Posted on by Fraser Cain

Ultra Cool Star Measured

Image credit: ESO
Using ESO’s Very Large Telescope at Paranal and a suite of ground- and space-based telescopes in a four-year long study, an international team of astronomers has measured for the first time the mass of an ultra-cool star and its companion brown dwarf. The two stars form a binary system and orbit each other in about 10 years.

The team obtained high-resolution near-infrared images; on the ground, they defeated the blurring effect of the terrestrial atmosphere by means of adaptive optics techniques. By precisely determining the orbit projected on the sky, the astronomers were able to measure the total mass of the stars. Additional data and comparison with stellar models then yield the mass of each of the components.

The heavier of the two stars has a mass around 8.5% of the mass of the Sun and its brown dwarf companion is even lighter, only 6% of the solar mass. Both objects are relatively young with an age of about 500-1,000 million years.

These observations represent a decisive step towards the still missing calibration of stellar evolution models for very-low mass stars.

Telephone number star
Even though astronomers have found several hundreds of very low mass stars and brown dwarfs, the fundamental properties of these extreme objects, such as masses and surface temperatures, are still not well known. Within the cosmic zoo, these ultra-cool stars represent a class of “intermediate” objects between giant planets – like Jupiter – and “normal” stars less massive than our Sun, and to understand them well is therefore crucial to the field of stellar astrophysics.

The problem with these ultra-cool stars is that contrary to normal stars that burn hydrogen in their central core, no unique relation exists between the luminosity of the star and its mass. Indeed, luminosities and surface temperatures of ultra-cool dwarf stars depend both on their age and their mass. An older, somewhat more massive ultra-cool dwarf can thus have exactly the same temperature as a younger, less massive one.

It is therefore a basic goal of modern astrophysics to obtain independently the masses of an ultra-cool dwarf star. This is in principle possible by studying such objects that are members in a binary system.

This is precisely what an international team of astronomers has now done in a four-year long study of a binary stellar system with an ultra-cool dwarf star, using a plethora of top telescopic facilities, including ESO’s Very Large Telescope, as well as Keck I and Gemini North in Hawaii and also the Hubble Space Telescope. This system – with the telephone number name of 2MASSW J0746425+2000321 – is located at a distance of 40 light-years.

The astronomers used high-angular-resolution imaging to see both stars in the binary system and to measure their motion over a four-year period. However, this is more easily said than done, as the separation on the sky between the two stars is quite small: between 0.13 and 0.22 arcsec. This corresponds to the size of a 1-Euro coin, seen at a distance of about 25 km.

This separation is so small that it is normally not possible to differentiate the two stars due to the blurring effect of atmospheric turbulence (the “seeing”). It is therefore necessary to use the technique of adaptive optics. This wonderful method is based on the measurement of the image quality in real-time and sending corresponding corrective signals up to 100 times every second to a small deformable mirror, located in front of the detector. As the mirror continuously modifies its shape, the disturbing effect of the turbulence is neutralised. Applied at the VLT, this technique has resulted in images which are at least ten times sharper than the “seeing” and which therefore show many more details in the observed objects.

At the Very Large Telescope, the astronomers used the state-of-the-art adaptive optics NACO instrument. Says Herv? Bouy, principal author of the paper presenting the results described here: “NACO offers the possibility to work in the infrared and is therefore ideally suited for the study of ultra-cool stars, which emit most of their light in this wavelength range. With the combination of the high efficiency of NACO and the VLT, and the excellent atmospheric conditions prevailing at Paranal, we were able to achieve very sharp images of this binary stellar system, almost as good as if the telescope were located in space.”

Ultra-cool and on diet
During their four-year long study, seven different relative positions of the two components of the binary system were measured and Herv? Bouy and his co-workers were able to determine with good precision the stellar orbits. They find that the two stars revolve around each other once every 10 years and that their physical separation is only 2.5 times the distance of the Earth to the Sun – as astronomers say, 2.5 Astronomical Units. Using Kepler’s laws, it is then straightforward to derive the total mass of the system. The obtained value is less than 15 % of the mass of the Sun.

