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New Horizons Hiccups, Goes into Safe Mode

Sun, 07/05/2015 - 08:45

Just 10 days before its history-making flyby of Pluto and its moons, NASA's New Horizons spacecraft briefly lost communication with Earth.

While most of us were celebrating Independence Day, scientists and flight engineers for NASA's New Horizons mission were hunkered down at the control center at Johns Hopkins University's Applied Physics Laboratory in Laurel, Maryland. They weren't celebrating much, because yesterday at 1:54 p.m. Eastern Daylight Time contact was lost with the spacecraft for about 1½ hours, until communication was restored at 3:15 p.m. EDT.

Trajectory of New Horizons past Pluto and its moons

Timing will be everything when New Horizons dashes past Pluto and its moons on July 14, 2015. Key events are shown in Universal Time.
NASA / JHU-APL

According to a NASA-issued announcement, "During that time the autonomous autopilot on board the spacecraft recognized a problem and — as it’s programmed to do in such a situation — switched from the main to the backup computer. The autopilot placed the spacecraft in 'safe mode,' and commanded the backup computer to reinitiate communication with Earth. New Horizons then began to transmit telemetry to help engineers diagnose the problem."

While the team figures out what went wrong (an "anomaly review board" is working on that), the spacecraft's planned observations have been suspended. New Horizons is still roughly 6½ million miles (10½ million km) from Pluto, and it's been taking a series of long-range navigation images to fine-tune the trajectory and timing during its dash through the Pluto system on July 14th. It's also been recording the state of the solar wind along its trajectory.

Pluto-Charon color rotation movie June 23-29

New Horizons scientists combined images taken with the craft's color and black-and-white cameras from June 23rd to 29th to create this time-lapse view of Charon circling around Pluto. During this interval the distance to these targets decreased from 15 million to 11 million miles (24 million to 18 million km). Both bodies were already showing many surface details.
NASA / JHU-APL / SwRI

Right now the spacecraft is nearly 3 billion miles (31.82 astronomical units) from Earth, so it takes 8.8 hours to complete a single, two-way communication. Pending the recommendations of the review board, the team expects to resume normal activities within the next few days. "We may lose a few appetizers off the planned menu," notes mission participant Richard Binzel (MIT), "but right now the focus is on delivering the main course."

That "main course" is an intense, tightly scripted sequence of observations centered on July 14th at 11:49:58 Universal Time (7:49:58 a.m. EDT), when the spacecraft will come within 7,800 miles (12,500 km) of Pluto's surface as it zips past at 8.6 miles (13.8 km) per second. Because of its great distance from Earth, the spacecraft will transmit its results at a low data rate, and mission scientists won't have all of the flyby data in hand until mid-November.

Emily Lakdawalla's 8-page preview article in the July issue provides all the info you'll need to prepare for New Horizons' historic flyby.

The post New Horizons Hiccups, Goes into Safe Mode appeared first on Sky & Telescope.

Categories: Astronomy Headlines

SOFIA Dashes into Pluto’s Shadow

Fri, 07/03/2015 - 14:18
The following post was written by Contributing Editor Govert Schilling, who lives in Amersfoort, The Netherlands, but frequently travels the world in search of great astronomy stories.

Despite an 11th-hour scramble due to an unexpected in predictions, NASA's flying observatory was in the right place at the right time on June 29th as distant Pluto briefly covered a 12th-magnitude star.

If you thought scientific data taking is a slow, boring process, you've never been on board SOFIA, the Stratospheric Observatory For Infrared Astronomy. On June 29th, SOFIA observed a stellar occultation by Pluto, and I was aboard to watch. It was the most exciting flight I've ever experienced.

SOFIA in Christchurch, New Zealand

NASA's Stratospheric Observatory for Infrared Astronomy on the runway in Christchurch, new Zealand.
Govert Schilling

As many S&T.com readers know, SOFIA is basically an old Boeing 747-SP airliner that NASA bought and transformed into a flying infrared observatory. A German-built 2.5-meter telescope peers out into space through a large rectangular opening in the plane's fuselage. From the plane's typical cruising altitude (near 40,000 feet or 12 km), high above the clouds and most of the atmosphere's infrared-absorbing water vapor, the telescope can make unique infrared observations.

