Posted on by Fraser Cain

Cassini Flies Past Mysterious Titan Again

With a diameter of 5,150 kilometres, Titan is the largest of Saturn?s family of moons; it’s even bigger than the planets Mercury or Pluto. It has an atmosphere of orange-yellow smog composed mostly of nitrogen with an abundance of hydrocarbon organic compounds including methane; although, it seems to have very few clouds. On October 26, Cassini passed close to Titan revealing a first glimpse of the moon’s strange surface. It discovered a rugged yet level landscape with few craters, meaning that the planet must be geologically active. Mysterious oily flows of cryogenic ice ooze across the surface. Planetary scientists have been thrilled by the results so far.

Titan is cold. Its surface temperature is -180? C – way too cold for liquid water, yet it’s close to the triple point of methane, where this hydrocarbon gas can exist in all three physical states at its surface: solid ice, liquid or gaseous.

Cassini turned its Ultraviolet Imaging Spectrograph (UVIS) towards the star Spica (Alpha Virginis), then Lambda Scorpi, and for the next 8 hours observed the stars as they were obscured by Titan’s atmosphere. This sensitive instrument is different from other types of spectrometers because it can take both spectral and spatial readings. It’s particularly adept at determining the composition of gases. Spatial observations take a wide-by-narrow view, only one pixel tall and 60 pixels across. The spectral dimension is 1,024 pixels per spatial pixel. Additionally, it’s capable of taking so many images that it can create movies to show the ways in which this material is moved around by other forces. This provided a vertical profile of the main constituents of the atmospheric layers that have a similar temperature profile to the Earth.

Close approach occurred before Cassini passed up through Saturn’s ring plane, and returned some of the best close up images of the ring system to date. Then Cassini began using its radar to map part of Titan’s surface terrain at a small solar phase angle. The experiment was looking for signs of hot spots on the moon’s surface that would indicate the presence of active cryo-volcanoes, and even lighting in Titan’s atmosphere.

The 2.6-meter Huygens lander probe will separate from its mother ship on Christmas Eve, travelling towards Titan and entering the moon’s atmosphere on 14 January. Much of Huygens’s science will take place during its atmospheric decent, which will be relayed to Cassini, and then transmitted back to Earth’s waiting scientists and the media. If Huygens actually lands successfully on Titan, it’ll be a major bonus for the mission.

Huygens will be attempting to determine the origin of Titan’s molecular nitrogen atmosphere. Planetary scientists want to answer the question: “Is Titan’s atmosphere primordial (accumulated as Titan formed) or was it originally accreted as ammonia, which subsequently broke down to form nitrogen and hydrogen?”

If nitrogen from the solar nebula (out of which our Solar System formed) was the source of nitrogen on Titan, then the ratio of argon to nitrogen in the solar nebula should be preserved. Such a finding would mean that we have truly found a sample of the “original” planetary atmospheres of our Solar System

Huygens will also try to detect lightning on Titan. The extensive atmosphere of Titan may host Earth-like electrical storms and lightning. Although no evidence of lightning on Titan has been observed so far, the Cassini Huygens mission provides the opportunity to determine whether such lightning exists. In addition to the visual search for lightning, the study of plasma waves in the vicinity of Titan may offer another method. Lightning discharges a broad band of electromagnetic emission, part of which can propagate along magnetic field lines as whistler-mode emission.

By Science Correspondent Richard Pearson

Posted on by Fraser Cain

What’s Up This Week – Dec 13 – Dec 19, 2004

Image credit: George Varros
Monday, December 13 – Tonight will be one of the most hauntingly beautiful and most mysterious displays of celestial fireworks all year – the Geminid meteor shower. First noted in 1862 by Robert P. Greg in England, and B. V. Marsh and Prof. Alex C. Twining of the United States in independent studies, the annual appearance Geminid stream was weak, producing no more than a few per hour, but it has grown in intensity during the last century and a half. By 1877 astronomers were realizing a new annual shower was occurring with an hourly rate of about 14. At the turn of the century it had increased to an average of over 20, and by the 1930s from 40 to 70 per hour. Only eight years ago observers recorded an outstanding 110 per hour during a moonless night… And now it is moonless again!