The astronomers then used the photometric data of each star obtained in several wavebands, as well as spectra obtained with the Hubble Space Telescope to study the two objects in more detail. Using the latest stellar models of the group of the Ecole Normale Sup?rieure de Lyon, they found that both stars have roughly the same surface temperature, around 1500 ?C (1800 K). For a star, this is ultra-cool indeed – by comparison, the surface temperature of the Sun is more than three times higher.

Using theoretical models, the team also found that the two stars are rather young (in astrophysical terms) – their age is between 500 and 1,000 million years only. The more massive of the two has a mass between 7.5 and 9.5% the mass of the Sun, while its companion has a mass between 5 and 7% of the solar mass.

Objects weighing less than about 7% of our Sun have been variously called “Brown Dwarfs”, “Failed Stars” or “Super Planets”. Indeed, since they have no sustained energy generation by thermal nuclear reactions in their interior, many of their properties are more similar to those of giant gas planets in our own solar system such as Jupiter, than to stars like the Sun.

The system 2MASSW J0746425+2000321 is thus apparently made up of a brown dwarf orbiting a slightly more massive ultra-cool dwarf star. It is a true “Rosetta stone” in the new field of low-mass stellar astrophysics and further studies will surely provide more valuable information about these objects in the transitional zone between stars and planets.

Original Source: ESO News Release

Posted on by Fraser Cain

Cassini Passes Phoebe

Image credit: NASA/JPL/Space Science Institute
Images collected during the Cassini-Huygens close fly-by of Saturn’s moon Phoebe give strong evidence that the tiny moon may be rich in ice and covered by a thin layer of darker material.

Its surface is heavily battered, with large and small craters. It might be an ancient remnant of the formation of the Solar System.

On Friday 11 June, at 21:56 CET, the Cassini-Huygens spacecraft flew by Saturn’s outermost moon Phoebe, coming within approximately 2070 kilometres of the satellite’s surface. All eleven on-board instruments scheduled to be active at that time worked flawlessly and acquired data.

The first high-resolution images show a scarred surface, covered with craters of all sizes and large variation of brightness across the surface.

Phoebe is a peculiar moon amongst the 31 known satellites orbiting Saturn. Most of Saturn’s moons are bright but Phoebe is very dark and reflects only 6% of the Sun’s light. Another difference is that Phoebe revolves around the planet on a rather elongated orbit and in a direction opposite to that of the other large moons (a motion known as ‘retrograde’ orbit).

All these hints suggested that Phoebe, rather than forming together with Saturn, was captured at a later stage. Scientists, however, do not know whether Phoebe was originally an asteroid or an object coming from the ‘Kuiper Belt’.

The stunning images obtained by Cassini’s high-resolution camera now seem to indicate that it contains ice-rich material and is covered by a thin layer of dark material, probably 300-500 metres thick.

Scientists base this hypothesis on the observation of bright streaks in the rims of the largest craters, bright rays radiating from smaller craters, grooves running continuously across the surface of the moon and, most importantly, the presence of layers of dark material at the top of crater walls.

“The imaging team is in hot debate at the moment on the interpretations of our findings,” said Dr Carolyn Porco, Cassini imaging team leader at the Space Science Institute in Boulder, USA.

“Based on our images, some of us are leaning towards the view that has been promoted recently, that Phoebe is probably ice-rich and may be an object originating in the outer solar system, more related to comets and Kuiper Belt objects than to asteroids.”

The high-resolution images of Phoebe show a world of dramatic landforms, with landslides and linear structures such as grooves, ridges and chains of pits. Craters are ubiquitous, with many smaller than one kilometre.

“This means, besides the big ones, lots of projectiles smaller than 100 metres must have hit Phoebe,” said Prof. Gerhard Neukum, Freie Universitaet Berlin, Germany, and a member of the imaging team. Whether these projectiles came from outside or within the Saturn system is debatable.

There is a suspicion that Phoebe, the largest of Saturn’s outer moons, might be parent to the other, much smaller retrograde outer moons that orbit Saturn. They could have resulted from the impact ejecta that formed the many craters on Phoebe.

Besides these stunning images, the instruments on board Cassini collected a wealth of other data, which will allow scientists to study the surface structures, determine the mass and composition of Phoebe and create a global map of it.

