BRIGHT EXPLOSION ON THE MOON
NASA
For the past 8 years, NASA astronomers have been monitoring the Moon
for signs of explosions caused by meteoroids hitting the lunar
surface. "Lunar meteor showers" have turned out to be more common
than anyone expected, with hundreds of detectable impacts occurring
every year. On March 17, 2013, an object about the size of a small
boulder hit the lunar surface in Mare Imbrium, exploding in a flash
nearly 10 times as bright as anything ever seen before. Anyone
looking at the Moon at the moment of impact could have seen the
explosion--no telescope required. For about one second, the impact
site was glowing like a 4th magnitude star. The 40 kg meteoroid
measuring 0.3 to 0.4 metres wide hit the Moon travelling 56,000 mph.
The resulting explosion packed as much punch as 5 tonne of TNT.
Scientists believe the lunar impact might have been part of a much
larger event. On the night of March 17, NASA and University of
Western Ontario all-sky cameras picked up an unusual number of deep-
penetrating meteors right here on Earth. These fireballs were
travelling along nearly identical orbits between Earth and the
asteroid belt. It’s likely that the two events are related, and that
this constitutes a short duration cluster of material encountered by
the Earth-Moon system. One of the goals of the lunar monitoring
program is to identify new streams of space debris that pose a
potential threat to the Earth-Moon system. The March 17th event seems
to be a good candidate.
Controllers of NASA's Lunar Reconnaissance Orbiter have been notified
of the strike. The crater could be as wide as 20 metres, which would
make it an easy target for LRO the next time the spacecraft passes
over the impact site. Comparing the size of the crater to the
brightness of the flash would give researchers a valuable "ground
truth" measurement to validate lunar impact models. Unlike Earth,
which has an atmosphere to protect it, the Moon is airless and exposed.
"Lunar meteors" crash into the ground with fair frequency. Since the
monitoring program began in 2005, NASA’s lunar impact team has
detected more than 300 strikes, most orders of magnitude fainter than
the March 17th event. Statistically speaking, more than half of all
lunar meteors come from known meteoroid streams such as the Perseids
and Leonids. The rest are sporadic meteors--random bits of comet and
asteroid debris of unknown parentage.
MARS CAMERA REVEALS HUNDREDS OF IMPACTS EACH YEAR
University of Arizona
Scientists using images from NASA's Mars Reconnaissance Orbiter, or
MRO, have estimated that the planet is bombarded by more than 200
small asteroids or bits of comets per year forming craters at least
3.9 meres across. Researchers have identified 248 new impact sites
on parts of the Martian surface in the past decade, using images from
the spacecraft to determine when the craters appeared. The 200-per-
year planetwide estimate is a calculation based on the number found in
a systematic survey of a portion of the planet. The High Resolution
Imaging Science Experiment, or HiRISE camera, took pictures of the
fresh craters at sites where before and after images had been taken.
This combination provided a new way to make direct measurements of the
impact rate on Mars and will lead to better age estimates of recent
features on Mars, some of which may have been the result of climate
change. These asteroids or comet fragments typically are no more than
1 to 2 metres in diameter. Space rocks too small to reach the ground
on Earth cause craters on Mars because the Red Planet has a much
thinner atmosphere.
HiRISE targeted places where dark spots had appeared during the time
between images taken by the spacecraft's Context Camera, or CTX, or
cameras on other orbiters. The impacts disturb the dust, creating
noticeable blast zones. In this part of the research, 44 fresh impact
sites were identified. The meteor over Chelyabinsk, Russia, in
February was about 10 times bigger than the objects that dug the fresh
Martian craters. Estimates of the rate at which new craters appear
serve as scientists' best yardstick for estimating the ages of exposed
landscape surfaces on Mars and other worlds. The researchers
calculated a rate for how frequently new craters at least 3.9 metres
in diameter are excavated. The rate is equivalent to an average of one
each year on each area of the Martian surface roughly the size of the
U.S. state of Texas. Earlier estimates pegged the cratering rate at
three to 10 times more craters per year. They were based on studies of
craters on the Moon and the ages of lunar rocks collected during
NASA's Apollo missions in the late 1960s and early 1970s. MRO has
been examining Mars with six instruments since 2006.
ACCURATE DISTANCE MEASUREMENT OF SS CYGNI
NRAO
Researchers used the National Science Foundation's Very Long Baseline
Array (VLBA) and the European VLBI Network (EVN) to precisely locate
one of the most-observed variable-star systems in the sky -- a double-
star system called SS Cygni -- at 370 light-years from Earth. This
new distance measurement meant that an explanation for the system's
regular outbursts that applies to similar pairs also applies to SS
Cygni. This is one of the best-studied systems of its type, but
according to our understanding of how these things work, it should not
have been having outbursts. The new distance measurement brings it
into line with the standard explanation. SS Cygni, in the
constellation Cygnus, is a dense white dwarf star in a close orbit
with a less-massive red dwarf. The strong gravity of the white dwarf
pulls material from its companion into a swirling disk surrounding the
white dwarf. The two stars orbit each other in only about 6.6 hours.
