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Friday, May 2, 2008

Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2008 May 2

Shaping NGC 6188
Credit & Copyright: John Ebersole

Explanation: Dark shapes with bright edges winging their way through dusty NGC 6188 are tens of light-years long. The emission nebula is found near the edge of an otherwise dark large molecular cloud in the southern constellation Ara, about 4,000 light-years away. Formed in that region only a few million years ago, the massive young stars of the embedded Ara OB1 association sculpt the fantastic shapes and power the nebular glow with stellar winds and intense ultraviolet radiation. The recent star formation itself was likely triggered by winds and supernova explosions, from previous generations of massive stars, that swept up and compressed the molecular gas. A false-color Hubble palette was used to create the this gorgeous wide-field image and shows emission from sulfur, hydrogen, and oxygen atoms in red, green, and blue hues. At the estimated distance of NGC 6188, the picture spans about 300 light-years.

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The End of Everything

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The Dark Ages, not a single star shines anywhere in the Universe.It can be said that humans have a bit of a short term view of things. We're concerned about the end of summer, the next school year, and maybe even retirement. But these are just a blink of an eye in cosmic terms. Let's really think big, stare forward in time, and think about what the future holds for the Universe. Look forward millions, trillions, and even 10100 years into the future. Let's consider the end of everything.

End of Humanity - 10,000 years
Modern humans originated in Africa about 200,000 years ago. Since then, we've gone on to inhabit every single corner of the globe. But this is just temporary. The vast majority of every species that has ever lived on Earth is now extinct. To think that humans can avoid the fate of every other creature is arrogant. Like all life on Earth, our time is limited. How long will we last?

There are many natural and man made disasters that could wipe us out. From an asteroid strike to worldwide pandemic; global warming to a nearby supernova detonation - there are many ways we could go. Perhaps we'll wrap it up in a mass extinction event, such as the one that killed the dinosaurs 65 million years ago, or "the Great Dying", 251 million years ago that killed 70% of land species and 96% of all marine species.

Perhaps another species (intelligent cockroaches, rats) will evolve, and out compete with us in our niche. Or maybe we'll engineer our robotic replacements.

But a species can last tens or even hundreds of millions of years. So how can we predict when our number will be up?

There's no way to know, but there's a calculation that can help. It's called the Doomsday Argument, developed in 1983 by astrophysicist Brandon Carter. According to Carter, if you assume that half of the humans who will ever live have already been born, you get approximately 60 billion people. If you assume that another 60 billion are yet to be born, our high population levels only give us another 9,000 years or so. Or more precisely, there's a 95% chance that humanity will have ended by the year 11,000.

There are other calculations, but they give similar amounts, ranging from a few thousand to a few million years.

That's a long time, but not long enough to appreciate the future the Universe has in store for itself.

Gobi Desert. Image credit: NASA

End of Life - 500 million years - 5 billion years
We thank the Sun for giving us energy. Without it, there'd be no life on Earth. It's ironic, then, that the Sun will eventually kill all life on Earth.

That's because the Sun is slowly heating up.

One of the most fascinating books about this topic is The Life and Death of Planet Earth by Peter Ward and Donald Brownlee. In their book, they chronicle how Sun's energy output is slowly increasing. In as soon as 500 million years, temperatures on Earth will rise to the point that most of the world will be a desert. The largest creatures won't be able to survive anywhere but the relatively cooler poles.

Over the course of the next few billion years, evolution will seem to go reverse. The largest organisms and least heat tolerant animals will die out, leaving hardy insects and bacteria. Finally, it'll be so hot on the surface of the Earth that the oceans will boil away. There'll be no place to hide from the terrible temperatures. Only the organisms that live deep underground will survive, as they have already for billions of years.

Red giant Betelgeuse. Image credit: Hubble Space Telescope

End of the Earth - 7.5 billion years
As mentioned above, we exist because of the Sun's good graces. But as our star nears the end of its lifetime, it'll take our planet out as it goes; one way or another.