So why are the Geminids such a mystery? Most meteor showers are historic, documented and recorded for hundred of years, and we know them as being cometary debris. When astronomers first began looking for the Geminids parent comet, they found none. After decades of searching, it wasn’t until October 11,1983 that Simon Green and John K. Davies, using data from NASA’s Infrared Astronomical Satellite, detected an orbital object that was the next night confirmed by Charles Kowal that matched the Geminid meteoroid stream. But this was no comet, it was an asteroid. Originally designated as 1983 TB, but later renamed 3200 Phaethon, this apparently rocky solar system member has a highly elliptical orbit that places it within 0.15 AU of the Sun about every year and half. But asteroids can’t fragment like a comet – or can they? Original hypothesis were that since Phaethon’s orbit passes through the asteroid belt, that it may have collided with other asteroids causing rocky debris. This sounded good, but the more we studied the more we realized the meteoroid “path” occurred when Phaethon neared the Sun. So now our asteroid is behaving like a comet, yet it doesn’t develop a tail.

So what exactly is this “thing”? Well, we do know that 3200 Phaethon orbits like a comet, yet has the spectral signature of an asteroid. By studying photographs of the meteor showers, scientists have determined that the meteors are more dense than cometary material and not as dense as asteroid fragments. This leads us to believe that Phaethon is probably an extinct comet that has gathered a thick layer of interplanetary dust during its travels, yet retains the ice-like nucleus. Until we are able to take physical samples of this “mystery”, we may never fully understand what Phaethon is, but we can fully appreciate the annual display it produces!

Thanks to the wide path of the stream, folks the world over get an opportunity to enjoy the show. The traditional peak time is tonight as soon as the constellation of Gemini appears around mid-evening and lasts through tomorrow morning. The radiant for the shower is right around bright star Castor, but meteors can originate from many points in the sky. From around 2:00 a.m. until dawn (when our local sky window is aimed directly into the stream) it is possible that we can see (animated clip by George Varros.) about one “shooting star” every 30 seconds. The most successful of observing nights are ones where you are comfortable, so be sure to use a reclining chair or pad the ground while looking up. Please get away from light sources when possible – it will triple the amount of meteors you see, dress warmly, take along refreshments and just enjoy the incredible and mysterious Geminids!

Tuesday, December 14 – So if you thought last night was great, then don’t plan on getting extra sleep tonight as we wait out the two-day old Moon to set and Orion to rise. Tonight we’re going to locate and explore Don Macholz tenth comet discovery – C/2004 Q2! This is definitely a “not to be missed” treat. Even the most modest of binoculars will reveal this spectacular comet. Located tonight on the Eridanus border you can easily locate Q2 by identifying the constellation of Lepus below Orion and simply sweeping the skies a short distance to the west. You cannot miss Macholz. It’s that bright and that easy!

Holding a rough magnitude of 5, Comet Macholz is visible to the naked eye at a dark site, but is sufficiently bright enough to be caught with small binoculars under less than perfect conditions. What can you expect to see? The coma (at the time of my observances prior to this article) is wonderfully huge and about the size of that great globular cluster, M13, yet it is definitely brighter! Veteran comet observers will appreciate its concentrated nucleus, extensive coma and twin dust and ion tails. For the novice, Macholz will indeed appear like a large, unresolvable globular cluster with a bright core, but look up, up and away at the stretch of tail. It’s the finest (in my humble opinion) since Ikeya/Zhang! If the constellation of Lepus is too low for your location, don’t worry. The wonderful Macholz will continue in the days ahead to climb northward until it reaches Taurus by month’s end. This one is awesome!

Wednesday, December 15, 2004 – For early evening viewers, tonight’s Moon will give a great opportunity to visit telescopically with some smaller features located within the fully disclosed Mare Crisium area. Near the terminator, look for two bright mountainous areas on the central western border of Crisium known as Olivium and Lavinium Promentoriums. Voyaging from this point toward the east across Crisium’s smooth floor, we will see the small punctuations of Craters Picard to the south and Pierce to the north. See how many nights you are still able to make out these features!