Path of the occultation by Pluto on June 29, 2015

A Pluto's-eye view of the path (shown in blue) cast during its occultation of a 12th-magnitude star on June 29, 2015.
MIT Planetary Astronomy Lab

Mobility can also play a role. SOFIA's home base is Palmdale, California, but this summer, it's been deployed to Christchurch, New Zealand, to observe objects in the southern sky for six weeks. Not coincidentally, Pluto's predicted cover-up of a 12th-magnitude star in Sagittarius would be observable from a broad zone that passed over part of Antarctica, the southern Indian Ocean, southeast Australia, and New Zealand.

"It's a very special occasion," says SOFIA's chief science advisor Eric Becklin (University of California at Los Angeles), who's one of the 30+ people on board. Now 75, Becklin is a pioneer of infrared astronomy and served as SOFIA's chief scientist during much of its development. "Everything has to go just right — there's no second chance. I wouldn't want to miss this opportunity."

The pressurized cabin of the aircraft has been transformed into a science lab. Nothing looks familiar. Although the telescope itself is hidden behind a big circular bulkhead, its attached science instruments jut far into the rear of the cabin and are an eye-catching sight. Tonight, two instruments will be used: FLITECAM (First-Light Infrared Test Camera) and HIPO (High-speed Imaging Photometer for Occultations). Since they can work in tandem, the combination has been christened FLIPO. A flying hippo — a plush toy outfitted with wings — serves as a mascot. They'll record the star's changing brightness as it disappears behind Pluto's disk and reappears about 90 seconds later. From these readings, scientists expect to learn more about the pressure and temperature profile of Pluto's extremely tenuous atmosphere and about the presence of particle hazes that might indicate cryovolcanic activity.

It seems like a simple task: fly through Pluto's "shadow, measure the light of a relatively bright star for a while, and then do some analysis. But in fact, it's a complicated endeavor. As Becklin said, everything has to work perfectly the first time — you can't ask Pluto to pass in front of the star again because a technical hiccup ruined your data. Moreover, SOFIA has to be in the right place at the right time, which turns out to be less straightforward than you might think.

Two "Uh-Oh" Moments SOFIA flight planners

Flight planners aboard SOFIA scramble to revise the plane's route after last-minute predictions showed a sizable shift in the planned intercept point.
Govert Schilling

We take off from Christchurch uneventfully at 10:09 p.m. local time and start heading toward our target point. The centerline's exact location has been calculated by colleagues at the Massachusetts Institute of Technology (MIT) in Cambridge, based on astrometry of Pluto carried out that same night by telescopes in Arizona and Chile. But 2 hours after takeoff, MIT sends a final set of coordinates — the centerline has shifted 225 km farther north than expected. With the plane already heading to the wrong location and the occultation just a few hours away, mission managers scramble to work up a revised flight plan.

Later, instrument scientist Jeffrey Van Cleve tells me that the flight plan actually needed revising a second time. "I checked the shifted central line with the set of MIT coordinates, and they didn't match," he says. "We hadn't taken Earth's curvature into account." Pluto's shadow was indeed 225 km farther north, but because the shadow doesn't hit Earth's surface perpendicularly, the offset was really 332 km. "It's a much bigger correction than we had expected," fretted HIPO scientist Ted Dunham (Lowell Observatory). And there was no time to double-check the new numbers. "We have to trust the new observations," he tells me. "We have no choice."

During the occultation Pluto casts a stellar shadow as wide as its diameter, about 2,300 km (1,430 miles). Observations from anywhere in the path would be scientifically useful, but everyone on board is very keen to hit the shadow path's centerline. When a distant star is exactly behind the center of a planet, a ring of refracted starlight produces a brief brightening in the light curve — a central flash. "We've learned what central-flash observations can tell you about atmospheric hazes," Dunham explains. Little wonder he's worried about whether SOFIA's new path will take his instrument to the right spot.

The computer monitor at the console where I'm seated shows 14th-magnitude Pluto and the occultation star (which is five times brighter) as seen through the telescope's guiding camera. As the hours pass, I can see Pluto closing in on the star at a rate of just under 1 arcsecond every 15 minutes. (Incidentally, this slow-motion merging has little to do with Pluto's inexorably slow orbital motion. Instead, it's almost completely a parallax effect due to Earth's orbital motion around the Sun.)

