Tag Archives: Hartley 2

Comet ice may come in three different flavors

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Jets spew out ice and carbon dioxide from one end of comet Hartley-2 in this EPOXI image, while water vapor gets released from the middle region. The differences suggest that the comet's core is made of at least two different ices. Ground-based measurements suggest the presence of a third ice. Credit: NASA/JPL-Caltech/UMD

A little ice, a little fizz, a few nuts and voila – we have a comet! We learned how to make a homemade comet the other week. This week NASA released a new study of Comet Hartley 2  that gives us an even better idea of a what goes into making one.

Using telescopes perched high in the mountains of Hawaii and Chile, Michael Mumma of NASA’s Goddard Space Flight Center and his team studied the comet’s coma—the envelope of gas, dust and ice particles that surrounds the core. What’s in the coma originates from the comet body itself, which is too small and shrouded in too much haze to study directly with earthbound telescopes. Astronomers deduce a comet’s composition by studying molecules buzzing around in its coma.

The location and spacing of bright lines in a spectrum act as a sort of bar code to tell us what kind of material is present.

Mumma’s team used a spectrograph to spread Hartley 2′s light into a detailed spectrum or rainbow of light. Bright and dark lines that resemble a bar code on a package of cookies stripes the rainbow from one end to the other. Each atom or molecule, through absorption and emission of light, imprints its own unique set of lines on the spectrum. By studying these ‘bar codes’, a scientist can tell you what made them, the amount present and even its temperature.

Ices in Hartley 2 are mostly made of water and carbon dioxide or dry ice. The team also discovered that the water ice contained methanol, a familiar form of alcohol that back here on Earth is mixed with gasoline or used straight to power racing cars. There were also indications of a third type of ice – ethane – a component of natural gas.

Myriads of fluffy snowballs caught up in vaporizing water and dry ice are shot into the coma by jets on the surface of 1.2-mile-long Comet Hartley 2 (right) in this photo taken by the Deep Impact probe during last November's close flyby. They range in size from pennies to basketballs. Credit: NASA/JPL-Caltech/UMD

Hartley 2′s surface is covered with small, volcano-like jets. When the comet is warmed by the sun, the jets shoot out a mix of vaporized water ice, dry ice and entrained rocky particles from the interior to create the coma. The researchers think that chunks of water ice are glued together in the comet’s core by the frozen carbon dioxide, which vaporizes before the water ice because it’s more sensitive to the sun’s heat. The carbon dioxide gas then drags along chunks of ice for the ride, which later vaporize to provide much of the water vapor in the coma.

In addition to Mumma’s studies, the EPOXI comet flyby mission last November also revealed that the carbon dioxide jets are not found at the large end of the comet, and in the middle region, water vapor is released without any carbon dioxide. And while water with methanol is released from all directions around the comet, ethane was released from just one direction. This uneven expression of ingredients across Hartley 2 may shed light on its origin. Did it condense from gas, dust and ices to form a single body or did a bunch of mini-comets of slightly different composition come together to create the comet? You can learn more about the group’s findings by clicking over to this NASA press release.

How to make your own comet

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My homemade 0.00152-km diameter comet threatens to destroy the Earth during an unannounced flyby last night. Photo: Bob King

I teach a community education astronomy class and since last night was the last class, I thought it would be fun to build a homemade comet. What with Comet Elenin in the news, the timing seemed appropriate.

Making a comet is much like making brownies – you just need the right ingredients. It also helps to see it done first by someone else, and what better place than Youtube (see below) for that. There are several variations but they all involve the following ingredients:

* A cup of water. One of the main ingredients of comets is water ice.
* Organics or carbon containing compounds. A cup of dirt will do the job. It also gives your comet a gritty, dirty appearance much like the real thing.
* A cup of sand to represent the silicate or rocky bits found in comets.
* A can of dark-colored soda. I used root beer. This represents organics and it also will darken the color of your comet. Real comets are mixtures of ice, dust and stony grits and as dark as charcoal.
* A few squirts of window cleaner. The ammonia in the cleaner is partly made of nitrogen, compounds of which are also found in comets.
* When you have placed all these in a metal bowl, add the final ingredient – a couple cups of dry ice pellets. Comets contain dry ice or frozen carbon dioxide. Cool, smoky vapor will now bubble up from your mixture.