Thursday, December 16 – Tonight the Moon is once again our prominent sky feature, so why not venture there and visit one of the oldest features left on our visible lunar side? Start by identifying two prominent craters in the southeast quadrant – Metius and Fabricus. While viewing the area around these craters, note that Frabricus’ walls actually intrude upon Metius pointing to its younger age of formation. Around Fabricus, but not including Metius is the boundary of a mountain walled plain that extends into the terminator. High power and stable conditions will reveal many breaks in its hexagonal walls and its floor will be marred by many smaller craters and fissures. This is crater Jannsen, and in all probability is one of the oldest craters left on the Moon. Look for three prominent interior craters as well as an ancient rimae that will be at the shadow’s edge. It may not seem exciting, but remember crater Jannsen could be as much as five billion years old!

Friday, December 17 – As we continue our lunar exploration tonight, look for the “three ring circus” of easily identified craters Theophilus, Cyrillus and Catherina. It is here that you will find a very unique highlight – a very conspicuous lunar feature that was never officially named! Cutting its way across Mare Nectaris from Theophilus to shallow crater Beaumont in the south will appear a long, thin, bright line. What you are looking at is an example of lunar dorsum – nothing more than a wrinkle or low ridge. Chances are probably good that this ridge is just a “wave” in the lava flow that congealed when Mare Nectaris was formed and it is quite striking tonight because of the lighting angle. Has it been named? Yes. It is unofficially known as the “Dorsae Beaumont”, but whatever it may be called, it is surely a distinct feature that I think you’ll enjoy!

Saturday, December 18 – There’s still plenty of Moon to explore tonight, so why don’t we try locating an area where many lunar exploration missions made their mark? Binoculars will easily reveal the fully disclosed areas of Mare Serenitatis and Mare Tranquillitatis, and it is where these two vast lava plains converge that we will set our sites. Telescopically you will see a bright “peninsula” westward of where the two conjoin that extends toward the east, just off that look for bright and small crater Pliny. It is near this rather inconspicuous feature that the remains Ranger 6 lay forever preserved when it crashed on February 2, 1964. Unfortunately, technical errors occured and it was never able to transmit lunar pictures. Not so Ranger 8! On a very successful mission to the same relative area, this time we received 7137 “postcards from the Moon” in the last 23 minutes before hard landing. On the “softer side”, Surveyor 5 also touched down near this area safely after two days of malfunctions on September 10, 1967. Incredibly enough, the tiny Surveyor 5 endured temperatures of up to 283 degrees F, but was able to spectrographically analyze the area’s soil… And by the way, it also managed to televise an incredible 18,006 frames of “home movies” from its distant lunar locale.

Sunday, December 19 – Tonight’s outstanding lunar feature is easily seen in binoculars and a treasure-trove of detail to the telescope. Located roughly one-third the way from south to north limb, Crater Albategnius will stand out in bold relief near the terminator. A fine challenge for binoculars will be to see if you can make out its bright central peak from its darker lava covered floor. Telescopically, Albategnius is a real treat! Also an ancient formation, look for the large number of younger craters in its ruined walls. The largest of these is Crater Klein, but there are myriad others. A nice test of your optics and abilities to discern small features is to look for three shallow depressions east of the central peak. Good luck!

Until next week? Remember that many deep sky objects are still visible despite the Moon, so keep looking up and enjoying the wonders of our own Universe! Wishing you clear, dark skies and light speed… ~Tammy Plotner

Posted on by Fraser Cain

Work Begins on Magellan Giant Telescope

The Carnegie Observatories of the Carnegie Institution, and the University of Arizona, Steward Observatory Mirror Lab, have signed an agreement to produce the first mirror for the Giant Magellan Telescope (GMT)?the first telescope of the next-generation of extremely large ground-based telescopes ( ELT) to begin mirror production. The telescope primary mirror will have a diameter of 83 feet (25.4 meters) with more than 4.5 times the collecting area of any current optical telescope.