SOFIA's flight path on June 29, 2015

After SOFIA departed Christchurch, New Zealand, for its encounter with Pluto's shadow, a late change to the event's predicted track forced revisions to the flight plan while en route — resulting in convoluted course changes during the 8½-hour-long flight.
FlightAware.com

Just under 7 hours into the flight, the magic moment finally occurs. We're over the eastern shore of New Zealand's South Island at an altitude of 11.9 km (39,100 feet), flying south-to-north at 986 km per hour; Pluto's shadow is racing across Earth's surface from east to west at almost 90,000 km per hour — a hundred times faster. At 16:53 Universal Time (4:53 a.m. on 30 June New Zealand time), SOFIA intercepts the shadow for just 90 seconds. Everyone around me cheers as they see the star fade, in exact agreement with those last-minute predictions.

SOFIA scientists after Pluto's occultation

Happy researchers, including former SOFIA chief scientist Eric Becklin (upper left), react to their successful observation of a star's occultation by Pluto on June 29, 2015.
NASA / SOFIA / Carla Thomas

The brightness recording from the guiding camera is available almost instantly, and the star's light curve shows a very pronounced central flash. The plot also shows two very narrow dips: a shallower one some 2 minutes before the start of the occultation and a deeper one about a half minute after the event's end. "I don't think they are real," says Dunham, who's been chasing Pluto's shadow for three decades. "They're probably due to some instrumental effect. On the other hand: they could indicate that Pluto is surrounded by partial rings, just like Neptune." When the FLITECAM data become available, his intuition proves to be correct: the two dips don't show up. If Pluto has a ring system, it must be incredibly thin.

All this scientific drama has added significance, given that NASA's New Horizons spacecraft will fly past Pluto at close range on July 14th. Its ultraviolet imaging spectrometer, called Alice, will record sunlight streaming through Pluto's thin atmosphere as the Sun sets behind the dwarf planet as seen from the spacecraft. Meanwhile, tracking antennas on Earth will measure how New Horizons' radio signal changes as the spacecraft ducks behind Pluto and reappears a short time later.

"The radio occultation samples the lowest parts of the atmosphere, and the UV occultation samples the very highest parts," explains MIT scientist Michael Person, a member of the HIPO team. "Measurements at visible and near-infrared wavelengths fill in the gap. Together, these observations will provide us with a comprehensive view of the structure of Pluto's atmosphere."

"We're very lucky that this occultation of a relatively bright star happens so close to the New Horizons encounter," says Dunham. Pluto's atmosphere has varied over the years, and given that Pluto is moving farther from the Sun in its orbit, there's been concern that those wisps of gas would have frozen completely and collapsed onto the surface by the time New Horizons arrived. Fortunately, Person says, the light curve clearly shows that "the atmosphere is still there."

SOFIA finally touches down at Christchurch International Airport at 6:33 a.m., nearly 8½ hours after it left. I see exhausted but relieved faces all around me. Becklin — a big smile on his face — shakes hands with scientists, telescope operators, mission managers, and flight engineers. Someone collects signatures and farewell messages on a plot of the much-debated flight plan, as a present for telescope operator Karen West, who had just made her last SOFIA flight. It was certainly a memorable one.

The post SOFIA Dashes into Pluto’s Shadow appeared first on Sky & Telescope.

Categories: Astronomy Headlines

This Week’s Sky at a Glance, July 3 – 11

Fri, 07/03/2015 - 02:05

Friday, July 3

Venus and Jupiter, low in the west at dusk, are now 1.5° apart: still strikingly close but widening every day.

Venus and Jupiter still make quite a sight. The blue 10° scale is about the size of your fist at arm's length.

Venus and Jupiter still make quite a sight. The blue 10° scale is about the size of your fist at arm's length.

Venus and Jupiter move wider apart (and lower) every day, as Regulus draws closer to them.

Venus and Jupiter move wider apart (and lower) every day, as Regulus draws closer to them.

Saturday, July 4

Out to watch fireworks? As dusk settles in, do a bit of astronomy outreach. Point out to people Venus and Jupiter still forming a striking pair low in the west (1.9° apart now). And fainter Regulus to their upper left.

