SOLAR 'TWIN'
ESO
'Solar twins', stars that are very similar to our Sun in mass,
temperature and chemical abundances, are rare. Very few have been
found since the first one was discovered in 1997. Now it has been
asserted, on the basis of spectra taken with the VLT in Chile, that
the ninth-magnitude star HD 197027, 250 light-years away in
Capricornus, is more like the Sun than any other solar twin. The
astronomers found HD 197027 to be the oldest solar twin known to date;
it is estimated to be 8.2 billion years old, compared to 4.6 billion
years for our own Sun. [Evidently 'twin' is a gross misnomer, then --
ED.] Observations suggest that HD 197027 may have rocky 'terrestrial'
planets. Studies of the star may offer a forecast of what may happen
to our own Sun when it reaches that age, and they have already
contributed one suggestion, concerning why does the Sun have such a
strangely low lithium content? Lithium, the third element in the
Periodic Table, was created in the Big Bang along with hydrogen and
helium. Astronomers have wondered why some stars appear to have less
lithium than others. With the new observations of HD 197027, they
appear to have taken a step towards solving that problem by finding a
correlation between a Sun-like star's age and its lithium content.
COLDEST BROWN DWARFS
Harvard-Smithsonian Center for Astrophysics.
Some astronomers are interested in hunting for ever-colder star-like
bodies, and two years ago a new class of objects was discovered by
researchers with the WISE space telescope. However, until now nobody
has known exactly how cool their surfaces really are -- some evidence
suggested they could be at room temperature. A new study shows that
while those brown dwarfs are indeed the coolest known star-like
bodies, they are warmer than previously thought, with temperatures
about 120-180°C. To reach such low surface temperatures after cooling
for billions of years means that the objects can have only about 5 to
20 times the mass of Jupiter. Their only source of energy is from
their own gravitational contraction, which depends directly on their
mass. If such an object were found orbiting a star, there is a good
chance that it would be called a planet, but because they probably
formed on their own and not in proto-planetary discs, astronomers call
the objects brown dwarfs even though they are of 'planetary mass'.
Characterizing them is difficult because they emit most of their light
at infrared wavelengths, and they are very faint owing to their small
size and low temperature. To get accurate temperatures, it is
necessary to know their distances; from parallaxes obtained with the
Spitzer space telescope, the team found that they are 20 to 50
light-years away. The observable properties of the objects do not
seem to correlate as strongly with temperature as those of warmer
brown dwarfs and stars. Perhaps other factors, such as convective
mixing, play increased roles in driving the chemistry at the surface.
The study reported here examined the initial sample of the coldest
brown dwarfs discovered in the WISE survey data. Additional objects
discovered in the past two years remain to be studied.
HUBBLE OBSERVES LARGEST KNOWN POPULATION OF GLOBULAR STAR CLUSTERS
NASA
Globular clusters, dense groups of hundreds of thousands of stars,
contain some of the oldest surviving stars in the Universe. Almost
95% of globular-cluster formation occurred within the first one or two
billion years after the Universe was born. An international team of
astronomers has now used the Hubble telescope to discover an
extraordinary population of globular clusters in the crowded core of
the rich grouping of galaxies Abell 1689. The team found that the
globular clusters are intimately intertwined with dark matter. The
study of Abell 1689 suggests how the relationship between globular
clusters and dark matter depends on the distance from centre of the
galaxy grouping. The Hubble study shows that most of the globular
clusters in Abell 1689 formed near the centre of the galaxy cluster,
which contains a deep well of dark matter. Their number decreases
outwards, and so does the amount of dark matter. The images showed
the visible-light glow of 10,000 globular clusters, some as dim as
29th magnitude; on the basis of that number, the team estimated that
there might be more than 160,000 globular clusters within a diameter
of 2.4 million light-years. For comparison, our Milky Way galaxy
has about 150 such clusters. Hubble can actually see only the
brightest clusters; the majority are estimated to be more like 31st
magnitude, out of reach of Hubble but not of the planned future
James Webb space telescope.
VOYAGER 1 HAS LEFT THE SOLAR SYSTEM
NASA
The Voyager 1 spacecraft is now officially the first man-made object
to enter true interstellar space. The 36-year-old probe is about 19
billion kilometres away. New data indicate that it has been moving
for about one year through the plasma, or ionized gas, that exists
in the space between stars. Voyager is in a transitional region
immediately outside the solar bubble, where some effects from the Sun
are still evident. It first detected the increased pressure of
interstellar space on the heliosphere, the bubble of charged particles
surrounding the Sun that reaches far beyond the outer planets, in
2004. Scientists then looked for evidence of the spacecraft's
interstellar arrival, knowing that the data analysis and interpret-
ation could take months or years. Voyager 1 has not got a working
plasma sensor, so the scientists needed a different way to measure
the spacecraft's plasma environment. A coronal mass ejection, a
massive burst of solar wind and magnetic fields that erupted from the
Sun in 2012 March, provided the data that they needed. When that blast
from the Sun eventually arrived at Voyager 1's location last April,
the plasma around the spacecraft began to vibrate like air in an organ
pipe. On April 9, Voyager 1's plasma-wave instrument detected the
movement. The frequency of the oscillations helped scientists to
estimate the density of the plasma. The spacecraft was found to be
bathed in plasma more than 40 times denser than it had encountered in
the outer parts of the heliosphere. Such a density is to be expected
in interstellar space. The plasma-wave team reviewed its data and
found an earlier, fainter set of oscillations in late 2012. After
considering both events, the team estimated that Voyager 1 first
reached interstellar space in 2012 August.