“If these additional data confirm that Phoebe is mostly ice, covered by layers of dust, this could mean that we are looking at a ‘leftover’ from the formation of the Solar System about 4600 million years ago,” said Dr Jean-Pierre Lebreton, ESA Huygens Project Scientist.

Phoebe might indeed be an icy wanderer from the distant outer reaches of the Solar System, which, like a comet, was dislodged from the Kuiper Belt and captured by Saturn when the planet was forming.

Whilst studying the nature of Phoebe may give scientists clues on the origin of the building blocks of the Solar System, more data are needed to reconstruct the history of our own neighbourhood in space.

With that aim, ESA’s Rosetta mission is on its way to study one of these primitive objects, Comet 67P/Churyumov-Gerasimenko, from close quarters for over a year and land a probe on it.

The fly-by of Phoebe on 11 June was the only one that Cassini-Huygens will perform with this mysterious moon. The mission will now take the spacecraft to its closest approach to Saturn on 1 July, when it will enter into orbit around the planet.

From there, it will conduct 76 orbits of Saturn over four years and execute 52 close encounters with seven other Saturnian moons. Of these, 45 will be with the largest and most interesting one, Titan. On 25 December, Cassini will release the Huygens probe, which will descend through Titan’s thick atmosphere to investigate its composition and complex organic chemistry.

Original Source: ESA News Release

Posted on by Fraser Cain

SpaceShipOne Set for Launch in a Week

In just one week Scaled Composite’s SpaceShipOne will make an attempt to become the first privately-built vehicle to reach space – an altitude of 100 km (62 miles). Thousands of people are expected to show up at the runway in California’s Mojave Desert to watch the launch and suborbital flight. This won’t be an official attempt to win the Ansari X Prize; however, but the company is planning to try for the $10 million prize if this flight is successful. Billionaire Paul Allen has contributed $20 million to the development of SpaceShipOne.

Posted on by Fraser Cain

Close Up on Phoebe Crater

Image credit: NASA/JPL/Space Science Institute
This eye-popping high-resolution image of Phoebe’s pitted surface taken very near closest approach shows a 13-kilometer (8-mile) diameter crater with a debris-covered floor. Part of another crater of similar size is visible at left, as is part of a larger crater at top and many scattered smaller craters. The radial streaks in the crater are due to downslope movements of loose fragments from impact ejecta. Also seen are boulders ranging from about 50 to 300 meters (160 to 990 feet) in diameter. The building-sized rocks may have been excavated by large impacts, perhaps from some other region of Phoebe rather than the craters seen here. There is no visible evidence for layering of ice and regolith or a hardened crust in this region, as on other parts of this moon.

Some of the relatively bright spots are from small impacts that excavated bright material from beneath the dark surface. Images like this provide information about impact and regolith processes on Phoebe.

This image was obtained at a phase, or Sun-Phoebe-spacecraft, angle of 78 degrees, and from a distance of 11,918 kilometers (7,407 miles). The image scale is approximately 18.5 meters (60.5 feet) per pixel. The illumination is from the right. No enhancement was performed on this image.

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 Office of Space Science, Washington, D.C. The imaging team is based at the Space Science Institute, Boulder, Colorado.

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: CICLOPS News Release

Posted on by Fraser Cain

Cassini Makes its Phoebe Flyby

Image credit: NASA/JPL/Space Science Institute
Phoebe?s true nature is revealed in startling clarity in this mosaic of two images taken during Cassini?s flyby on June 11, 2004. The image shows evidence for the emerging view that Phoebe may be an ice-rich body coated with a thin layer of dark material. Small bright craters in the image are probably fairly young features. This phenomenon has been observed on other icy satellites, such as Ganymede at Jupiter. When impactors slammed into the surface of Phoebe, the collisions excavated fresh, bright material — probably ice — underlying the surface layer. Further evidence for this can be seen on some crater walls where the darker material appears to have slid downwards, exposing more light-colored material. Some areas of the image that are particularly bright ? especially near lower right ? are over-exposed.

An accurate determination of Phoebe?s density ? a forthcoming result from the flyby ? will help Cassini mission scientists understand how much of the little moon is comprised of ices.