?This agreement is historic for the future of astronomy,? stated Dr. Richard Meserve, president of the Carnegie Institution. ?It is the first of many milestones that we and our partners look forward to?both in constructing an enormous ground-based telescope and in the scientific discoveries that will result. Everyone in the eight-member GMT consortium is extremely excited by this step,? he added. The consortium includes the Carnegie Observatories, Harvard University, Smithsonian Astrophysical Observatory, University of Arizona, University of Michigan, Massachusetts Institute of Technology, University of Texas at Austin, and Texas A&M University.

The GMT is slated for completion in 2016 at a site in Northern Chile. Viewing conditions in Chile, such as at Carnegie’s Las Campanas Observatory, are some of the best in the world. The GMT will have ten times the resolution of the Hubble Space Telescope. With its powerful resolution and enormous collecting area, the GMT will be able to probe the secrets of planets that have formed around other stars in the Milky Way, peer back in time toward the Big Bang with unprecedented clarity, delve into the nature of dark matter and dark energy, and explore the formation of black holes?the most important questions in astronomy today.

?The Giant Magellan Telescope will allow an unprecedented view of extrasolar planets as well as a window out to the largest scales and back to the earliest moments of the universe. We plan to complete the GMT so that it will work in tandem with the future generation of planned ground- and space-based telescopes,? stated Dr. Wendy Freedman, director of the Carnegie Observatories. ?The real distinction of the GMT, however, is that it is building on a heritage of successful technology developed for the twin 6.5-meter Magellan telescopes at Las Campanas. Their performance has far exceeded our expectations. The Magellan telescopes have proven to be the best natural imaging telescopes on the ground, due in large part to the genius of its Project Scientist, Carnegie Observatories? Stephen Shectman, and Roger Angel and his team at the Steward Mirror Lab,? she continued.

The mirrors for the GMT will be made using the existing infrastructure at Steward that made the 6.5-meter Magellan mirrors and the 8.4-meter Large Binocular Telescope mirrors on Mt. Graham. The new telescope will be composed of seven, 8.4-meter primary mirrors, arranged in a floral pattern. One spare off-axis mirror will also be made. Seven of the eight mirrors will be off-axis and require new techniques in casting and polishing. The first off-axis mirror will be cast this coming summer (2005) to address the new challenges. ?The upcoming decade promises to be a very exciting one for astronomy. The National Academy of Sciences Astronomy and Astrophysics Survey Committee Report (2001) ranked the science for extremely large telescopes as the highest priority for ground-based optical astronomy,? said Jeremy Mould, Director of the National Optical Astronomy Observatory. Site testing at the Las Campanas Observatory is also underway along with many other aspects of the project. Detailed information about the design of the GMT and the science that it will perform is located at http://www.gmto.org/.

Original Source: Carnegie News Release

Posted on by Fraser Cain

Dark Side of the Rings

As Cassini swung around to the dark side of the planet during its first close passage after orbit insertion, the intrepid spacecraft spied three ring moons whizzing around the planet.

Visible in this image are: Mimas (398 kilometers, or 247 miles across) brightest and above center; Janus (181 kilometers, or 112 miles across) second brightest at upper left; and Prometheus (102 kilometers, or 63 miles across) just above the main rings at upper left.

The normally bright B ring appears very dark from this vantage point. Regions with smaller concentrations of particles, such as the Cassini division (bright near center) transmit more sunlight and thus are brighter.

The image was taken in visible light with the Cassini spacecraft wide angle camera on Oct. 27, 2004, at a distance of 757,000 kilometers (470,000miles) from Saturn. The image scale is about 42 kilometers (26 miles) per pixel.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA’s Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team is based at the Space Science Institute, Boulder, Colo.

For more information about the Cassini-Huygens mission, visit http://saturn.jpl.nasa.gov and the Cassini imaging team home page, http://ciclops.org.

