MINI-COMETS EJECTED BY COMET HOLMES IN ITS OUTBURST
RAS
Astronomers have discovered that multiple fragments were ejected
during the outburst of Comet 17P/Holmes in 2007 October, when the
small (3.6-km) body brightened by nearly a millionfold in less than a
day. The astronomers examined a sequence of images, taken with the
Canada-France-Hawaii telescope in Hawaii on nine nights in 2007
November; they used a digital filter, called a Laplacian filter, that
enhances sharp discontinuities within images and made it possible to
recognize faint small-scale features that would otherwise be hard to
detect against the bright background of the expanding comet. They
found that numerous small objects had moved radially away from the
nucleus at speeds up to 125 m/s. The objects were too bright to be
just bare rocks, but were more like mini-comets, creating their own
dust clouds as ice sublimated from their surfaces.
While cometary outbursts are common, their causes are unknown. One
possibility is that internal pressure built up as the comet moved
closer to the Sun and sub-surface ices evaporated. The pressure
eventually became too great and part of the surface broke away,
releasing a cloud of dust and gas, as well as larger fragments.
However, the solid nucleus of Comet Holmes survived the outburst and
continued on its orbit, seemingly unperturbed, just as it did in the
analogous outburst that led to its discovery in 1892. Comet Holmes
has an orbital period of about 6 years, and travels from the inner
edge of the asteroid belt to beyond Jupiter. The comet is now moving
away from the Sun but will return to perihelion in 2014.
NEW IMAGES OF MARS
JPL, Pasadena, California
Thousands of new images from the Mars Reconnaissance Orbiter show a
wide range of gullies, dunes, craters, areological layering, and other
features. The orbiter recorded the images from April until early
August of this year. The camera team at the University of Arizona,
Tucson, makes several new images available each week and periodically
releases much larger sets, such as the batch posted on September 2.
Each complete image from the camera covers a strip of Martian ground
6 km wide, about 2--4 km long, and show details as small as 1 metre
across.
JUPITER CAPTURED COMET FOR 12 YEARS IN MID-20TH CENTURY
RAS
Comet 147P/Kushida--Muramatsu was captured as a temporary satellite of
Jupiter in the mid-20th century and remained trapped in an irregular
orbit for about twelve years. There are only a few comets known to
have been captured as temporary satellites, and the capture duration
in the case of Kushida-Muramatsu, which orbited Jupiter between 1949
and 1961, is the third-longest. An international team modelled the
trajectories of 18 'quasi-Hilda comets' -- objects with the potential
to go through a temporary capture by Jupiter that results in them
either leaving or joining the Hilda group of objects in the asteroid
belt. Most of the cases of temporary capture in the modelled
trajectories were fly-bys, where the comets did not complete a full
orbit. However, the team used recent observations that tracked
Kushida-Muramatsu over nine years to narrow down the diversity of the
possible orbital paths for the comet over the previous century.
In all the variants, Kushida--Muramatsu completed two full revolutions
of Jupiter, making it only the fifth captured orbiter to be identified.
Asteroids and comets can sometimes be distorted or fragmented by tidal
effects induced by the gravitational field of a capturing planet, or
may even impact upon the planet. The most famous victim of both those
effects was Comet Shoemaker--Levy 9, which was torn apart on passing
close to Jupiter and whose fragments then collided with that planet in
1994. Previous computational studies have shown that Shoemaker--Levy
9 may well have been a quasi-Hilda comet before its capture by
Jupiter. The object that impacted upon Jupiter this July, causing the
new dark spot, may also have been a member of the class, even if it
did not suffer tidal disruption like Shoemaker--Levy. The team has
also confirmed a future moon of Jupiter. Comet 111P/Helin--Roman-
Crockett, which orbited Jupiter three times between 1967 and 1985,
will complete six laps of the planet between 2068 and 2086.
CRATERS ON VESTA AND CERES COULD HOLD KEY TO JUPITER'S AGE
RAS
Crater patterns on Vesta and Ceres could help to show when Jupiter
began to form during the evolution of the early Solar System. A study
modelling the cratering history of the two largest objects in the
asteroid belt, which are believed to be among the oldest in the Solar
System, indicates that the type and distribution of craters would show
marked changes at different stages of Jupiter's development. The
study, carried out by scientists at the Italian National Institute for
Astrophysics in Rome, explored the hypothesis that one or both objects
formed during Jupiter's formation by modelling their cratering
histories. Their simulation described Jupiter's formation in three
stages: an initial accretion of its core followed by a stage of rapid
gas accretion, in turn followed by a phase where the gas accretion
slowed down while the planet reached its final mass. During the last
two phases Jupiter's gravity started to affect more and more distant
objects. For each of the phases, the team simulated how Jupiter
affected the orbits of asteroids and comets from the inner and outer
Solar System, and the likelihood of them being moved onto collision
paths with Vesta or Ceres.
