Faraway Eris is Pluto's Twin

In November 2010, the distant dwarf planet Eris passed in front of a faint background star, an event called an occultation. These occurrences are very rare and difficult to observe as the dwarf planet is very distant and small. The next such event involving Eris will not happen until 2013. Occultations provide the most accurate, and often the only, way to measure the shape and size of a distant Solar System body.

The candidate star for the occultation was identified by studying pictures from the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory. The observations were carefully planned and carried out by a team of astronomers from a number of (mainly French, Belgian, Spanish and Brazilian) universities using — among others — the TRAPPIST [1] (TRAnsiting Planets and PlanetesImals Small Telescope, eso1023) telescope, also at La Silla.

“Observing occultations by the tiny bodies beyond Neptune in the Solar System requires great precision and very careful planning. This is the best way to measure Eris’s size, short of actually going there,” explains Bruno Sicardy, the lead author.

Observations of the occultation were attempted from 26 locations around the globe on the predicted path of the dwarf planet’s shadow — including several telescopes at amateur observatories, but only two sites were able to observe the event directly, both of them located in Chile. One was at ESO’s La Silla Observatory using the TRAPPIST telescope, and the other was located in San Pedro de Atacama and used two telescopes [2]. All three telescopes recorded a sudden drop in brightness as Eris blocked the light of the distant star.

The combined observations from the two Chilean sites indicate that Eris is close to spherical. These measurements should accurately measure its shape and size as long as they are not distorted by the presence of large mountains. Such features are, however, unlikely on such a large icy body.

Eris was identified as a large object in the outer Solar System in 2005. Its discovery was one of the factors that led to the creation of a new class of objects called dwarf planets and the reclassification of Pluto from planet to dwarf planet in 2006. Eris is currently three times further from the Sun than Pluto.

While earlier observations using other methods suggested that Eris was probably about 25% larger than Pluto with an estimated diameter of 3000 kilometres, the new study proves that the two objects are essentially the same size. Eris’s newly determined diameter stands at 2326 kilometres, with an accuracy of 12 kilometres. This makes its size better known than that of its closer counterpart Pluto, which has a diameter estimated to be between 2300 and 2400 kilometres. Pluto’s diameter is harder to measure because the presence of an atmosphere makes its edge impossible to detect directly by occultations. The motion of Eris’s satellite Dysnomia [3] was used to estimate the mass of Eris. It was found to be 27% heavier than Pluto [4]. Combined with its diameter, this provided Eris’s density, estimated at 2.52 grams per cm3 [5].

“This density means that Eris is probably a large rocky body covered in a relatively thin mantle of ice,” comments Emmanuel Jehin, who contributed to the study [6].

The surface of Eris was found to be extremely reflective, reflecting 96% of the light that falls on it (a visible albedo of 0.96 [7]). This is even brighter than fresh snow on Earth, making Eris one of the most reflective objects in the Solar System, along with Saturn’s icy moon Enceladus. The bright surface of Eris is most likely composed of a nitrogen-rich ice mixed with frozen methane — as indicated by the object's spectrum — coating the dwarf planet’s surface in a thin and very reflective icy layer less than one millimetre thick.

“This layer of ice could result from the dwarf planet’s nitrogen or methane atmosphere condensing as frost onto its surface as it moves away from the Sun in its elongated orbit and into an increasingly cold environment,” Jehin adds. The ice could then turn back to gas as Eris approaches its closest point to the Sun, at a distance of about 5.7 billion kilometres.

The new results also allow the team to make a new measurement for the surface temperature of the dwarf planet. The estimates suggest a temperature for the surface facing the Sun of -238 Celsius at most, and an even lower value for the night side of Eris.

Trusty Comet Garradd - Observing Highlights

Comet Garradd and M71
Comet Garradd passed M71 in Sagitta on the evening of August 26th.

Comet Elenin has been getting a lot of press in recent months — and now it seems almost certain to be a total bust. Meanwhile, people in the know have been following a comet that has never been hyped much, but seems almost certain to be a fine, though not spectacular performer for many months to come.

We're talking about Comet C/2009 P1 Garradd. At the end of August it was already a fine sight through telescopes of all sizes, shining high in the evening sky at 7th magnitude. It has a bright head, a sharp starlike nucleus, and a tail that's stubby but well defined. And it's forecast to shine near its 6th-magnitude best all the way from October to mid-March. Not since Hale-Bopp has any comet remained so bright for so long.

Comet Garradd passes through the Coathanger asterism on the evening of Friday, September 2nd, then crosses the northwest corner of Sagitta, and soon enters Hercules, where it will remain until February. That means it’s high in the west after nightfall in October, lower but still in good view in November, and near the west-northwest horizon at the end of twilight around Christmas. But by then it’s already up higher in the east before the first light of dawn; the best viewing tips from evening to morning on December 16th.



Path of Comet Garradd Oct 2011 - Jan 2012
Comet Garradd crosses Vulpecula and Sagitta in early September, then remains in Hercules for the next six months.

As the comet climbs high in the early-morning sky of January and February, it will pass the Keystone of Hercules, skim ½° by the globular cluster M92 on the morning of February 3rd (mark your calendar), then sail northward past the head of Draco. It should stay bright all the way into spring as it returns to the evening sky.

Why is it changing so slowly? Comet Garradd is unusually large and distant as 6th-magnitude comets go. It never comes closer to the Sun than Mars’s average distance; at perihelion on December 23rd it’s 1.55 astronomical units from the Sun. Nor does the comet ever come near Earth; it’s about 2 a.u. from us all through October and November, and when closest next March 5th it will still be 1.27 a.u. away. Too bad! Garradd might have qualified for “Great Comet” status if had been on a trajectory to pass close to the Sun and if Earth weren’t on the wrong side of its orbit at the time.