In approximately 5 billion years from now, the Sun will begin the final stage of its life, consuming the last of its hydrogen fuel supply. At that point, gravity will force the Sun to collapse, and only a small amount of hydrogen will remain in a shell wrapped around the star's core. It will then expand into a red giant star, consuming each of the inner planets: first Mercury, then Venus, and finally encompassing even the orbit of Earth.

There is a controversy about whether or not a red giant Sun will actually burn up the Earth. In some scenarios, the change in the Sun's density as it expands causes the Earth to spiral out away from the Sun, keeping out of reach. In another scenario, the Sun's outer envelope will enclose the Earth. The additional friction will slow the Earth down, causing it to spiral down into the Sun.

Whatever the outcome, the Earth will be scorched to a cinder, and effectively destroyed, 5 billion years from now.

Ring Nebula. A vision of our Sun

End of the Sun - 7.5 billion - 1 trillion years
When the Sun becomes a red giant, that's only the beginning of the end. With the end of its hydrogen, the Sun will have switched to fusing helium, then carbon, and finally oxygen. At that point, our Sun will lack the gravity to continue the fusion process. It will shut down, and shed its outside layers to form a planetary nebula, such as the ring nebula we can see in the night sky. It'll then settle down to live out the rest of its days as a white dwarf.

It will still retain most of its mass, but have a size no larger than the Earth's diameter. Once yellow-hot with the heat of fusion, the Sun will slowly cool down over time. Eventually, its temperature will match the background temperature of the Universe and it will become a cold black dwarf - an inert chunk of matter floating in the darkness of space.

Even the oldest white dwarfs still radiate at several thousand degrees Kelvin, so the Universe hasn't been around long enough for black dwarfs to exist.. yet. But give the Sun another 1 trillion years or so, and it should finally become a cold black dwarf.

Artist impression of a disk of material around a white dwarf star. Image credit: Gemini Observatory

End of the Solar System
Even though the Sun will have burned out billions of years from now, the planets that weren't consumed will remain. Perhaps even Earth will join that group. Certainly Jupiter, Saturn, the rest of the outer planets and the Kuiper belt objects will remain orbiting for eons.

A recently discovery, published in the journal Science, reported that astronomers had discovered a disk of rapidly rotating metallic material orbiting a white dwarf. Researchers built a simulation where they put hypothetical planets in orbit around a dying star, and found that the star's death wreaked havoc on the stability of a star system. Changes in the mass of a star causes planets to collide, and rearrange their orbits. Some spiral into their star, while others are ejected into interstellar space.

Once all these new gravitational interactions are worked out, all that might remain of our solar system is the white dwarf remnant of our Sun and the rapidly rotating disk of planetary wreckage surrounding it. Everything else will be lost to interstellar space.

Hubble Deep Field survey shows many many galaxies. Image credit: Hubble

End of Cosmology - 3 Trillion Years from Now
The Universe acts as a natural time machine. Since light moves at the speed of, well, light, we can look at distant objects and see them how they looked in the past. Look to the very ends of the visible Universe, and you see light that was emitted billions of years ago, shortly after the Big Bang.

It's handy, but there's a problem. That mysterious dark energy force, which is accelerating the expansion of the Universe is making the most distant galaxies move faster and faster away from us. Eventually, they will cross an event horizon and appear to be moving away from us faster than the speed of light. At this point, any light emitted by the galaxy will cease to reach us. Any galaxy that crosses this horizon will fade away from view, until its last photon reaches us. All galaxies will disappear from view forever.

According to a new research paper by Lawrence M. Krauss and Robert J. Scherrer, future astronomers living 3 trillion years from now will only see our own galaxy when they look into the night sky.

This accelerating expansion has another consequence as well. The cosmic microwave background radiation, which astronomers used to discover evidence of the Big Bang will have faded away too. Not only that, but the abundance of chemicals, which precisely match the amounts theorized for the Big Bang will be hidden by subsequent generations of stars.

