Three Potentially Habitable Worlds Found Around Nearby Dwarf Star

The constellation of Aquarius can be seen low in the SW Sky just before dawn. Next to the star  Rio Aqurii Astronomers using the TRAPPIST telescope at ESO’s La Silla Observatory have discovered three planets orbiting an ultracool dwarf star just 40 light-years from Earth. These worlds have sizes and temperatures similar to those of Venus and Earth and are the best targets found so far for the search for life outside the Solar System. They are the first planets ever discovered around such a tiny and dim star. The new results were published in the journal Nature today.

A team of astronomers led by Michaël Gillon, of the Institute d’Astrophysique et Géophysique at the University of Liège in Belgium, have used the Belgian TRAPPIST telescope to observe the star 2MASS J23062928-0502285, now also known as TRAPPIST-1. They found that this dim and cool star faded slightly at regular intervals, indicating that several objects were passing between the star and the Earth. Detailed analysis showed that three planets with similar sizes to the Earth were present.

TRAPPIST-1 is an ultracool dwarf star — it is much cooler and redder than the Sun and barely larger than Jupiter. Such stars are both very common in the Milky Way and very long-lived, but this is the first time that planets have been found around one of them. Despite being so close to the Earth, this star is too dim and too red to be seen with the naked eye or even visually with a large amateur telescope. It lies in the constellation of Aquarius (The Water Carrier).

Emmanuël Jehin, a co-author of the new study, is excited: “This really is a paradigm shift with regards to the planet population and the path towards finding life in the Universe. So far, the existence of such ‘red worlds’ orbiting ultra-cool dwarf stars was purely theoretical, but now we have not just one lonely planet around such a faint red star but a complete system of three planets!”

 

Michaël Gillon, lead author of the paper presenting the discovery, explains the significance of the new findings: "Why are we trying to detect Earth-like planets around the smallest and coolest stars in the solar neighbourhood? The reason is simple: systems around these tiny stars are the only places where we can detect life on an Earth-sized exoplanet with our current technology. So if we want to find life elsewhere in the Universe, this is where we should start to look."

Astronomers will search for signs of life by studying the effect that the atmosphere of a transiting planet has on the light reaching Earth. For Earth-sized planets orbiting most stars this tiny effect is swamped by the brilliance of the starlight. Only for the case of faint red ultra-cool dwarf stars — like TRAPPIST-1 — is this effect big enough to be detected.

Follow-up observations with larger telescopes, including the HAWK-I instrument on ESO’s 8-metre Very Large Telescope in Chile, have shown that the planets orbiting TRAPPIST-1 have sizes very similar to that of Earth. Two of the planets have orbital periods of about 1.5 days and 2.4 days respectively, and the third planet has a less well determined period in the range 4.5 to 73 days.

"With such short orbital periods, the planets are between 20 and 100 times closer to their star than the Earth to the Sun. The structure of this planetary system is much more similar in scale to the system of Jupiter’s moons than to that of the Solar System," explains Michaël Gillon.

Although they orbit very close to their host dwarf star, the inner two planets only receive four times and twice, respectively, the amount of radiation received by the Earth, because their star is much fainter than the Sun. That puts them closer to the star than the habitable zone for this system, although it is still possible that they possess habitable regions on their surfaces. The third, outer, planet’s orbit is not yet well known, but it probably receives less radiation than the Earth does, but maybe still enough to lie within the habitable zone.

"Thanks to several giant telescopes currently under construction, including ESO’s E-ELT and the NASA/ESA/CSA James Webb Space Telescope due to launch for 2018, we will soon be able to study the atmospheric composition of these planets and to explore them first for water, then for traces of biological activity. That's a giant step in the search for life in the Universe," concludes Julien de Wit, a co-author from the Massachusetts Institute of Technology (MIT) in the USA.

This work opens up a new direction for exoplanet hunting, as around 15% of the stars near to the Sun are ultra-cool dwarf stars, and it also serves to highlight that the search for exoplanets has now entered the realm of potentially habitable cousins of the Earth. The TRAPPIST survey is a prototype for a more ambitious project called SPECULOOS that will be installed at ESO’s Paranal Observatory.

Upcoming: Aldebaran & Mercury Lunar occultations

ALDEBARAN (Alpha Tauri) will be occulted by the slender crescent Moon on 8 May beginning at 08h 42m UT. The event is visible from N. & NE Africa, Southern Europe, the Middle East, Russia, China, and Japan.

The next planetary occultation will be on 3 June when Mercury will pass behind the slender crescent Moon at 09h 47m UT. This event is only visible in the southern hemisphere from parts of Antarctica, S. Africa, and Madagascar.

Comet 252P/Linear puts on a show

At magnitude +7 COMET 252P/Linear continues to put on a show in the constellation Ophiuchus after midnight.

This image of the comet by Ivan Gendorf on 28 April shows its nucleus and green Coma nicely.

The BAA Comet section Director Nick James said that 252P/LINEAR made a close pass (0.0356 au) on March 21 and reached 4th magnitude, but is now fading. 

