Did SOHO’s comet survive? The hunt is on!

Newly-named Comet C/2015 D1 (SOHO) will share the sky with Venus and Mars at dusk. For the next few nights it will be quite low and nearly impossible to see. Its situation improves over time as the comet moves rapidly northward into Pegasus and Andromeda. Tick marks show the comet's position each evening. Stars are shown to magnitude +6.5. Created with Chris Marriott's SkyMap software

Newly-named Comet C/2015 D1 (SOHO) will share the sky with Venus and Mars at dusk. For the next few nights it will be quite low and nearly impossible to see. Its situation improves over time as the comet moves rapidly northward into Pegasus and Andromeda. Tick marks show the comet’s position each evening. Stars are shown to magnitude +6.5. Created with Chris Marriott’s SkyMap software

There’s no telling whether anyone will yet see the “phoenix comet” that survived a close encounter with the Sun last week and continued to keep it together until it exited the coronagraph on the orbiting Solar and Heliospheric Observatory (SOHO).

The last we heard, SOHO-2875, now formally named C/2015 D1 (SOHO), still glowed around magnitude +4 on Feb. 21.

Now the hunt is on to see if enough of the comet remains after its perilous journey to make an appearance in evening twilight. After many position measurements from photos taken by SOHO, an orbit has been calculated that was just published today. Using those numbers, I made a map showing the comet’s nightly progress as it travels up from the western horizon not far from Venus and Mars through the constellations Pisces and Andromeda over the next couple weeks.

Don’t expect to see it tonight. It’s likely no brighter than the naked eye limit (magnitude +6) and swamped in the glow of twilight very low in the western sky. Still, by good fortune, it just happens to hover very close to the star Gamma in Pisces about 5° above the horizon near the end of dusk.Those with crystal clear skies and an open view to the west should give it a try.

The situation soon improves as SOHO’s northward movement carries it higher into a darker sky. It’s uncertain if the comet is a tight, dense ball or a ballooning bag of dust. Until the first observations come in, we won’t know how the comet’s fairing. Also keep in mind that the orbit is preliminary, meaning C/2015 D1 may not exactly follow the path shown. Do sweeps around the positions, moving left and right and up and down from each nightly spot.

I’m as eager as you to see our new visitor.

Moon meets Uranus next / Oddball comet update

Wow! What a fine photo from last night's conjunction. Venus, Mars and waxing crescent Moon over sculpture "Calling The Power" by Larry Bechtel at Vic Thomas Park. Credit: Terry Aldhizer

Wow! What a fine photo from last night’s conjunction. Venus, Mars and the waxing crescent Moon over the sculpture “Calling The Power” by Larry Bechtel at Vic Thomas Park in Roanoke, Virginia. Credit: Terry Aldhizer

Close-up of the moon, Venus, Mars gathering last night. The earthshine on the moon is amazing! Credit: Terry Aldhizer

Close-up of the moon, Venus, Mars gathering last night. The earthshine on the moon is amazing! Credit: Terry Aldhizer

We were  hopelessly cloudy for last night’s conjunction. You were luckier I hope. Don’t forget, tonight’s thicker crescent passes very close to the planet Uranus, occulting it from the far northeastern U.S. and southeastern Canada. Venus and Mars will also be in conjunction today and a smidge closer that they were yesterday evening.

Wide view of Uranus and the moon on tonight (Feb. 21)  as seen from the Midwest about an hour and a quarter after sunset. Source: Stellarium

For the Central Time Zone, Uranus will lie 0.5° west of the moon in twilight, 1° away the Mountain States and 1.5° for the West Coast. What a great opportunity to spot the 7th planet in binoculars. Not only that, but a simple time exposure with a tripod-mounted camera will easily show it. Wait till late twilight and try a range of exposures starting around 5 seconds at ISO 800 with the lens wide open to f/2.8 or 3.5.