And so, 3 trillion years from now, there won't be any trace of the Big Bang. No clues for future cosmologists to recognize that the Universe we live in started from a single point, and has been expanding ever since. The Universe will seem static and unchanging.

The core of the Milky Way seen in Infrared. Image credit: Spitzer

End of the Milky Way
Galaxies collide. All you have to do is look out into space with a telescope and see the fate that awaits our galaxy. In all directions we can see the interactions between the gravity of various galaxies. At first the encounters are violent; galaxies tear at each other, stripping off material, and generating huge swaths of star formation. The dormant supermassive black holes at their centres spring to live and become active galactic nuclei, gobbling up the newly delivered material.

Our future merger partner is barreling towards us right now: Andromeda. In approximately 2 billion years from now, our two galaxies will collide, and then pull apart. Then they'll collide again and again until they settle down into a new, larger galaxy: Milkomedia. The twin supermassive black holes will orbit one another, and eventually merge together into an even more massive black hole.

Our position in the galaxy will change; we'll probably be pushed out to the outer reaches of the galaxy's halo - at least 100,000 light years from the centre. Since the Sun will still have billions of years left, some future form of life on Earth might be around to watch these events unfold.

The merger process will be complete approximately 7 billion years from now.

That's not the end of the galaxy, though. It will still be an island in space, with stars orbiting a central core. Over a long period of time, though, estimated to be between 1019 1020 years. The galaxy will erode, with all the stars escaping into intergalactic space.

Artist impression of a white dwarf, surrounded by shed material. Image credit: STSCI

End of Stars - 100 trillion years from now
We can look out into the Milky Way and see stars forming all around us. There is still enough remaining gas and dust in the Milky Way to create whole new generations of stars. But when we look at other galaxies, we can see older, elliptical galaxies which have already used up their free gas and dust. Instead of the bright, hot stars we see in star forming regions, these aging red galaxies are cooling down.

One day there won't be newly forming stars at all. And then one day, the last star will use up the last of its hydrogen fuel, become a red giant and then fade away to a white dwarf. Even the dimmest stars, the cool red dwarfs will use up their fuel - although, it might taken another 10 trillion years or so. They too will turn into black dwarfs.

And so, in about 100 trillion years from now, every star in the Universe, large and small, will be a black dwarf. An inert chunk of matter with the mass of a star, but at the background temperature of the Universe.

Artist illustration of a black hole. Image credit: NASA

The End of Regular Matter - 1030 years
So now we have a Universe with no stars, only cold black dwarfs. There will also be neutron stars and black holes left over from the time where there were stars in the Universe. The Universe will be completely dark.

A future observer might notice the occasional flash, when some object interacts with a black hole. Its matter will spread out into an accretion disk around the black hole. And for a brief period, it will flare up, emitting radiation. But then it too will be added to the mass of the black hole. And everything will go dark again.

Chunks of matter and binary black dwarfs will merge together creating new black holes, and these black holes will be consumed by even larger black holes. It might be that in the far future, all matter will exist in a few, truly massive black holes.

But even if matter escapes this fate, it's doomed eventually. Some theories of physics predict that protons are unstable over long periods of time. They just can't last. Any matter that wasn't consumed by a black hole will start to decay. The protons will turn into radiation, leaving a fine mist of electrons, positrons, neutrinos and radiation to spread out into space.

Theorists anticipate that all protons in the Universe will decay over the course of 1030 years.

Artist impression of a black hole consuming a star.

End of Black Holes - 10100 Years
Black holes were thought to be one-way streets. Matter goes in, but it doesn't come out. But famed astrophysicist Stephen Hawking turned that concept on its head with his theory that black holes can evaporate. It's not much, and it's not fast, but black holes release a tiny amount of radiation back into space.

As it releases this radiation, the black hole actually loses mass, finally evaporating away entirely. The amount of radiation increases as the black hole loses mass. It's actually possible that it could generate a final burst of X-rays and gamma rays as it disappears completely. Future observers (who survived their protons decaying) might see the occasional flash in an otherwise dark universe.