It is a large, diffuse, object and may be difficult to see from sites affected by light pollution.  It is best seen in the morning sky.  It brightened rapidly and, as is often the case with such objects, there were reports that the comet was in outburst, however the degree of condensation remained low.  A linear light curve fits the observations best, with the comet at peak output 29 days after perihelion.  The rate of brightening was unusually rapid.  Marco Goiato reported that the coma was 150' across around the time of closest approach and visible to the naked eye.

Click on the images to enlarge

3 Questions: The origin of the cosmos’ heaviest elements

Dark Energy Survey image of the region surrounding the faint dwarf galaxy Reticulum II. The nine brightest known stars in the galaxy are marked with red circles. Spectra showing the unique chemical content of three stars are shown.
Unique galaxy offers clues to the extreme conditions that yield the heavies of the periodic table.

MIT Kavli Institute for Astrophysics and Space Research
March 21, 2016

Reticulum II is an ancient and faint dwarf galaxy discovered in images taken as part of the Dark Energy Survey. It orbits the Milky Way galaxy about 100,000 light years away from us. Though the galaxy looks unassuming at first, the chemical content of its stars may hold the key to unlocking a 60-year-old mystery about the cosmic origin of the heaviest elements in the periodic table. Today in the journal Nature, a team of astronomers at MIT’s Kavli Institute for Astrophysics and Space Research and the Observatories of the Carnegie Institution of Washington report on observations of this unique galaxy using the Magellan telescopes at the Las Campanas Observatory in Chile’s Atacama Desert. Lead author and MIT physics graduate student Alex Ji explains more.

Q: How are the heaviest elements in the periodic table created in the cosmos?

A: Carl Sagan popularized the notion that we are all made of star stuff. He could say so with confidence because we actually know where nearly every element in the periodic table is made in the universe. But there's a hole in our understanding. The heaviest elements are made in what is called the "rapid neutron-capture process," or "r-process" for short, in which heavy elements are quickly built up from lighter seed nuclei. Gold, platinum, and uranium are r-process elements, as are more exotic elements like europium, neodymium, and gadolinium.

The nuclear physics of the r-process was mostly worked out by 1957, but for almost 60 years astronomers have debated about the astrophysical site that could provide the extreme conditions for the r-process to occur. Synthesizing the r-process elements requires environments with a very large number of neutrons. The two best candidate sites are supernovae and merging neutron stars. Supernovae are the explosions that mark the end of a massive star's life. They often leave behind a remnant called a neutron star. During the formation of a neutron star, a large amount of neutrons is released. If two of these neutron stars happen to be orbiting each other, they will eventually merge to form one giant neutron star. During that explosion neutrons are released and r-process elements can form.

Q: How does this dwarf galaxy help us understand the site of the r-process?

A: Reticulum II is not the first ancient dwarf galaxy to have its chemical content examined; it's actually the 10th. But its chemical composition differs completely from those other galaxies. The stars in those first nine galaxies have unusually low amounts of r-process elements. Reticulum II, on the other hand, is chock-full of r-process elements. Its stars display some of the highest r-process enhancements we have ever seen. It's almost literally a gold mine.

What this means is that a single rare event produced a rather large amount of this r-process material. All those elements were then incorporated into the surrounding gas and from there into the next generations of stars. It is those stars that we can still observe today. The single, prolific r-process event in this galaxy implies that a neutron star merger could have produced these elements in the early universe. A normal supernova would have produced less, and the observed enhancement could not have been as high, though it's also hypothesized that rare, magnetically-driven supernovae might be able to produce much more r-process material.

Interestingly, there is indirect evidence that neutron star mergers do also synthesize r-process elements in the universe today. So it looks like neutron star mergers could be the primary r-process sites throughout cosmic time. It's amazing to think that Reticulum II preserved a signature of that extraordinary event for more than 12 billion years, just waiting for us to dig it up.

Q: What was it like to be at the telescope and realize what you had found?

A: Based on studying the other ultra-faint galaxies, I had expected to find stars with essentially none of these r-process elements and to further establish that these types of dwarf galaxies are devoid in these elements. So we had a plan to get some really good, low upper limits on the r-process content to push this issue. When we realized the stars in this galaxy were the complete opposite, and instead full of r-process elements, I was certain I had screwed something up. From the telescope in Chile I called my advisor Anna Frebel in Cambridge [Mass.] in the middle of the night to urgently talk about what was going on. Telescope time is precious and expensive after all and shouldn't be wasted.

During the hour-long discussions that followed, I kept observing more stars while carrying out preliminary analyses of the data at hand to ensure that this was a real signal. At the observatory, astronomers work all night and sleep during the day. But after seeing the r-process elements in the first few stars, I couldn't sleep anymore; all I could do was stare out the window and hope the incoming clouds would go away again and the wind would die down.

We were very lucky that it ended up being clear most of the four nights we had available. My last night there, the weather forecast, as translated from Spanish by Google, read "rain and wind." So I prepared myself to get no data that night. But it turned out Google had translated the word "despejado" incorrectly and in fact it was supposed to be "clear and wind." An important translation to get right, especially for astronomers!

