RSSArchive for September, 2010

Hubble’s greatest galaxy images

Since its launch in 1990, the NASA/ESA Hubble Space Telescope has taken thousands of images of astronomical bodies near and far.

Many of its targets have been galaxies or groups of galaxies. A galaxy is a collection of millions or billions of stars—many of which might have planets—plus clouds of gas and dust.

Galaxies come in range of different shapes and sizes. There are spiral galaxies like our Milky Way, plus elliptical galaxies, lenticular galaxies, dwarf galaxies and irregular galaxies.

Prominent galaxies have been given proper names; others are simply known by their catalogue number. One of the better-known catalogues of astronomical objects is the imaginatively named New General Catalogue, or NGC.

Here’s our selection of Hubble’s Top 15 galaxy images.

1. NGC 5866

NGC 5866 is around 50 million light-years away. Sometimes called the Spindle Galaxy, is either a spiral galaxy seen edge on, or a type known as lenticular—halfway between a spiral and an elliptical galaxy. A lane of dark dust clouds is prominent.

ACS Image of NGC 5866

Hubble Space Telescope view of galaxy NGC 5866.

2. The Cartwheel Galaxy

This amazing looking galaxy is located 500 million light-years from Earth. Its strange shape is thought to be the result of a collision between two galaxies. It has a nucleus in the centre, a ring of young stars around the outside, and wisps of material connecting the two. It is classed as a ring galaxy.

The Cartwheel Galaxy

Hubble Space Telescope view of the Cartwheel Galaxy.

3. NGC 7049

About 100 million light-years from Earth lies NGC 7049, a large (150 million light-year-wide) half-spiral, half-elliptical galaxy that has dramatic dust clouds ringing its nucleus. This is another example of a structure that might have arisen from a galactic collision.

NGC 7049

Hubble Space Telescope view of the galaxy NGC 7049.

4. NGC 1512

NGC 1512 is a barred-spiral galaxy about 30 million light-years from Earth. Barred spiral means that it has an elongated section of stars coming out of either side of the nucleus, from which the spiral arms extend and wrap themselves around the galaxy.

NGC 1512

A combined ultraviolet-visible-infrared image of NGC 1512.

5. AM 0644-741

Another ring galaxy, AM 0644-741 is about 300 million light-years from Earth and is a bit wider than our Milky Way galaxy. Again, thought to be the result of a collision between two galaxies, it has nucleus of old stars and a ring of younger stars, whose birth was prompted by the collision.

AM 0644-741

Hubble Space Telescope view of the ring galaxy AM 0644-741.

6. NGC 4921

This is a magnificent image of the face-on spiral galaxy NGC 4921, put together from 80 separate Hubble images. It is 320 million light-years from Earth.

NGC 4921

Hubble Space Telescope view of galaxy NGC 4921.

7. NGC 1316

Dark clouds of dust are spread throughout this galaxy, which is just over 60 million light-years from Earth. Classified as a lenticular (half-spiral, half-elliptical) galaxy, it’s yet a further example of a galaxy that is thought to have undergone a merger with another galaxy.

NGC 1316

Hubble Space Telescope view of galaxy NGC 1316.

8. Hoag’s Object

This amazing ring galaxy is similar to the Cartwheel and AM 0644-741, but is more perfectly formed. A ring of bright, young stars surrounds its nucleus of older stars. A background galaxy can be seen at about the one o’clock position. Hoag’s Object is about 600 million light-years from Earth.

Hoag's Object

Hubble Space Telescope view of Hoag's Object.

9. UGC 10214

Nicknamed the “Tadpole”, this remarkable galaxy has been distorted and “stretched” by the gravity of a passing galaxy (the small, blue galaxy in the top-left corner of the Tadpole). It is 420 million light-years from Earth.

UGC 10214

Hubble Space Telescope view of the "Tadpole" galaxy, UGC 10214.

10. NGC 4622

A spiral galaxy that spins the wrong way? That’s the conclusion reached by astronomers studying NGC 4622. A normal spiral galaxy has arms that trail around behind as the galaxy slowly spins. But with NGC 4622, the tips of the spiral arms actually point in the direction of rotation. (This has now been established by two different types of observations). This galaxy is 111 million light-years away.

NGC 4622

Hubble Space Telescope view of galaxy NGC 4622.

11. NGC 3314

This odd looking galaxy is actually two galaxies, one in front of the other. Together, the two galaxies are known by the catalogue number NGC 3314. The pair are about 140 million light-years from Earth.

NGC 3314

Hubble Space Telescope view of galaxy pair NGC 3314.

