mercredi 16 août 2017

Robotic Arm Reaches Out and Grapples Dragon & dock at Station

SpaceX - CRS-12 Dragon Mission patch.

August 16, 2017

Image above: The SpaceX Dragon cargo craft is pictured approaching the International Space Station on Wednesday morning. Image Credit: NASA TV.

While the International Space Station was traveling over the Pacific Ocean north of New Zealand, NASA astronaut Jack Fischer and ESA (European Space Agency) astronaut Paolo Nespoli captured the Dragon spacecraft at 6:52 a.m. EDT using the station’s robotic arm. It then will be installed on the station’s Harmony module.

Dragon Installed to Station for Month of Cargo Swaps

Image above: Flying over South Australian coasts, SpaceX Dragon docked at Space Station, altitude: 415,70 Km / speed: 27'582 Km/h. Image captured (by Roland Berga from Aerospace) with EarthCam from ISS - International Space Station (via ISS HD Live application) on August 16, 2017 at 16:00 UTC. Image Credits: Mentioned.

The SpaceX Dragon cargo spacecraft was berthed to the Harmony module of the International Space Station at 9:07 a.m. EDT. The hatch between the newly arrived spacecraft and the Harmony module of the space station is scheduled to be opened as soon as later today.

CRS-12 is scheduled to deliver more than 6,400 pounds of supplies and payloads to the station, including a sweet treat for the astronauts: ice cream. The small cups of chocolate, vanilla and birthday cake-flavored ice cream are arriving in freezers that will be reloaded with research samples for return to Earth when the Dragon spacecraft departs the station mid-September.

Image above: Four spaceships are parked at the space station including the SpaceX Dragon cargo craft, the Progress 67 resupply ship and two Soyuz crew ships. Image Credit: NASA.

For more information about the SpaceX CRS-12 mission, visit Join the conversation on Twitter by following @Space_Station.

Space Station Research and Technology:

International Space Station (ISS):

Images (mentioned), Text, Credits: NASA/Mark Garcia/ Aerospace/Roland Berga.

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Sneak peek in colour

ESA - Gaia Mission patch.

16 August 2017

While surveying the positions of over a billion stars, ESA's Gaia mission is also measuring their colour, a key diagnostic to study the physical properties of stars. A new image provides a preview of Gaia's first full-colour all-sky map, which will be unleashed in its highest resolution with the next data release in 2018.

Image above: Preliminary map of Gaia's sky in colour. Image Credits: ESA/Gaia/DPAC/CU5/CU8/DPCI/F. De Angeli, D.W. Evans, M. Riello, M. Fouesneau, R. Andrae, C.A.L. Bailer-Jones.

Stars come in a variety of colours that depend on their surface temperature, which is, in turn, determined by their mass and evolutionary stage.

Massive stars are hotter and therefore shine most brightly in blue or white light, unless they are approaching the end of their life and have puffed up into a red supergiant. Lower-mass stars, instead, are cooler and tend to appear red.

Measuring stellar colours is important to solve a variety of questions, ranging from the internal structure and chemical composition of stars to their evolution.

Gaia, ESA's astrometry mission to compile the largest and most precise catalogue of stellar positions and motions to date, has also been recording the colour of the stars it observes. The satellite was launched in December 2013 and has been collecting scientific data since July 2014.

A special effort in the Gaia Data Processing and Analysis Consortium (DPAC) is dedicated to the challenging endeavour of extracting stellar colours from the satellite data. While these measurements will be published with Gaia's second data release in April 2018, a preview of the Gaia sky map in colour demonstrates that the ongoing work is progressing well.

The new map, based on preliminary data from 18.6 million bright stars taken between July 2014 and May 2016 [1], shows the middle value (median) of the colours of all stars that are observed in each pixel. It is helpful to look at it next to its companion map, showing the density of stars in each pixel, which is higher along the Galactic Plane – the roughly horizontal structure that extends across the image, corresponding to the most densely populated region of our Milky Way galaxy – and lower towards the poles.

Image above: Star density map. Image Credits: ESA/Gaia/DPAC/CU5/CU8/DPCI/F. De Angeli, D.W. Evans, M. Riello, M. Fouesneau, R. Andrae, C.A.L. Bailer-Jones.