Lazarus comet from Oort Cloud brings clues about the origin of the Solar System

Astronomers have found a unique object that appears to be made of inner Solar System material from the time of Earth’s formation, which has been preserved in the Oort Cloud far from the Sun for billions of years. Observations with ESO’s Very Large Telescope, and the Canada France Hawaii`I Telescope, show that C/2014 S3 (PANSTARRS) is the first object to be discovered on a long-period cometary orbit that has the characteristics of a pristine inner Solar System asteroid. It may provide important clues about how the Solar System formed.

In a paper to be published today in the journal Science Advances, lead author Karen Meech of the University of Hawaii`i’s Institute for Astronomy and her colleagues conclude that C/2014 S3 (PANSTARRS) formed in the inner Solar System at the same time as the Earth itself, but was ejected at a very early stage.

Their observations indicate that it is an ancient rocky body, rather than a contemporary asteroid that strayed out. As such, it is one of the potential building blocks of the rocky planets, such as the Earth, that was expelled from the inner Solar System and preserved in the deep freeze of the Oort Cloud for billions of years.

Karen Meech explains the unexpected observation: “We already knew of many asteroids, but they have all been baked by billions of years near the Sun. This one is the first uncooked asteroid we could observe: it has been preserved in the best freezer there is.”

C/2014 S3 (PANSTARRS) was originally identified by the Pan-STARRS1 telescope as a weakly active comet a little over twice as far from the Sun as the Earth. Its current long orbital period (around 860 years) suggests that its source is in the Oort Cloud, and it was nudged comparatively recently into an orbit that brings it closer to the Sun.

The team immediately noticed that C/2014 S3 (PANSTARRS) was unusual, as it does not have the characteristic tail that most long-period comets have when they approach so close to the Sun. As a result, it has been dubbed a Manx comet, after the tailless cat. The alternative name is Lazarus comet. Within weeks of its discovery, the team obtained spectra of the very faint object with ESO’s Very Large Telescope in Chile.

Learn more about the Asteroid belt & Lazarus Comets, please watch again my program from December 2013

Careful study of the light reflected by C/2014 S3 (PANSTARRS) indicates that it is typical of asteroids known as S-type, which are usually found in the inner asteroid main belt. It does not look like a typical comet, which are believed to form in the outer Solar System and are icy, rather than rocky. It appears that the material has undergone very little processing, indicating that it has been deep frozen for a very long time. The very weak comet-like activity associated with C/2014 S3 (PANSTARRS), which is consistent with the sublimation of water ice, is about a million times lower than active long-period comets at a similar distance from the Sun.

The authors conclude that this object is probably made of fresh inner Solar System material that has been stored in the Oort Cloud and is now making its way back into the inner Solar System.

A number of theoretical models are able to reproduce much of the structure we see in the Solar System. An important difference between these models is what they predict about the objects that make up the Oort Cloud. Different models predict significantly different ratios of icy to rocky objects. This first discovery of a rocky object from the Oort Cloud is therefore an important test of the different predictions of the models. The authors estimate that observations of 50–100 of these Manx comets are needed to distinguish between the current models, opening up another rich vein in the study of the origins of the Solar System.

Co-author Olivier Hainaut (ESO, Garching, Germany), concludes: “We’ve found the first rocky comet, and we are looking for others. Depending how many we find, we will know whether the giant planets danced across the Solar System when they were young, or if they grew up quietly without moving much.”

Missing sulphur in Venus ‘Dark Stripes’

The beautiful dark stripes on ultraviolet images of Venus's disc are in no way connected with the crystalline sulphur particles in its atmosphere – the ultraviolet is absorbed by another substance. This has been proven by data obtained from the first ever model of the distribution of sulphur in Venus's gaseous envelope, which has been developed by the head of MIPT's Laboratory of High Resolution Infrared Spectroscopy of Planetary Atmospheres, Prof. Vladimir Krasnopolsky.

The results of the study have been published in the scientific journal Icarus. Viewed through a normal optical telescope, Venus is a dull, yellowish-white sphere without any other distinguishing features. However, in the ultraviolet range, the picture changes drastically – dark and light areas appear on the disc, reflecting the dynamics of the atmosphere.

"These areas mean that somewhere in the upper cloud layer there is a substance that is absorbing UV radiation. Over the past 30 years there have been a wide range of hypotheses as to what this substance could be. Many scientists believed that sulphur particles were responsible for the absorption.

But now we will have to abandon this hypothesis," says Krasnopolsky.

He first questioned the "sulphur hypothesis" in 1986 by demonstrating that the amount was not enough to explain the effect of UV absorption. In the new paper, Krasnopolsky presents the first photochemical model of the formation of sulphur particles in Venus's clouds. In particular, the model includes certain processes of the breakdown of sulphur compounds under the influence of light that had not been factored into previous models. The resulting profile compiles the concentration of sulphur aerosol at various altitudes.