This spectacular view was obtained at a phase, or Sun-Phoebe-spacecraft, angle of 84 degrees, and from a distance of approximately 32,500 kilometers (20,200 miles). The image scale is approximately 190 meters (624 feet) per pixel. No enhancement was performed on this image.

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 Office of Space Science, Washington, D.C. The imaging team is based at the Space Science Institute, Boulder, Colorado.

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: CICLOPS News Release

Posted on by Fraser Cain

Youngest Black Hole Found?

Image credit: NRAO
Astronomers using a global combination of radio telescopes to study a stellar explosion some 30 million light-years from Earth have likely discovered either the youngest black hole or the youngest neutron star known in the Universe. Their discovery also marks the first time that a black hole or neutron star has been found associated with a supernova that has been seen to explode since the invention of the telescope nearly 400 years ago.

A supernova is the explosion of a massive star after it exhausts its supply of nuclear fuel and collapses violently, rebounding in a cataclysmic blast that spews most of its material into interstellar space. What remains is either a neutron star, with its material compressed to the density of an atomic nucleus, or a black hole, with its matter compressed so tightly that its gravitational pull is so strong that not even light can escape it.

A team of scientists studied a supernova called SN 1986J in a galaxy known as NGC 891. The supernova was discovered in 1986, but astronomers believe the explosion actually occurred about three years before. Using the National Science Foundation’s Very Long Baseline Array (VLBA), Robert C. Byrd Green Bank Telescope (GBT), and Very Large Array (VLA), along with radio telescopes from the European VLBI Network, they made images that showed fine details of how the explosion evolves over time.

“SN 1986J has shown a brightly-emitting object at its center that only became visible recently. This is the first time such a thing has been seen in any supernova,” said Michael Bietenholz, of York University in Toronto, Ontario. Bietenholz worked with Norbert Bartel, also of York University, and Michael Rupen of the National Radio Astronomy Observatory (NRAO) in Socorro, New Mexico, on the project. The scientists reported their findings in the June 10 edition of Science Express.

“A supernova is likely the most energetic single event in the Universe after the Big Bang. It is just fascinating to see how the smoke from the explosion is blown away and how now after all these years the fiery center is unveiled. It is a textbook story, now witnessed for the first time,” Bartel said.

Analysis of the bright central object shows that its characteristics are different from the outer shell of explosion debris in the supernova.

“We can’t yet tell if this bright object at the center is caused by material being sucked into a black hole or if it results from the action of a young pulsar, or neutron star,” said Rupen.

“It’s very exciting because it’s either the youngest black hole or the youngest neutron star anybody has ever seen,” Rupen said. The youngest pulsar found to date is 822 years old.

Finding the young object is only the beginning of the scientific excitement, the astronomers say.

“We’ll be watching it over the coming years. First, we hope to find out whether it’s a black hole or a neutron star. Next, whichever it is, it’s going to give us a whole new view of how these things start and develop over time,” Rupen said.

For example, Rupen explained, if the object is a young pulsar, learning the rate at which it is spinning and the strength of its magnetic field would be extremely important for understanding the physics of pulsars.

The scientists point out that it will be important to observe SN 1986J at many wavelengths, not just radio, but also in visible light, infrared and others.

In addition, the astronomers also now want to look for simiilar objects elsewhere in the Universe.

The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

Original Source: NRAO News Release

Posted on by Fraser Cain

Cassini Will Reach Phoebe Today

Image credit: NASA/JPL/Space Science Institute
As Cassini sails toward its rendezvous with Phoebe, details on the small, dark moon are coming into view at a dizzying pace. The images shown here were taken only 13 hours apart on June 10, 2004, just one day prior to closest approach, and show a dramatic increase in detail between these two views. On Phoebe, the spin axis points up and approximately 13 degrees to the left of the boundary between day and night. Phoebe completes one rotation about its spin axis in 9 hours and 16 minutes. We are looking at opposite hemispheres in these two views.

A large crater, roughly 50 km (31 miles) across, is visible in the image on the left. The image on the right shows a body heavily pitted with craters of varying sizes, including very large ones, and displaying a substantial amount of variation in surface brightness. Features that appear to be cliffs may in fact be the boundaries between large craters. Despite its exaggerated topography, Phoebe is more round than irregular in shape.