Even though this map is only meant as an appetizer to the full treat of next year's release, which will include roughly a hundred times more stars, it is already possible to spot some interesting features.

The reddest regions in the map, mainly found near the Galactic Centre, correspond to dark areas in the density map: these are clouds of dust that obscure part of the starlight, especially at blue wavelengths, making it appear redder – a phenomenon known as reddening.

It is also possible to see the two Magellanic Clouds – small satellite galaxies of our Milky Way – in the lower part of the map.

The task of measuring colours is performed by the photometric instrument on Gaia. This instrument contains two prisms that split the starlight into its constituent wavelengths, providing two low-resolution spectra for each star: one for the short, or blue, wavelengths (330-680 nm) and the other for the long, or red, ones (640-1050 nm). Scientists then compare the total amount of light in the blue and red spectra to estimate stellar colours.

To precisely calibrate these spectra, however, it is necessary to know the position of each source on Gaia's focal plane to very high accuracy – in fact, to an accuracy that only Gaia itself can provide.

As part of the effort to extract physical parameters from the data sent back by the satellite, scientists feed them to an iterative algorithm that compares the recorded images of stars to models of how such images should look: as a result, the algorithm provides a first estimate of the star's parameters, such as its position, brightness, or colour. By collecting more data and feeding them to the algorithm, the models are constantly improved and so are the estimated parameters for each star.

Image above: Artist's impression of Gaia. Credits: ESA/ATG medialab; background image: ESO/S. Brunier.

The first Gaia data release, published in September 2016, was based on less than a quarter of the total amount of data that will be collected by the satellite over its entire five-year mission, which is expected to observe each star an average of 70 times. This first release, listing unprecedentedly accurate positions on the sky for 1.142 billion stars, along with their brightness, contained no information on stellar colours: by then, it had not been possible to run enough iterations of the algorithm to accurately estimate additional parameters.

As the satellite continues to observe more stars, scientists have now had more time to feed data to the iterative algorithm to obtain estimates of stellar colours, like the ones shown in the new map. These estimates will be validated, over the coming months, as part of the overall data processing effort leading to the second Gaia data release.

Since the first data release, scientists across the world have been using Gaia's brightness measurements – which are obtained over the full G-band, from 330 to 1050 nm – along with datasets from other missions to estimate stellar colours. These studies have been applied to a variety of subjects, from variable stars and stellar clusters in our Galaxy to the characterisation of stars in the Magellanic Clouds.

Next year, the second release of Gaia data will include not only the position and G-band brightness, but also the blue and red colour for over a billion stars – in addition to the long-awaited estimates of stellar parallaxes and proper motions based on Gaia measurements for all the observed stars [2]. This extraordinary dataset will allow scientists to delve into the secrets of our Galaxy, investigating its composition, formation and evolution to an unparalleled degree of detail.


[1] The preliminary colour map shows a sample of stars that have been selected randomly from all Gaia stars with G-band magnitudes brighter than 17 and for which both colour measurements (from the blue and the red channels of Gaia's photometric instrument) are available.

[2] Gaia's goal is to measure the parallax (a small, periodic change in the apparent position of a star caused by Earth's yearly revolution around the Sun, which depends on the star's distance from us) and proper motion (the motion of stars across the plane of the sky caused by their physical movement through the Galaxy) for over one billion stars. In the process, Gaia will measure also the brightness and colour of these stars, take spectra for a subset of them, and observe a variety of other celestial objects, from asteroids in our own Solar System to distant galaxies beyond the Milky Way.

Related links:

Data Processing and Analysis Consortium (DPAC):

Photometric instrument:

First Gaia data release:

Preliminary View of the Gaia sky in colour:

ESA Gaia:

Images (mentioned), Text, Credit: European Space Agency (ESA).

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mardi 15 août 2017

NASA Studies CubeSat Mission to Solve Venusian Mystery

NASA - Goddard Space Flight Center logo.

Aug. 15, 2017

Venus looks bland and featureless in visible light, but change the filter to ultraviolet, and Earth’s twin suddenly looks like a different planet. Dark and light areas stripe the sphere, indicating that something is absorbing ultraviolet wavelengths in the planet’s cloud tops.