The model showed that sulphur aerosol is predominantly found in the lower cloud layer. Its mass constitutes approximately one-tenth of the layer and it is not externally visible. However, observations in the near UV radiation range obtained from the Soviet interplanetary station Venera 14 indicate that absorption in this range occurs in the upper cloud layer at an altitude of approximately 60 km.

"This means that sulphur aerosol cannot be the cause of absorption of Venus's atmosphere in the near UV range," says Krasnopolsky.

In his opinion, the main absorber and source of the stripes on Venus's disc could be ferric chloride (FeCl3), which was discovered in the planet's atmosphere by the X-ray fluorescence spectrometer on board Venera 12.

The cloud layer of Venus is mainly composed of liquid droplets of sulphuric acid (H2SO4). In 1981, the Space Research Institute of the RAS conducted laboratory tests of the reflection coefficient in the near UV range for a 1 percent solution of ferric chloride in sulphuric acid, and their results are fully consistent with the observations of the present study.

"We can therefore consider this mixture of sulphuric acid and ferric chloride to be the most likely substance causing this very mysterious UV absorption," says Krasnopolsky.

AN EVENING WITH SIR PATRICK MOORE

 

Sir Patrick Alfred Caldwell-Moore CBE, FRS, FRAS (4 March 1923 – 9 December 2012)

Acknowledgements: BBC | CHANNEL 4

This specially extended program celebrates the life of a TV icon. Sir Patrick Moore presented the BBC Sky at Night program for 55 years, and introduced a generation of people from all walks of life to take up Astronomy as a hobby, and in many cases a profession.

The original program celebrating Sir Patrick Moore was 55 minutes in duration and was broadcast on 1 January 2015. This new edition has been specially extended to 1 hour 20 minutes, the length of a movie, so please sit back, reminisce, and enjoy and evening with your favourite TV astronomer.

Understanding Spica (Alpha Virginis)

<-- Left | A schematic of the binary stars in Spica, showing four stages of an orbital period. Massive binary stars often have a "mass discrepancy problem," meaning that the masses derived from orbital and evolutionary methods disagree. Credit: A. Tkachenko et al.

The familiar star Spica (Alpha Virginis) is the fifteenth brightest star in the night sky, in part because it is relatively nearby, only about 250 light-years away. It is easy to find by following the arc of the Big Dipper's handle and using the mnemonic,"Arc to Arcturus (Alpha Bootes) and then spike to Spica." In fact Spica is a "spectroscopic" binary, two stars orbiting each other and too close together to separate visually. They were revealed as being a binary pair in 1890 when spectroscopic observations discovered that the stellar lines were doubled due to each star having a slightly different velocity and corresponding Doppler shift. The stars in Spica are, moreover, an unusual pair: They are very close, separated by about twenty-eight solarradii, and orbit each other in only 4.01 days. This puts them so close together that their mutual gravity tidally distorts their atmospheres, with the result that the stars are not spherical. Oh, and the more massive star pulses in size and luminosity.

Binary stars play a critical role for astronomers studying stars. Because mass and gravity determine the dynamics of orbital behaviour, in a binary system it is possible to get at the stars' masses by studying the orbital motions, something that nominally can be done with great accuracy. In contrast, for a single star the mass must be inferred from a much more complicated set of stellar properties and evolutionary models, although even so these models are thought to be excellent and reliable. Sometimes, however, the mass determined from spectroscopy (kinematics) differs from that determined from evolutionary modelling. In the case of massive binary stars (and Spica's two stars are both massive, at 11.4 and 7.2 solar masses, respectively) this is known as the "mass discrepancy problem."

Enter CA astronomer Dimitra Sasselov who is part of a team trying to resolve the mass discrepancy problem. In previous work on massive binaries, the team found that the single-star evolutionary models were slightly in error, in particular for the smaller partner. For their analysis of Spica, the astronomers obtained 1731 high-quality spectra and broadband measurements over the course of nearly twenty-three days. They were able to refine all of the system parameters, and realized that the pulsations in the larger star are actually tidally induced, the first such case found for a massive binary. They also found, somewhat surprisingly, that there was no mass discrepancy problem for Spica - the stellar masses derived in both ways are actually consistent, although there are large uncertainties introduced by the complex nature of the Spica system. The research program continues, and the astronomers plan to observe and analyse another few dozen systems in a consistent way, to improve their insights into the nature of the mass discrepancy problem for massive stars.

launch of Sentinel-1B

 

 

 

 

 

 

 

 

Kourou's Jupiter control room as launch approached. ESA CubeSats/Fly Your Satellite! and Sentinel-1B teams at the CGS spaceport's controls.

The launch of Sentinel-1B provides an opportunity for the three education CubeSats to get a ride into space. They were developed by teams of university students through ESA Education's ‘Fly Your Satellite!’ programme in collaboration with European universities. The other satellite that is piggybacking a ride is Microscope from the French space agency, CNES.