Astronomer Gordon J. Garradd discovered the comet at 17th magnitude on August 13, 2009, at Australia’s Siding Spring Observatory while hunting for — ironically — near-Earth objects.

Supernova in M101

Supernova in M101. The new Type Ia supernova is still brightening in M101, the Pinwheel Galaxy off the Big Dipper's handle. Supernova 2011fe was discovered on August 24th at magnitude 17.2, reached 13.8 on the 25th, 12.5 on the 27th, and 11.6 on the 29th. By then it was easier to see than the galaxy itself in amateur telescopes through suburban light pollution. On the evening of August 31st it was up to about magnitude 10.8.

A normal Type Ia supernova at M101's distance, 23 million light-years, should reach magnitude 10.0 at its peak, assuming none of its light is lost to interstellar absorption in M101 itself. It's well within visual reach in a 4-inch scope. You'll be using the supernova to find the galaxy, not the other way around!

Moonlight will increasingly return to the evening sky starting around September 3rd or 4th.

Opportunity Reaches Its New Home

The west rim of Endeavour Crater, seen from outside
A portion of the west rim of Endeavour crater sweeps southward in this color view
from NASA's Mars Exploration Rover Opportunity. Click for full-size image.
It shouldn't be long until Opportunity is peering over the rim inside.

Yet another adventure is about to begin for NASA’s Mars Exploration Rover B, more famously known as Opportunity. The golf-cart-size, solar-powered vehicle reached the outer hills of its new destination, Endeavour crater, on August 9th after a 3-year, 13-mile (21-km) trek across the flat Martian terrain from the previous big crater it explored, Victoria.

The site from which Opportunity radioed Earth has been named “"Spirit Point,” in memory of its smaller twin, Spirit, that collected data on Mars from January 2004 to March 2011.

Launched in 2003, both Spirit and Opportunity were expected to complete a 90-Martian-day mission; instead, Opportunity has endured 30 times longer and traversed a total of 20.81 miles. Since touching down south of the Martian equator on January 25, 2004, the rover has explored five craters: Eagle, Endurance, Erebus, Victoria, and Santa Maria.

Endeavour, its new destination, is huge: 14 miles (22 km) wide — 25 times as large as Victoria, whose geology the rover studied for two years. A spectrometer aboard NASA’s Mars Reconnaissance Orbiter has detected clay minerals inside Endeavour, which likely formed during a warmer and wetter period on the Red Planet.

"We're soon going to get the opportunity to sample a rock type the rovers haven't seen yet," says science-team member Matthew Golombek (Jet Propulsion Laboratory, Pasadena, California).

Mission scientists will next choose the safest way to have Opportunity descend into the crater. There’s no word on how long they expect this epic exploration to last.

However, we do know that Opportunity's larger successor, Curiosity, will soon be launched on its way to Gale crater on another side of Mars.

2011 Perseid Meteor Showers

The Perseid meteor shower is an annual meteor shower that is extremely regular in its timing and can potentially be visible for weeks in the late summer sky, depending on weather and location.

The Perseid meteor shower is named after the constellation Perseus, which is located in roughly the same point of the night sky where the Perseid meteor shower appears to originate from. This is a useful naming convention, but not very accurate!

The source of the Perseid meteor shower is actually debris from the comet Swift-Tuttle. Every year, the earth passes through the debris cloud left by the comet when the earth's atmosphere is bombarded by what is popularly known as "falling stars."

When and where to look for Perseids in 2011

In 2011, visibility (the weather also notwithstanding) will be somewhat limited by a full moon on August 13 which will likely wipe out fainter meteors from view.

Because of the way the earth hits this debris cloud, the Perseid meteor shower is much more visible in the Northern hemisphere.

People in Canada, for instance, can see the meteor shower by mid-July, but generally there isn't much activity at such an early date. Throughout Europe, the US and the rest of North America, meteor shower activity usually peaks sometime around August 12th, when it is not unusual to see at least 60 meteors per hour streaking across the Northeast sky.

The meteors are certainly bright, but they are actually only tiny objects, usually no more than a grain of sand. However, as they travel at speeds of up to 71 kilometers per second, these small particles put on quite a brilliant show.

The Perseid meteor showers were observed as far back as two thousand years ago, and in ancient Europe, the Perseid meteor shower was known as the "Tears of St. Lawrence."

How to view Perseids

Today, the best place to observe the Perseid meteor shower (or any meteor shower for that matter), is somewhere dark, away from light pollution, and with the moon out of the field of vision. The less light visible, the more brilliant the meteor shower will be.

Telescope or camera?

While mostly viewable to the naked eye, the annual Perseid meteor show may be partially obstructed by the moon, clouds or night mist, so amateur astronomers may want to carry along a pair of binoculars or a camera with a telescopic lens. Even on clear nights, some kind of viewing aid comes in handy for catching sight of even the faintest of falling stars, aptly named "telescopic" meteors. Experts usually just advise to forget the telescope, and simply look up toward the northeast sky.

For photographing the annual event, a digital camera mounted on a tripod helps to steady the images that swiftly move across the sky. A quick trigger finger also helps. Even random clicks during the height of Perseid "prime-time" will guarantee that you'll catch something! Be sure to have the camera focused on infinity and, if your camera permits, leave the shutter open for several minutes for the most spectacular photographic effects.

New Video Showing Water Maybe Flowing on Mars

Scientists believe they have found water on Mars after finding that lines on the planet's surface are more visible in warm seasons.