Uranus in early twilight (left) just before its dramatic disappearance behind the earth-lit edge of the moon tonight Feb. 21 as seen from Portland, Maine. 36 minutes later Uranus emerges at the bright crescent’s edge. Both disappearance and reappearance occur in a dark enough sky to see in a small telescope. Source: Stellarium

Map showing where the occultation of Uranus by the moon will be visible. Between the white lines, it’ll be visible in a dark sky. Blue is twilight and the red dotted line is daytime. Uranus is too faint to see in the daytime sky. Click the map to get a list of disappearance and reappearance times for a variety of cities. Credit: IOTA/Occult

Most of the time the moon occults stars along its path since there are a lot more of those than planets. Because they’re so remote, stars are little more than points of light; as the moon moves over them they disappear with surprisingly suddenness. Since Uranus displays a real, measurable disk it will take a second or two to disappear behind the moon’s edge.

SOHO-8275 comet about 7 east of the Sun this morning at 9:06 a.m. (CST) this morning. Credit: NASA/ESA

SOHO-2875 comet about 7 east of the Sun this morning at 9:06 a.m. (CST) this morning. Credit: NASA/ESA

The little comet we discussed yesterday continues trekking away from the Sun after its searing encounter two days ago. SOHO-2875 still shows a short tail and hangs in there around magnitude +3.5. It reached the edge of the field of view of the Solar and Heliospheric Observatory’s C3 wide-field coronagraph this morning headed east-northeast.

Since the field of view of the coronagraph is 15°, the comet’s presently about 7° east of the Sun, too close to spot yet in twilight. Give it 4-5 more days and someone may see it in a telescope in evening twilight. As soon as an orbit becomes available I’ll put together a chart to help you find it.

New Fast-Moving Comet May Become Visible from Earth

SOHO-2875 seen SOHO's LASCO C3, wide-field coronograph called LASCO C3 at 11:02 a.m. (CST) today Feb. 20. It's already moved a good distance to the east-southeast of the Sun and still displays a short tail. Credit: NASA/ESA

SOHO-2875 seen SOHO’s LASCO C3, wide-field coronograph called LASCO C3 at 11:02 a.m. (CST) today Feb. 20. It’s already moved a good distance to the east-southeast of the Sun and still displays a short tail. Credit: NASA/ESA

A newly-discovered comet may soon make an appearance in the evening sky. Dubbed SOHO-2875, it was spotted in photos taken by the Solar and Heliospheric Observatory (SOHO) earlier this week. Astronomer Karl Battams, who maintains the Sungrazer Project website, originally thought this little comet would dissipate after its close brush with the Sun. To his and our delight, it’s now outperforming expectations. Given the comet’s rapid movement away from the Sun, we won’t have to wait long to find out whether it might be visible in a telescope.

Composite of Comet SOHO-2875 crossing the C2 coronagraph field yesterday. Credit: NASA/ESA/Barbara Thompson

Composite of Comet SOHO-2875 crossing the C2 coronagraph field yesterday. Credit: NASA/ESA/Barbara Thompson

Most sungrazing comets discovered by SOHO are members of the Kreutz family, a group of icy fragments left over from the breakup of a single much larger comet centuries ago. We know they’re all family by their similar orbits. The newcomer, SOHO’s 2,875th comet discovery, is a “non-group” comet or one that’s unrelated to the Kreutz family or any other comet club for that matter. According to Battams these mavericks appear several times a year.

Photo taken at 20:00 UT (2 pm. CST) Feb. 19 with the SOHO C2 coronagraph, a device that blocks the Sun, allowing a view of the area close by. A faint tail can be seen just below the comet's bright head. Credit: NASA/ESA

Photo taken at 20:00 UT (2 pm. CST) Feb. 19 with the SOHO C2 coronagraph, a device that blocks the Sun, allowing a view of the area close by. A faint tail can be seen just below the comet’s bright head. Credit: NASA/ESA

What’s unusual about #2,875 is how bright it is. At least for now, it appears to have survived the Sun’s heat and gravitational tides and is turning around to the east headed for the evening sky. I’m no expert but having looked at many SOHO photos over the years, I’d estimate the comet is presently about magnitude +2.5 and some 5° from the Sun. No one can say for sure whether it has what it takes to hang on, so don’t get your hopes up just yet.