ESA | Herschel’s Galactic panorama

This new video from ESA’s Herschel space observatory reveals in stunning detail the intricate pattern of gas, dust and star-forming hubs along the plane of our Galaxy, the Milky Way.

Against the diffuse background of the interstellar material, a wealth of bright spots, wispy filaments and bubbling nebulas emerge, marking the spots where stars are being born in the Galaxy.

The video was compiled by stitching together several hundred hours of Herschel observations. It spans a vast portion – almost 40% – of the plane of the Milky Way, where most of the stars in the Galaxy form and reside.

 <-- Herschel’s view of the Eagle Nebula

Our disc-shaped Galaxy has a diameter of about 100 000 light-years and the Solar System is embedded in it about half way between the centre and periphery. From our vantage point, this huge disc of stars, gas and dust appears as a circular strip winding around the sky, familiar as the Milky Way in the night sky.

Denser portions of the interstellar medium, the mixture of gas and dust that pervades the Galaxy, are visible in orange and red, popping up against the background in this false-colour view. These concentrations of matter, often arranged in long, thread-like structures, are the sites where future generations of stars will form.

The tiny white spots that appear sprinkled over the filaments are denser clumps of gas and dust, embedding the seeds of stars that are slowly taking shape.

In contrast, the glowing blue and violet gas is set ablaze by the powerful light emitted by new-born stars in their vicinity. This signature of full-fledged stars completes the inventory of all stages in the process of stellar formation that are portrayed in this stunning panorama.

A set of individual images extracted from the video reveal several jewels nestled in the Galactic Plane, such as the Eagle Nebula, the Cat’s Paw Nebula, and the War and Peace Nebula.

Herschel’s view of the War and Peace and Cat’s Paw nebulas

These billowing clouds are home to clusters of young stars that are shining brightly and driving powerful winds, which are in turn carving cavities in the surrounding material, while at the same time the nebulas are ceaselessly witnessing the birth of new stars within them.

 <-- Herschel’s view of RCW 120

The image of RCW 120 tells another tale of relentless star formation: a star at the centre, invisible at these infrared wavelengths, has blown a beautiful bubble around itself with the mighty pressure of the light it radiates.

The pressure is so strong that it has compressed the material at the edge of the bubble, causing it to collapse and triggering the birth of new stars.

 

More interesting sights are revealed near the Galactic Centre, where the density of stars is greater than elsewhere in the Milky Way.

<-- Herschel’s view of the Galactic Centre

There, the clouds of gas and dust appear distributed along a giant, twisted ring, over 600 light-years wide, encompassing the supermassive black hole sitting at the Galaxy’s core.

Herschel obtained these unprecedented views by peering at the Milky Way in far infrared light to detect the glow of cosmic dust – a minor but crucial component of the interstellar mixture from which stars are born.

Is there life in the Venusian atmosphere?

Click on the images to enlarge.

Recent results from ESA's Venus Express before it plummeted down through the planet's atmosphere have revealed it to be rippling with atmospheric waves – and, at an average temperature of -157°C, is colder than anywhere on Earth.

This hints that there maybe a ‘goldilocks’ region where there are warm temperatures beneficial for life to live and evolve, although the strong winds and air currents would make life difficult for any alien life flourish, it is not impossible.

The chemical spectral signatures of Venus, Earth and Mars, shown above, are very similar. This gives astronomers a guide to what to look for in the atmospheres of far away planets orbiting distant stars in the hunt for alien life. 

As well as telling us much about Venus' previously-unexplored Polar Regions and improving our knowledge of our planetary neighbour, the experiment holds great promise for ESA's ExoMars mission, which is currently winging its way to the Red Planet. The findings were published in the journal Nature Physics on 11 April 2016.

ESA's Venus Express arrived at Venus in 2006. It spent eight years exploring the planet from orbit, vastly outliving the mission's planned duration of 500 days, before running out of fuel. The probe then began its descent, dipping further and further into Venus' atmosphere, before the mission lost contact with Earth and officially ended in December 2014.

When Müller-Wodarg and colleagues of Imperial College London, UK, gathered their observations, Venus Express was orbiting at an altitude of between 130 and 140 kilometres near Venus' Polar Regions, in a portion of Venus' atmosphere that had never before been studied before.

These new measurements, taken as part of the Venus Express Atmospheric Drag Experiment (VExADE) from 24 June to 11 July 2014 reveal several surprises.

For one, the polar atmosphere is up to 70 degrees colder than expected, with an average temperature of -157°C (114 K). The polar atmosphere is also not as dense as expected; at 130 and 140 km in altitude, it is 22% and 40% less dense than predicted, respectively. When extrapolated upward in the atmosphere, these differences are consistent with those measured previously by VExADE at 180 km, where densities were found to be lower by almost a factor of two.

"This is in-line with our temperature findings, and shows that the existing model paints an overly simplistic picture of Venus' upper atmosphere," added Müller-Wodarg. "These lower densities could be at least partly due to Venus' polar vortices, which are strong wind systems sitting near the planet's poles. Atmospheric winds may be making the density structure both more complicated and more interesting!"