Scientists announced on Thursday that they saw dark, finger-like features appearing and extending down some Martian slopes during late spring through summer, which fade in the winter and return during next spring.

These recurring features were located on several steep slopes in Mars' southern hemisphere, according to NASA, and are believed to be briny water.

"The best explanation for these observations so far is the flow of briny water," said Alfred McEwen of the University of Arizona, Tucson, in a statement. McEwen is the principal investigator for the orbiter's High Resolution Imaging Science Experiment, or HiRISE, and the lead author of a report about the recurring flows published in Thursday's edition of the journal Science.

Water flows that appear in spring and summer on a slope inside Mars' Newton crater are shown in this combination of orbital imagery with 3-D modeling in this NASA handout photo released to Reuters August 4, 2011. This image has been reprojected to show a view of a slope as it would be seen from a helicopter inside the crater, with a synthetic Mars-like sky. The source observation was made May 30, 2011, by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter. Color has been enhanced.


This series of images shows warm-season features that might be evidence of salty liquid water active on Mars today. Evidence for that possible interpretation is presented in a report by McEwen et al. in the Aug. 5, 2011, edition of Science. These images come from observations of Horowitz crater, at 32 degrees south latitude, 141 degrees east longitude, by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter. In time, the series spans from late summer of one Mars year to mid-summer of two years later. The images taken from oblique angles have been adjusted so that all steps in the sequence show the scene as if viewed from directly overhead. The features that extend down the slope during warm seasons are called recurring slope lineae. They are narrow (one-half to five yards or meters wide), relatively dark markings on steep (25 to 40 degree) slopes at several southern hemisphere locations. Repeat imaging by HiRISE shows the features appear and incrementally grow during warm seasons and fade in cold seasons. They extend downslope from bedrock outcrops, often associated with small channels, and hundreds of them form in rare locations. They appear and lengthen in the southern spring and summer from 48 degrees to 32 degrees south latitudes favoring equator-facing slopes. These times and places have peak surface temperatures from about 10 degrees below zero Fahrenheit to 80 degree above zero Fahrenheit (about 250 to 300 Kelvin). Liquid brines near the surface might explain this activity, but the exact mechanism and source of the water are not understood.


This series of images shows warm-season features that might be evidence of salty liquid water active on Mars today. Evidence for that possible interpretation is presented in a report by McEwen et al. in the Aug. 5, 2011, edition of Science. These images come from observations of a steep crater slope in the Terra Cimmeria region of Mars, at 38.8 degrees south latitude, 159.5 degrees east longitude, by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter. In time, the series spans from the end of summer of one Mars year to mid-summer of two years later. The images taken from oblique angles have been adjusted so that all steps in the sequence show the scene as if viewed from directly overhead. The features that extend down the slope during warm seasons are called recurring slope lineae. They are narrow (one-half to five yards or meters wide), relatively dark markings on steep (25 to 40 degree) slopes at several southern hemisphere locations. Repeat imaging by HiRISE shows the features appear and incrementally grow during warm seasons and fade in cold seasons. They extend downslope from bedrock outcrops, often associated with small channels, and hundreds of them form in rare locations. They appear and lengthen in the southern spring and summer from 48 degrees to 32 degrees south latitudes favoring equator-facing slopes. These times and places have peak surface temperatures from about 10 degrees below zero Fahrenheit to 80 degree above zero Fahrenheit (about 250 to 300 Kelvin). Liquid brines near the surface might explain this activity, but the exact mechanism and source of the water are not understood.


This series of images shows warm-season features that might be evidence of salty liquid water active on Mars today. Evidence for that possible interpretation is presented in a report by McEwen et al. in the Aug. 5, 2011, edition of Science. These images come from observations of Newton crater, at 41.6 degrees south latitude, 202.3 degrees east longitude, by the High Resolution Imaging Science Experiment (HiRISE) camera on NASA's Mars Reconnaissance Orbiter. In time, the series spans from early spring of one Mars year to mid-summer of the following year. The images have been adjusted to correct those taken from oblique angles to show how the scene would look from directly overhead. The features that extend down the slope during warm seasons are called recurring slope lineae. They are narrow (one-half to five yards or meters wide), relatively dark markings on steep (25 to 40 degree) slopes at several southern hemisphere locations. Repeat imaging by HiRISE shows the features appear and incrementally grow during warm seasons and fade in cold seasons. They extend downslope from bedrock outcrops, often associated with small channels, and hundreds of them form in rare locations. They appear and lengthen in the southern spring and summer from 48 degrees to 32 degrees south latitudes favoring equator-facing slopes. These times and places have peak surface temperatures from about 10 degrees below zero Fahrenheit to 80 degree above zero Fahrenheit (about 250 to 300 Kelvin). Liquid brines near the surface might explain this activity, but the exact mechanism and source of the water are not understood. The series is timed to dwell two seconds on the first and last frames and one second on intermediate frames, though network or computer performance may cause this to vary.

Earth's Traveling Companion

Earth's Trojan asteroid
Not much to look at, the asteroid 2010 TK7 nonetheless represents Earth's first Trojan asteoid. NASA's WISE spacecraft captured the view at top in October 2010 at the infrared wavelength of 4 microns. Then, in April 2011, a follow-up image was recorded by the Canada-France-Hawaii Telescope in Hawaii. M. Connors & P. Wiegert (top); C. Veillet (bottom)

There's something deeply intriguing about the interplanetary objects known as Trojan asteroids.

The great French dynamicist Joseph-Louis Lagrange predicted in 1772 that small bodies might be sharing Jupiter's orbit, in gravitationally stable sweet spots (now called Lagrange points) located ahead of and behind the planet by 60°. But it wasn't until 1906 that the first of these, 588 Achilles, was spotted. Today more than 4,800 Jupiter Trojans are known, with roughly two-thirds in the preceding "Greek camp" (L4) and a third in the trailing "Trojan camp" (L5).