Left to right, the two views were obtained at a phase, or Sun-Phoebe-spacecraft, angle of 87 degrees, and from distances ranging from 956,000 kilometers (594,000 miles) to 658,000 kilometers (409,000 miles). The image scale ranges from 5.7 to 3.9 kilometers (3.5 to 2.4 miles) per pixel. To aid visibility, the images were magnified three times via linear interpolation; no contrast enhancement was performed.

Phoebe is approximately 220 kilometers (137 miles) wide. Its many secrets await as Cassini draws close to its only flyby with this mysterious outer moon of Saturn at 1:56 pm PDT on June 11, 2004.

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 Office of Space Science, Washington, D.C. The imaging team is based at the Space Science Institute, Boulder, Colorado.

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: CICLOPS News Release

Posted on by Fraser Cain

It Doesn’t Get Much Hotter Than Io

Image credit: NASA/JPL
The hottest spot in the solar system is neither Mercury, Venus, nor St. Louis in the summer. Io, one of the four satellites that the Italian astronomer Galileo discovered orbiting Jupiter almost 400 years ago, takes that prize. The Voyager spacecraft discovered volcanic activity on Io over 20 years ago and subsequent observations show that Io is the most volcanically active body in the solar system. The Galileo spacecraft, named in honor of the astronomer Galileo, found volcanic hot spots with temperatures as high as 2,910 Fahrenheit (1,610 Celsius).

Now computer models of volcanic eruptions on Io performed by researchers at Washington University in St. Louis show that the lavas are so hot that they are vaporizing sodium, potassium, silicon and iron and probably other gases as well into its atmosphere.

Using an updated version of MAGMA, a versatile computer program he developed 15 years ago with a Harvard University colleague, Bruce Fegley, Jr., Ph.D., professor of earth and planetary sciences in Arts & Sciences at Washington University in St. Louis, found that some of these elements are vaporized at least partly as single-atom gases. Others are vaporized in different molecular forms, for instance, silicon monoxide, silicon dioxide and iron monoxide.

“Reaction of these gases with sulfur and chlorine species in volcanic gases could lead to the formation of such unusual gases as sodium chloride, potassium chloride, magnesium dichloride and iron dichloride, ” Fegley said.

In 2000, Fegley and former Washington University colleague Mikhail Zolotov, Ph.D., now at Arizona Sate University, predicted formation of sodium chloride and potassium chloride vapor in volcanic gases on Io. Three years later astronomers found sodium chloride gas on Io. However, these observations were not sensitive enough to detect the less abundant potassium chloride vapor.

Now Fegley has found that sodium and potassium in Ionian volcanic gases are being vaporized from the hot lavas. Fegley and research assistant Laura Schaefer of Washington University used data from the Galileo mission and Earth-based observations from high-powered telescopes in their NASA-funded research. They published their results in the May 2004 issue of Icarus, the leading planetary science journal.

“We’re basically doing geology on Io using data from telescopes on Earth, which shows that observations like this can compete with expensive space missions,” said Fegley. “It’s amazing how hot and how volcanically active Io is. It is 30 times more active than Earth. It’s the hottest body outside of the sun in the solar system.”

The innermost of the four major satellites of Jupiter – there are at least 16 – Io gets its high rate of volcanism from tidal interactions with Jupiter, which has the strongest magnetic field of all the planets. Over 100 active volcanoes have been identified on Io. Hotspots there have temperatures as high as 1,600 degrees Celsius. This is several hundred degrees hotter than terrestrial volcanoes like Kilauea in Hawaii, which has a temperature of about 1,000 Celsius (1,830 Fahrenheit).

Fegley and Schaefer found that silicon monoxide is the major silicon-bearing gas over the lavas.

“The interesting thing about this is that astronomers have observed silicon monoxide in other environments in interstellar space, most notably in the atmospheres of cool stars,” said Fegley.

Astronomical observations of actively erupting volcanoes on Io may be able to detect the silicon monoxide gas in its atmosphere.

Fegley and Schaefer recommend an Io volcanic probe mission to directly measure the pressure, temperature and composition of gases of Pele, one of Io’s most active volcanoes. Such an endeavor is “feasible using present technology,” Fegley said. “It would vastly expand our knowledge of the most volcanically active body in the solar system.”