Also the two brightest stars of summer: Arcturus, 37 light-years away, very high toward the southwest, and Vega, 25 light-years distant, nearly as high in the east. It's amazing the number of people who don't quite grasp you can see such things for yourself, with your own eyes.

Sunday, July 5

Vega is the brightest star high in the east. Barely to its lower left after dark is one of the best-known multiple stars in the sky: 4th-magnitude Epsilon (ε) Lyrae, the Double-Double. It forms one corner of a roughly equilateral triangle with Vega and Zeta (ζ) Lyrae. The triangle is less than 2° on a side, hardly the width of your thumb at arm's length. Binoculars easily resolve Epsilon, and a 4-inch telescope at 100× or more should resolve each of Epsilon's wide components into a tight pair.

Zeta Lyrae is also a double star for binoculars; much tougher, but easily split with any telescope. Delta (δ) Lyrae, a similar distance below Zeta, is much wider and easier to separate.

Monday, July 6

After nightfall, Altair shines in the east-southeast. It's the second-brightest star on the eastern side of the sky, after Vega high to its upper left. Above Altair by a finger-width at arm's length is little orange Tarazed. And a bit more than a fist-width to Altair's lower left is Delphinus, the Dolphin, leaping leftward.

Tuesday, July 7

The Big Dipper, high in the northwest after dark, is turning around to "scoop up water" through the evenings of summer and early fall.

Wednesday, July 8

Last-quarter Moon (exact at 4:24 p.m. EDT). It rises around 1 a.m. tonight, in Pisces far below the Great Square of Pegasus (and close below dim Uranus!).

Have you ever explored the Small Sagittarius Star Cloud, between M23 and M25? On these moonless late evenings, work through this area with Sue French's Deep-Sky Wonders story, chart, and photos in the July Sky & Telescope, page 56.

And stretch your observing skills with Rod Mollise's Deep-Sky Summer tour, page 62.

Got a big scope? Explore the galaxy cluster above the back of Draco with Ken Hewitt-White's Going Deep, page 59.

Thursday, July 9

This month two spacecraft are imaging the two brightest dwarf planets, Ceres and Pluto, up close. Find Ceres, magnitude 7.7, just below Capricornus using the chart in the July Sky & Telescope, page 50. Pluto is a daunting 14th magnitude, but its chart starts on page 52. Coincidentally, both are in the late-night southern sky about 25° apart.

The waning crescent Moon passes the "fall and winter" stars of Taurus in the dawns of July. Look early!

The waning crescent Moon passes the "fall and winter" stars of Taurus in the dawns of July. Look early! The Moon is plotted for its position at the time of twilight near the middle of North America: at latitude 40° north, longitude 90° west.

Friday, July 10

If you have a dark enough sky, the Milky Way now forms a magnificent arch high across the whole eastern sky after nightfall is complete. It runs all the way from below Cassiopeia in the north-northeast, up and across Cygnus and the Summer Triangle (crowned by bright Vega) in the east, and down past the spout of the Sagittarius Teapot in the south.

Saturday, July 11

As dawn brightens on Sunday morning the 12th, look east for the thin waning crescent Moon not far from Aldebaran, as shown here. Observers in Japan, northeast Siberia, and the far north of North America can see the Moon occult (cover) the orange-giant star. Far to Aldebaran's lower left (out of the scene here) is brighter Mercury.

__________________________

Want to become a better astronomer? Learn your way around the constellations. They're the key to locating everything fainter and deeper to hunt with binoculars or a telescope.

This is an outdoor nature hobby. For an easy-to-use constellation guide covering the whole evening sky, use the big monthly map in the center of each issue of Sky & Telescope, the essential guide to astronomy.

Once you get a telescope, to put it to good use you'll need a detailed, large-scale sky atlas (set of charts). The standards are the little Pocket Sky Atlas, which shows stars to magnitude 7.6; the larger and deeper Sky Atlas 2000.0 (stars to magnitude 8.5); and once you know your way around, the even larger Uranometria 2000.0 (stars to magnitude 9.75). And read how to use sky charts with a telescope.

You'll also want a good deep-sky guidebook, such as Sue French's Deep-Sky Wonders collection (which includes its own charts), Sky Atlas 2000.0 Companion by Strong and Sinnott, the bigger Night Sky Observer's Guide by Kepple and Sanner, or the beloved if dated Burnham's Celestial Handbook.