Within the past two decades, astronomers have found four Trojan asteroids sharing the orbit of Mars and seven accompanying Neptune. They've looked for companions to Earth as well, but the geometry is all wrong: Earth's Trojans would spend most of their time in the daylight sky.

But the odds tipped back in observers' favor with the 2009 launch of NASA's Wide-field Infrared Survey Explorer (WISE), which recorded big swaths of sky 90° away from the Sun. Late last year, Canadian astronomers Martin Connors (Athabasca University) and Paul Wiegert (University of Western Ontario) picked through the spacecraft's scans and identified one object, designated 2010 TK7, that seemed to have an Earthlike orbit. Follow-up was needed, but that wasn't possible until this past April, when it was swept up by two observers in Hawaii.

Their suspicions confirmed, Connors, Wiegert, and Christian Veillet (Canada-France-Hawaii Telescope) report the discovery in July 28th's Nature.

This little body is tied to Earth's preceding Lagrange point. But if you're imagining it circling the Sun in lock step with our planet, think again. The orbit of 2010 TK7 is distinctly eccentric (0.19) and inclined (21°). In fact, it's never actually at L4. Instead, it vacillates widely — almost wildly — in a 400-year-long epicyclic pattern that at times brings it relatively near Earth (though still many times the Moon's distance) and at others places it on the far side of the Sun from us, near the L3 point.



Orbit of asteroid 2010 TK₇
The small asteroid designated 2010 TK7 is locked in an orbital resonance with Earth. This plot shows the range of separation between the asteroid and our planet over a 400-year period. The red line is its average orbit, which is pinned to the L4 Lagrange point that precedes Earth by 60°.

Earth's little buddy is so wide ranging that it might even occasionally spend some time resonating around the distant L3 point. In fact, gravitational influences from Jupiter make the orbit chaotic, and there's no way to know with certainty where 2010 TK7 was or will be when its orbit is tracked for more than 10,000 years.

Unfortunately, even though Earth probably has other Trojans in its entourage, WISE won't be able to see them. The spacecraft ran out of its cryogenic coolant last October, and on February 17th principal investigator Ned Wright sent a command to turn off WISE's transmitter for good. Word is that the spacecraft will remain in hibernation, awaiting a possible wake-up call in the future.

Massive Meteorite Found in China

Xinjiang metoerite
Chinese researchers measure a huge iron meteorite found in a remote mountainous region in July 2011. The oblong metallic object has an estimated mass of 25 tons or more.

As the meteorite specialist for the Beijing Planetarium, Baolin Zhang gets all kinds of unusual reports — like the dramatic (but ultimately specious) tale of a peasant woman who recently found a blue-ice "meteorite" in her yard.


Map of China's Xinjiang region
Although the exact location of the newly found meteorite has not been announced, its general location is the mountainous border region of China's Xinjiang Uyghur province.

But credible reports of a massive, oddly shaped and colored stone in the remote Altai Mountains of Xinjiang Uygur province (in northwest China) got his attention. So earlier this month he assembled a small team to check it out firsthand. The trek was cold and arduous, involving a rented jeep, borrowed horses, and even a camel to cross rugged terrain and rivers still swollen with snowmelt.

On the afternoon of July 16th, after reaching a mountainous crest 9,500 feet (2,900 m) up Zhang and his team finally spotted their objective: a large dark-brown stone jutting from the ground. It took only moments for him to realize what they'd found. "This is a huge iron meteorite," he exulted as cameras recorded the scene.

Based on the size of the oblong portion above ground, 7.5 feet (2.3 m) long and about half as wide, Zhang thinks its mass is roughly 25 tons — and it could perhaps top 30 tons. Such an enormous find would rank as one of the largest meteorites known, perhaps even surpassing China's current record-holder, the 28-ton Armanty iron, found in the same region in 1898.

Apparently the big stone's existence has been well known among locals for decades. A few scrawls of graffiti have been cut into the exterior, which also bears "saw marks" that expose the interior. As Zhang reports, "The surface was shiny silver, and I can clearly see exposed not only the iron-nickel composition but also the unique grid lines," called a Widmanstätten pattern, that are common among iron meteorites.

Interestingly, the meteorite is wedged beneath an even larger granite slab, and apparently both were dragged to their current locations long ago by glaciers. It's not yet clear when or how the massive Xinjiang stone will be excavated — though this would seem too magnificent a prize to simply leave in place. The Armanty iron is on display outside the Xinjiang Geology and Mineral Museum in Urumqi, the region's capital city.


Graffiti in Xinjiang meteorite
A dozen names, some dating to 1980, are carved into the Xinjiang meteorite.

Conceivably, the Xinjiang and Armanty meteorites are part of the same fall; tests should soon establish whether they are siblings or just happen to be enormous unrelated hunks of meteoritic metal that fell to Earth from interplanetary space.

Dawn Arrives at Vesta

After gently cruising through interplanetary space for over four years, Dawn, NASA’s asteroid probe, will enter orbit around asteroid 4 Vesta at 1 a.m. EDT on July 16th. The arrival marks the beginning of a yearlong study of the second-largest object in the belt of rocky bodies between Mars and Jupiter.

Yesterday NASA released an image of Vesta taken on July 9th, when Dawn was only 26,000 miles (42,000 km) from the asteroid. As chief engineer Marc Rayman noted earlier this month, the spacecraft's destination looks, "wrinkled, ancient, wizened, with a tremendous amount of character that bears witness to some fascinating episodes in the solar system's history."