Additionally, the polar region was found to be dominated by strong atmospheric waves, a phenomenon thought to be key in shaping planetary atmospheres – including our own.

"By studying how the atmospheric densities changed and were perturbed over time, we found two different types of wave: Atmospheric gravity waves and planetary waves," explained co-author Sean Bruinsma of the Centre National D'Etudes Spatiales (CNES), France. "These waves are tricky to study, as you need to be within the atmosphere of the planet itself to measure them properly. Observations from afar can only tell us so much."

Shown of the right: Venus’ Polar vortex

Atmospheric gravity waves are similar to waves we see in the ocean, or when throwing stones in a pond, only they travel vertically rather than horizontally. They are essentially a ripple in the density of a planetary atmosphere – they travel from lower to higher altitudes and, as density decreases with altitude, become stronger as they rise. The second types, planetary waves, are associated with a planet's spin as it turns on its axis; these are larger-scale waves with periods of several days.

We experience both types on Earth. Atmospheric gravity waves interfere with weather and cause turbulence, while planetary waves can affect entire weather and pressure systems. Both are known to transfer energy and momentum from one region to another, and so are likely to be hugely influential in shaping the characteristics of a planetary atmosphere.

"We found atmospheric gravity waves to be dominant in Venus' polar atmosphere," added Bruinsma. "Venus Express experienced them as a kind of turbulence, a bit like the vibrations you feel when an aeroplane flies through a rough patch. If we flew through Venus' atmosphere at those heights we wouldn't feel them because the atmosphere just isn't dense enough, but Venus Express' instruments were sensitive enough to detect them."

Venus Express found atmospheric waves at an altitude of 130-140 km that the team think originated from the upper cloud layer in Venus' atmosphere, which sits at and below altitudes of approximately 90 km, and a planetary wave that oscillated with a period of five days. "We checked carefully to ensure that the waves weren't an artefact of our processing," said co-author Jean-Charles Marty, also of CNES.

This is not just a first for Venus Express; while the aero braking technique has been used for Earth satellites, and was previously used on NASA-led missions to Mars and Venus, it had never before been used on any ESA planetary mission.

However, ESA's ExoMars Trace Gas Orbiter, which launched earlier this year, will use a similar technique. "During this activity we will extract similar data about Mars' atmosphere as we did at Venus," added Håkan Svedhem, project scientist for ESA's ExoMars 2016 and Venus Express missions.

"For Mars, the aero braking phase would last longer than on Venus, for about a year, so we'd get a full dataset of Mars' atmospheric densities and how they vary with season and distance from the Sun," added Svedhem. "This information isn't just relevant to scientists; it's crucial for engineering purposes as well. The Venus study was a highly successful test of a technique that could now be applied to Mars on a larger scale – and to future missions after that."

Comet Linear seen in the skies over Chile

The illuminated dome of the Residencia at ESO's Paranal Observatory in Chile produces no light pollution to impede the work of the telescopes. Above, the magical arc of the Milky Way spans the sky. To the left, near the horizon, is the comet 252P/LINEAR, which made only its fifth closest approach to the Earth in recorded history on 22–23 March 2016. This wonderful image was taken during ESO's Fulldome Expedition to Chile, a team of photographers collecting spectacular images for use in planetarium presentations.

 

Credit:

ESO/P. Horálek

ESO's VLT Survey Telescope Captures the Fornax Cluster

This new image from the VLT Survey Telescope (VST) at ESO’s Paranal Observatory in Chile captures a spectacular concentration of galaxies known as the Fornax Cluster, which can be found in the southern hemisphere constellation of Fornax (The Furnace). The cluster plays host to a menagerie of galaxies of all shapes and sizes, some of which are hiding secrets.

 

Galaxies, it seems, are sociable animals and they like to gather together in large groups, known as clusters. Actually it’s gravity that holds the galaxies in the cluster close together as a single entity, with the pull of gravity arising from large amounts of dark matter, as well as from the galaxies we can see. Clusters can contain anything between about 100 and 1000 galaxies and can be between about 5 and 30 million light-years across.

 

Galaxy clusters do not come in neatly defined shapes so it is difficult to determine exactly where they begin and end. However, astronomers have estimated that the centre of the Fornax Cluster is in the region of 65 million light-years from Earth. What is more accurately known is that it contains nearly sixty large galaxies, and a similar number of smaller dwarf galaxies. Galaxy clusters like this one are commonplace in the Universe and illustrate the powerful influence of gravity over large distances as it draws together the enormous masses of individual galaxies into one region.

 

At the centre of this particular cluster, in the middle of the three bright fuzzy blobs on the left side of the image, is what is known as a cD galaxy — a galactic cannibal. cD galaxies like this one, called NGC 1399, look similar to elliptical galaxies but are bigger and have extended, faint envelopes. This is because they have grown by swallowing smaller galaxies drawn by gravity towards the centre of the cluster.