Original Source: NASA News Release

Posted on by Fraser Cain

NASA Administrator Set to Resign

NASA Administrator Sean O’Keefe is set to resign this week from the agency, after heading it up for three years. President Bush is considering five men to take over, with the former leader of the Pentagon Missile Defense Agency, Air Force Lt. Gen. Ronald Kadish, widely considered to be the top candidate. Other people being considered for the position are former Congressman Robert Walker and former shuttle astronauts Ron Sega, Charles Bolden and Robert Crippen. O’Keefe is said to be considering a new position as the chancellor of Louisiana State University.

Posted on by Fraser Cain

The Winter Solstice Approaches

Image credit: NASA
To understand the Winter Solstice (and by contrast the Summer Solstice) we must first understand a fundamental fact about the earth. Earth?s axis of rotation is tilted approximately 23.5? from vertical. This means that as earth orbits the sun, it points first one hemisphere, then the other toward the sun. This tilt causes sunlight to strike the surface of earth at different angles at different times of year. In the summer, the sun is high overhead for the Northern hemisphere and the heat energy is concentrated over a smaller area. In the winter, when the angle of the sun is low, the energy covers a much larger area and therefore heats less efficiently.

Imagine pointing a flashlight directly at a piece of paper. It’ll create a bright circle of light concentrated in an area. Now tilt the flashlight so that it’s hitting the paper on an angle. The same amount of light is coming out of the flashlight, but it’s spread out over a much larger area of paper. It’s this changing of our angle towards the Sun that gives us seasons.

The season we call ?winter? begins on the Winter Solstice. The word Solstice means ?sun still?. But to understand the significance of the Winter Solstice, we must first go back in time to the Summer Solstice, or first day of summer. Starting on June 21st, the sun gradually loses altitude in the sky as seen at noon. By September 22nd, the sun?s noon time altitude is significantly lower in the sky. The process continues until December 21st. Around this date, the sun seems to hold its position in the sky and then slowly begins to climb northward again; hence the term ?sun still.? To ancient peoples, the Solstice was a significant point in the year.

Because ancient peoples knew nothing of the earth?s tilt, the southward march of the sun was a troubling time. There was fear that one day the sun might continue moving south until was lost entirely. Many cultures conducted rituals to encourage the sun to move north again and when it did there were great celebrations. These celebrations, regardless of culture, all had a common theme that of rekindled light.

Not surprising then that many of the traditions and customs of ancient Solstice celebrations have survived to the present day. Although we know that the sun will begin moving north without any encouragement from humans, we still use this time of cold and darkness to celebrate the theme of rekindled light. From the Hanukah Menorah, to the Scandinavian Yule log, to the lights of the Christmas tree, during this season we seek to push back the darkness with light. Although the forms have evolved over the centuries, we cans still see the spirit of many of the old ways in our present day Solstice celebrations.

Now here is an interesting question to ponder, if the earth were not tilted and we had no seasons, would we celebrate the holidays in the same way?

Written by Rod Kennedy

Posted on by Fraser Cain

Planetary Systems Seen Forming

Two of NASA’s Great Observatories, the Spitzer Space Telescope and the Hubble Space Telescope, have provided astronomers an unprecedented look at dusty planetary debris around stars the size of our sun.

Spitzer has discovered for the first time dusty discs around mature, sun-like stars known to have planets. Hubble captured the most detailed image ever of a brighter disc circling a much younger sun-like star. The findings offer “snapshots” of the process by which our own solar system evolved, from its dusty and chaotic beginnings to its more settled present-day state.

“Young stars have huge reservoirs of planet-building materials, while older ones have only leftover piles of rubble. Hubble saw the reservoirs and Spitzer, the rubble,” said Dr. Charles Beichman of NASA’s Jet Propulsion Laboratory (JPL), Pasadena, Calif. He is lead author of the Spitzer study. “This demonstrates how the two telescopes complement each other,” he added.

The young star observed by Hubble is 50 to 250 million years old. This is old enough to theoretically have gas planets, but young enough that rocky planets like Earth may still be forming. The six older stars studied by Spitzer average 4 billion years old, nearly the same age as the sun. They are known to have gas planets, and rocky planets may also be present. Prior to the findings, rings of planetary debris, or “debris discs,” around stars the size of the sun had rarely been observed, because they are fainter and more difficult to see than those around more massive stars.