Launched on September 27, 2007, Dawn carries high-resolution cameras, spectrometers, and other instruments to investigate the true nature of two alien worlds: Vesta and 1 Ceres. After exploring Vesta for a year, Dawn will set sail for Ceres in late 2012. Scientists believe that these two objects, which formed early in the life of the solar system, carry important clues to the formation of the terrestrial planets.

What we know about Vesta is fascinating but incomplete. Images from the Hubble Space Telescope reveal that the object was pummeled early in the history of the solar system. Radioisotope studies of meteorites presumed to be fragments of Vesta show evidence that the object accreted in a span of 5 to 15 million years, possibly in the same manner as the terrestrial planets. Then it got hot enough (due to the decay of radioisotopes in its interior) to melt at least partially . Researchers believe that Vesta might have an iron-nickel core and a metal-rich mantle beneath its rocky exterior.

Beginning tomorrow, dynamicists will use measurements of Vesta's influence on the craft's motion to determine the asteroid’s mass and deduce the distribution of mass in its interior. And as Dawn dips closer and closer to Vesta’s surface to make these measurements, the spacecraft will also send home some of the best pictures of the asteroid.

Initially the craft will hover about 9,000 miles from the surface. Then, in mid-August, Dawn’s ion propulsion system will reduce the separation to about 2,800 miles, and then to only 110 miles early next year. The resolution of the pictures will increase from about 500 m per pixel in late July to about 30 m per pixel in 2012.

Dawn’s science team has extended the scope of the mission, and is even planning to look for moons.

Observing Vesta

Fortunately for observers, the Dawn's arrival happens at a time when Vesta is readily visible through binoculars and telescopes. It now shines in Capricornus at magnitude 6.0, almost as bright as it ever gets. The only bad news is that the just-past-full Moon is also in Capricornus, brightening the sky tremendously.

For Northern Hemisphere observers, Vesta climbs to a point fairly low in the southeast by midnight, rising higher in the hours before dawn. You can pinpoint its location using our finder chart. Of course, even the most powerful backyard telescope will only reveal Vesta as a simple point of light — and the spacecraft is far too small to spot across such a huge distance.

Springtime at Mars’ south pole

ESA’s Mars Express celebrates eight years in space with a new view of ice in the southern polar region of Mars. The poles are closely linked to the planet’s climate and constantly change with the seasons. Their study is an important scientific objective of the mission.

	Ulyxis Rupes in context

About two-thirds of the image is covered by part of the southern polar ice cap and other scattered ice deposits, near a feature known as Ulyxis Rupes. The left side of the image is dominated by the polar cap’s ice shield, which is covered by dark dusty material that hides the bright ices beneath.

At this location, further than 1000 km from the south pole itself, the ice is relatively thin: radar data indicate it is only about 500 m thick, whereas near the south pole it can reach more than 3.7 km.

Features near Ulyxis Rupes

However, on the north-facing cliffs the layers of ice and dust are discernible. These form part of the polar, layered deposits. The cliffs are often curved, which could mean that they are shaped by underlying impact craters.

The elevation of this region decreases markedly from south to north, dropping in steps by about 1500 m in total from left to right across the image.

	Elevation of Ulyxis Rupes

Just northward of the ice shield, about halfway across the image, there are large ice deposits that are heavily covered by overlying material blown into long dunes by the prevailing winds in this region. The orientation of the dunes suggests the wind must come predominantly from the northwest.

Ulyxis Rupes in high resolution

With increasing distance from the south pole, ice becomes confined to larger impact craters, such as the one in the top right of the image. These provide the best shelter. The ice itself is slightly offset towards the north because, with the sunlight coming from the north, the southern walls of the crater tend to warm up more, causing the ice to melt.

Ulyxis Rupes is a large cliff and is the only named feature in this image (‘rupes’ is the Latin term for cliff). With a length of 390 km and a height of up to 1 km, it is just visible at the top right of this image where it intrudes on the immediate left of the crater there.

	Ulyxis Rupes in perspective

Puzzling parallel structures in the martian dust can be seen in the bottom right quarter of the image. Although their origin is uncertain, it is possible that they are the result of underlying ice deposits, permanently frozen because they are protected by overlying dust and rocks.

The image was taken in January 2011, during the southern spring on Mars. At the moment it is summer there, but when the southern winter begins in March 2012, the temperatures will drop again and more ice will accumulate. Mars Express will be waiting.

Ulyxis Rupes in 3D

June 15th's Deep, Eastern Lunar Eclipse

During June 15th's total lunar eclipse, the lunar disk will spend
100 minutes completely inside Earth's umbral shadow.

We're in the midst of an interesting eclipse trifecta. Partial solar eclipses occur during the New Moons on June 1st and July 1st, which are sandwiched around a total lunar eclipse during the Full Moon on June 15th.

Unfortunately, none of these are really visible from North America — which is too bad, because this lunar eclipse should be a doozy. Those of you in Europe and elsewhere in the Eastern Hemisphere are in for a real treat.

The Moon will plunge deeply into Earth's shadow, passing almost directly through its center. Consequently, totality lasts a whopping 100 minutes — the longest umbral immersion since July 2000, and nearly 40 minutes longer than the well-observed lunar eclipse last December 21st.

The lunar disk first encroaches into the penumbra, Earth's partial shadow, at 17:25 Universal Time, but don't expect to see any dusky shading along its easternmost edge for at least 30 minutes afterward. The first nibble from Earth's deep umbra comes at 18:23 UT, with the black bite taking a full hour to creep steadily across the disk.

Totality begins at 19:22:30 UT and ends at 21:02:42 UT, with the moment of greatest eclipse at 20:12:37 UT.