We’ll be watching and waiting. I’ll have an update on SOHO-2875’s progress soon.

Sweet Valentine’s Day closeups of Rosetta’s Comet

Four image mosaic of Comet 67P/Churyumov-Gerasimenko comprising images taken on 14 February at 14:15 GMT from a distance of 8.9 km from the surface. The image scale is 0.76 m/pixel and the mosaic measures 1.35×1.37 km across. The image focuses on the stunning features of the Imhotep region, on the comet’s large lobe. Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

Four image mosaic of Comet 67P/Churyumov-Gerasimenko taken on Feb. 14 at 8:15 a.m. (CST) from a distance of 5.5 miles from the surface. The view measures 0.8 miles across. The image focuses on the stunning features of the Imhotep region, on the comet’s large lobe. Click for a large version. Credits: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

If you’ve ever wondered what it would be like to fly over a comet in a jet airplane, soak in these photos. This past Saturday, the Rosetta spacecraft swung within 3.7 miles (6 km) of the surface of Comet 67P/C-G. Pictures for the mosaic image (above) were taken from an altitude of just 29,000 feet, nearly 10,000 feet lower than a typical transatlantic flight.

It really is like looking out the plane window especially when you consider you’re viewing a chunk of landscape barely a mile across. You and I could walk across that mosaic in less than 20 minutes! Assuming no obstacles of course.

Cropped, close-in view of several raised circular structures near the center of the mosaic. Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

Cropped, close-in view of several raised circular structures near the center of the mosaic. Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

Closest approach occurred over the Imhotep region on the comet’s large lobe. What caught my eye was the long, layered mesa-like feature in the lower left of the frame. In the cropped version, you can make out the outlines of several raised, near-circular structures with smooth floors. Boulders, ranging in size from 12 feet (a few meters) to a 35 feet (10 meters) lie scattered across the whole surface of the comet. The big boulder near the top of the mosaic and seen up close below is named Cheops. It’s 148 feet or about 45 meters across.

Close-up view of the large, mesa-like formation photographed by Rosetta on Saturday from 29,000 feet. Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

Close-up, rotated view of the layered formation photographed by Rosetta on Saturday from 29,000 feet. Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

The big boulder Cheops casts a shadow at upper right. It's not far from what looks like a section of the comet's crust that's collapsed. Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

The big boulder Cheops (upper right) sits in its shadow. It’s not far from what looks like a collapsed area of the comet’s crust (left). Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

What appears to be a collapsed section of a cliff or wall. Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

What appears to be a collapsed section of a cliff or wall. Credit: ESA/Rosetta/NAVCAM – CC BY-SA IGO 3.0

Download the mosaic to your desktop and take a few minutes to explore it. You’ll find flows, depressions, more of of those circular features and a delightful assortment of boulders of all shapes and sizes. What are those things – dust-covered ice chunks?

Rosetta is now moving out for a far view of the comet and will reach a distance of about 158 miles (255 km) from the comet’s center tomorrow. Stop by in a day or two for new pictures of 67P’s atmosphere or coma. In the meantime you can download a zip file of the 16 individual frames comprising the mosaic here.

Glowing comets and deep fried ice cream

Comet 67P/C-G taken on February 6 from a distance of 77 miles (124 km) to the comet center. We see the comet’s small lobe on the left and larger lobe to the right. The image has been processed to bring out the details of the comet’s jetting activity. Exposure time was 6 seconds. Credits: ESA/Rosetta/NAVCAM

Everything’s glowing! This new photo from the Rosetta spacecraft was taken on Feb. 6 and processed to show jets active all over comet 67P. They’re brightest and most effervescent in the Hapi region, the name given to the neck of the comet, but take a look around the nucleus. You can see that the entire comet is swathed in a soft, nebulous halo of vaporizing ices and dust motes.

Some of the grainy white spots around the comet’s nucleus are background noise brought to the fore as a consequence of processing the images to see the jets more clearly, but some is actual comet stuff jetted into 67P’s coma like dried leaves from a leafblower.