Voyager 1 and its twin, Voyager 2, were launched 16 days apart in
1977. Both spacecraft flew by Jupiter and Saturn. Voyager 2 also
flew by Uranus and Neptune. Voyager 2, launched before Voyager 1, is
the spacecraft that has been continuously operating the longest.
Voyager mission controllers still talk to or receive data from both
Voyagers every day, though the emitted signals are very faint, being
emitted a long way away at a power of about 23 watts. Data from
Voyager 1's instruments are transmitted typically at 160 bits per
second, and received by 34-m and 70-m Deep Space Network stations.
The signal from Voyager 1 takes about 17 hours to reach us.
Scientists do not know when Voyager 1 will reach the undisturbed part
of interstellar space where there is no influence from the Sun, or
when Voyager 2 may cross into interstellar space.
'DEEP IMPACT' COMET MISSION ENDS
NASA
The Jet Propulsion Laboratory has pronounced the mission at an end
after being unable to communicate with the spacecraft since early
August. Launched in 2005 January, the spacecraft first travelled
about 431 million kilometres to the vicinity of Comet Tempel 1. On
2005 July 3, the spacecraft deployed an impactor into the path of the
comet, to be run into by its nucleus on July 4. That caused material
from below the comet's surface to be blasted out into space where it
could be examined by the telescopes and instrumentation of the flyby
spacecraft. Sixteen days after the encounter, the Deep Impact team
placed the spacecraft on a trajectory to fly back past the Earth in
late 2007 to put it on course to visit another comet, Hartley 2. The
spacecraft's extended mission culminated in the successful encounter
with the comet on 2010 Nov. 4. Along the way, it also observed six
different stars to confirm the motion of planets orbiting them, and
took images and data concerning the Earth, the Moon and Mars. The
data helped to confirm the existence of water on the Moon, and
attempted to confirm the methane signature in the atmosphere of Mars.
In 2012 January, Deep Impact performed imaging and determined the
composition of the distant comet C/2009 P1 (Garradd). It took images
of comet ISON this year. After losing contact with the spacecraft
last month, mission controllers spent several weeks trying to uplink
commands to reactivate its onboard systems. Although the exact cause
of the loss is not known, analysis has uncovered a potential problem
with computer time-tagging that could have led to a loss of control
of Deep Impact's orientation. That would then affect the positioning
of its radio antennae, making communication difficult, as well as its
solar arrays, which would in turn prevent the spacecraft from getting
power and allow low temperatures to ruin onboard equipment, freezing
its battery and propulsion systems.
ROSETTA'S WAKE-UP CALL AND ACTIVITY SCHEDULE FOR TARGET COMET
RAS
After a journey of almost ten years, the Rosetta mission has just a
few months left to cruise before beginning its rendezvous with Comet
67P/Churymov-Gerasimenko, a 'dirty snowball' of ice and dust that
preserves material from the formation of the Solar System. During
2014, Rosetta will both start to orbit the comet's nucleus and deploy
a small laboratory of scientific instruments, Philae, to land on the
comet's surface. To aid Rosetta, an international group of scientists
is using ground-based telescopes in an effort to understand the
behaviour of the comet as it approaches the Sun and begins to form its
tail. Astronomers would like to understand the formation and
evolution of dust coma structures at all scales, from tiny filaments
only visible close to the surface of the nucleus, to large structures
extending tens of thousands of kilometres in the coma. Comet 67P
appears to behave in a very consistent way, at least over the last two
orbits. The southern hemisphere is more active than the northern and
there are three major active regions from where there emerge gas jets,
which can eject dust particles at around 50 km/h.
To safeguard the spacecraft during its long, cold journey through deep
space, Rosetta was placed into hibernation in 2011. Research suggests
that 67P will start emitting gas and dust by next March, two months
after the spacecraft receives its wake-up call on 2014 January 20.
The scientists have based their predictions on 31 sets of images that
enabled them to study changes in brightness and hence the activity
levels of the comet was at different points in its orbit. They
estimated that the comet would start to form its tail at distance of
around 450 million kilometres from the Sun, when it would become warm
enough for water ice to sublimate. Instead, it became active much
further out, at 650 million kilometres. Water will still be frozen
solid at that distance from the Sun, so some other gas must be
responsible for the earlier activity that has been observed.
Topic: September Astronomy Bulletin (Read 1097 times)