On an average of once every 49 days, a powerful outburst greatly
brightens the system. This type of system is called a dwarf nova,
and, based on other examples, scientists proposed that the outbursts
result from changes in the rate at which matter moves through the disk
onto the white dwarf. At high rates of mass transfer from the red
dwarf, the rotating disk remains stable, but when the rate is lower,
the disk can become unstable and undergo an outburst.
This mechanism seemed to work for all known dwarf novae except for SS
Cygni, based on previous distance estimates. Hubble Space Telescope
measurements in 1999 and 2004 put SS Cygni at a distance of about 520
light-years. That was a problem. At that distance, SS Cygni would
have been the brightest dwarf nova in the sky, and should have had
enough mass moving through its disk to remain stable without any
outbursts. The closer distance measured with the radio telescopes
means that the system is intrinsically less bright, and now fits the
characteristics outlined in the standard explanation for dwarf-nova
outbursts. The astronomers made the new distance measurement using
the VLBA and EVN, both of which use widely-separated radio telescopes
that work together as a single, extremely precise, telescope. These
systems are capable of the most accurate measurements of positions in
the sky available in astronomy. By observing SS Cygni when Earth is
on opposite sides of its orbit around the Sun, astronomers can measure
the subtle shift in the object's apparent position in the sky,
compared to the background of more-distant objects. This effect,
called parallax, allows scientists to directly measure an object's
distance by applying simple, high-school trigonometry.
The astronomers knew that SS Cygni emits radio waves during its
outbursts, so they made their radio-telescope observations after
receiving reports from amateur astronomers that an outburst was
underway. They observed the object during outbursts from 2010 to 2012.
The difference in the distance measurements between the Hubble visible
-light and the radio observations may have several causes. The radio
observations were made against a background of objects far beyond our
own Milky Way Galaxy, while the Hubble observations used stars within
our Galaxy as reference points. The more-distant objects provide a
better, more stable, reference, the astronomers pointed out. The radio
observations, they added, are immune to other possible sources of
error as well. Discovered in 1896, SS Cygni is a popular object for
amateur astronomers. According to the American Association of Variable
Star Observers, not a single outburst of SS Cygni has been missed
since its discovery. It has been observed nearly half a million times,
and its brightness variations carefully tracked, making it one of the
most intensely studied astronomical objects of the past century.
HIDDEN POPULATION OF EXOTIC NEUTRON STARS
NASA
Magnetars -- the dense remains of dead stars that erupt sporadically
with bursts of high-energy radiation -- are some of the most extreme
objects known in the Universe. A major campaign using NASA's Chandra
X-ray Observatory and several other satellites shows magnetars may be
more diverse -- and common -- than previously thought. When a massive
star runs out of fuel, its core collapses to form a neutron star, an
ultradense object about 10 to 15 miles wide. The gravitational energy
released in this process blows the outer layers away in a supernova
explosion and leaves the neutron star behind. Most neutron stars are
spinning rapidly -- a few times a second -- but a small fraction have
a relatively low spin rate of once every few seconds, while generating
occasional large blasts of X-rays. Because the only plausible source
for the energy emitted in these outbursts is the magnetic energy
stored in the star, these objects are called "magnetars." Most
magnetars have extremely high magnetic fields on their surface that
are ten to a thousand times stronger than for the average neutron
star. New observations show that the magnetar known as SGR 0418+5729
(SGR 0418 for short) doesn't fit that pattern. It has a surface
magnetic field similar to that of mainstream neutron stars.
The researchers monitored SGR 0418 for over three years using Chandra,
ESA's XMM-Newton as well as NASA's Swift and RXTE satellites. They
were able to make an accurate estimate of the strength of the
external magnetic field by measuring how its rotation speed changes
during an X-ray outburst. These outbursts are likely caused by
fractures in the crust of the neutron star precipitated by the buildup
of stress in a relatively strong, wound-up magnetic field lurking just
beneath the surface. By modelling the evolution of the cooling of the
neutron star and its crust, as well as the gradual decay of its
magnetic field, the researchers estimated that SGR 0418 is about
550,000 years old. This makes SGR 0418 older than most other
magnetars, and this extended lifetime has probably allowed the surface
magnetic field strength to decline over time. Because the crust
weakened and the interior magnetic field is relatively strong,
outbursts could still occur. The case of SGR 0418 may mean that there
are many more elderly magnetars with strong magnetic fields hidden
under the surface, implying that their birth rate is five to ten times
higher than previously thought.