Rosetta’s moved out from the comet temporarily as part of an orbit change that will see it fly just 3.7 miles (6 km) of its surface this Saturday. Photos from the flyby will be downlinked to Earth Sunday and Monday.

In other comet news, a team of astronomers have simulated what happens to a comet’s ices when it nears the Sun. It’s not what you’d expect.

Familiar ice like that on ponds, lakes and clinking as ice cubes in a glass are made of neatly ordered arrangements of water molecules. Amorphous ice is also made of water but its structure is disordered. Credit: Wikipedia

“A comet is like deep fried ice cream,” said Murthy Gudipati of NASA’s Jet Propulsion Laboratory in Pasadena, Calif., corresponding author of a recent study appearing in The Journal of Physical Chemistry. “The crust is made of crystalline ice, while the interior is colder and more porous. The organics are like a final layer of chocolate on top.”

Comets, which formed in the bitter cold of the outer solar system, are probably composed of amorphous ice, a form of water ice where the H20 molecules are randomly arranged instead of packed into neat lattices like those that make up the more familiar crystalline ice we use to chill our sodas.

Researchers at NASA’s Jet Propulsion Laboratory use a cryostat instrument, nicknamed “Himalaya,” to study the icy conditions under which comets form. Image credit: NASA/JPL-Caltech

First, the team flash-froze water vapor infused with carbon compounds called polycyclic aromatic hydrocarbons (PAHs) at -405° F to preserve the disorderly states of the water molecules and create amorphous ice. Then, using a cryostat instrument nicknamed “Himalaya”, they gradually raised the temperature of the mixture from -405° F to -190° F (-243° C to -123° C).

The PAHS stuck together and were expelled by the ice as it crystallized. With the carbon compounds now gone, the water molecules moved into the empty spaces linking up to form more compact crystalline ice.

That rings true when it comes to the Rosetta mission’s Philae lander which hit 67P/C-G with a big bounce proving it had a hard surface. Some of the dust seen around 67P might well have been “squeezed” out when amorphous ice transformed into the crystalline variety.

Similar to the fried ice cream pictured here, comets have cold, icy interiors surrounded by a crust of organics expelled when amorphous ice warms to become crystalline ice. Credit: Wikipedia

“What we saw in the lab — a crystalline comet crust with organics on top — matches what has been suggested from observations in space,” said Gudipati. Deep fried ice cream is really the perfect analogy, because the interior of the comets should still be very cold and contain the more porous, amorphous ice.”

Rosetta sees fascinating changes in Comet 67P

A new jet issues from a fissure in the rugged, dusty surface of Rosetta’s comet. Credit: ESO/Rosetta/Navcam; processed by Elisabetta Bonora & Marco Faccin

It only makes sense. Sunlight heats a comet and causes ice to vaporize. This leads to changes in the appearance of surface features. For instance, the Sun’s heat can eat away at the ice on sunward-facing cliffs, hollowing them out and eventually causing them to collapse in heaps of shards. Solar heating can also warm the ice that’s beneath the surface. When it becomes a vapor, it pushes against the ice above, cracking it and releasing sprays of gas and dust as jets.

Take a look at this photo taken on December 9 of a part of the neck of the comet called Hapi. I’ve labeled a boulder and three prominent cracks. Sunlight is coming from top and behind in this image. Compare to the photo below shot on Jan. 8. Credit: ESA/Rosetta/Navcam

Recent photos taken by the Rosetta spacecraft reveal possible changes on the surface of 67P/Churyumov-Gerasimenko that are fascinating to see and contemplate. In a recent entry of the Rosetta blog, the writer makes mention of horseshoe-shaped features in the smooth neck region of the comet called “Hapi”. An earlier image from Jan. 8 may show subtle changes in the region compared to a more recent image from Jan. 22. We’ll get to those in a minute, but there may be examples of more vivid changes.