A team of scientists and engineers working at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, has received funding from the agency’s Planetary Science Deep Space SmallSat Studies, or PSDS3, program to advance a CubeSat mission concept revealing the nature of this mysterious absorber situated within the planet’s uppermost cloud layer.

Called the CubeSat UV Experiment, or CUVE, the mission would investigate Venus’ atmosphere using ultraviolet-sensitive instruments and a novel, carbon-nanotube light-gathering mirror.

Similar in structure and size to Earth, Venus spins slowly in the opposite direction of most planets. Its thick atmosphere, consisting mainly of carbon dioxide, with clouds of sulfuric acid droplets, traps heat in a runaway greenhouse effect, making it the hottest planet in our solar system with surface temperatures hot enough to melt lead.

Image above: The cloud-enshrouded Venus appears featureless, as shown in this image taken by NASA’s MESSENGER mission. In ultraviolet, however, the planet takes on a completely different appearance as seen below. Image Credit: NASA.

Although NASA and other international space programs have dispatched multiple missions to Venus, “the exact nature of the cloud top absorber has not been established,” said CUVE Principal Investigator Valeria Cottini, a researcher at the University of Maryland who is leading a team of experts in the composition, chemistry, dynamics, and radiative transfer of the planet’s atmosphere. “This is one of the unanswered questions and it’s an important one,” she added.

Past observations of Venus show that half of the solar energy is absorbed in the ultraviolet by an upper layer of the sulfuric-acid clouds, giving the planet its striped dark and light features. Other wavelengths are scattered or reflected into space, which explains why the planet looks like a featureless, yellowish-white sphere in the optical — wavelengths visible to the human eye.

Theories abound as to what causes these streaked, contrasting features, Cottini said. One explanation is that convective processes dredge the absorber from deep within Venus’ thick cloud cover, transporting the substance to the cloud tops. Local winds disperse the material in the direction of the wind, creating the long streaks. Scientists theorize the bright areas as observed in the ultraviolet are probably stable against convection and do not contain the absorber, while the dark areas do.

“Since the maximum absorption of solar energy by Venus occurs in the ultraviolet, determining the nature, concentration, and distribution of the unknown absorber is fundamental,” Cottini said. “This is a highly-focused mission — perfect for a CubeSat application.”

Image above: As seen in the ultraviolet, Venus is striped by light and dark areas indicating that an unknown absorber is operating in the planet’s top cloud layer. The image was taken by NASA’s Pioneer-Venus Orbiter in 1979. Image Credit: NASA.

To learn more about the absorber, the CUVE team, which includes Goddard scientists as well researchers affiliated with the University of Maryland and Catholic University, is leveraging investments Goddard has made in miniaturized instruments and other technologies. In addition to flying a miniaturized ultraviolet camera to add contextual information and capture the contrast features, CUVE would carry a Goddard-developed spectrometer to analyze light over a broad spectral band — 190-570 nanometers — covering the ultraviolet and visible. The team also plans to leverage investments in CubeSat navigation, electronics, and flight software.

“A lot of these concepts are driven by important Goddard research-and-development investments,” said Tilak Hewagama, a CUVE team member who has worked with Goddard scientists Shahid Aslam, Nicolas Gorius, and others to demonstrate a CubeSat-compatible spectrometer. “That’s what got us started.”

One of the other novel CUVE adaptations is the potential use of a lightweight telescope equipped with a mirror made of carbon nanotubes in an epoxy resin. To date, no one has been able to make a mirror using this resin.

Such optics offer several advantages. In addition to being lightweight and highly stable, they are relatively easy to reproduce. They do not require polishing — a time-consuming and often-times expensive process that assures a smooth, perfectly shaped surface.

Developed by Goddard contractor Peter Chen, the mirror is made by pouring a mixture of epoxy and carbon nanotubes into a mandrel, or mold, fashioned to meet a specific optical prescription. Technicians then heat the mold to cure and harden the epoxy. Once set, the mirror is coated with a reflective material of aluminum and silicon dioxide.

Study Objectives

The team plans to further enhance the mission’s technologies and evaluate technical requirements to reach a polar orbit around Venus as a secondary payload. The team believes it would take CUVE one-and-a-half years to reach its destination. Once in orbit, the team would gather data for about six months.