And then in about 10100 years, the last black hole will be gone. All that remains is the radiation emitted.

The Dark Ages, not a single star shines anywhere in the Universe.

The End of Everything - 10100 years and beyond
When the last black hole evaporates, all that will remain in the Universe are photons of radiation, and elementary particles that escaped capture by black holes. The temperature of the entire Universe will reach a final temperature just above absolute zero.

Dark energy may play some future role, continuing the expansion of the Universe, accelerating each of these elementary particles and photons away from each other until they're effectively cut off from one another. No future gravity will bring them together again.

Perhaps there will be another Big Bang someday. Perhaps the Universe is cyclical and the whole process will start up again.

Perhaps it won't, and this bleak future of a cold, dead Universe is all that awaits us. It's not happy, but it's awe inspiring to consider the long future ahead, and helps us appreciate the vibrant age we live in today.

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Titan Launch Pad Tower Blown Up at Cape Canaveral (Gallery)

Cape Canaveral's Titan launch pad gantry was demolished on Sunday. The tower was built in the 1990's to support the US Air Force Titan 4 rocket program. The site has been used for NASA missions as well, including the launch of the Cassini-Huygens Saturn mission in 1997. Launch Complex 40 is being demolished and then refurbished to make way for the new SpaceX Falcon rocket launch facility. Now the gantry is rubble, the clean up operation can begin…

At 9am on Sunday, April 27th, 200 pounds of high explosives brought the Complex 40 mobile service tower crashing down. The tower was responsible for housing and preparing the highly successful Titan rockets for launch. Mainly used for military payloads, the Titan 4 series also sent the NASA Cassini probe on its way to Saturn on October 15th, 1997. A Titan 4 rocket was also used to send the ill-fated Mars Observer mission to the Red Planet on September 25th, 1992. Mission controllers lost contact with Observer when it was three days away from orbital insertion.

A Titan 4 rocket pre-launch as the gantry rolls back. The lightning protection system surrounds the pad (Justin Ray/Spaceflight Now)

The gantry weighed nearly 6500 tonnes and was installed with an advanced satellite processing clean room. The tower supported a total of 17 launches, deploying sophisticated surveillance and communication satellites for the US government. Two of these launches were devoted to the NASA interplanetary missions. The last Titan 4 was launched three years ago, handing heavy launch duties over to the modern Atlas 5 and Delta 4 rocket systems. In its glory days Titan 4 was the largest rocket available carrying the heaviest payloads into space.

Now that the tower has been removed, Space Exploration Technologies (SpaceX) can begin to set up the commercial launch site as the East Coast base of operations for its Falcon 9 rocket system which is currently under development. But why can't the tower be renovated for SpaceX launches? The Falcon 9 rocket system will be assembled horizontally and rolled to the launch pad shortly before launch; the gantry is therefore superfluous to the company's needs at Complex 40.

SpaceX Falcon 1 rocket system in 2004 (SpaceX)

However, not all the infrastructure of the site will be removed. The launch pad's concrete deck and flame duct, water deluge system, electrical systems, lightning towers and instrumentation in the bay under the pad will be reused. The existing office space will also be renovated for SpaceX use. Since last October, SpaceX employees have been working at the site, removing any equipment not compatible with the Falcon system. The site will be up and running in time to begin supplying the International Space Station when NASA's Space Shuttle fleet is retired in 2010. Complex 40 will live on, minus gantry, for NASA contracted launches and other commercial satellite orbital insertions by SpaceX.

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Mars Express in orbit around Mars

ESA's Mars Express in orbit around Mars
Mars Express in orbit around Mars


29 April 2008
Artificial intelligence (AI) being used at the European Space Operations Centre is giving a powerful boost to ESA's Mars Express as it searches for signs of past or present life on the Red Planet.

Since January 2004, Mars Express has been using its sophisticated instruments to study the atmosphere, surface and subsurface of Mars, confirming the presence of water and looking for other signatures of life on and below the Red Planet's rocky terrain.