Can a computerized telescope replace charts? Not for beginners, I don't think, and not on mounts and tripods that are less than top-quality mechanically (meaning heavy and expensive). As Terence Dickinson and Alan Dyer say in their Backyard Astronomer's Guide, "A full appreciation of the universe cannot come without developing the skills to find things in the sky and understanding how the sky works. This knowledge comes only by spending time under the stars with star maps in hand."

This Week's Planet Roundup

Mercury is low in the glow of dawn, brightening from magnitude –0.4 to –1.0 this week. Look for it 30 or 40 minutes before sunrise just above the east-northeast horizon, very far down to the lower right of Capella. Binoculars help, especially as dawn grows bright. (Don't confuse Mercury with 1st-magnitude Aldebaran far to its upper right.)

On June 30th, Damian Peach imaged Venus at high resolution in the near-infrared shortly before sunset. "No details were seen."

Shortly before sunset on June 30th, Damian Peach imaged Venus at high resolution in the near-infrared. "No details were seen."

Venus and Jupiter are the two bright "stars" low in the west at dusk. They're moving apart after their June 30th conjunction and are getting lower every week. But they still make an impressive pair, shining at an magnitude –4.7 and –1.8, respectively. Jupiter is the one on the right. They're separated by 1.5° on July 3rd and 4.1° by the 10th. Look for fainter Regulus to their upper left.

Mars is hidden deep in the glare of sunrise.

Saturn (magnitude +0.3, in Libra upper right of the head of Scorpius) is highest in the south at dusk. Lower left of Saturn by 13° twinkles fiery orange Antares, not quite as bright. Delta Scorpii is the star roughly midway between them.

Uranus (magnitude +5.8, in Pisces) and Neptune (magnitude +7.9, in Aquarius) are in the southeast and south, respectively, just before dawn begins to brighten. Finder charts.

Jupiter map by Peach, March 2015

Damian Peach assembled this complete map of Jupiter from images he took in March 2015. Click here for full-size version. North is up. Among other notable features are Red Spot Junior (Oval BA) to the lower right of the Great Red Spot, eleven white ovals all at the same latitude just a bit farther south in the South Temperate Belt, and the strikingly dark red, but transient, barge in the broken North Temperate Belt. Here's a rotating Jupiter globe that Peach made from this map.

__________________________

All descriptions that relate to your horizon — including the words up, down, right, and left — are written for the world's mid-northern latitudes. Descriptions that also depend on longitude (mainly Moon positions) are for North America.

Eastern Daylight Time (EDT) is Universal Time (UT, UTC, or GMT) minus 4 hours.

__________________________

“This adventure is made possible by generations of searchers strictly adhering to a simple set of rules. Test ideas by experiments and observations. Build on those ideas that pass the test. Reject the ones that fail. Follow the evidence wherever it leads, and question everything. Accept these terms, and the cosmos is yours.”

— Neil deGrasse Tyson, 2014

 

The post This Week’s Sky at a Glance, July 3 – 11 appeared first on Sky & Telescope.

Categories: Astronomy Headlines

Venus and Jupiter

Thu, 07/02/2015 - 09:02

The post Venus and Jupiter appeared first on Sky & Telescope.

Categories: Astronomy Headlines

Photos of the Venus-Jupiter Conjunction

Wed, 07/01/2015 - 11:26
Rick Fienberg shot last night’s epic planet pairing with an 80-mm f/6 Explore Scientific refractor and a Canon Rebel T3i camera. This image is a composite of a short exposure (1/160s, ISO 400) that showed Venus and Jupiter, and a second, longer exposure (0.8s, ISO 400) that brought out Jupiter’s moons (from left to right: Ganymede, Io and Europa merged into one image, and Callisto. The field of view is cropped to two-thirds of a degree.

Rick Fienberg shot last night’s epic planet pairing with an 80-mm f/6 Explore Scientific refractor and a Canon Rebel T3i camera. This image is a composite of a short exposure (1/160s, ISO 400) that showed Venus and Jupiter, and a second, longer exposure (0.8s, ISO 400) that brought out Jupiter’s moons (from left to right: Ganymede, Io and Europa merged into one image, and Callisto). The field of view is cropped to two-thirds of a degree.