Although the viewing angle and lighting geometry has changed some between this photo, taken Jan. 8, and the one above, it certainly appears that the three cracks have virtually disappeared in a month’s time. The same boulder is flagged in both photos. Credit: ESA/Rosetta/Navcam

I did some digging around and found what appears to be variations in terrain between photos of the same Hapi region on Dec. 9 and Jan.8. Naturally, we have to be careful when comparing photos taken under different lighting conditions and viewing geometry. Rarely does one view precisely match another. That has to be taken into account when deciding whether a change in a feature is real or due to change in lighting or perspective.

Side by side comparison of the two image from Dec. 9, 2014 (left) and Jan. 8, 2015. Credit: ESA/Rosetta/Navcam

But take a look at those cracks in the December image that appear to be missing in January’s. The change, if real, is dramatic. If they did disappear, how? Are they buried in dust released by jets that later drifted back down to the surface?

Comparison of Jan. 22 and Jan. 9 photos of the “horseshoes” or depressions in 67P’s Hapi region. Outside of differences in lighting, do you see any changes? Credit: ESA/Rosetta/Navcam

Now back to those horseshoe features. Again, the viewing angles are somewhat different, but I can’t see any notable changes in the scene. Perhaps you can. While comets are expected to change, it’s exciting when it seems to be happening right before your eyes.

Four-image mosaic shows the overall view of the comet on January 22 photographed 17.4 miles (28 km) from its center. The larger of the two lobes is at left; Hapi is the smooth region at the transition between the lobes. Credit: ESA/Rosetta/Navcam

Comatose on Rosetta’s comet, a follow-up on Philae

The likely position of Philae in a visualization of a topographic modeling of the comet’s surface. Credit: ESA/Rosetta/Philae/CNES/FD

As the good folks behind the Rosetta mission take a well-deserved year-end break this week and next, it’s a good time to check in on Philae, the little lander that that finally settled down for good on Comet 67P/Churyumov-Gerasimenko after bouncing three times on that gravity-deprived world.

Philae’s view of “Perihelion Cliff” at its final resting place on the comet’s surface. Reprocessing brings out more detail in rocks. The two bright spots above and below center are reflections from the metal surfaces of the spacecraft. Philae landed in a shadowed area of the comet where its solar panels can’t presently get enough sunshine to recharge the probe’s batteries. Without power, it’s now in hibernation. Credit: ESA/Rosetta/Philae/CIVA

No one yet knows exactly where the craft has landed, but using the few images taken by Philae before its batteries went dead, mission control specialists have constructed the visual (above) showing the lander tucked into a little valley alongside a steep cliff. Scientists still hope to re-activate the lander by February or March when the comet’s position relative to the Sun will allow sunlight to once again strike its solar panels and recharge the batteries.

Philae’s blurred first photo taken after its first bounce off the comet’s surface on November 12, 2014. Credit: ESA/Rosetta/Philae/CIVA

The original landing site for Philae would have been perfect for science studies up until about February or March, but too hot for the lander as the comet approached its August perihelion. Its more sheltered location fortuitously shields it from sunlight long enough to allow Philae to study the comet around and after perihelion – the reason the big rock or cliff about 3 feet away is dubbed Perihelion Cliff.

Same simulation as above showing Philae’s location but this version has the photo of Perihelion Cliff superimposed to better show where on the cliff the photo was taken. The cliff is composed of blocky, fractured ice. Credit: ESA/Rosetta/Philae/CNES/FD

The atmosphere of the comet contains a great variety of molecules, while the majority of the surface layer away from the active regions holds between 5 and 7 percent organic material.

Rosetta’s instruments have already detected a rich assortment of molecules in the hazy, temporary atmosphere of the comet called the coma. Credit: ESA

The flatter plains appear to be covered in an ice-free layer of dust and gravel about half-an-inch (1 cm) made up of low iron minerals, silicates, organics and carbon. The deeper you go, the icier it gets with a mix of dust, ice and organic materials about 4-8 inches (10-20 cm) thick. Water amounts in the coma vary according to time of day and position of the comet. Carbon dioxide emission are more constant throughout. Significant amounts of sulfur have also been found.