12. Sombrero Galaxy

One of the most famous galaxies and a favourite target of recreational astronomers, the Sombrero Galaxy (also known by its catalogue number, M104) is a spiral galaxy seen edge on and encircled by thick dust lanes. M104 has a mass of 800 billion suns, is 50,000 light-years across and is located 28 million light-years from Earth.

Sombrero Galaxy

Hubble Space Telescope view of the Sombrero Galaxy, also known as M104.

13. NGC 7742

This pretty, face-on spiral galaxy hides a dark secret—at its core there probably lives a black hole. (Mind you, the same can probably be said of pretty much all galaxies.) The central part of the galaxy contains lots of older stars. Surrounding that is ring where stars are being born, followed by tightly wrapped spiral arms. On the outside is a dimmer region that was once probably the site of star formation.

NGC 7742

Hubble Space Telescope view of galaxy NGC 7742.

14. Stephan’s Quintet

A clash of cosmic proportions is underway in this grouping of galaxies. Although five major galaxies are visible (the galaxy to the right of middle is actually two galaxies interacting), the one in the top left is actually a ring in—known as NGC 7320, it is seven times closer to us than the others, and so is not involved. Three of the others are distorted and disturbed by coming close to each other. NGC 7320 is 40 million light-years from Earth, while the others are around 300 million light-years away.

Stephan's Quintet

Hubble Space Telescope view of the group of galaxies known as Stephan's Quintet.

15. Abell 1689

The huge cluster of galaxies known as Abell 1689 acts as a cosmic lens, distorting our view of other galaxies that lie beyond it. Those other galaxies show up as arcs of light. The gravitational lens effect was predicted by Einstein’s theory of gravity.

Abell 1689

Hubble Space Telescope view of galaxy cluster Abell 1689, which acts as a gravitational lens.

Image credits:

NGC 5866: NASA / ESA/ Hubble Heritage Team (STScI / AURA)

The Cartwheel Galaxy: Curt Struck, Philip Appleton (Iowa State University) / Kirk Borne (Hughes STX Corporation) / Ray Lucas (STScI) / NASA / ESA

NGC 7049: NASA / ESA / W. Harris (McMaster University, Canada)

NGC 1512: NASA / ESA / Dan Maoz (Tel-Aviv University, Israel, and Columbia University, USA)

AM 0644-741: NASA / ESA / Hubble Heritage Team (AURA / STScI)

NGC 4921: NASA / ESA / K. Cook (Lawrence Livermore National Laboratory, USA)

NGC 1316: NASA / ESA/ Hubble Heritage Team (STScI / AURA)

Hoag’s Object: NASA / ESA/ Hubble Heritage Team (STScI / AURA)

UGC 10214: NASA / Holland Ford (JHU) / ACS Science Team / ESA

NGC 4622: NASA / ESA/ Hubble Heritage Team (STScI / AURA)

NGC 3314: NASA / ESA/ Hubble Heritage Team (STScI / AURA)

Sombrero Galaxy: NASA / ESA/ Hubble Heritage Team (STScI / AURA)

NGC 7742: NASA / ESA/ Hubble Heritage Team (STScI / AURA)

Stephan’s Quintet: NASA / ESA / Hubble SM4 ERO Team

Abell 1689: NASA / N. Benitez (JHU), T. Broadhurst (The Hebrew University), H. Ford (JHU), M. Clampin (STScI), G. Hartig (STScI), G. Illingworth (UCO/Lick Observatory) / ACS Science Team / ESA.

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“Goldilocks” planet discovered

Artist's impression of a planet orbiting Gliese 581

An artist's impression of a planet orbiting the red dwarf star Gliese 581. Astronomers have just discovered one in the star's "habitable zone", where temperatures could be right for liquid water to exist.

  • Gliese 581g orbits in its star’s “Goldilocks” zone
  • Temperature okay for liquid water – not too hot, not too cold
  • If confirmed, will be first potentially habitable planet yet found

A team of planet-hunting astronomers, utilising the HIRES spectrometer on the W.M. Keck Observatory’s Keck I Telescope, has announced the discovery of an Earth-sized planet orbiting a nearby red dwarf star.

The new planet, known as Gliese 581g, is at a distance that places it squarely in the middle of the star’s “habitable zone” where liquid water could exist on the planet’s surface.

If confirmed, this would be the most Earth-like exoplanet and the first bona fide potentially habitable one yet discovered.

To astronomers, a “potentially habitable” planet is one that could sustain life—even the simplest of life—and not necessarily one that humans would consider a nice place to live. Habitability depends on many factors, but liquid water and an atmosphere are among the most important.