“CUVE is a targeted mission, with a dedicated science payload and a compact bus to maximize flight opportunities such as a ride-share with another mission to Venus or to a different target,” Cottini said. “CUVE would complement past, current, and future Venus missions and provide great science return at lower cost.”

Small satellites, including CubeSats, are playing an increasingly larger role in exploration, technology demonstration, scientific research and educational investigations at NASA, including: planetary space exploration; Earth observations; fundamental Earth and space science; and developing precursor science instruments like cutting-edge laser communications, satellite-to-satellite communications and autonomous movement capabilities.

Related links:


Small Satellite Missions:

For more technology news, go to

Images (mentioned), Text, Credits: NASA/Lynn Jenner/Goddard Space Flight Center, by Lori Keesey.


Tracking a solar eruption through the Solar System

ESA- European Space Agency logo / NASA - National Aeronautics and Space Administration logo.

15 August 2017

Ten spacecraft, from ESA’s Venus Express to NASA’s Voyager-2, felt the effect of a solar eruption as it washed through the Solar System while three other satellites watched, providing a unique perspective on this space weather event.

Tracking a solar eruption through the Solar System

Scientists working on ESA’s Mars Express were looking forward to investigating the effects of the close encounter of Comet Siding Spring on the Red Planet’s atmosphere on 19 October 2014, but instead they found what turned out to be the imprint of a solar event.

While this made the analysis of any comet-related effects far more complex than anticipated, it triggered one of the largest collaborative efforts to trace the journey of an interplanetary ‘coronal mass ejection’ – a CME – from the Sun to the far reaches of the outer Solar System.

SOHO’s view

Although Earth itself was not in the firing line, a number of Sun-watching satellites near Earth – ESA’s Proba-2, the ESA/NASA SOHO and NASA’s Solar Dynamics Observatory – had witnessed a powerful solar eruption a few days earlier, on 14 October.

NASA’s Stereo-A not only captured images of the other side of the Sun with respect to Earth, but also collected in situ information as the CME rushed passed.

Thanks to the fortuitous locations of other satellites lying in the direction of the CME’s travel, unambiguous detections were made by three Mars orbiters – ESA’s Mars Express, NASA’s Maven and Mars Odyssey – and NASA’s Curiosity Rover operating on the Red Planet’s surface, ESA’s Rosetta at Comet 67P/Churyumov–Gerasimenko, and the international Cassini mission at Saturn.

Hints were even found as far out as NASA’s New Horizons, which was approaching Pluto at the time, and beyond to Voyager-2. However, at these large distances it is possible that evidence of this specific eruption may have merged with the background solar wind.

“CME speeds with distance from the Sun is not well understood, in particular in the outer Solar System,” says ESA’s Olivier Witasse, who led the study.

In the firing line

“Thanks to the precise timings of numerous in situ measurements, we can better understand the process, and feed our results back into models.”

The measurements give an indication of the speed and direction of travel of the CME, which spread out over an angle of at least 116º to reach Venus Express and Stereo-A on the eastern flank, and the spacecraft at Mars and Comet 67P Churyumov–Gerasimenko on the western flank.

From an initial maximum of about 1000 km/s estimated at the Sun, a strong drop to 647 km/s was measured by Mars Express three days later, falling further to 550 km/s at Rosetta after five days. This was followed by a more gradual decrease to 450–500 km/s at the distance of Saturn a month since the event.

Multispacecraft view

The data also revealed the evolution of the CME’s magnetic structure, with the effects felt by spacecraft for several days, providing useful insights on space weather effects at different planetary bodies. The signatures at the various spacecraft typically included an initial shock, a strengthening of the magnetic field, and increases in the solar wind speed.

In the case of ESA’s Venus Express, its science package was not switched on because Venus was ‘behind’ the Sun as seen from Earth, limiting communication capabilities.

A faint indication was inferred from its star tracker being overwhelmed with radiation at the expected time of passage.

Furthermore, several craft carrying radiation monitors – Curiosity, Mars Odyssey, Rosetta and Cassini ­­– revealed an interesting and well-known effect: a sudden decrease in galactic cosmic rays. As a CME passes by, it acts like a protective bubble, temporarily sweeping aside the cosmic rays and partially shielding the planet or spacecraft.