Another implication of the model is that the surface magnetic field
of SGR 0418 should have once been very strong at its birth a half
million years ago. This, plus a possibly large population of similar
objects, could mean that the massive progenitor stars already had
strong magnetic fields, or these fields were created by rapidly
rotating neutron stars in the core collapse that was part of the
supernova event. If large numbers of neutron stars are born with
strong magnetic fields then a significant fraction of gamma-ray bursts
might be caused by the formation of magnetars rather than black holes.
Also, the contribution of magnetar births to gravitational wave
signals -- ripples in space-time -- would be larger than previously
thought. SGR 0418 is located in the Milky Way galaxy at a distance of
about 6,500 light years from Earth.
MEGA-GALAXY IS MISSING LINK IN HISTORY OF COSMOS
University of California
Two young galaxies that collided 11 billion years ago are rapidly
forming a massive galaxy about 10 times the size of the Milky Way.
Capturing the creation of this type of large, short-lived object is
extremely rare -- the equivalent of discovering a missing link
between winged dinosaurs and early birds. The new mega-galaxy, dubbed
HXMM01, is the brightest, most luminous and most gas-rich
submillimeter-bright galaxy merger known. HXMM01 is fading away as
fast as it forms, a victim of its own cataclysmic birth. As the two
parent galaxies smashed together, they would quickly exhaust the
hydrogen gas in the following hundreds of million years and lead to
the new galaxy's slow starvation for the rest of its life.
.
The discovery solves a riddle in understanding how giant elliptical
galaxies developed quickly in the early Universe and why they stopped
producing stars soon after. Other astronomers have theorized that
giant black holes in the heart of the galaxies blew strong winds that
expelled the gas. But the team has found definitive proof that cosmic
mergers and the resulting highly efficient consumption of gas for
stars are causing the quick burnout. The new galaxy was initially
observed as a bright blob in images of the so-called cold cosmos --
areas where gas and dust come together to form stars -- recorded by
the European Space Agency's Herschel telescope. Follow-up views at a
variety of wavelengths were obtained at more than a dozen ground-based
observatories, particularly the W. M. Keck Observatory in Hawaii.
MARS ROVER PASSES 40-YEAR-OLD RECORD
NASA/Jet Propulsion Laboratory
While Apollo 17 astronauts Eugene Cernan and Harrison Schmitt visited
the Moon for three days in December 1972, they drove their mission's
Lunar Roving Vehicle 35.744 kilometres. That was the farthest total
distance for any NASA vehicle driving on a world other than Earth
until last month. The team operating NASA's Mars Exploration Rover
Opportunity received confirmation that Opportunity's total odometry
since landing on Mars in January 2004 has now exceeded that figure.
The international record for driving distance on another world is
still held by the Soviet Union's remote-controlled Lunokhod 2 rover,
which travelled 37 kilometres on the surface of the Moon in 1973.
Opportunity has began a multi-week trek from an area where it has
been working since mid-2011, the "Cape York" segment of the rim of
Endeavour Crater, to an area about 2.2 kilometres away, "Solander
Point."
NASA TO EXAMINE BEGINNINGS OF UNIVERSE
NASA
When did the first stars and galaxies form in the Universe? How
brightly did they burn their nuclear fuel? Scientists will seek to
gain answers to these questions with the launch of the Cosmic Infrared
Background ExpeRIment (CIBER) on a suborbital sounding rocket on June
4. The first massive stars to form in the Universe produced copious
ultraviolet light that ionized gas from neutral hydrogen. CIBER
observes in the near infrared, as the expansion of the Universe
stretched the original short ultraviolet wavelengths to long near-
infrared wavelengths today. CIBER investigates two telltale signatures
of first star formation -- the total brightness of the sky after
subtracting all foregrounds, and a distinctive pattern of spatial
variations. The objectives of the experiment are of fundamental
importance for astrophysics, to probe the process of first galaxy
formation, but the measurement is also extremely difficult
technically. This will be the fourth flight for CIBER on a NASA
sounding rocket. The previous launches were in 2009, 2010, and 2012
from the White Sands Missile Range, New Mexico. After each flight the
experiment or payload was recovered for post-calibrations and
re-flight.
For this flight CIBER will fly on a larger and more powerful rocket
than before. This will loft CIBER to a higher altitude than those
previously obtained, thus providing longer observation time for the
instruments. The experiment, which will safely splash down in the
Atlantic Ocean more than 400 miles off the Virginia coast, will not
be recovered. The 70-foot tall four-stage Black Brant XII rocket will
carry CIBER to an altitude of about 350 miles. Our experience in the
near-infrared waveband is that we see appreciable emission from the
atmosphere up to 150 miles.
Topic: Early June Astronomy Bulletin (Read 1974 times)