The discovery by the team, led by astronomers at the University of California, Santa Cruz, and the Carnegie Institution of Washington DC, is based on 11 years of observations made at the Keck Observatory atop Mauna Kea mountain on the Big Island of Hawaii.

“Our findings offer a very compelling case for a potentially habitable planet,” said Steven Vogt, professor of astronomy and astrophysics at UC Santa Cruz. “The fact that we were able to detect this planet so quickly and so nearby tells us that planets like this must be really common.”

Diagram showing a star's habitable zone

Earth is in our Solar System's habitable or "Goldilocks" zone (blue band) where the temperature is neither too hot nor too cold for liquid water to exist on a rocky planet's surface. For hotter (whiter) or cooler (red) stars (shown at left), the zone is at a different distance.

“Advanced techniques combined with old-fashioned ground-based telescopes continue to lead the exoplanet revolution,” added Paul Butler of the Carnegie Institution.

“Our ability to find potentially habitable worlds is now limited only by our telescope time.”

Vogt and Butler lead the Lick-Carnegie Exoplanet Survey. The team’s new findings are reported in a paper to be published in the Astrophysical Journal.

Planet of perpetual night and day

The astronomers have deduced that the planet is tidally locked to the star, meaning that one side is always facing the star and basking in perpetual daylight, while the side facing away from the star is in perpetual darkness.

One effect of this is to stabilise the planet’s surface climates, according to Vogt. The most habitable zone on the planet’s surface would be the line between shadow and light (known as the “terminator”), with surface temperatures decreasing toward the dark side and increasing toward the light side.

“Any emerging life forms would have a wide range of stable climates to choose from and to evolve around, depending on their longitude,” Vogt said.

The researchers estimate that the average surface temperature of the planet is between -31 to -12 degrees Celsius. Actual temperatures would range from blazing hot on the side facing the star to freezing cold on the dark side.

Artist's impression of planets orbiting Gliese 581

It is now thought there are six planets circling the star Gliese 581, making it the most Solar System-like place discovered so far in the cosmos.

If Gliese 581g has a rocky composition similar to the Earth’s, its diameter would be about 1.2 to 1.4 times that of the Earth. The surface gravity would be about the same or slightly higher than Earth’s, so that a person could easily walk upright on the planet, Vogt said.

In fact, the scientists have reported the discovery of not one but two new planets circling Gliese 581. This brings to six the number of known planets around this star, the most yet discovered in a planetary system other than our own solar system.

Like our Solar System, the planets of Gliese 581 have nearly circular orbits. Gliese 581g has a mass 3 to 4 times that of the Earth and an orbital period of just under 37 days. Its mass indicates that it is probably a rocky planet with a definite surface and that it has enough gravity to hold on to an atmosphere, according to Vogt.

A difficult discovery

Although the planets themselves can’t be seen, the effect of their gravitational pull on their parent star can be measured. It shows up as a slight movement, or radial velocity change, in the star.

Multiple planets induce complex wobbles in the star’s motion, and astronomers use sophisticated analyses to distinguish the effects of the planets and determine their orbits and masses.

“It’s really hard to detect a planet like this,” Vogt said. “Every time we measure the radial velocity, that’s an evening on the telescope, and it took more than 200 observations with a precision of about 1.6 meters per second to detect this planet.”

W.M. Keck Observatory

Domes of the twin giant telescopes of the W.M. Keck Observatory on Mauna Kea mountain on the Big Island of Hawaii.

To get that many radial velocity measurements (238 in total), Vogt’s team combined their HIRES observations with published data from another group led by the Geneva Observatory (HARPS, the High Accuracy Radial velocity Planetary Search project).

In addition to the radial velocity observations, co-authors Gregory Henry and Michael Williamson of Tennessee State University made precise night-to-night brightness measurements of the star with one of Tennessee State University’s robotic telescopes.

“Our brightness measurements verify that the radial velocity variations are caused by the new orbiting planet and not by any process within the star itself,” Henry said.

How many habitable planets are out there?

Given the relatively small number of stars that have been carefully monitored by planet hunters, this discovery has come surprisingly soon.

“If these are rare, we shouldn’t have found one so quickly and so nearby,” Vogt said.

“The number of systems with potentially habitable planets is probably on the order of ten or 20 percent, and when you multiply that by the hundreds of billions of stars in the Milky Way, that’s a large number. There could be tens of billions of these systems in our galaxy.”