“The new Hubble image gives us the best look so far at reflected light from a disc around a star the mass of the sun,” said Hubble study lead author, Dr. David Ardila of the Johns Hopkins University, Baltimore. “Basically, it shows one of the possible pasts of our own solar system,” he said.

Debris discs around older stars the same size and age as our sun, including those hosting known planets, are even harder to detect. These discs are 10 to 100 times thinner than the ones around young stars. Spitzer’s highly sensitive infrared detectors were able to sense their warm glow for the first time.

“Spitzer has established the first direct link between planets and discs,” Beichman said. “Now, we can study the relationship between the two.” These studies will help future planet-hunting missions, including NASA’s Terrestrial Planet Finder and the Space Interferometry Mission, predict which stars have planets. Finding and studying planets around other stars is a key goal of NASA’s exploration mission.

Rocky planets arise out of large clouds of dust that envelop young stars. Dust particles collide and stick together, until a planet eventually forms. Sometimes the accumulating bodies crash together and shatter. Debris from these collisions collects into giant doughnut-shaped discs, the centers of which may be carved out by orbiting planets. With time, the discs fade and a smaller, stable debris disc, like the comet-filled Kuiper Belt in our own solar system, is all that is left.

The debris disc imaged by Hubble surrounds the sun-like star called HD 107146, located 88 light-years away. John Krist, a JPL astronomer, also used Hubble to capture another disc around a smaller star, a red dwarf called AU Microscopii, located 32 light-years away and only 12 million years old. The Hubble view reveals a gap in the disc, where planets may have swept up dust and cleared a path. The disc around HD 107146 also has an inner gap.

Beichman and his colleagues at JPL and the University of Arizona, Tucson, used Spitzer to scan 26 older sun-like stars with known planets, and found six with Kuiper Belt-like debris discs. The stars range from 50 to 160 light-years away. Their discs are about 100 times fainter than those recently imaged by Hubble, and about 100 times brighter than the debris disc around the sun. These discs are also punctuated by holes at their centers.

Both Hubble images were taken with the advanced camera for surveys. They will be published in the Astronomical Journal and the Astrophysical Journal Letters. The Spitzer observations are from the multiband imaging photometer and will appear in the Astrophysical Journal.

The Space Telescope Science Institute (STScI) is operated by the Association of Universities for Research in Astronomy, Inc. (AURA), for NASA, under contract with the Goddard Space Flight Center, Greenbelt, MD. The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency (ESA).

Original Source: Hubble News Release

Posted on by Fraser Cain

Tarantula Nebula in Detail

The Tarantula Nebula is one of the most impressive views in the Southern sky, cf. ESO Press Photos 14a-g/02. Visible to the unaided eye in the Large Magellanic Cloud (LMC), a satellite galaxy of the Milky Way that is located in the direction of the southern constellation Doradus at a distance of about 170,000 light-years, this huge nebula is the prototype of what astronomers refer to as a “Giant HII region”. In this complex of glowing gas and very hot and luminous stars, the gas is mainly composed of protons and electrons, which are kept apart by energetic photons emitted by the stars in this area.

The Tarantula Nebula (also designated 30 Doradus) owes its name to the arrangement of its brightest patches of nebulosity that somewhat resemble the legs of a spider. They extend from a central “body” where a cluster of hot stars (designated “R136”) resides that illuminate the nebula. This name, of the biggest spiders on the Earth, is also very fitting in view of the gigantic proportions of the celestial nebula – it measures nearly 1,000 light years across!

While the central regions of 30 Doradus may be compared to a tarantula, the entangled filaments in the outskirts of this nebula – some of which are seen in PR Photo 34a/04 – could well be likened with its cobweb. They testify to an ongoing history of very vigorous activity and make this spectacular sky region a showcase of dramatic effects caused by the tremendous output of energy from the most massive stars known.