Who’d a- thunk it? Mercury May Have Meteor Showers Too

Artist’s concept of Mercury crossing the debris trail of Comet Encke, sparking a recurring meteor shower. New evidence from the MESSENGER mission suggests the planet receives regular doses of Mercurial dust. Credit: NASA’s Goddard Space Flight Center

Of course, of course, it only makes sense. We’re so caught up in watching meteor showers on our own planet, who ever thinks about meteors at Mercury? Or Venus for that matter? This week NASA announced that regular spikes in the amount of calcium in Mercury’s upper atmosphere bespeak a cyclical source. The likely culprit? Comet 2P/Encke.

Like breadcrumbs dropped to mark a path, dust and fragile bits of rock are released through vaporization of a comet’s ice and pushed back by the pressure of sunlight to form a tail. The larger pieces are left behind to fan out along the comet’s orbit. If by good fortune Earth’s orbit happens to intersect the debris trail, we see a shower of meteors in the sky.

This photo, made by NASA’s Spitzer Space Telescope in infrared light, shows Comet Encke’s glowing nucleus/nuclear region and a trail of warm dust and pebbly debris shed by the comet along its orbital path. Credit: NASA

Most recently, the Geminids put on a great show, although their origin lies with the peculiar rock-shedding asteroid 3200 Phaethon.

Comet 109P/Swift-Tuttle brings us the familiar Perseid meteor shower, while 2P/Encke gives rise to several meteor streams – the Southern and Northern Taurids, showers that peak in October and November, and the daytime Beta Taurids in June and July.

Measurements taken by MESSENGER’s Mercury Atmospheric and Surface Composition Spectrometer have revealed seasonal surges of calcium that occurred regularly over the first nine Mercury years (1 year = 88 Earth days) since MESSENGER began orbiting the planet in March 2011. Just as we saw huge spikes in the amounts of metals like magnesium and iron in Mars’ upper atmosphere during Comet’s Siding Spring’s brush with the planet last October, MESSENGER’s instrument detected periodic spikes in the amount of calcium – although with a twist.

A color- enhanced view of Mercury, assembled from images taken at various wavelengths by the cameras on board the MESSENGER spacecraft, shows a cratered composed with a surface composed of a variety of minerals. The circular, orange area near the center-top of the disk is the enormous Caloris impact basin. Credit: NASA / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington

Mercury’s has only a whiff of an atmosphere, what astronomers term an exosphere, the last thing you could call an atmosphere before encountering the vacuum of space. The shower of small dust particles peppering interplanetary space pass right down to the surface and strike the planet’s rocks, knocking calcium-bearing molecules free from the surface, which are then free to rise to greater heights. This process, called impact vaporization, continually renews the gases in Mercury’s exosphere as interplanetary dust and meteoroids rain down on the planet.

These type of impacts happen all the time, but scientists noticed a pattern in the calcium spikes that pointed to a repeating source. Sounds like a perfect time for a comet to step in. Examination of the handful of comets in orbits that would permit their debris to cross Mercury’s orbit indicated that the likely source of the spikes was Encke.

The Jupiter family of comets were all once long-period objects in the Kuiper the orbits of which were changed to short-period by close passes by Jupiter. The green circle is Jupiter’s orbit, the purple is Earth’s. Notice that when farthest from the Sun, the comets about as far as Jupiter is from the Sun. Credit: Wikipedia with additions by the author

“If our scenario is correct, Mercury is a giant dust collector,” said Joseph Hahn, a planetary dynamist in the Austin, Texas, office of the Space Science Institute and coauthor of the study. “The planet is under steady siege from interplanetary dust and then regularly passes through this other dust storm, which we think is from comet Encke.”

To test their hypothesis, Han and crew created detailed computer simulations and discovered that the MESSENGER were offset from the expected results but in a way that made sense due to changes in Encke’s orbit over time from the gravitational pull of Jupiter and other planets.