Gliese 581, located 20 light years away from Earth, has a somewhat chequered history of habitable-planet claims. Two previously detected planets in the system lie at the edges of the habitable zone…one on the hot side (planet c) and one on the cold side (planet d).

While some astronomers still think planet d may be habitable if it has a thick atmosphere with a strong greenhouse effect to warm it up, others are sceptical. The newly discovered planet g, however, lies right in the middle of the habitable zone.

“It’s the Goldilocks planet,” Vogt said. “That’s a well-worn analogy, but in this case it fits. We had planets on both sides of the habitable zone—one too hot and one too cold—and now we have one in the middle that’s just right.”

Adapted from information issued by W.M. Keck Observatory / ESO / L. Calçada / NASA / ESA / G. Bacon (STScI).

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Another close shave for Earth

Artist's impression of an asteroid.

Artist's impression of an asteroid. A new telescope called Pan-STARRS has just discovered one that will come within 6.5 million kilometres of Earth in mid-October 2010.

The Panoramic Survey Telescope & Rapid Response System (Pan-STARRS) PS1 telescope has discovered an asteroid that will come within 6.5 million kilometres of Earth in mid-October.

The object is about 45 metres in diameter and was discovered in images acquired on September 16, when it was about 32 million kilometres away.

It is the first “potentially hazardous object” (PHO) to be discovered by the Pan-STARRS survey and has been given the designation “2010 ST3.”

“Although this particular object won’t hit Earth in the immediate future, its discovery shows that Pan-STARRS is now the most sensitive system dedicated to discovering potentially dangerous asteroids,” said Robert Jedicke, a University of Hawaii member of the PS1 Scientific Consortium, who is working on the asteroid data from the telescope.

2010 ST3

Two images of asteroid 2010 ST3 (circled in green) taken by PS1 about 15 minutes apart on the night of September 16 show the asteroid moving against the background field of stars and galaxies.

“This object was discovered when it was too far away to be detected by other asteroid surveys,” Jedicke noted.

Most of the largest PHOs have already been catalogued, but scientists suspect that there are many more under a kilometre across that have not yet been discovered. These could cause devastation on a regional scale if they ever hit our planet.

Such impacts are estimated to occur once every few thousand years.

Pan-STARRS expects to discover tens of thousands of new asteroids every year with sufficient precision to accurately calculate their orbits around the Sun.

Any sizable object that looks like it may come close to Earth within the next 50 years or so will be labelled “potentially hazardous” and carefully monitored.

NASA experts believe that, given several years warning, it should be possible to organise a space mission to deflect any asteroid that is discovered to be on a collision course with Earth.

Pan-STARRS has broader goals as well. PS1 and its bigger brother, PS4, which will be operational later in this decade, are expected to discover a million or more asteroids in total, as well as more distant targets such as variable stars, supernovae, and mysterious energy bursts from galaxies across more than half the universe. PS1 became fully operational in June 2010.

Adapted from information issued by the Harvard-Smithsonian Centre for Astrophysics / Rob Ratkowski / PS1SC / NASA, ESA, and G. Bacon (STScI).

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Saturn’s eerie polar light show

False-colour image of Saturn

This false-colour composite of 65 Cassini spacecraft images shows the glow of the aurora over Saturn's north polar region. The colours are: blue is sunlight reflecting from the rings and from one of Saturn's cloud levels; red shows where heat is coming from the interior of the planet; and green shows the aurora.

  • Mini-movie made of Saturn’s polar aurora
  • Uses data from NASA’s Cassini spacecraft
  • Aurora pulses in time with the Sun and Saturn’s spin

As if its rings weren’t spectacular enough, Saturn also puts on a light show for anyone who can see at the right wavelengths.

In this case, those eyes belong to NASA’s Cassini spacecraft, which has been orbiting the Ringed Planet since 2004.

Scientists using Cassini’s visual and infrared mapping spectrometer (VIMS) instrument have been studying Saturn’s aurora, the equivalent of Earth’s Northern and Southern Lights.

Aurorae occur when particles in the solar wind are directed along magnetic field lines towards a planet’s poles. Funnelling down into the atmosphere, they strike gas molecules and cause an eerie, but very pretty, glow.

“Cassini’s instruments have been imaging the aurora in magnificent detail, but to understand the overall nature of the auroral region we need to make a huge number of observations—which can be difficult because Cassini observation time is in high demand,” says Dr Tom Stallard of the University of Leicester in the UK.

Time-lapse video of Saturn's aurora

This time-lapse video covers 20 Earth hours—just under two whole Saturnian days. Parts of the aurora seem synchronised with the direction of the Sun (left-hand side), while other parts appear orchestrated with Saturn's magnetic field.