The long duration of this event is due in part to Earth being near aphelion in its orbit and the Moon being near perigee in its orbit. Since the lunar disk passes 5.3 arcminutes north of the umbra's center, observers might see the northern limb appear a little brighter than the southern limb.

In any case, this has the makings of a very dark eclipse. Observers rate totality's darkness using a five-point scale developed by French astronomer André Danjon, ranging from L = 0 (nearly invisible) to L = 5 (bright copper-red or orange disk). (Click here to learn more about the Danjon scale and other eclipse tips.)

David Dunham points out that the darkened lunar disk makes it very easy to watch the Moon cover up stars along its path. "Especially good will be the occultation of 4.8-magnitude 51 Ophiuchi," he notes. "Perhaps a naked-eye event, it will be spectacular as seen with binoculars or any small telescope." Click here to get a full listing of stars to be occulted during the eclipse.

The total lunar eclipse on June 15, 2011, favors observers in the Eastern Hemisphere. Click on the map for a larger version.

As the map at right shows, the ringside seats for Wednesday's event will be centered around 50°E in longitude. This favors eastern Africa (near moonrise), Asia, and western Australia (near moonset). Those of you in Europe will see most everything, though the early stages occur before the Moon rises. Only northern Scotland and Scandinavia miss out — but, then again, they were favored for the partial solar eclipse on June 1st.

Those of us stuck in North America won't see any of this eclipse by eyeball, but we'll be able to watch it vicariously thanks to the following live webcasts:

• AstroNation.net from Hessen, Germany
  
  
• Bareket Observatory from Maccabim, Israel
  
  
• ECA (Eclipse Chasers Athaenium) from Delhi, India
  

Later this year, on December 10th, most North Americans will have a chance to see a total lunar eclipse. Let's hope for clear weather!

Do Planets Outnumber Stars ?

An artist's portrayal of a rogue planet drifting alone through interstellar space, lit only by starlight.

Ask an astronomer how many stars populate the Milky Way, and the usual answer will be 200 to 400 billion. It's not that all those suns have actually been counted; instead, it's a statistical guesstimate based on the census in our immediate interstellar surroundings.

But a new study, published in today's issue of Nature, suggests that a complete census of "big bodies" drifting loose in our galaxy might actually total nearly one trillion — because Jupiter-mass "planets" in interstellar space might well outnumber the stars themselves.

The evidence for this sudden glut of planet-mass objects results from a dedicated search by two teams of observers: the Microlensing Observations in Astrophysics (MOA) Collaboration and the Optical Gravitational Lensing Experiment (OGLE) Collaboration.

In 2006-07, the MOA and OGLE teams used telescopes in New Zealand and Chile, respectively, to monitor the brightnesses of 50 million stars located in the huge stellar bulge surrounding the Milky Way's center. Instruments recorded the brightness of each star at least once per hour. After boiling down all that data, the teams found that 474 stars had briefly surged in brightness in a way that indicated gravitational lensing of their light by unseen foreground objects passing nearly front of them. During these incidental syzygies, the gravity of the foreground object bends and concentrates the light from the background star — an event known as microlensing.

Microlensing searches aren't new: they've long been used to search for massive dim or dark objects in the galaxy. But the MOA and OGLE teams found that 10 of these little surges lasted less than two days — too short to be caused by stars but just right for Jupiter-mass objects. Based on these statistics, the teams estimate that big planets must be far more common than believed and in fact must outnumber all the Milky Way's normal stars by about two to one.

Surprisingly, during these 10 brief events there were no corresponding lensing surges to betray the presence of nearby stars. So the observers conclude that these "Jupiters" must either be at least 10 astronomical units from their host stars (at least Saturn's distance from the Sun), or they are orphans drifting freely across interstellar space. They're more likely to be free-floaters, because previous direct-imaging searches found that giant planets rarely exist in very wide orbits.

"The implications of this discovery are profound," notes lensing specialist Joachim Wambsganss (Heidelberg University) in an accompanying Nature perspective.

Theorists are chuckling, "We told you so!" They've argued for years that the galaxy should teem with unbound planets. Some have proposed that objects with masses almost as low as Jupiter's form the way normal stars do, directly from collapsing clouds of gas and dust. Think of these as undersized brown dwarfs. Others point out that the chaos that seems to prevail in many just-formed solar systems must cause many close encounters among planets that yield "winners" (those that remain in orbit) and "losers" (those that get flung out of the system entirely).

Taken at face value, the MOA-OGLE statistics imply that most of the loose planet-mass objects aren't just low-mass stellar wannabes — there are too many of them. Instead, the researchers believe they're finding bodies that have been ejected from unstable planetary families — and, by extension, that planetary systems should be the norm, not the exception, for the Milky Way's hundreds of billions of stars.

This also implies that early chaos in planetary systems is common. Exoplanet researchers had already concluded that this is the case from the large number of explanets that have been left in highly eccentric orbits, which they could not have formed with.

The Four-Planet Dance of 2011

If you can find a spot with a completely unobstructed eastern horizon, you can watch an extraordinary sky show from late April 2011 through the end of May. Every morning just before sunrise, four planets combine to form fascinating and ever-changing patterns. This is the tightest grouping of bright planets that has occurred yet in the 21st century.

If you live in the Southern Hemisphere, you can watch the whole show with your unaided eyes, but you will need binoculars to appreciate it properly from mid-northern latitudes. Go outside 45 minutes before sunrise and scan the eastern horizon until you find a planet. You're sure to spot either Venus or Jupiter first, because these are by far the brightest of the four.