Pantheon Fossae – The striking troughs of Mercury’s Pantheon Fossae, the feature that MESSENGER scientists first called “The Spider” when they discovered it. Credit: NASA / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington

Comets get nudged by planets routinely, especially if they pass near Jupiter, the outer Solar System’s gravitational goliath. Jupiter, with the help of Neptune, has re-worked the orbits of countless bodies that once resided in the distant Kuiper Belt into shorter-period comets that swing around the Sun in 20 years or less. Called the Jupiter-family, there are about 400 known and Encke is one of them with an orbital period of just 3.3 years.

Who knows how many other meteor showers might pepper Mercury in a year, but scientists will be looking for potential signs of them in planet’s atmosphere in the months ahead. While they may not leave bold streaks of light as they do on Earth, they create something almost as amazing – a shower of particles that goes up instead of down.

Rosetta’s comet – colorful personality but gray as a foggy day

A color photo of Comet 67P/Churyumov-Gerasimenko composed of three images taken by Rosetta’s scientific imaging system OSIRIS in the red, green and blue filters. The images were taken on August 6, 2014 from a distance of 75 miles (120 km) from the comet. Click to enlarge. Credti: ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/SSO/INTA/UPM/DASP/IDA

Is this really a color photo? Yes! And it shows how remarkably gray and colorless the comet truly is. This is just how you’d see 67P/C-G if you could piggyback on Rosetta and whirl around it for a few orbits.

“As it turns out, 67P/C-G looks dark grey, in reality almost as black as coal,” says the instrument’s Principal Investigator Holger Sierks from the Max Planck Institute for Solar System Research.

Pictures of Enceladus, the Earth, the Moon, and Comet 67P/C-G showing their relative brightness. Saturn’s icy moon Enceladus reflects back nearly 100% of the sunlight it receives, Earth, 31% , the moon, 12% and 5% for 67P/C-G. Images not to scale. Credit: NASA/JPL/Space Science Institute (Enceladus); ESA/Rosetta/MPS for OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/ UPM/DASP/IDA and Gordan Ugarkovich (Earth); Robert Vanderbei, Princeton University (Moon); ESA/Rosetta/NAVCAM (67P/C-G).

The intensity of the images has been enhanced to span the full range from black to white and bring out surface details, but the colors haven’t been altered. Shadows are deep black because there’s no atmosphere to significantly diffuse the light as on Earth. Some photos however do show shadow detail from sunlight reflecting off other parts of the comet and into shadowed regions.

The comet – at least at this distance – is nothing more than a hundred shades of gray. A careful analysis shows a small amount of excess red light reflected from 67P due to fine-grained dust on its surface. We can’t see this hint of rouge because our eyes are much more sensitive to the greens, yellows and blues of sunlight, but a camera recording light reflected from the comet through multiple color filters can.

Before Rosetta moved in close to 67P/C-G, Earth-based telescopes had also shown the comet’s gray nature, but I think it’s safe to say scientists were surprised that even close-up, the comet remains a monotonous monochrome megalith.

For instance, any ice on the surface should appear brighter in the blue filter, leading to the appearance of blue-ish patches. This photo contains no indication of any such icy patches, consistent with observations made by some of Rosetta’s instruments.

So what’s up? The same thing that dulls the shine on your computer monitor coats the surface the comet: dust. Dark dust is everywhere and mission scientists are in the process of determining what it’s made of. Ice is surely there – Philae detected ice at its landing site and Rosetta’s MIRO instrument found the comet shedding 2 cups of water a second as icy vapor.

Jets of carbon dioxide blast from beneath the surface of Comet Hartley 2 in this photo taken during the flyby by NASA’s EPOXI/Deep Impact spacecraft in 2010. The jets carry water ice in the form of large snowballs (white dots) and dust particles. Credit: NASA

It would seem logical to assume that some of the dust embedded in that vaporizing ice drifts back down to the surface, quickly covering any exposed material. I’ve seen something similar happen during a winter here in Duluth, Minnesota. Grit and sand accumulate atop fresh snow. Over time, the snow compacts, melts and refreezes to form ice covered by black gunk.