So Dr Stallard and his colleagues turned to other Cassini images that weren’t specifically targeted at the aurora—but which nevertheless happened to serendipitously capture it—to compile a short video that shows the aurora’s behaviour as Saturn rotates.

“Sometimes the aurora can be clearly seen, sometimes we have to add multiple images together to produce a signal,” Dr Stallard said.

The video shows the aurora changing considerably during the over the course of Saturn’s day, which is around 10 hours and 47 minutes in Earth time.

On the left-hand side of the video—the direction towards the Sun—the aurora brightens, indicating that it is being influenced by the Sun.

Other parts of the aurora seem more connected with the planet below; specifically, with the orientation of Saturn’s magnetic field as the planet spins.

“Saturn’s aurora are very complex and we are only just beginning to understand all the factors involved,” said Stallard.

Story by Jonathan Nally, editor

Images courtesy NASA / JPL / University of Leicester / University of Arizona.

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Spiral galaxy seen in a new light

Infrared view of NGC 1365

An infrared view of NGC 1365, a beautiful barred spiral galaxy in the Fornax cluster of galaxies, about 60 million light-years from Earth.

  • Galaxy NGC 1365, the Great Barred Spiral Galaxy
  • 200,000 light-years wide, with two huge spiral arms
  • Located 60 million light-years from Earth

A new image taken with the powerful HAWK-I camera on the European Southern Observatory’s (ESO’s) Very Large Telescope in Chile shows the beautiful “barred spiral” galaxy NGC 1365 in infrared light.

NGC 1365 is a member of the Fornax cluster of galaxies, about 60 million light-years from Earth.

NGC 1365 is one of the best-known and most-studied barred spiral galaxies, sometimes nicknamed the Great Barred Spiral Galaxy because of its strikingly perfect form…a straight “bar” or middle section and two prominent outer spiral arms.

Closer to the centre there is also a second spiral structure, and the galaxy as a whole is laced with delicate lanes of interstellar dust.

Astronomers consider it an excellent “laboratory” for studying how spiral galaxies form and evolve.

The new infrared images from HAWK-I are less affected by the dust that obscures parts of the galaxy than images taken at visible light wavelengths, and they reveal very clearly the glow from vast numbers of stars in both the bar and the spiral arms.

The images were acquired to help astronomers understand the complex flow of gas within the galaxy and how it affects the reservoirs of gas from which new stars can form.

The huge bar disturbs the shape of the gravitational field of the galaxy and this leads to regions where gas becomes compressed, triggering the formation of new stars.

See the full-size, high-resolution version of the image (suitable for PC wallpaper) here.

Visible light and infrared views of NGC 1365

This comparison shows a visible-light image (left) of NGC 1365 along with the new infrared view (right). The infrared view "peels away" the veil of dust to reveal more stars beneath.

Black hole hidden in the core

Many huge young star clusters are visible in the main spiral arms, each containing hundreds or thousands of bright young stars that are less than 10 million years old.

The galaxy is too remote for single stars to be seen—most of the tiny clumps visible in the picture are really clusters of stars.

While the bar of the galaxy comprises mainly older stars long past their prime, many new stars are born in stellar “nurseries” of gas and dust in the inner spiral close to the core. Over the whole galaxy, stars are forming at a rate of about three times the mass of our Sun per year.

The bar also funnels gas and dust gravitationally into the very centre of the galaxy, where astronomers have found evidence for the presence of a super-massive black hole, well hidden among myriads of intensely bright new stars.

Here’s a video that zooms in on NGC 1365, alternating between visible wavelength and infrared wavelength views:

NGC 1365, including its two huge outer spiral arms, is around 200,000 light-years across. Different parts of the galaxy take different times to make a full rotation around the core, with the outer parts of the bar taking about 350 million years to complete one circuit.

NGC 1365 and other galaxies of its type have come to more prominence in recent years with new observations indicating that our Milky Way galaxy could also be of the barred spiral type.

Such galaxies are quite common—two thirds of spiral galaxies are barred according to recent estimates. Studying them can help astronomers understand our own galactic home.

Adapted from information issued by ESO / P. Grosbøl.

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Dive into the Lagoon Nebula

Close-up shot of the centre of the Lagoon Nebula

This close-up shot of the centre of the Lagoon Nebula clearly shows the delicate structures formed when the powerful radiation of young stars interacts with the hydrogen cloud from which they formed.