The volcanic probe mission would represent an advance in the effort to unveil some of Io’s mysteries, such as how the satellite, about the size of our own Moon, can maintain its high magma temperatures without being nearly totally molten, and how does Io maintain a strong enough lithosphere to support mountains higher than Mount Everest?

Original Source: WUSTL News Release

Posted on by Fraser Cain

Wallpaper: Flood Plains on Mars

Image credit: ESA
These images of fluvial surface features at Mangala Valles on Mars were obtained by the High Resolution Stereo Camera (HRSC) on board the ESA Mars Express spacecraft.

The HRSC has imaged structures several times which are related to fluvial events in the past on Mars.

The region seen here is situated on the south-western Tharsis bulge and shows the mouth of the Mangala Valles and Minio Vallis outflow channels.

The source of the outflow channel is related to the Mangala Fossa, a fissure running east-west for several hundred kilometres.

One theory about its formation is related to a process known on Earth as ?dyke emplacement?.

This is when hot molten rock finds its way to the surface through a fissure, releasing large amounts of water by the melting of subsurface ice.

It is still unclear for how long and to what extent water, mud or even ice masses and wind have carved the channel here.

This theory on its formation has several analogues on Earth. Events like the one proposed for Mangala Valles occur on Earth, for example in Iceland, where volcanic activity causes episodic releases of water from subsurface reservoirs, causing catastrophic floods.

Along the channel troughs, areas with so-called ?chaotic terrain? features favour the idea of the existence of subsurface ice.

The small-scale chaotic terrain is characterised by isolated blocks of surface material which have been randomly arranged during the release of subsurface water and subsequent collapse of the surface.

Huge areas of chaotic terrain can be found near the source areas of the outflow channels around Chryse Planitia, such as Kasei, Maja and Ares Valles.

Beside the large outflow channels, a variety of smaller ?dendritic? valley networks with a number of tributary valleys can be seen near the main channels. This indicates possible precipitation.

The images were taken during orbit 299 with a resolution of 28 metres per pixel. The image centre is located at 209? E longitude and 5? S latitude. For practical use on the internet, the images have been reduced in resolution.

The red/cyan 3D anaglyph image was created using the stereo- and nadir channels of the HRSC. The perspective view was calculated from the digital terrain model derived from the stereo and colour information of the image data.

Original Source: ESA News Release

Posted on by Fraser Cain

Gemini Goes Silver

Image credit: Gemini
To investors looking for the next sure thing, the silver coating on the Gemini South 8-meter telescope mirror might seem like an insider’s secret tip-off to invest in this valuable metal for a huge profit. However, it turns out that this immense mirror required less than two ounces (50 grams) of silver, not nearly enough to register on the precious metals markets. The real return on Gemini’s shiny investment is the way it provides unprecedented sensitivity from the ground when studying warm objects in space.

The new coating-the first of its kind ever to line the surface of a very large astronomical mirror-is among the final steps in making Gemini the most powerful infrared telescope on our planet. “There is no question that with this coating, the Gemini South telescope will be able to explore regions of star and planet formation, black holes at the centers of galaxies and other objects that have eluded other telescopes until now,” said Charlie Telesco of the University of Florida who specializes in studying star- and planet-formation regions in the mid-infrared.

Covering the Gemini mirror with silver utilizes a process developed over several years of testing and experimentation to produce a coating that meets the stringent requirements of astronomical research. Gemini’s lead optical engineer, Maxime Boccas who oversaw the mirror-coating development said, “I guess you could say that after several years of hard work to identify and tune the best coating, we have found our silver lining!”

Most astronomical mirrors are coated with aluminum using an evaporation process, and require recoating every 12-18 months. Since the twin Gemini mirrors are optimized for viewing objects in both optical and infrared wavelengths, a different coating was specified. Planning and implementing the silver coating process for Gemini began with the design of twin 9-meter-wide coating chambers located at the observatory facilities in Chile and Hawaii. Each coating plant (originally built by the Royal Greenwich Observatory in the UK) incorporates devices called magnetrons to “sputter” a coating on the mirror. The sputtering process is necessary when applying multi-layered coatings on the Gemini mirrors in order to accurately control the thickness of the various materials deposited on the mirror’s surface. A similar coating process is commonly used for architectural glass to reduce air-conditioning costs and produce an aesthetic reflection and color to glass on buildings, but this is the first time it has been applied to a large astronomical telescope mirror.