Venus appears at just about the same spot every morning in May — just 2° or 3° above the horizon 45 minutes before sunrise for observers at mid-northern latitudes, and rising 3° higher each 15 minutes after that. If you pay attention to its location, you can probably continue to see it without optical aid long after the Sun rises.

Jupiter is very low at the beginning of May, but it passes Venus on May 11th and ends the month more than 12° above the horizon 45 minutes before sunrise. So by mid-May, you're likely to spot Jupiter before Venus despite the fact that it's less than one-quarter as bright.

Mercury is the 3rd-brightest planet in the grouping, but it's five to ten times fainter than Jupiter, and quite low in the sky. So you're likely to need binoculars to spot it. It tracks Venus's motion, staying a few degrees to the lower left of the brighter planet throughout this period.

Mars is quite faint, just one-hundredth as bright as Venus. It starts May very low in the sky, but catches up with the Venus-Mercury pairing around mid-month.

A thin crescent Moon joins the show from April 29th to May 2nd and again on May 29-31.

See the Eta Aquarid Meteor Shower

Here's the Eta Aquarid's radiant as seen from latitude 30° north
(Houston, Cairo, Delhi, Shanghai) 90 minutes before sunrise. Farther
north, the radiant is even lower when the sky starts to get light. But
Eta Aquarids are occasionally seen as far north as the Mid-Atlantic
States.

The Eta Aquarids might be the best meteor shower that you've never heard of. This shower is caused by flecks of dust released from the nucleus of Halley's Comet. It stays near full strength for five days — longer than any comparably intense shower — and its meteors are bright and plentiful.

So why isn't it better known?

If you live in the Southern Hemisphere, where this is arguably the year's best meteor shower, you've very likely heard of it. But relatively few Eta Aquarids are visible from mid-northern latitudes, where the lion's share of amateur astronomers live. Still, this shower puts on quite a respectable show in the southernmost tier of the United States. And because the meteors are so bright, they're occasionally seen much farther north than that during morning twilight — and even broad daylight.

Conditions are ideal for the Eta Aquarids this year, because the Moon is absent from the sky during the predawn hours. The shower is forecast to peak on the morning of Friday, May 6th, with good activity from the 4th through the 8th.

As the name Eta Aquarids suggests, all of this shower's meteors appear to radiate from a spot near the northeastern corner of the constellation Aquarius. The higher a shower's radiant is in the sky, the more meteors you can see, and you won't see any meteors at all when the radiant is significantly below the horizon.

In the case of the Eta Aquarids, the radiant doesn't rise until long after midnight, and it reaches its highest in the sky well after sunrise. So the best time to watch for meteors is anywhere from one to two hours before sunrise. Earlier than that, the radiant is too low — any later, the sky is too bright.

So-So Prospects for Comet Elenin

Comet Elenin on March 14, 2011

Last December, comet-lovers got a bit of an adrenaline rush when they learned that a new object, Comet Elenin (C/2010 X1), might reach naked-eye brightness a week or so after it reaches perihelion on September 10th.

It's still early in the game, but reports from visual and photographic observers over the past few weeks have tempered expectations somewhat.

Those looking for Comet Elenin by eye have found it elusive. Only two observers — Jakub Koukal, using a 9½-inch (24-cm) reflector in the Czech Republic; and Juan José González Suárez, using an 8-inch Schmidt-Cassegrain in Spain — feel certain they glimpsed it by eye in early April. But it was a no-show for comet-hunter Alan Hale, who had a larger telescope at a pitch-black site 7200 feet (2200 m) up.

Another consideration is that the visual estimates (magnitude 15.3 and 14.9, respectively) are at odds with CCD observations suggesting something no brighter than magnitude 16. Such differences would make sense if Comet Elenin were somewhat diffuse, but everyone agrees that it's strongly condensed and almost stellar in appearance.

Koukal and González are veteran observers who carefully checked their sightings against faint nearby stars. Even so, former S&T columnist John Bortle, who's watched comets come and go for more than 50 years, is skeptical of visual sightings made at the hairy edge of a telescope's capability. "I can cite many instances of 'positive' observations turning out to be spurious, even when made by experienced observers," he notes.

For now, who can or can't see it doesn't matter much, as the interloper is still heading inward and won't get seriously worked up for several months. But the comet cognoscenti have already started calling it "intrinsically faint," and it's becoming clear that hopes for a nice eyeball-easy showing have dimmed considerably.

Best guesstimates now suggest that Comet Elenin's total brightness might peak near magnitude 6 in mid-September — a nice binocular object — presuming that it survives its dash through perihelion just 45 million miles (0.48 astronomical unit) from the Sun.

Meanwhile, you have my permission to ignore or refute any of the wacky postings about the supposed danger posed by Comet Elenin. All this nonsense seems to have started back in January, when edge-of-reality blogger Laura Knight Jadczyk made provocative warnings — all based on information from a member of her research team who's "an astronomer at a large observatory". (Yea, right.) It's not even worth giving you a link to her ramblings.

MESSENGER: Mercury Surface,

Click on image to enlarge.

Early this morning, at 5:20 am EDT, MESSENGER captured this historic image of Mercury. This image is the first ever obtained from a spacecraft in orbit about the Solar System's innermost planet. Over the subsequent six hours, MESSENGER acquired an additional 363 images before downlinking some of the data to Earth. The MESSENGER team is currently looking over the newly returned data, which are still continuing to come down. Tomorrow, March 30, at 2 pm EDT, attend the NASA media telecon to view more images from MESSENGER's first look at Mercury from orbit.