Their model suggested that the stage of Jupiter's development made
big differences to the speed of impacts and the origin of potential
impactors. When Jupiter's core approached its critical mass, it
caused a sharp increase in low-velocity impacts from small, rocky
bodies orbiting nearby to Vesta and Ceres, which led to intense and
uniform crater-distribution patterns. Such low-speed collisions may
have helped Vesta and Ceres gather mass. Once Jupiter's core had
formed and the planet rapidly accreted gas, it deflected more distant
objects onto a collision course with Ceres and Vesta and the impacts
became more energetic. Although rocky objects from the inner Solar
System were the dominant impactors at that stage, the higher energies
of collisions with icy bodies from the outer Solar System made the
biggest mark.
The third stage of Jupiter's formation was complicated by a period
known as the Late Heavy Bombardment, which occurred around 4.1 to 3.8
billion years ago. During that time a significant number of objects
from the outer Solar System were injected into orbits crossing those
of the giant planets and may have reached the asteroid belt. In
addition, Jupiter is thought to have migrated in its orbit around that
time, which would have caused an addition flux of impactors on Vesta
and Ceres. The team will have occasion to reconsider their results
when the 'Dawn' space mission reaches Vesta in 2011 and then continues
for a rendezvous with Ceres in 2015. Dawn will gather information on
the structure and surface morphologies of the two asteroids and send
back high-resolution images of the crater patterns.
SPOT ON DWARF PLANET
RAS
Haumea is an object that orbits the Sun beyond Neptune, in the
so-called Kuiper belt. It is the fourth-largest known Kuiper-belt
object (KBO) after Eris, Pluto and Makemake. Those large KBOs,
together with main-belt asteroid Ceres, are now officially called
dwarf planets. Because it is so far away, Haumea is visible only as
a rather uninformative point of light. Most of what little we know
about it is derived from its brightness variations. Because of its
rotation and elongated shape, Haumea brightens and dims periodically
as it reflects more and less sunlight. The period tells us that it
rotates in 3.9 hours -- faster than any other large object in the
Solar System -- and the amplitude of the variation tells us how
elongated Haumea is. It is estimated to be approximately an
ellipsoid, 2000 by 1600 by 1000 km, whose shape balances gravitational
and rotational accelerations. Haumea may have been spun up by a
massive impact in the distant past. The shape and rotation period
imply that Haumea has a density 2.5 times that of water. Since we
know from spectroscopic observations that Haumea is covered in water
ice, its high density implies that Haumea must have a rocky interior,
in contrast to its icy surface. Photometry shows two maxima and
two minima of brightness as the oblong object rotates. They are not
exact duplicates of one another, as would be expected from a uniform
surface: there must be differences in reflectivity between different
azimuths. The differences are not quite the same in infrared
wavelengths as they are in visible light, and show that the darker
azimuth is slightly redder than the rest of the surface. The reason
for the non-uniformity of surface brightness and colour is presently
a matter for speculation.
NASA APPROVES X-RAY SPACE MISSION
Science Daily
NASA recently said that it hopes to launch the 'Nuclear Spectroscopic
Telescope Array' (NuSTAR) mission in 2011 August. NuSTAR will carry
the first high-energy-X-ray focusing telescopes into orbit, providing
deeper, clearer views of energetic phenomena such as black holes and
supernova explosions than previous X-ray instruments.
HUBBLE OPENS NEW EYES ON THE UNIVERSE
NASA
The first pictures have been received from the refurbished Hubble
telescope; astronauts installed new instruments during the servicing
mission in May. The new imaging camera can observe across a wide
swath of the electromagnetic spectrum, from the ultraviolet to the
near infrared. The telescope's new instruments are more sensitive and
can observe significantly more efficiently than previous generations
of Hubble instruments.
ASTRONOMER'S SUPERNOVA-DISCOVERY RECORD
A Suffolk astronomer has broken a long-standing world record by
identifying his 125th supernova. Tom Boles, 65, runs the Coddenham
Observatory, which has three robotically-controlled telescopes that
scour the night sky looking for supernovae. The eccentric Bulgarian-
born astronomer Fritz Zwicky has held the record for 36 years after
discovering 123 supernovae before his death in 1974 but Mr Boles has
now leapt ahead of him after his latest discovery in August. He said
his state-of-the-art equipment regularly examined a range of 12,000
galaxies, and his long-standing ambition was to find a supernova in or
close to our own Galaxy.