The spacecraft generates huge volumes of scientific data, which must be downloaded to Earth at the right time and in the correct sequence, otherwise data packets can be permanently lost when the limited on-board memory is overwritten by newly collected data.

Traditionally, data downloading was managed using human-operated scheduling software to generate command sequences sent to Mars Express, telling it when to dump specific data packets. "This is tedious, time-consuming and never really eliminated the occasional loss - forever - of valuable science data," says Alessandro Donati, Head of the Advanced Mission Concepts and Technologies Office at ESA's Space Operations Centre (ESOC), Darmstadt, Germany.

Complex, dynamic problem

Donati says the downloading problem involves several constantly changing variables, including spacecraft orientation, ground station availability, space-ground communication bandwidth, on-board storage availability and the varying amounts of data generated by each of the seven on-board instruments. All these must be optimised in very short time - often with as little as a few hours between ground station passes.

But since 2005, AI researchers at Italy's Institute for Cognitive Science and Technology (ISTC-CNR) led by Dr Amedeo Cesta and mission planners and computer scientists at ESOC have been developing a solution to the complex Mars Express scheduling problem by applying artificial intelligence (AI) techniques to the problem. These are similar to those used to solve scheduling and optimisation problems faced by airlines, shipping companies and large construction projects.


ESA-developed artificial intelligence tool used for Mars Express

MEXAR2 in action
AI for Mars Express: MEXAR2

The result of this work is a new 'smart' tool, dubbed MEXAR2 ('Mars Express AI Tool'), which has successfully passed initial testing and validation and is now an integral part of the Mars Express mission planning system.

MEXAR2 works by considering the variables that affect data downloading - including the overall science observation schedule for all Mars Express instruments - and then intelligently projecting which on-board data packets might be later lost due to memory conflicts. It then optimises the data download schedule and generates the commands needed to implement the download. "With MEXAR2, any loss of stored data packets has been largely eliminated," says Fred Jansen, ESA's mission manager for Mars Express.


Europe's first deep-space mission to fly with AI

MEXAR2 has reduced the mission planning team's workload considerably - by 50 percent compared to the old manual method - for generating workable downlink plans. "And because it optimises bandwidth used to receive data on Earth, we have been able to free expensive ground station time for other missions," says Michel Denis, Mars Express Spacecraft Operations Manager at ESOC.


ESTRACK S- & X-band ground station at Perth

ESA's Perth station tracks Mars Express
MEXAR2 recently won the 'best application' award at ICAPS 2007, a benchmark international conference for AI planning & scheduling technology.

"During MEXAR2's development, the flexibility of AI-based technology let us capture many specific requirements that would otherwise have needed costly software specification redesign" says Nicola Policella, AI research fellow at ESOC. "The use of a model-based approach enabled us to rapidly improve the software prototype to produce an effective mission planning application in a short time."


“MEXAR2 is a trailblazer in bringing AI technology to spacecraft operations.”
MEXAR2 on the global stage

MEXAR2 was also recently mentioned as one of three outstanding AI applications for mission operations by Dr Ari Kristinn Jónsson, formerly a research scientist at NASA's Ames Research Center, in a keynote speech given at iSAIRAS 2008, the 9th International Symposium on Artificial Intelligence, Robotics and Automation for Space.

"It should be noted, that - like the very few other AI tools in spacecraft operations - MEXAR2 is a trailblazer in bringing AI technology to spacecraft operations. The effectiveness of the tool and the benefits it has provided are therefore significant accomplishments in themselves," says Dr Jónsson, who is now Dean of the School of Computer Science at Reykjavik University, Iceland.


Artificial intelligence provides solutions for complex problems, and has now entered the space mission operations field as a value-adding technology. "Mars Express is the first European deep-space exploration mission to fly using an AI tool on the ground, and the technology is boosting science return while reducing time and resource costs," adds Donati.


ExoMars rover

EXOMARS rover - artist's concept
AI can help solve other mission operations problems

With MEXAR2's proven success, scientists at both ESOC and ISTC-CNR are working to apply the current AI technology to other problems.