The dominant rayed crater in the upper portion of the image is Debussy. The smaller crater Matabei with its unusual dark rays is visible to the west of Debussy. The bottom portion of this image is near Mercury's south pole and includes a region of Mercury's surface not previously seen by spacecraft. Compare this image to the planned image footprint to see the region of newly imaged terrain, south of Debussy. Over the next three days, MESSENGER will acquire 1185 additional images in support of MDIS commissioning-phase activities. The year-long primary science phase of the mission will begin on April 4, and the orbital observation plan calls for MDIS to acquire more than 75,000 images in support of MESSENGER's science goals.

On March 17, 2011 (March 18, 2011, UTC), MESSENGER became the first spacecraft to orbit the planet Mercury. The mission is currently in its commissioning phase, during which spacecraft and instrument performance are verified through a series of specially designed checkout activities. In the course of the one-year primary mission, the spacecraft's seven scientific instruments and radio science investigation will unravel the history and evolution of the Solar System's innermost planet. Visit the Why Mercury? section of this website to learn more about the science questions that the MESSENGER mission has set out to answer.

Messenger: Mercury's New Moon

Some 96 million miles away, the Messenger spacecraft had fired its braking rocket and thrusters for nearly 15 minutes, a long burn begun at 8:45 p.m. EDT that slowed the hurtling craft by 1,929 miles per hour (0.86 km per second). A few anxious minutes later, an interplanetary communiqué arrived at the mission's control center at Johns Hopkins University's Applied Physics Laboratory. The burn had gone flawlessly, placing the spacecraft safely in a looping 12-hour polar orbit.

It's taken more than 6½ years for Messenger to reach its new home away from home. Along the way was one flyby of Earth, two of Venus, and three of Mercury itself — all part of an intricate, carefully designed interplanetary billiard shot designed to deliver the biggest payload possible on a Delta II 7925 launch vehicle. As planetary scientist Mike Brown commented via Twitter, "Clearly, physics works really really well."

NASA's science team packed eight instruments onto Messenger (a contraction for Mercury Surface, Space Environment, Geochemistry and Ranging).

But don't expect a glut of images to start flowing into the computers at mission control right away. For the moment, however, the ball is in the engineers' court to ensure that onboard systems are working properly. Most instruments won't be turned on until March 23rd, the cameras five days later.

The planned year-long study of all things Mercurian will begin in earnest on April 4th, along with gravity studies derived from the craft's motion and radio transmissions. According to project scientist Sean Solomon (Carnegie Institution of Washington), the first full-up release of results won't occur until May 10th.

In the meantime, check the mission website for progress reports and to see images of Mercury obtained during the trio of earlier flybys. You'll also get a kick touring the planet at close range by downloading this Mercury KML file for use with Google Earth.

Finally, don't miss the chance to spot the innermost planet making its best evening appearance of the year.

The scars of impacts on Mars

Elongated crater on Mars

 
4 March 2011
ESA’s Mars Express has returned new images of an elongated impact crater in the southern hemisphere of Mars. Located just south of the Huygens basin, it could have been carved out by a train of projectiles striking the planet at a shallow angle.
 
 

	Elongated crater, south of Huygens crater

The large Huygens basin (not visible in the main image but seen in the wider contextual image) is about 450 km in diameter and lies in the heavily cratered southern highlands. In this area there are many impact scars but none perhaps are more intriguing than the ‘elongated craters’.

One of these craters is seen in this new image, which covers an area of 133 x 53 km at 21°S / 55°E. The scene was captured on 4 August 2010 and the smallest objects distinguishable by the camera are about 15 m across.
 
 

Features in the elongated crater

This unnamed elongated crater sits just to the south of the much larger Huygens basin. It is about 78 km in length, opens from just under 10 km wide at one end to 25 km at the other, and reaches a depth of 2 km.
 
 

	Elevation of the elongated crater

Impact craters are generally round because the projectiles that create them push into the ground before the shockwave of the impact can explode outwards. So why is this one elongated?

The clue comes from the surrounding blanket of material, thrown out in the initial impact. This ‘ejecta blanket’ is shaped like a butterfly’s wings, with two distinct lobes. This hints that two projectiles, possibly halves of a once-intact body, slammed into the surface here.
 
 

Elongated crater in high resolution

 
In the crater itself, there are three deeper areas that could be evidence for more than two projectiles. In addition, a second elongated crater lies to the north-northwest. It can be seen in the wider contextual image and is in line with the one seen here, reinforcing the notion that these structures were the result of a train of projectiles.
 
 

Perspective view of elongated crater

 
In the early 1980s, scientists proposed that elongated impact craters were formed by incoming chains of orbital debris following trajectories that decayed with time. As the debris spiralled downwards, it eventually struck the planet at shallow angles, gouging out the elongated craters.
 
 

Perspective view of elongated crater

 
This particular ejecta blanket contains many smaller craters, indicating that the original formed a relatively long time ago and then itself become a target.

In addition, there are several small channels on the blanket, suggesting that the strike took place into a surface rich in volatiles, perhaps even water, that were melted by the heat of impact and flowed away.
 
 

Perspective view of elongated crater

Below the eastern crater rim are two well-formed and relatively deep craters. They have punched through the ejecta blanket and so must have appeared after the formation of the large crater. Despite their sizes of 4 km and 5 km, these smaller craters show no indication of the presence of water.
 
 

	Perspective view of elongated crater

To the north there is another crater that must be older because the butterfly-ejecta blanket has partially flowed into it. Several landslides have modified the steep crater rim. This can be most clearly seen on the two smaller craters on the rim, which are only partially preserved, parts of them having fallen away.
 
 

Elongated crater in 3D

The formation of these elongated features is not over: the martian moon Phobos will plough into the planet in a few tens of millions of years, breaking up in the process, and likely creating new chains across the surface.