Often, water escapes as a plume of dust-laden vapor through a vent in its surface like a geyser. I’d love to see a close-up of one of those. Imagine the amount and ubiquity of fine dust deposited over millions of years every time the comet swings by the Sun and gets cooked.

If we just get somebody up there to sweep the floor.

Ho-ho-ho! Comet Lovejoy Q2 brings Christmas joy

Comet Lovejoy Q2 on December 12th shows a big glowing coma and faint, 2° long gas tail. The comet becomes an easy binocular object this week for northern skywatchers. Credit: Rolando Ligustri

A naked eye comet for Christmas? Yes, Bobby there is a Santa Claus. Comet C/2014 Q2 Lovejoy, which has been slowly pushing into northern skies this month, has brightened in recent days to magnitude +6.5. That’s the cusp of naked eye visibility. A few observers in Australia, where Lovejoy hovers nearly overhead, reported seeing it faintly with the naked eye last night.

Most of us will view Q2 with ease in binoculars, especially once it gains a bit more elevation at night. Right now, the comet’s in Puppis the Stern, a gangly constellation south of the more familiar Canis Major the Greater Dog, home to Sirius, the brightest star. When best placed for observing around 1-1:30 a.m. local time it climbs to an altitude of 10° (one fist held at arm’s length) for observers in the central U.S. but just 3-5° for the northern states.

Comet Lovejoy Q2 begins its northward trek slowly but picks up speed with each passing night. On the night of December 28-29, the comet will pass 1/3° from the bright globular cluster M79 in Lepus. This map shows the sky and comet’s position facing south from 42° north latitude around 1:30 a.m. CST. Click to enlarge. Source: Chris Marriott’s SkyMap software

That will change soon as Lovejoy swings rapidly northward and rises earlier and earlier in the coming nights. By Christmas, the comet will be even brighter and stand 20° high from places like Kansas City, Denver and Indianapolis shortly before midnight.

Lovejoy Q2 is Australian amateur astronomer Terry Lovejoy’s fifth find. He snagged it while making a photographic search for comets with his 8-inch (20-cm) last August. Q2’s been on a beeline toward the Sun since that time and brightened from a 15th magnitude smudge to a robust, glowing ball with a skinny-necktie tail.

Comet Lovejoy Q2 is a long-period comet, dropping in toward the Sun with an orbital period of about 11,500 years. Here it’s shown when nearest Earth and brightest in early January 2015. The comet follows a steeply tilted orbit that takes it high over the plane of the planets. Credit: NASA/JPL HORIZONS

As it approaches Earth this month and next, Q2’s expected to brighten to 5th magnitude, putting it within naked eye range from the outer ‘burbs of a mid-sized city. Binoculars will provide a clear view of the fat, fuzzy coma and telescopes will add a faint ion tail composed of vaporizing gases fluorescing in solar UV. Cool!

Closest approach to Earth happens on January 7-8th when Lovejoy will be 43.6 million miles (70.2 million km) away. A little more than 3 weeks later on January 30, the comet passes perihelion to the Sun at a distance of 120 million miles.

I’ve included three maps to find and track Comet Lovejoy through early January. The first (top) is a wide view showing the “big picture” to help you get oriented. The others go in tighter and show black stars against a white background. I prefer them for a couple reasons – they use far less ink when making printouts and are cleaner and easier to read at the telescope. Click each to download a larger version.

Detailed map showing the comet night-by-night path starting tomorrow December 14th through December 27th in the early morning hours (CST). Stars shown to magnitude +8.0. Source: Chris Marriott’s SkyMap software

Because Comet Lovejoy moves rapidly into the evening sky by mid-late December, its position on this map is shown for 10 p.m. (CST) nightly. Mark your calendars for the close approach to M79 on Dec. 28-29. Source: Chris Marriott’s SkyMap software

I’m hoping we get out from under our week-long battle with clouds, so I can see the comet for myself. I’ll be updating all along and would love to include your observations in future blogs.