 

Indeed, there is evidence that this process is happening before our eyes — if you look closely enough. Recent work by a team of astronomers led by Enrichetta Iodice (INAF – Osservatorio di Capodimonte, Naples, Italy), using data from ESO’s VST, has revealed a very faint bridge of light between NGC 1399 and the smaller galaxy NGC 1387 to its right. This bridge, which has not been seen before (and is too faint to show up in this picture), is somewhat bluer than either galaxy, indicating that it consists of stars created in gas that was drawn away from NGC 1387 by the gravitational pull of NGC 1399. Despite there being little evidence for ongoing interactions in the Fornax Cluster overall, it seems that NGC 1399 at least is still feeding on its neighbours.

 

Towards the bottom right of this image is the large barred spiral galaxy NGC 1365. This is a striking example of its type, the prominent bar passing through the central core of the galaxy, and the spiral arms emerging from the ends of the bar. In keeping with the nature of cluster galaxies, there is more to NGC 1365 than meets the eye. It is classified as a Seyfert Galaxy, with a bright active galactic nucleus also containing a supermassive black hole at its centre.

 

This spectacular image was taken by the VLT Survey Telescope (VST) at ESO’s Paranal Observatory in Chile. At 2.6 metres in diameter, the VST is by no means a large telescope by today’s standards, but it has been designed specifically to conduct large-scale surveys of the sky. What sets it apart is its huge corrected field of view and 256-megapixel camera, called OmegaCAM, which was specially developed for surveying the sky. With this camera the VST can produce deep images of large areas of sky quickly, leaving the really big telescopes — like ESO’s Very Large Telescope (VLT) — to explore the details of individual objects.

NASA plans to Hitch a ride on an asteroid

CERES IS A FACINATING WORLD that still has mysteries to be solved, NASA/JPL now plan to send a space robe to hitch a ride on an asteroid, and follow it around the Sun.

 

Catching a ride from one solar system body to another isn't easy. You have to figure out how to land your spacecraft safely and then get it on its way to the next destination. The landing part is especially tricky for asteroids and comets, which have low gravitational pull.

A concept called Comet Hitchhiker, developed at NASA's Jet Propulsion Laboratory, Pasadena, California, puts forth a new way to get into orbit and land on comets and asteroids, using the kinetic energy -- the energy of motion -- of these small bodies. Masahiro Ono, the principal investigator based at JPL, had "Hitchhiker's Guide to the Galaxy" in mind when dreaming up the idea.

"Hitchhiking a celestial body is not as simple as sticking out your thumb, because it flies at an astronomical speed and it won't stop to pick you up. Instead of a thumb, our idea is to use a harpoon and a tether," Ono said. Ono is presenting results about the concept at the American Institute of Aeronautics and Astronautics SPACE conference on September 1.

A reusable tether system would replace the need for propellant for entering orbit and landing, so running out wouldn't be an issue, according to the concept design.

While closely flying by the target, a spacecraft would first cast an extendable tether toward the asteroid or comet and attach itself using a harpoon attached to the tether. Next, the spacecraft would reel out the tether while applying a brake that harvests energy while the spacecraft accelerates.

This technique is analogous to fishing on Earth. Imagine you're on a boat on a lake with a fishing pole, and want to catch a big fish. Once the fish bites, you would release more of the line with a moderate tension, rather than holding it tightly. With a long enough line, the boat will eventually catch up with the fish.

Once the spacecraft matches its velocity to the "fish" -- the comet or asteroid in this case -- it is ready to land by simply reeling in the tether and descending gently. When it's time to move on to another celestial target, the spacecraft would use the harvested energy to quickly retrieve the tether, which accelerates the spacecraft away from the body.

"This kind of hitchhiking could be used for multiple targets in the main asteroid belt or the Kuiper Belt, even five to 10 in a single mission," Ono said.

Ono and colleagues have been studying whether a harpoon could tolerate an impact of this magnitude, and whether a tether could be created strong enough to support this kind of manoeuvre. They used supercomputer simulations and other analyses to figure out what it would take.

Researchers have come up with what they call the Space Hitchhike Equation, which relates the specific strength of the tether, the mass ratio between the spacecraft and the tether, and the change in velocity needed to accomplish the manoeuvre.

In missions that use conventional propellant, spacecraft use a lot of fuel just to accelerate enough to get into orbit.

"In Comet Hitchhiker, accelerating and decelerating do not require propellant because the spacecraft is harvesting kinetic energy from the target," Ono said.

For any spacecraft landing on a comet or asteroid, being able to slow down enough to arrive safely is critical. Comet Hitchhiker requires a tether made from a material that can withstand the enormous tension and heat generated by a rapid decrease in speed for getting into orbit and landing. Ono and colleagues calculated that a velocity change of about 0.9 miles (1.5 kilometres) per second is possible with some materials that already exist: Zylon and Kevlar.

"That's like going from Los Angeles to San Francisco in less than seven minutes," Ono said.

But the bigger the velocity change required for orbit insertion, the shorter the flight time needed to get from Earth to the target -- so if you want to get to a comet or asteroid faster, you need even stronger materials. A 6.2 mile-per-second (10 kilometre-per-second) velocity change is possible, but would require more advanced technologies such as a carbon nanotube tether and a diamond harpoon.