The coating is built up in a stack of four individual layers to assure that the silver adheres to the glass base of the mirror and is protected from environmental elements and chemical reactions. As anyone with silverware knows, tarnish on silver reduces the reflection of light. The degradation of an unprotected coating on a telescope mirror would have a profound impact on its performance. Tests done at Gemini with dozens of small mirror samples over the past few years show that the silvered coating applied to the Gemini mirror should remain highly reflective and usable for at least a year between recoatings.

In addition to the large primary mirror, the telescope’s 1-meter secondary mirror and a third mirror that directs light into scientific instruments were also coated using the same protected silver coatings. The combination of these three mirror coatings as well as other design considerations are all responsible for the dramatic increase in Gemini’s sensitivity to thermal infrared radiation.

A key measure of a telescope’s performance in the infrared is its emissivity (how much heat it actually emits compared to the total amount it can theoretically emit) in the thermal or mid-infrared part of the spectrum. These emissions result in a background noise against which astronomical sources must be measured. Gemini has the lowest total thermal emissivity of any large astronomical telescope on the ground, with values under 4% prior to receiving its silver coating. With this new coating, Gemini South’s emissivity will drop to about 2%. At some wavelengths this has the same effect on sensitivity as increasing the diameter of the Gemini telescope from 8 to more than 11 meters! The result is a significant increase in the quality and amount of Gemini’s infrared data, which allows detection of objects that would otherwise be lost in the noise generated by heat radiating from the telescope. It is common among other ground-based telescopes to have emissivity values in excess of 10%

The recoating procedure was successfully performed on May 31, and the newly coated Gemini South mirror has been re-installed and calibrated in the telescope. Engineers are currently testing the systems before returning the telescope to full operations. The Gemini North mirror on Mauna Kea will undergo the same coating process before the end of this year.

Why Silver?
The reason astronomers wish to use silver as the surface on a telescope mirror lies in its ability to reflect some types of infrared radiation more effectively than aluminum. However, it is not just the amount of infrared light that is reflected but also the amount of radiation actually emitted from the mirror (its thermal emissivity) that makes silver so attractive. This is a significant issue when observing in the mid-infrared (thermal) region of the spectrum, which is essentially the study of heat from space. ?The main advantage of silver is that it reduces the total thermal emission of the telescope. This in turn increases the sensitivity of the mid-infrared instruments on the telescope and allows us to see warm objects like stellar and planetary nurseries significantly better,? said Scott Fisher a mid-infrared astronomer at Gemini.

The advantage comes at a price however. To use silver, the coating must be applied in several layers, each with a very precise and uniform thickness. To do this, devices called magnetrons are used to apply the coating. They work by surrounding an extremely pure metal plate (called the target) with a plasma cloud of gas (argon or nitrogen) that knocks atoms out from the target and deposits them uniformly on the mirror (which rotates slowly under the magnetron). Each layer is extremely thin; with the silver layer only about 0.1 microns thick or about 1/200 the thickness of a human hair. The total amount of silver deposited on the mirror is approximately equal to 50 grams.

Studying Heat Originating from Space
Some of the most intriguing objects in the universe emit radiation in the infrared part of the spectrum. Often described as “heat radiation,” infrared light is redder than the red light we see with our eyes. Sources that emit in these wavelengths are sought after by astronomers since most of their infrared radiation can pass through clouds of obscuring gas dust and reveal secrets otherwise shrouded from view. The infrared wavelength regime is split into three main regions, near- , mid- and far-infrared. Near-infrared is just beyond what the human eye can see (redder than red), mid-infrared (often called thermal infrared) represents longer wavelengths of light usually associated with heat sources in space, and far-infrared represents cooler regions.

Gemini’s silver coating will enable the most significant improvements in the thermal infrared part of the spectrum. Studies in this wavelength range include star- and planet-formation regions, with intense research that seeks to understand how our own solar system formed some five billion years ago.

Original Source: Gemini News Release