For the last few weeks, countless numbers of the world’s 7 billion people watched the western evening sky as the two brightest planets, Venus and Jupiter, edged closer and closer to one another. Last night, June 30th, they reached their least separation: 0.3° apart (at the time of twilight for the Americas). But the story is far from over. Tonight they’ll still be only about 0.6° apart for the Americas; tomorrow night 1.0°.  But they are saying goodbye and will journey apart as they sink deeper into the afterglow of sunset throughout July.

We’ve been lavished with lovely photos from our readers and thought we’d share a few below. Check out our online Photo Gallery to see them all, and submit your own!

Enjoy.

2015-06-22_558852dc41bc7_Venus_Moon_Jupiter.png

Jupiter, Venus, and the waxing crescent Moon on June 19th, taken by Mario_RSC in Toledo, Spain, with an Olympus SP520.

2015-06-22_558780b04397f_sunsetcelestialshow_jeffdai.jpg

Sunset celestial show by Jeff Dai, taken in Yamdrok Lake, Tibet, on June 21st with a Canon 6D Lens: Nikkor 14-24mm f/2.8G.g.

2015-06-30_5592b02b50356_4VenusJupiterSPeter_Campidoglio_29june2015b_s.jpg

Venus and Jupiter close to conjunction shine above Saint Peter’s in Rome, by Gianluca Masi on June 29th. Taken with a Canon 7D mark II and a 17-55mm lens.

2015-06-30_5592a817d6d3c_1-DSC07107.jpg

Venus and Jupiter Close Conjunction at Dusk by Adam Hazique Zakwan Luhat, taken in Belaga, Sarawak, Malaysia, on June 30th, the evening of the closest pairing. The photographer used a Sony NEX-5 camera with 18-50 mm zoom lens, and a Celestron 70 mm f/5.6 telescope for the close up.

2015-07-01_559353b95ecc4_venus_jupiter_june_30_2015_ST.jpg

Venus and Jupiter at Twilight by Frankie Lucena, in Cabo Rojo, Puerto Rico, on June 30th. Taken with an Olympus 3000 mounted on a tripod.

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Venus-Jupiter by sel3ulba@yahoo.com, taken with a 150 mm Reflective Newtonian, Canon 60D, in Urmia, Iran, on June 30th.

The post Photos of the Venus-Jupiter Conjunction appeared first on Sky & Telescope.

Categories: Astronomy Headlines

Rosetta Spots Sinkholes on Comet

Wed, 07/01/2015 - 10:13

The spacecraft orbiting Comet 67P/Churyumov-Gerasimenko has found 18 holes in the nucleus's surface.

Being a comet is the pits. Sure, there’s the fame — if you’re bright enough — but being a cold, porous pile of rubble has its disadvantages.

Take, for instance, the lack of stability. Comet 67P/Churyumov-Gerasimenko is a denizen of the Kuiper Belt, the outer frozen reaches of our solar system. But it’s not left to mind its own business: it’s a Jupiter-family comet, meaning the king of the planets exerts influence on its orbit. Jupiter ungraciously yanked Comet 67P out of its old orbit in 1959, when a close encounter between the two bodies moved the comet’s closest approach to the Sun twice as near as it was before, to only 1.2 times the Earth-Sun distance.

sinkhole inforgraphic

Rosetta mission scientists have identified 18 pits on Comet 67P that look like sinkholes. But the smallest of the sinkholes is 50 meters wide, so don't pack the golf clubs just yet. Click here for a larger version of the infographic.
Credit: ESA / Rosetta / MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA; graphic from J.-B. Vincent et al. (Nature 2015)

The closer perihelion will expose the comet’s nucleus to far more solar radiation than before. Sunlight heats the nucleus, evaporating its ices and creating the gigantic, diffuse coma and tails that are a comet’s trademark.

ESA’s Rosetta spacecraft has been orbiting Comet 67P since August 2014, gathering amazing observations of the funny-looking nucleus, which is shaped rather like a dog’s head (or maybe a Star Trek phaser, depending on how nerdy you are) and has a surface that looks like it’s been inconsistently smeared with putty.