  • Lagoon Nebula, a famous “starbirth” region
  • Located 4,000 to 5,000 light-years away
  • Evidence that stars and planets are forming within

The Advanced Camera for Surveys (ACS) on Hubble Space Telescope has captured a dramatic view of gas and dust sculpted by intense radiation from hot young stars deep in the heart of the Lagoon Nebula (also known as Messier 8).

This spectacular object is named after the wide, lagoon-shaped dust lane that crosses the glowing gas of the nebula.

This dust lane structure is prominent in wide-field images, but cannot be seen in this close-up. However the strange billowing shapes and sandy texture visible in this image make the Lagoon Nebula’s watery name eerily appropriate from this viewpoint too.

Here’s a video pan across the new Hubble image:

Located 4,000 to 5,000 light-years away, Messier 8 is a huge region of star birth that stretches across 100 light-years. Clouds of hydrogen gas are slowly collapsing to form new stars, whose bright ultraviolet rays then light up the surrounding gas in a distinctive shade of red.

See the full-size, high-resolution version of the image here.

Wide-field image of the Lagoon Nebula

A wide-field image shows the entirety of the Lagoon Nebula. The Hubble image focuses on a tiny portion in the heart of the Nebula, just below and to the right of centre.

The wispy tendrils and beach-like features of the nebula are not caused by the ebb and flow of tides, but rather by ultraviolet radiation’s ability to erode and disperse the gas and dust into the distinctive shapes that we see.

In recent years astronomers probing the secrets of the Lagoon Nebula have found the first unambiguous evidence that star formation by accumulation of matter from the gas cloud is ongoing in this region.

Young stars that are still surrounded by a swirling cloud of gas and dust occasionally shoot out long tendrils of matter from their poles. Several examples of these jets, known as Herbig-Haro objects, have been found in this nebula in the last five years, providing strong support for astronomers’ theories about star formation in such hydrogen-rich regions.

Watch this impressive zoom-in video, which takes us from the outer reaches of the Milky Way and into the Lagoon:

The Lagoon Nebula is faintly visible to the naked eye on dark nights as a small patch of grey in the heart of the Milky Way. Without a telescope, the nebula looks underwhelming because human eyes are unable to distinguish clearly between colours at low light levels.

Charles Messier, the 18th century French astronomer, studied the nebula and included it in his famous astronomical catalogue, from which the nebula’s alternative name comes. But his relatively small refracting telescope would only have hinted at the dramatic structures and colours now visible thanks to Hubble.

Adapted from information issued by / NASA / A. Caulet (ST-ECF, ESA) / Hunter Wilson.

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Aussie amateur spots Mercury’s tail

The planet Mercury

The planet Mercury. Astronomers have known that the planet has a tenuous gas tail stretching into space, but now an Australian amateur astronomer has spotted it in data from a Sun-monitoring satellite.

Scientists from Boston University’s Centre for Space Physics reported today that NASA satellites designed to view the escaping atmosphere of the Sun have also recorded evidence of escaping gas from the planet Mercury.

The STEREO mission has two satellites placed in the same orbit around the Sun that the Earth has, but at locations ahead and behind it. This configuration offers multi-directional views of the electrons and ions that make up the escaping solar wind.

Movie of Mercury's tail

This movie covers a 4-day period and shows Mercury's tail as seen by the STEREO A spacecraft on 6-9 February 2008. The faint tail can be seen streaming off to the top left of the planet.

On occasion, the planet Mercury appears in the field of view of one or both satellites. In addition to its appearance as a bright disc of reflected sunlight, a ‘tail’ of emission can be seen in some of the images.

Of special interest is the way the tail feature was spotted in the STEREO data. It was not found by the Boston University team, but by Dr Ian Musgrave, a medical researcher in Adelaide, South Australia, who has a strong interest in astronomy.

Viewing the on-line database of STEREO images and movies, Dr Musgrave recognised the tail and sent news of it to Boston asking the BU team to compare it with their observations.

“A joint study was started and now we have found several cases, with detections by both STEREO satellites,” explained Jeffrey Baumgardner, Senior Research Associate in the Centre for Space Physics at Boston University, and the designer of the optical instruments that discovered the exceptionally long sodium tail.

Schematic representation of the viewing geometry that allows the STEREO camera systems to make observations of Mercury’s tail

One of the twin STEREO spacecraft was in the right position to see the tail sweeping away from Mercury in a direction opposite to the Sun.

Tail blown by sunlight

This new method of observing Mercury and efforts to try to understand the nature of the gases that might make up this tail feature were topics presented at the European Planetary Science Congress meeting in Rome today.

It has been known that Mercury exhibits comet-like features, with a coma, or cloud, of tenuous gas surrounding the planet and a very long tail extending in the anti-sunward direction.