Successful recent work includes the reverse problem of how to optimise the upload of commands to Mars Express, in a project dubbed - somewhat tongue-in-cheek - as 'RAXEM' - for the 'Reverse of MEXAR'.

ESA-developed AI technology will also be applied to the 'Advanced Planning & Scheduling Initiative', which is designed to provide AI benefits to other areas and missions, including long term observation for ESA's Integral, an orbiting space observatory mission.

"It's possible to apply the same AI concepts to future missions, like ExoMars, Europe's first planetary rover mission to the Red Planet," says Donati, adding, "Today's achievement is the starting point for implementing new on-board autonomy concepts for ESA's challenging missions of the future."


Editor's note

See related article "Artificial intelligence for robotic exploration: Q&A with Ari Kristinn Jónsson" via link at top right.
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Astronomers get closer view of black hole jet

An artist's conception of the blazar BL Lacertae at it spurts out jets of charged particles accelerated by corkscrew magnetic field lines.
Marscher et al., Wolfgang Steffen, Cosmovision, NRAO/AUI/NSF
An artist's conception of the blazar BL Lacertae at it spurts out jets of charged particles accelerated by corkscrew magnetic field lines.

While we may never know what it looks like inside a black hole, astronomers recently obtained one of the closest views yet. The sighting allowed scientists to confirm theories about how these giant cosmic sinkholes spew out jets of particles travelling at nearly the speed of light.

Ever since the first observations of these powerful jets, which are among the brightest objects seen in the universe, astronomers have wondered what causes the particles to accelerate to such great speeds. A leading hypothesis suggested the black hole's gigantic mass distorts space and time around it, twisting magnetic field lines into a coil that propels material outward.

Now researchers have observed a jet during a period of extreme outburst and found evidence that streams of particles wind a corkscrew path away from the black hole, as the leading hypothesis predicts.

"We got an unprecedented view of the inner portion of one of these jets and gained information that's very important to understanding how these tremendous particle accelerators work," said Boston University astronomer Alan Marscher, who led the research team. The results of the study are detailed in the April 24 issue of the journal Nature.

The team studied a galaxy called BL Lacertae (BL Lac), about 950 million light years from Earth, with a central black hole containing 200 million times the mass of our Sun. Since this supermassive black hole's jets are pointing nearly straight at us, it is called a blazar (a quasar is often thought to be the same as a blazar, except its jets are pointed away from us).

The new observations, taken by the National Science Foundation's Very Long Baseline Array (VLBA) radio telescope, along with NASA's Rossi X-ray Timing Explorer and a number of optical telescopes, show material moving outward along a spiral channel, as the scientists expected.

These data support the suggestion that twisted magnetic field lines are creating the jet plumes. Material in the center of the galaxy, such as nearby stars and gas, gets pulled in by the black hole's overwhelming gravity and forms a disk orbiting around the core (the material's inertia keeps it spiraling in a disk rather than falling straight into the black hole). The distorted magnetic field lines seem to pull charged particles off the disk and cause them to gush outward at nearly the speed of light.

"We knew that material was falling in to these regions, and we knew that there were outbursts coming out," said University of Michigan astronomer Hugh Aller, who worked on the new study. "What's really been a mystery was that we could see there were these really high-energy particles, but we didn't know how they were created, how they were accelerated. It turns out that the model matches the data. We can actually see the particles gaining velocity as they are accelerated along this magnetic field."

The astronomers also observed evidence of another phenomenon predicted by the leading hypothesis — that a flare would be produced when material spewing out in the jets hit a shock wave beyond the core of the black hole.

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Compact Galaxies in Early Universe Pack a Big Punch

The full news release story:

Compact Galaxies in Early Universe Pack a Big PunchView this image

Imagine receiving an announcement touting the birth of a baby 20 inches long and weighing 180 pounds. After reading this puzzling message, you would immediately think the baby's weight was a misprint.