Cosmic ray drop

A drop of about 20% in cosmic rays was observed at Mars – one of the deepest recorded at the Red Planet – and persisted for about 35 hours. At Rosetta a reduction of 17% was seen that lasted for 60 hours, while at Saturn the reduction was slightly lower and lasted for about four days. The increase in the duration of the cosmic ray depression corresponds to a slowing of the CME and the wider region over which it was dispersed at greater distances.

“The comparison of the decrease in galactic cosmic ray influx at three widely separated locations due to the same CME is quite novel,” says Olivier. “While multispacecraft observations of CMEs have been done in the past, it is uncommon for the circumstances to be such to include so many spread across the inner and outer Solar System like this.

“Finally, coming back to our original intended observation of the passage of Comet Siding Spring at Mars, the results show the importance of having a space weather context for understanding how these solar events might influence or even mask the comet’s signature in a planet’s atmosphere.”

Notes for Editors:

“Interplanetary coronal mass ejection observed at Stereo-A, Mars, comet 67P/Churyumov–Gerasimenko, Saturn and New Horizons en route to Pluto. Comparison of its Forbush decreases at 1.4, 3.1 and 9.9 AU,” by O. Witasse et al. is published in Journal of Geophysical Research: Space Physics, a journal of the American Geophysical Union.

Journal of Geophysical Research: Space Physics:

Related links:

ESA's SOHO home page:

Mars Express:



Venus Express:

Curiosity Rover (MSL):

Mars Odyssey:


New Horizons:

Solar Dynamics Observatory:



Images, Videos, Text, Credits: ESA/Markus Bauer/Olivier Witasse/SDO/NASA; SOHO (ESA & NASA); NASA/Stereo; ESA/Royal Observatory of Belgium.

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lundi 14 août 2017

Study Finds Drought Recoveries Taking Longer

JPL - Jet Propulsion Laboratory logo.

Aug. 14, 2017

As global temperatures continue to rise, droughts are expected to become more frequent and severe in many regions during this century. A new study with NASA participation finds that land ecosystems took progressively longer to recover from droughts in the 20th century, and incomplete drought recovery may become the new normal in some areas, possibly leading to tree death and increased emissions of greenhouse gases.

In results published Aug. 10 in the journal Nature, a research team led by Christopher Schwalm of Woods Hole Research Center, Falmouth, Massachusetts, and including a scientist from NASA’s Jet Propulsion Laboratory, Pasadena, California, measured recovery time following droughts in various regions of the world. They used projections from climate models verified by observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument on NASA’s Terra satellite and ground measurements. The researchers found that drought recovery was taking longer in all land areas. In two particularly vulnerable regions -- the tropics and northern high latitudes -- recovery took ever longer than in other regions.

Image above: Global patterns of drought recovery time, in months. The longest recovery times are depicted in shades of blue and pink, with the shortest recovery times in yellow. White areas indicate water, barren lands, or regions that did not experience a drought during the study period. Image Credits: Woods Hole Research Center.

Schwalm noted that in model projections that assumed no new restrictions on greenhouse gas emissions (the so-called business-as-usual scenario), "Time between drought events will likely become shorter than the time needed for land ecosystems to recover from them.”

”Using the vantage point of space, we can see all of Earth’s forests and other ecosystems getting hit repeatedly and increasingly by droughts,“ said study co-author Josh Fisher of JPL. “Some of these ecosystems recover, but, with increasing frequency, others do not. Data from our ‘eyes’ in space allow us to verify our simulations of past and current climate, which, in turn, helps us reduce uncertainties in projections of future climate.”

The scientists argue that recovery time is a crucial metric for assessing the resilience of ecosystems, shaping the odds of crossing a tipping point after which trees begin to die. Shorter times between droughts, combined with longer drought recovery times, may lead to widespread tree death, decreasing the ability of land areas to absorb atmospheric carbon.

The research is funded by the National Science Foundation and NASA. Other participating institutions include Northern Arizona University, Flagstaff; the University of Utah, Salt Lake City; Carnegie Institution for Science, Stanford, California; the University of New Mexico, Albuquerque; the U.S. Forest Service, Ogden, Utah; Arable Labs Inc., Princeton, New Jersey; the National Snow and Ice Data Center, Boulder, Colorado; Oak Ridge National Laboratory, Oak Ridge, Tennessee; the University of Maine, Orono; Pacific Northwest National Laboratory, Richland, Washington; the University of Illinois, Urbana; the University of Nevada, Reno; and Auburn University, Auburn, Alabama.