From Earth, observations of both of these features can be done using light from sodium gas sputtered off the surface of Mercury. The Sun’s radiation pressure then pushes many of the sodium atoms in the direction away from the Sun, creating a tail that extends many hundreds of times the physical size of Mercury.

“We have observed this extended sodium tail to great distances using our telescope at the McDonald Observatory in Texas,” Boston University graduate student Carl Schmidt explained, “and now the tail can also be seen from satellites near Earth.”

Much closer to Mercury, several smaller tails composed of other gases, both neutral and ionised, have been found by NASA’s MESSENGER satellite as it flew by Mercury in its long approach to entering into a stable orbit there.

Adapted from information issued by Europlanet / Boston University / NASA.

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Sailing the Solar System

Artist’s illustration of the data clipper concept

An artist’s illustration of the data clipper concept, with solar sail-powered spacecraft continually circling through the Solar System, receiving and delivering data from planetary probes.

A new golden age of sailing may be about to begin…in space. Future missions to explore the outer planets could employ fleets of ‘data-clippers’—manoeuvrable spacecraft equipped with solar sails—to ship vast quantities of scientific data to back Earth.

Although the memory capacities of spacecraft have increased dramatically over the decades, “a full high-res map of, say, Europa or Titan, would take several decades to download from a traditional orbiter, even using very large antennae,” says Joel Poncy of the Thales Alenia Space company, who undertook a study of the concept.

The concept is for a “clipper”—propelled along by the gentle pressure of the Sun’s solar wind— to pass close to a spacecraft investigating another planet, upload its data, and then fly back past Earth, at which point terabytes of data could be downloaded to a ground station.

“Downloading data is the major design driver for interplanetary missions,” adds Poncy. “We think that data clippers would be a very efficient way of overcoming this bottleneck.”

IKAROS spacecraft

The Japanese IKAROS spacecraft, launched on May 21, 2010, is testing solar sail propulsion.

A fleet of data clippers continually cruising around the Solar System could provide support for an entire suite of planetary missions.

The technology could be ready in time to support missions planned to the moons of Jupiter and Saturn.

The Japanese Space Agency, JAXA, is currently testing a solar sail mission called IKAROS.

“Using the Sun as a propulsion source has the considerable advantage of requiring no propellant on board,” says Poncy. “The use of data clippers could lead to a valuable downsizing of exploration missions and lower ground operation costs—combined with a huge science return.”

The orbiting spacecraft would still download some samples of their data directly to Earth to enable real-time discoveries and interactive mission operations.

But the bulk of the data is less urgent and is often processed by scientists much later. Data clippers could provide an economy delivery service from the outer Solar System, over and over again, Poncy says.

Adapted from information issued by Europlanet / Thales Alenia Space.

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Lunar lander aims for pole position

Artist's impression of the ESA lunar south pole explorer

Artist's impression of the European Space Agency's unmanned lunar south pole explorer, due for launch in 2018.

The first mission aiming to visit the Moon’s south pole has taken a significant step forward, with the European Space Agency signing a design study contract with the EADS-Astrium company in Berlin, Germany.

The unmanned mission is intended to land in the mountainous and heavily cratered terrain of the lunar south pole in 2018.

This could be a prime location for future human explorers because it offers almost continuous sunlight for power and potential access to vital resources such as water ice.

To reach the surface safely, the lander must precisely find its way to a mountain peak or crater rim, carefully avoiding boulders and steep slopes, before gently setting down to take in one of the most spectacular views in the Solar System.

The Moon is a favoured target for the human exploration missions outlined in the ‘Global Exploration Strategy’ by 14 space agencies around the world. The strategy supports international space exploration and calls for further studies of the Moon and Mars—places where humans will one day live and work.

The new study is important because now, following the preliminary planning and feasibility studies, the mission’s design will be continued and some of the key technologies will be developed and tested for the first time.

First, the most recent topographic data covering the Moon’s south pole will be analysed in detail to find promising landing sites. The target area is poorly understood and only now are scientists beginning to get the information needed to consider landing and operating a mission there.

Then, the robotic lander will be designed down to the level of its various subsystems, such as propulsion and navigation.

The study will culminate in a ‘Preliminary System Requirements Review’ in 2012, which will provide the basis for the final design of the mission and lander.

Adapted from information issued by ESA.

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Saturn’s Siamese twin moons

Saturn's moon Epimetheus

Saturn's moon Epimetheus, as seen by the Cassini spacecraft narrow-angle camera from a distance of approximately 37,400 kilometres. A large crater dominates one half of the 116km-wide moon.