Astronomers looking at galaxies in the universe's distant past received a similar perplexing announcement when they found nine young, compact galaxies, each weighing in at 200 billion times the mass of the Sun. The galaxies, each only 5,000 light-years across, are a fraction of the size of today's grownup galaxies but contain approximately the same number of stars. Each galaxy could fit inside the central hub of our Milky Way Galaxy.

Astronomers used NASA's Hubble Space Telescope and the W.M. Keck Observatory on Mauna Kea, Hawaii, to study the galaxies as they existed 11 billion years ago, when the universe was less than 3 billion years old.

"Seeing the compact sizes of these galaxies is a puzzle," said Pieter G. van Dokkum of Yale University in New Haven, Conn., who led the study. "No massive galaxy at this distance has ever been observed to be so compact. It is not yet clear how they would build themselves up to become the large galaxies we see today. They would have to change a lot over 11 billion years, growing five times bigger. They could get larger by colliding with other galaxies, but such collisions may not be the complete answer."

To determine the sizes of the galaxies, the team used the Near Infrared Camera and Multi-Object Spectrometer on Hubble. The Keck observations were carried out with assistance of a powerful laser to correct for image blurring caused by the Earth's atmosphere. "Only Hubble and Keck can see the sizes of these galaxies because they are very small and far away," van Dokkum explained.

Van Dokkum and his colleagues studied the galaxies in 2006 with the Gemini South Telescope Near-Infrared Spectrograph, on Cerro Pachon in the Chilean Andes. Those observations provided the galaxies' distances and showed that the stars are a half a billion to a billion years old. The most massive stars had already exploded as supernovae.

"In the Hubble Deep Field, astronomers found that star-forming galaxies are small," said Marijn Franx of Leiden University, The Netherlands. "However, these galaxies were also very low in mass. They weigh much less than our Milky Way. Our study, which surveyed a much larger area than in the Hubble Deep Field, surprisingly shows that galaxies with the same weight as our Milky Way were also very small in the past. All galaxies look really different in early times, even massive ones that formed their stars early."

The ultradense galaxies might comprise half of all galaxies of that mass 11 billion years ago, van Dokkum said, forming the building blocks of today's largest galaxies.

How did these small, crowded galaxies form? One way, suggested van Dokkum, involves the interaction of dark matter and hydrogen gas in the nascent universe. Dark matter is an invisible form of matter that accounts for most of the universe's mass. Shortly after the Big Bang, the universe contained an uneven landscape of dark matter. Hydrogen gas became trapped in puddles of the invisible material and began spinning rapidly in dark matter's gravitational whirlpool, forming stars at a furious rate.

Based on the galaxies' masses, which are derived from their color, the astronomers estimated that the stars are spinning around their galactic disks at roughly 890,000 to 1 million miles an hour (400 to 500 kilometers a second). Stars in today's galaxies, by contrast, are traveling at about half that speed because they are larger and rotate more slowly than the compact galaxies.

These galaxies are ideal targets for the Wide Field Camera 3, which is scheduled to be installed aboard Hubble during Servicing Mission 4 in the fall of 2008. "We hope to use the Wide Field Camera 3 to find thousands of these galaxies. The Hubble images, together with the laser adaptive optics at Keck and similar large telescopes, should lead to a better understanding of the evolution of galaxies early in the life of the universe," said Garth Illingworth of the University of California, Santa Cruz, and Lick Observatory.

The findings appeared in the April 10 issue of The Astrophysical Journal Letters.

The authors of the science paper are Pieter van Dokkum (Yale University), Marijn Franx (Leiden University, The Netherlands), Mariska Kriek (Princeton University), Bradford Holden, Garth Illingworth, Daniel Magee, and Rychard Bouwens (University of California, Santa Cruz, and Lick Observatory), Danilo Marchesini (Yale University), Ryan Quadri (Leiden University), Greg Rudnick (National Optical Astronomical Observatory, Tucson), Edward Taylor (Leiden University), and Sune Toft (European Southern Observatory, Germany).