Related links:


Jet Propulsion Laboratory (JPL):

Image (mentioned), Text, Credits: NASA/Tony Greicius/JPL/Andrew Good/Woods Hole Research Center/Dave McGlinchey.


ATLAS observes direct evidence of light-by-light scattering

CERN - European Organization for Nuclear Research logo.

14 Aug 2017

Physicists from the ATLAS experiment at CERN have found the first direct evidence ofhigh energy light-by-light scattering, a very rare process in which two photons – particles of light – interact and change direction. The result, published today in Nature Physics, confirms one of the oldest predictions of quantum electrodynamics (QED).

"This is a milestone result: the first direct evidence of light interacting with itself at high energy,” says Dan Tovey(University of Sheffield), ATLAS Physics Coordinator. “This phenomenon is impossible in classical theories of electromagnetism; hence this result provides a sensitive test of our understanding of QED, the quantum theory of electromagnetism."

Image above: A light-by-light scattering event measured in the ATLAS detector (Image: ATLAS/CERN).

Direct evidence for light-by-light scattering at high energy had proven elusive for decades – until the Large Hadron Collider’s second run began in 2015. As the accelerator collided lead ions at unprecedented collision rates, obtaining evidence for light-by-light scattering became a real possibility. “This measurement has been of great interest to the heavy-ion and high-energy physics communities for several years, as calculations from several groups showed that we might achieve a significant signal by studying lead-ion collisions in Run 2,” says Peter Steinberg (Brookhaven National Laboratory), ATLAS Heavy Ion Physics Group Convener.

Heavy-ion collisions provide a uniquely clean environment tostudy light-by-light scattering. As bunches of lead ions are accelerated, an enormous flux of surrounding photons is generated. When ions meet at the centre of the ATLAS detector, very few collide, yet their surrounding photons can interact and scatter off one another. These interactions are known as ‘ultra-peripheral collisions’.

Studying more than 4 billion events taken in 2015, the ATLAS collaboration found 13 candidates for light-by-light scattering. This result has a significance of 4.4 standard deviations, allowing the ATLAS collaboration to report the first direct evidence of this phenomenon at high energy.

“Finding evidence of this rare signature required the development of a sensitive new ‘trigger’ for the ATLAS detector,” says Steinberg. “The resulting signature — two photons in an otherwise empty detector — is almost the diametric opposite of the tremendously complicated eventstypically expected from lead nuclei collisions. The new trigger’s success in selecting these events demonstrates the power and flexibility of the system, as well as the skill and expertise of the analysis and trigger groups who designed and developed it.”

 Large Hadron Collider (LHC). Animation Credit: CERN

ATLAS physicists will continue to study light-by-light scattering during the upcoming LHC heavy-ion run, scheduled for 2018. More data will further improve the precision of theresult and may open a new window to studies of new physics. In addition, the study of ultra-peripheral collisions should play a greater role in the LHC heavy-ion programme, as collision rates further increase in Run 3 and beyond.​


CERN, the European Organization for Nuclear Research, is one of the world’s largest and most respected centres for scientific research. Its business is fundamental physics, finding out what the Universe is made of and how it works. At CERN, the world’s largest and most complex scientific instruments are used to study the basic constituents of matter — the fundamental particles. By studying what happens when these particles collide, physicists learn about the laws of Nature.

The instruments used at CERN are particle accelerators and detectors. Accelerators boost beams of particles to high energies before they are made to collide with each other or with stationary targets. Detectors observe and record the results of these collisions.

Founded in 1954, the CERN Laboratory sits astride the Franco–Swiss border near Geneva. It was one of Europe’s first joint ventures and now has 22 Member States.

Related links:

ATLAS experiment:

Large Hadron Collider (LHC):

For more information about European Organization for Nuclear Research (CERN), Visit:

Image (mentioned), Animation (mentioned), Text, Credits: CERN/Katarina Anthony.

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Cassini Prepares to Say Goodbye to a True Titan & Cloudy Waves

NASA & ESA - Cassini-Huygens Mission to Saturn & Titan patch.