  • Epimetheus and Janus circle Saturn at almost the same distance
  • Every four years they swap orbits with each other
  • Both moons are probably loose piles of rocky, icy rubble

Epimetheus (pronounced ep-ee-MEE-thee-us) and the neighbouring moon Janus have been referred to as the Siamese twins of Saturn because they circle Saturn in nearly the same orbit. This co-orbital condition (also called a 1:1 resonance) confused astronomers, who at first could not believe that two moons could share nearly identical orbits without colliding.

The two moons lie amongst Saturn’s rings at a distance from Saturn of roughly 151,500 kilometres (94,100 miles). One moon orbits 50 kilometres (31 miles) higher (farther away from the planet) and consequently moves slightly slower than the other. The slight velocity difference means the inner satellite catches up to the other in approximately four Earth years.

When this happens, the gravitational interaction between the two pulls the inner moon it to a higher orbit. At the same time, the catching-up inner moon drags the leading outer moon backward so that it drops into a lower orbit. The result is that the two exchange places, and the nearest they approach is within 15,000 kilometres (6,200 miles).

At their most recent trade in early 2010, Epimetheus’ orbital radius dropped by approximately 80 kilometres (50 miles) while Janus’ orbit increased by only approximately 20 kilometres (12.4 miles). Janus’ orbit changed only a quarter of that of Epimetheus because Janus is four times more massive than Epimetheus.

Both of the moons are “phase locked” with Saturn, which means that one side always faces toward the planet. And being so close to Saturn, they orbit around it in less than 17 hours.

Saturn's moon Epimetheus

The Cassini spacecraft snapped this image of Epimetheus from a distance of about 107,000 kilometres.

Rubble piles

Epimetheus and Janus may have formed by the break-up of one moon. If so, it would have happened early in the life of the Saturnian system because both moons have ancient cratered surfaces, many with soft edges because of dust. They also have some grooves (similar to grooves on the Martian moon Phobos) suggesting some glancing blows from other bodies.

They are both thought to be composed of largely of water ice, but their density of less than 0.7 is much less than that of water. Thus, they are probably “rubble piles”—each a collection of numerous pieces held together loosely by gravity.

Each moon has dark, smoother areas, along with brighter areas of terrain. One interpretation of this is that the darker material evidently moves down slopes, leaving shinier material such as water ice on the walls of fractures.

Their temperature is approximately -195 degrees Celsius (-319 degrees Fahrenheit). Their reflectivity (or albedo) of 0.7 to 0.8 in the visual range again suggests a composition largely of water ice.

Epimetheus has several craters larger than 30 kilometres, including Hilaeira and Pollux.

Saturn's moon Janus

Janus, seen here, swaps orbits with Epimetheus every four Earth years.


French astronomer Audouin Dollfus spotted a moon of Saturn on December 15, 1966, for which he proposed the name “Janus.” On December 18 of the same year, Richard Walker made a similar observation, now credited as the discovery of Epimetheus.

At the time, astronomers believed that there was only one moon, unofficially known as “Janus,” in the given orbit.

Twelve years later, in October 1978, Stephen M. Larson and John W. Fountain realised that the 1966 observations were best explained by two distinct objects (Janus and Epimetheus) sharing very similar orbits. Observations by the Voyager I spacecraft confirmed this in 1980, and so Larson and Fountain officially share the discovery of Epimetheus with Walker.

The Cassini spacecraft has made several close approaches and provided detailed images of the moon since it achieved orbit around Saturn in 2004.

Nineteenth-century English astronomer John Herschel suggested that the moons of Saturn be associated with mythical brothers and sisters of Kronus, known to the Romans as Saturn. (The International Astronomical Union now controls the official naming of astronomical bodies.)

The name Epimetheus comes from the Greek god (or titan) Epimetheus (or hindsight) who was the brother of Prometheus (foresight). Together, they represented humanity. The craters on Epimetheus include Hilaeira (who was a priestess of Artemis and Athena) and Pollux (who was a warrior in The Illiad and who carried off Hilaeira).

Astronomers also refer to Epimetheus as Saturn XI and as S/1980 S3, and they refer to Janus as Saturn X and as S/1980 S1.

Epimetheus data:

  • Discovered: 1966 by R. Walker
  • Distance from Saturn: 151,422 km
  • Period of orbit around Saturn: 16.7 hours
  • Diameter: 138 x 110 x 110 km
  • Mass: 5.3 x 10^17 kg

Adapted from information issued by NASA / JPL / Space Science Institute.

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