Intricate colours
The marvellous richness of the filament colours is due to the varying conditions in the interstellar gas in this region, cf. PR Photo 34b/04. The red in these images is caused by emission of excited hydrogen atoms, the green shades correspond to emission from oxygen atoms from which two electrons (“doubly-ionized oxygen”) have been “knocked off” by the energetic radiation of hot stars in the R136 cluster, that is located beyond the lower right corner of this photo. The intensity of this emission increases towards R136, explaining the yellowish colour near the edge of the photo.

A blue colour is contributed by singly-ionized atoms of oxygen. Other atoms like nitrogen and sulfur at different levels of ionization also add to the emission of the nebula at specific wavelengths. The observed colours thus probe the physical condition of the emitting gas and the temperature of the star(s) that excite(s) it. The intricate appearance of the filaments is mostly a consequence of turbulence in the interstellar gas, of the magnetic fields, and of the energy input by the massive stars in the neighbourhood.
Supernovae blow interstellar “bubbles”

The large ring-shaped nebula slightly to the lower-left (South-East) of the centre of PR 34a/04 is known as DEM L 299 [1]. Detailed investigations show that it represents an “interstellar bubble” which was “blown” by supernovae explosions, most probably happening millions of years ago, as massive stars near the centre of this structure ended their comparatively short lives in glorious flashes.

A closer inspection shows that another supernova exploded somewhat later near the rim, forming a bright and more compact nebula known as SNR 0543-689 (PR 34c/04). Other supernovae in this general field exploded even more recently, such as the one that created the remnant B0544-6910 (PR 34d/04) only a few tens of thousands of years ago, a blink of an eye by all astronomical standards.

Nebulae with built-in powerhouses
Not all the nebulae seen in this region are caused by supernovae, however. The glow of N 164 [1], a bright, extended red-yellow nebula just below DEM L 299, is mostly due to its own “private” powerhouse, that consists of several massive stars deeply embedded in its interior (PR Photo 34e/04).

The same holds for DEM L 297, the somewhat smaller and fainter nebula to the right of DEM L 299 (PR Photo 34f/04). It is divided into two half-circle formed segments by a dark lane of interstellar dust in front of it. Indeed, within the Tarantula complex many such dark and dusty clouds are seen in silhouette as they obscure bright nebulosity behind them.

Many stellar clusters
The outskirts of the Tarantula Nebula are also rich in stellar clusters. One of them, NGC 2093 [1], cf. PR Photo 34g/04, has relatively few stars and is relatively young, just a few tens of millions of years. It appears that its stars have already excavated a sizeable cavity around them that is now relatively void of gas.

An older and much more compact cluster, NGC 2108, is seen near the bottom of PR Photo 34h/04 and reproduced in full in PR Photo 34a/04. It resembles the globular clusters in our own Galaxy, but it formed much more recently, about 600 million years ago. Still, NGC 2108 is much older than the Tarantula complex and it is quite possible that in its “youth” it was the core of another giant HII region that has since dissolved into interstellar space.

The images for this release were produced by two ESO astronomers who are impressed by this sky region. Nausicaa Delmotte did the observations for her thesis and notes that: “many of the nebulae and clusters seen in these photos would stand out prominently if they were located elsewhere in the sky and not this close to the core of the spectacular Tarantula complex.”. She is echoed by her colleague, Fernando Comeron: “This amazing concentration of clusters, HII regions, supernova remnants, and extremely hot and luminous stars in a single region makes the Tarantula in the LMC a unique celestial object, unrivalled in our own Galaxy and other nearby galaxies!”.
Note

[1]: The designation “DEM L 299” indicates that this object is no. 299 in the list of nebulae in the Large and Small Magellanic Clouds published in 1976 by astronomers R.D.Davies, K.H.Elliott and J.Meaburn. “N” refers to a list of bright nebulae in these galaxies that was compiled in 1956 by K.G.Henize. “NGC” stands for the “New General Catalogue” published in 1888 by J.L.E. Dreyer.

Original Source: ESO News Release

Posted on by Fraser Cain

Streaks Across Dione

A gorgeous Dione poses for Cassini, with shadowed craters and bright, wispy streaks first observed by the Voyager spacecraft 24 years ago. The wispy areas will be imaged at higher resolution in mid-December 2004. Subtle variations in brightness across the surface of this moon are visible here as well. Dione’s diameter is 1,118 kilometers, (695 miles).