Aug. 14, 2017

A World Unveiled: Cassini at Titan

Video above: Saturn’s giant, hazy moon Titan has been essential to NASA’s Cassini mission during its 13 thrilling years of exploration there. Video Credit: NASA/JPL-Caltech/Space Science Institute.

Mere weeks away from its dramatic, mission-ending plunge into Saturn, NASA's Cassini spacecraft has a hectic schedule, orbiting the planet every week in its Grand Finale. On a few orbits, Saturn's largest moon, Titan, has been near enough to tweak Cassini's orbit, causing the spacecraft to approach Saturn a bit closer or a bit farther away. A couple of those distant passes even pushed Cassini into the inner fringes of Saturn's rings.

Titan will be waiting once again when the road runs out in September. A last, distant encounter with the moon on Sept. 11 will usher Cassini to its fate, with the spacecraft sending back precious science data until it loses contact with Earth.

But this gravitational pushing and shoving isn't a new behavior for Titan. It's been doing that all along, by design.

The True Engine of the Mission

Repeated flybys of Titan were envisioned, from the mission's beginning, as a way to explore the mysterious planet-size moon and to fling Cassini toward its adventures in the Saturn system. Scientists had been eager for a return to Titan since NASA's Voyager 1 spacecraft flew past in 1980 and was unable to see through the dense, golden haze that shrouds its surface.

Image above: These two views of Saturn's moon Titan exemplify how NASA's Cassini spacecraft has revealed the surface of this fascinating world. Image Credits: NASA/JPL-Caltech/Space Science Institute.

Titan is just a bit larger than the planet Mercury. Given its size, the moon has significant gravity, which is used for bending Cassini's course as it orbits Saturn. A single close flyby of Titan could provide more of a change in velocity than the entire 90-minute engine burn the spacecraft needed to slow down and be captured by Saturn's gravity upon its arrival in 2004.

The mission's tour designers -- engineers tasked with plotting the spacecraft's course, years in advance -- used Titan as their linchpin. Frequent passes by the moon provided the equivalent of huge amounts of rocket propellant. Using Titan, Cassini's orbit could be stretched out, farther from Saturn -- for example, to send the spacecraft toward the distant moon Iapetus. With this technique, engineers used Titan flybys to change the orientation of Cassini's orbit many times during the mission; for example, lifting the spacecraft out of the plane of the rings to view them from high above, along with high northern and southern latitudes on Saturn and its moons.

What We've Learned

Over the course of its 13-year mission at Saturn, Cassini has made 127 close flybys of Titan, with many more-distant observations. Cassini also dropped off the European Space Agency's Huygens probe, which descended through Titan's atmosphere to land on the surface in January 2005.

Successes for Cassini during its mission include the revelation that, as researchers had theorized, there were indeed bodies of open liquid hydrocarbons on Titan's surface. Surprisingly, it turned out Titan's lakes and seas are confined to the poles, with almost all of the liquid being at northern latitudes in the present epoch. Cassini found that most of Titan has no lakes, with vast stretches of linear dunes closer to the equator similar to those in places like Namibia on Earth. The spacecraft observed giant hydrocarbon clouds hovering over Titan's poles and bright, feathery ones that drifted across the landscape, dropping methane rain that darkened the surface. There were also indications of an ocean of water beneath the moon's icy surface.

Image above: Cassini spacecraft looks toward the night side of Saturn's moon Titan in a view that highlights the extended, hazy nature of the moon's atmosphere. Image Credits: NASA/JPL-Caltech/Space Science Institute.

Early on, Cassini's picture of Titan was spotty, but every encounter built upon the previous one. Over the course of the entire mission, Cassini's radar investigation imaged approximately 67 percent of Titan's surface, using the spacecraft's large, saucer-shaped antenna to bounce signals off the moon's surface. Views from Cassini's imaging cameras, infrared spectrometer, and radar slowly and methodically added details, building up a more complete, high-resolution picture of Titan.

"Now that we've completed Cassini’s investigation of Titan, we have enough detail to really see what Titan is like as a world, globally," said Steve Wall, deputy lead of Cassini's radar team at NASA's Jet Propulsion Laboratory in Pasadena, California.

Scientists now have enough data to understand the distribution of Titan's surface features (like mountains, dunes and seas) and the behavior of its atmosphere over time, and they have been able to begin piecing together how surface liquids might migrate from pole to pole.