Researchers also estimated that the tether would need to be about 62 to 620 miles long (100 to 1,000 kilometres) for the hitchhiking manoeuvre to work. It would also need to be extendable, and capable of absorbing jerks on it, while avoiding being damaged or cut by small meteorites.

The next steps for studying the concept would be to do more high-fidelity simulations and try casting a mini-harpoon at a target that mimics the material found on a comet or asteroid.

Comet Hitchhiker is in Phase I study through the NASA Innovative Advanced Concepts (NIAC) Program. NIAC is a program of NASA's Space Technology Mission Directorate, located at the agency's headquarters in Washington. Professor David Jewitt at the University of California, Los Angeles, partnered in this research. JPL is managed by the California Institute of Technology in Pasadena for NASA.

John Palisa, the most successful visual discoverer of asteroids

Palisa was director of the Pola observatory in Vienna from 1872 until 1880. He discovered 28 minor planets and one comet during that time. In 1880, he took a position at the new Vienna observatory. Here, he discovered further 94 minor planets, all by visual observations. His most famous discovery is probably the Amor-type asteroid (719) Albert. Today, Palisa remains the most successful visual discoverer of asteroids.

 

Johann Palisa was born on December 6, 1848 in Troppau, Silesia (now Czech Republic). From 1866 to 1870 he studied mathematics and astronomy at the University of Vienna, but did not graduate until 1884. Already in 1870, he became assistant at the University observatory in Vienna, and in the following year, he took a position at the observatory in Geneva.

Only 24 years old, Palisa became director of the Austrian Naval Observatory in Pola in 1872. Pola (now Pula) was harbor of the Austrian Navy from 1850 until the empire of Austria-Hungary collapsed at the end of World War I.

When the new Vienna observatory was inaugurated in 1880 by Emperor Franz Joseph I, he was offered a position as "Adjunkt", comparable to a modern night assistant. Palisa gave up his position as director of the Naval Observatory and accepted the subordinate employment, only because he was able to routinely use the large 27" refractor in Vienna, at that time the largest telescope in the world. To handle this telescope of 10.54m focal length, and the dome, 14m in diameter, two assistants were usually provided to aid the observer. The story goes that Palisa used to send his assistants to bed at midnight, but continued to observe until the break of dawn, handling the instrument all alone. Palisa discovered further 94 asteroids at Vienna, all by visual observations, using the 27" and the 12" refractor. In addition, Palisa discovered eight objects that were included by Dreyer in the NGC catalogue, as well as four nebulae listed in the IC.

In 1883, he joined the expedition of the French academy to observe the total solar eclipse on May 6 of that year. During the eclipse, he searched for the proposed planet Vulcan, which was supposed to circle the sun within the orbit of Mercury. In addition to observing the eclipse, Palisa collected insects for the Natural History Museum in Vienna. When he returned, he named minor planet (235) Carolina after the atoll of the Line Islands, 450 miles northwest of Tahiti, where this expedition set up the instruments to observe the eclipse.

In 1885, Palisa offered to sell the naming right for minor planet (244) for £50 to raise funds for his expedition to the total solar eclipse of August 29, 1886. Apparently, this was not successful, as Palisa did not travel to the eclipse, and the minor planet was later named after the Indian goddess Sita.

 

At that time, there were no star charts available to support his search for new minor planets, so Palisa used to draw the maps on his own. At the end of the 19th century, Johann Palisa and Max Wolf in Heidelberg joined forces and worked on the Palisa-Wolf-Sternkarten. This work, which is the first photographic star atlas, was published between 1900 and 1908. Two years later, Palisa published his Sternenlexikon, a star catalogue covering the sky between declinations -1° and +19°. In 1908, Palisa became vice director of the Vienna observatory.

He retired in 1919, with the right to continue his observations at the observatory. For his work, Palisa was awarded with the Great Price of the Paris Academy. He was also honoured by minor planet (914) Palisana, discovered and named by Max Wolf, and by a lunar crater 33km in diameter. Palisa died in Vienna on May 2, 1925. With 122 minor planets, Palisa is still the most successful Austrian discoverer of asteroids, as well as the most successful visual discoverer in the history of minor planet research.

Palisa's discoveries remain targets of modern research: Minor planet (153) Hilda is the prototype of the Hilda asteroids, orbiting the sun in 3:2 resonances with Jupiter. Asteroid (216) Kleopatra hit the headlines in 2000, when observations with the Arecibo Planetary Radar fount it to have an unusual dog-bone shape. In 1993, the Galileo spacecraft flew by (243) Ida, the NEAR spacecraft passed by (253) Mathile in 1997, and asteroid (140) Siwa will be fly-by target of ESAs Rosetta mission in 2008. Palisa's discovery is probably asteroid (719) Albert. Being only the second NEA found, it was lost only a few days after its discovery. The Amor-type asteroid was finally recovered in 2000 by the Space watch project.