The image shows primarily the trailing hemisphere of Dione, which is the side opposite the moon’s direction of motion in its orbit. The image has been rotated so that north is up.

The image was taken in visible light with the Cassini spacecraft narrow angle camera on Oct. 27, 2004, at a distance of about 1.2 million kilometers (746,000 miles) from Dione and at a Sun-Dione-spacecraft, or phase, angle of 28 degrees. The image scale is 3.5 kilometers (2.2 miles) per pixel.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA’s Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team is based at the Space Science Institute, Boulder, Colo.

For more information about the Cassini-Huygens mission, visit http://saturn.jpl.nasa.gov and the Cassini imaging team home page, http://ciclops.org .

Original Source: NASA/JPL/SSI News Release

Posted on by Fraser Cain

People, Not Robots, Should Upgrade Hubble

To ensure continuation of the extraordinary scientific output of the Hubble Space Telescope and to prepare for its eventual de-orbiting, NASA should send a space shuttle mission, not a robotic one, says a new congressionally requested report from the National Academies’ National Research Council. The agency should consider launching the manned mission as early as possible after the space shuttle is deemed safe to fly again, because some of the telescope’s components could degrade to the point where it would no longer be usable or could not be safely de-orbited, said the committee that wrote the report.

“A shuttle servicing mission is the best option for extending the life of the Hubble telescope and ultimately de-orbiting it safely,” said committee chair Louis J. Lanzerotti, distinguished research professor at the New Jersey Institute of Technology, Newark, and consultant, Bell Laboratories, Lucent Technologies, Murray Hill, N.J. “NASA’s current planned robotic mission is significantly more technologically risky, so a robotic mission should be pursued only for the eventual removal of the Hubble telescope from orbit, not for an attempt to upgrade it. Also, a shuttle mission could be used to place equipment on the telescope to make a robotic de-orbiting mission more feasible.”

The Hubble telescope, which has operated continuously in orbit for the past 14 years, was designed to be serviced regularly by astronauts. Four servicing missions replaced nearly all the key components while increasing the telescope’s capabilities. The fifth and final mission — to replace aging batteries, fine-guidance sensors, gyroscopes, and two scientific instruments — was originally intended to be completed by a shuttle crew as well, but NASA is currently planning an unmanned mission to service the telescope robotically.

The committee’s principal concerns about a robotic mission are the risk of failing to develop it in time and the risk of a mission failure, as well as the possibility that the robot could critically damage the telescope. A robotic mission would face significant challenges in using its grapple system to perform autonomous close-proximity maneuvers and the final capture of the space telescope — activities that have no precedent in the history of the space program and whose chances of success are low, according to the committee.

“Our detailed analyses showed that the proposed robotic mission involves a level of complexity that is inconsistent with the current 39-month development schedule,” said Lanzerotti. “The design of such a mission, as well as the immaturity of the technology involved and the inability to respond to unforeseen failures, make it highly unlikely that NASA will be able to extend the scientific lifetime of the telescope through robotic servicing.”

The committee assessed the safety risks of a shuttle servicing mission by comparing shuttle missions to the International Space Station — to which NASA plans to send 25 to 30 more shuttle flights — and shuttle missions to the Hubble telescope. The differences between the risks faced by the crew of a single shuttle mission to the space station and the risks faced by the crew of a mission to the Hubble telescope are very small, the committee concluded.

Also, a shuttle crew would be able to successfully carry out unforeseen repairs to the Hubble telescope and develop innovative procedures for unexpected challenges in orbit, the report notes. Such contingencies have been successfully addressed on three of the four prior missions to the telescope. A robotic mission, on the other hand, might not be able to repair failures that it is not designed to address, possibly stalling the mission in its early stages.

“With the replacement of aging components and the installation of new science instruments, Hubble is expected to generate as many new discoveries about stars, extra-solar planets, and the far reaches of the universe as it has already produced so far, with images 10 times more sensitive than ever before,” Lanzerotti said.

Original Source: National Academies of Science