As part of that observing campaign, scientists with the mission have now found 18 pits in the nucleus’s surface. They’re not the first holes seen on comet nuclei, but they are the first that look like this. The pits tend to cluster together in small groups, and they range from 50 to 310 meters (160 to 1,020 feet) wide. Some pits are cylindrical and deep; others are shallow. The deep ones seem to be “active,” with dusty jets spewing from their walls or floors. The deepest one reaches more than 200 meters below the surface.

The holes can’t be from erosion, because erosion wouldn’t create such nicely circular, relatively narrow holes. Nor can eruptions explain them, because estimates of the amount of stuff exhumed by outbursts from the nucleus (based on what Rosetta has detected) suggest such plumes only contain a thousandth as much material as a typical large, active pit would have expelled.

sinkhole on comet

These close-up images show the same pit, Seth_01, from two different angles. Seth_01 is the most active of the 18 pits discovered on Comet 67P and is 220 meters wide and 185 meters deep. A new study suggests that the pits could be sinkholes, formed by a surface-collapse process similar to the way these features form on Earth.
Vincent et al. / Nature Publishing Group

Instead, Jean-Baptiste Vincent (Max Planck Institute for Solar System Research, Germany) and colleagues think the pits are sinkholes. Somehow, the team writes in the July 2nd Nature, cavities form beneath the comet’s surface. Once the cavity’s ceiling becomes too thin to support its own weight, it will collapse, creating deep, circular pits like those observed. The collapse would expose fresh material in the pit’s sides, which would then sublimate away and thereby fill the pit with debris.

That would explain both why deep, cylindrical pits seem to be active (they’re younger) and why non-active pits more often have had their sides eaten away and their bottoms filled with rubble (they’re older). Younger parts of the surface will thus look ragged, with many pits, whereas older areas will be more eroded and smoother.

Why the cavities form remains unclear. The team offers three possibilities:

  1. Cavities could be innate: Comet 67P’s nucleus is incredibly porous, with 75-85% of its interior just empty space. If the nucleus formed when pieces slowly collided and stuck together (very plausible), the process would have created and preserved cavities.
  2. Stuff underground could sublimate: heat conducted to subsurface ices such as carbon dioxide could cause these volatile ices to sublimate, even though they’re not directly exposed to sunlight, and thereby create a cavity underground.
  3. Water ice could release energy when it changes its molecular structure: water ice can have two structures, crystalline (nice, orderly lattices) and amorphous (more helter-skelter). Amorphous ice generally forms in environments far colder than those on Earth — like the solar system’s distant suburbs. If the ice transitioned from amorphous to crystalline as it's heated (expected), that transition would release energy, sublimating the underground ice and creating a cavity.

We don’t know which of these is correct (maybe more than one?), but each option is reasonable, given what we know about comets.

 

Reference: J.-B. Vincent et al. “Large heterogeneities in comet 67P as revealed by active pits from sinkhole collapse.” Nature. July 2, 2015.

Read more about the Rosetta mission in Sky & Telescope's August 2014 issue.

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Categories: Astronomy Headlines

Fireflies, Green Stars, and Chromatic Duos

Wed, 07/01/2015 - 08:12

July nights bring the green flicker of fireflies and a question —  are there any green stars we can see in our telescopes? The answer may surprise you.

Jove watches over the fireflies

Fireflies flash green light as they zig, streak, and squiggle in this time exposure taken several summers ago. Jupiter glares at upper left.
Bob King

Like many kids growing up in the Midwest, I collected fireflies on warm June and July nights. We'd cup the slow-flying beetles in our hands and transfer them to a jar with holes poked in the lids for air. All the while they tried to escape by crawling back up our hands and arms. Later, I'd set the jar by the bedside and fall asleep to their silent flashes.

I still love fireflies, though I don't catch them anymore. More often they catch me. By surprise. When out observing, I've mistaken them for meteors, Iridium satellites, and once, a supernova. That happened when one crawled into the bottom of my focuser and let off a blast of light while I was in middle of making a variable star estimate. Ka-boom!

Stellar color dash

Stars come in many colors — mostly appearing in pale hues to our eyes — but you won't find a green one in this time exposure of star trails above three of the four auxiliary telescopes of the Very Large Telescope Interferometer (VLTI) at the European Space Observatory’s Paranal Observatory in Chile.
ESO / Babak Tafreshi

The color of light emitted by the luciferin molecule responsible for the firefly's beacon can vary from green to yellow to red. In my neighborhood, most flash green, a color never exhibited by the stars overhead. It's true. We've all seen white, pale blue, yellow, orange, and red stars, but you'll search in vain for true green.