CONTACT

Donna Weaver/Ray Villard
Space Telescope Science Institute, Baltimore, Md.
410-338-4493/4514
dweaver@stsci.edu/villard@stsci.edu


Pieter van Dokkum
Yale University, New Haven, Conn.
203-432-3019
pieter.vandokkum@yale.edu

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Space war would leave destructive legacy

If war ever breaks out in space it's not the loss of individual satellites that will do the damage, but the debris this produces. It will stay in orbit and go on harming satellites for decades, according to two studies presented at the American Physical Society meeting in St Louis, Missouri, last week.

A commission chartered by the US Congress in 2000 warned that the US military's dependence on satellites would mean a space attack could be crippling. Last year, China heightened fears of a space war by testing an anti-satellite weapon, while earlier this year the US destroyed one of its own defunct satellites using a missile.

Now it seems that the immediate impact of a space war, at least on the US military, would be limited. "We have built up such high redundancy to space assets that we're almost invulnerable," says Geoff Forden of the Massachusetts Institute of Technology, who assessed the risk posed by China to the US. He found that only a few of the US's low-Eart-orbit satellites are over China at any one time, and that higher-orbiting satellites used for GPS, communications and surveillance could only be destroyed by multistage missiles, for which China has only three launch pads.

Crucially, any space attack would increase debris, which can have a long-lasting effect on satellites. David Wright of the Union of Concerned Scientists in Washington DC reports that destruction of one 10-tonne spy satellite in low-Earth orbit "would double or triple the debris" in that zone. Every new collision produces even more debris, triggering a cascade of satellite break-ups with time.



Galaxy Evolution Explorer Celebrates Five Years in Space

ultraviolet view of M106 An ultraviolet view of galaxy M106 Image credit: NASA/JPL-Caltech
› Full image and caption
Since its launch five years ago, the Galaxy Evolution Explorer has photographed hundreds of millions of galaxies in ultraviolet light. M106 is one of those galaxies, and from 22 million light years away, it strikes a pose in blue and gold for this new commemorative portrait.

The galaxy's extended arms are the blue filaments that curve around its edge, creating its outer disk. Tints of blue in M106's arms reveal hot, young massive stars. Traces of gold toward the center show an older stellar population and indicate the presence of obscuring dust.

"We see these arms in optical-light images, but they are very faint and diffuse. These structures really pop out in the ultraviolet due to the exquisite sensitivity of the detectors on the Galaxy Evolution Explorer," said Mark Seibert of the Carnegie Institution of Washington in Pasadena, Calif.

From 24 million light-years away, neighboring galaxy NGC 4248 also makes a memorable appearance, sitting just right of M106. The irregular galaxy looks like a yellow smudge, with a bluish-white bar in the center. The galaxy's outer golden glow indicates a population of older stars, while the blue central region shows a younger stellar demographic.

Dwarf galaxy UGC 7365 emerges at the bottom center of this image, as a faint yellow smudge directly below M106. This galaxy is not forming any new stars, and looks much smaller than M106 despite being closer to Earth, at 14 million light-years away.

Over the past five years, the Galaxy Evolution Explorer has imaged half a billion objects over 27,000 square degrees of sky – equivalent to an area that would be covered by 138,000 full moons. The telescope orbits Earth every 94 minutes and travels approximately 408,470 million miles per day. Its overarching question is: how do galaxies grow and change over 10 billion years of cosmic history?

"Frankly we have only begun to scratch the surface of this vast data set. Astronomers will be mining the telescope's data archive for the next decade," said Chris Martin, of the California Institute of Technology, Pasadena, Calif. He is principal investigator for the Galaxy Evolution Explorer mission, which is managed by NASA's Jet Propulsion Laboratory in Pasadena, Calif.

M106, also known as NGC 4258, is located in the constellation Canes Venatici. This image is a two-color composite, where far-ultraviolet light is blue, and near-ultraviolet light is red.
Written by Linda Vu/Spitzer Science Center
Media contact: Whitney Clavin/JPL 818-354-4673
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