Among the things that remain uncertain is exactly how the methane in Titan's atmosphere is being replenished, since it's broken down over time by sunlight. Scientists see some evidence of volcanism, with methane-laden water as the "lava," but a definitive detection remains elusive.

Cassini's long-term observations could still provide clues. Researchers have been watching for summer rain clouds to appear at the north pole, as their models predicted. Cassini observed rain clouds at the south pole in southern summer in 2004. But so far, clouds at high northern latitudes have been sparse.

Image above: During its final targeted flyby of Titan on April 22, 2017, Cassini's radar mapper got the mission’s last close look at the moon's surface. Image Credits: NASA/JPL-Caltech/ASI.

"The atmosphere seems to have more inertia than most models have assumed. Basically, it takes longer than we thought for the weather to change with the seasons," said Elizabeth Turtle, a Cassini imaging team associate at Johns Hopkins Applied Physics Laboratory, Laurel, Maryland.

The sluggish arrival of northern summer clouds may match better with models that predict a global reservoir of methane, Turtle said. "There isn't a global reservoir at the surface, so if one exists in the subsurface that would be a major revelation about Titan." This points to the value of Cassini's long-term monitoring of Titan's atmosphere, she said, as the monitoring provides data that can be used to test models and ideas.

Results from the Last Close Pass

Cassini made its last close flyby of Titan on April 22. That flyby gave the spacecraft the push it needed to leap over Saturn's rings and begin its final series of orbits, which pass between the rings and the planet.

During that flyby, Cassini's radar was in the driver's seat -- its observation requirements determining how the spacecraft would be oriented as it passed low over the surface one last time at an altitude of 608 miles (979 kilometers). One of the priorities was to have one last look for the mysterious features the team dubbed "magic islands," which had appeared and then vanished in separate observations taken years apart. On the final pass there were no magic islands to be seen. The radar team is still working to understand what the features might have been, with leading candidates being bubbles or waves.

Most interesting to the radar team was a set of observations that was both the first and last of its kind, in which the instrument was used to sound the depths of several of the small lakes that dot Titan's north polar region. Going forward, the researchers will be working to tease out information from these data about the lakes' composition, in terms of methane versus ethane.

As Cassini zoomed past on its last close brush with Titan, headed toward its Grand Finale, the radar imaged a long swath of the surface that included terrain seen on the very first Titan flyby in 2004. "It's pretty remarkable that we ended up close to where we started," said Wall. "The difference is how richly our understanding has grown, and how the questions we're asking about Titan have evolved."

Cloudy Waves (False Color)

Image above: Clouds on Saturn take on the appearance of strokes from a cosmic brush thanks to the wavy way that fluids interact in Saturn's atmosphere. Image Credits: NASA/JPL-Caltech/Space Science Institute.

Neighboring bands of clouds move at different speeds and directions depending on their latitudes. This generates turbulence where bands meet and leads to the wavy structure along the interfaces. Saturn’s upper atmosphere generates the faint haze seen along the limb of the planet in this image.

This false color view is centered on 46 degrees north latitude on Saturn. The images were taken with the Cassini spacecraft narrow-angle camera on May 18, 2017 using a combination of spectral filters which preferentially admit wavelengths of near-infrared light. The image filter centered at 727 nanometers was used for red in this image; the filter centered at 750 nanometers was used for blue. (The green color channel was simulated using an average of the two filters.)

The view was obtained at a distance of approximately 750,000 miles (1.2 million kilometers) from Saturn. Image scale is about 4 miles (7 kilometers) per pixel.

The Cassini mission is a cooperative project of NASA, ESA (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 mission for NASA's Science Mission Directorate, Washington. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging operations center is based at the Space Science Institute in Boulder, Colorado.

The Cassini-Huygens mission is a cooperative project of NASA, ESA (European Space Agency) and the Italian Space Agency. NASA's Jet Propulsion Laboratory, a division of Caltech in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington. JPL designed, developed and assembled the Cassini orbiter.

Grand Finale:

For more information about the Cassini-Huygens mission visit and . The Cassini imaging team homepage is at and ESA's website:

Images (mentioned), Video (mentioned), Text, Credits: NASA/Martin Perez/Tony Greicius/JPL/Preston Dyches.

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