Here's the problem. Stars emit light across the visible spectrum with hotter stars radiating more light at shorter, bluer wavelengths and cooler stars at longer, redder wavelengths. Ruddy Antares in Scorpius has a surface temperature of 3600° Kelvin (6000° F) with a peak emission in the near-infrared end of the spectrum. The Sun is hotter at 5700° K (9800° F) with a blue-green peak. Yet Antares is certainly not "infrared colored" nor is the Sun blue-green.

Light curves by class

The x-axis shows depicts the wavelength or star color, the y-axis relative brightness. Curves show light emitted by several classes of stars. The peak of each curve is the color or wavelength at which a star emits most of its light.
Wikipedia, with additions by the author

Even though a star's peak emission may lie anywhere in the spectrum, it also pours out lots of light at other wavelengths. Blended together, this makes most stars, including the Sun, appear white or, at the very least, weakly color-saturated. Green stars are absent for the same reason. Any star hot enough to emit a significant amount of green light will also radiate blue, red, and all the rest, effectively masking the green. Flooded with every color of the spectrum, we see white. Kermit had it right all along: "It isn't easy being green."

A white Sun of many colors

The Sun may peak in the blue-green part of the spectrum, but because it pours out light of every color the net effect is snowy white.
Bob King

So are the twinkles of fireflies as close as we'll come to seeing green on a July night? Not if you take advantage of color contrast in double stars.

Double stars where the brighter primary star is vividly orange or red will cause its fainter companion to assume the complementary color. Red stars "push" their fainter companions towards green, while yellow stars make us see the secondary as blue. This is all the more interesting when you realize that most of these companions are far too dim to excite the eye's cone cells responsible for color vision. Seeing can be a very subjective thing.

Looking over older double star observations, you'll sometimes come across descriptions of "apple green" or "emerald green". There's even a single star, Libra's Zubeneschmali (Beta Librae), that some observers claim looks green, though to my eye, it appears white.

Just as we find artificial flavors better than no flavor at all, let's embrace the greens of double stars, even if they're nothing but ocular artifice. Below you'll find a few to peruse the next clear night. The "color pushing" just described is most easily seen with a brief look. Stare a while and the hues might just disappear.  Be sure to check out the links, too. Some point you to beautiful digital sketches of double stars by Jeremy Perez. Good luck!

Green treat

Alpha Herculis is one of a few doubles where the fainter companion star looks green.
Bob King

* Izar (Epsilon Boötis) — Mags. 2.7, 5.1, separation 2.9″. Orange primary with a secondary some observers see as pale green.
* Rasalgethi (Alpha Herculis) — Mags. 3.1 and 5.4, sep. 4.9″. Lovely red-orange primary. This one works for me — I see green ... briefly!
* 95 Herculis — Mags. 4.8 and 5.2, sep. 6″. A real gem. Maybe because the magnitudes are so similar, this one presents an obvious color contrast for many, my eye included. A 19th-century amateur astronomer described them as "apple-green and cherry red."
* Graffias (Beta Scorpii) — Mags. 2.9 and 5.1, sep. 13.6”. One of the prettiest doubles of summer. I see two white stars, but some observers report the secondary as being slightly greenish.
* Antares (Alpha Scorpii) — Mags.1.0 and 5.4, sep. 2.5”. Probably the most famous example of color contrast. A tough challenge requiring excellent seeing, but if you can crack it, the companion looks distinctly green nestled next to its brilliant, orange-red primary.
* Zeta Lyrae — Mags. 4.3 and 5.9, sep. 44″. Ruddy primary and watery green secondary.
* Gamma Delphini — Mags. 4.4 and 5.0, sep. 9.1″. Awesome double! I see two yellowish stars but others see one yellow and one pale emerald.

Sky & Telescope has lots of great articles on double stars, as well as an observing guide for observing doubles with small scopes. Eagle Creek Observatory offers an extended list of color contrasted doubles if you'd like to explore further.

The post Fireflies, Green Stars, and Chromatic Duos appeared first on Sky & Telescope.

Categories: Astronomy Headlines

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