A real comet! This picture of Comet Hartley 2 was taken a flyby on November 4, 2010 by the EPOXI spacecraft. Vaporizing ice at right sends streamers of dust and water into space. Credit: NASA.

Needless to say, it’s important to wear protective glasses and good gloves when handling dry ice, since it’s 109 degrees below zero. I bought two pounds of pellets for just under $5 from our local industrial gas and welding supply shop (Praxair) here in Duluth, Minn. Kept in a cooler, the ice easily lasted though the afternoon and into the night.

Once all your ingredients are in place, reach into the bowl with your gloved hands and crush the stuff together, holding the mass tightly for a maybe a half minute. When you lift it out for all to see, you’ll be holding an ugly, lumpy, vaporizing, crackling plug of homemade comet. In real comets, sunlight causes ice to vaporize or sublimate, going directly from solid to gas. You’ll see the same in your homemade version. If you blow on it to simulate the sun and solar wind, a misty tail will point away from the ‘sun’ (your mouth).

As a comet swings inward toward the sun, heat, light pressure and the solar wind cause the vaporizing ice and its entrained dust to 'blow' back behind the comet and form two tails, one of dust and the other gas. Credit: NASA

My first attempt with the class was an instructive failure. I lifted the comet out of the bowl and it cracked into a couple pieces. Since real comets are friable objects, they do sometimes fall apart under the heat and gravitational stress caused by the sun.

But serendipity was on my side last night, because after letting the mix sit for 15 minutes without touching it, a very fine comet – with spoon stuck inside – spontaneously formed on its own! I lifted it into view at the end of class, and we were all pleasantly surprised. What a beauty. And just for fun, my buddy Will Wiethoff and I simulated a near collision with the Earth using one of the paintings hanging in the planetarium entryway.

You can simulate the solar wind and light pressure from the sun by blowing over your comet or over a spoonful of dry ice. Photo: Bob King

So if you’d like to make a comet, collect your ingredients, taking special care with dry ice, and have at it. It’s important to use a metal bowl so it won’t crack when you fill it with the bitter cold ice. And if you want to keep your comet for show-and-tell, bag it and store it in a freezer. Creating a comet helps us to better visualize and understand what we’re talking about when we see one through a telescope or in pictures. I’ve included one of the videos below to help you along the way. And here’s a link to another.

Kickin’ around in the snow

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The forest at Hawk Ridge in Duluth wore a cover of snow Sunday. Photo: Bob King

I took a great walk in the woods yesterday. Many trees were still blanketed with snow from our Thanksgiving Day storm. As I kicked through the fluffy white clods, I thought of Comet Hartley 2 and its carbon dioxide-propelled geysers shooting out chunks of snow into outer space. Were they soft and airy like the stuff rubbing against my boots? Whatever the texture, you probably couldn’t build a snowman with Hartley 2 snow. In the cold, near-zero pressure environment of outer space, it would hardly make “good packing”.

A beautiful scene! Comet Hartley 2 was caught by amateur astronomer Hap Griffin of Sumter, South Carolina between the star clusters M46 (left) and M47 in Puppis Saturday. Credit: Hap Griffin

This weekend the comet did a do-si-do between two very pretty binocular star clusters in the constellation of Puppis (PUP-is) the Stern located just east of the more familiar Canis Major the Greater Dog. We know Canis Major primarily through Sirius, the brightest star in the sky and one of the easiest to find. Shoot a line southward (to the left) of Orion’s Belt and you’ll hit Sirius straightaway. With the moon rising in the wee hours for the remainder of the week, you can see the two clusters – M46 and M47 – if you’re up and about at midnight or later. M47′s the brighter and visible to the naked eye from a dark sky location. It’s neighbor, M46, is about the same size but looks misty because it has far more fainter stars than bright ones. These side by side star clusters are two of the 110 objects in Charles Messier’s famous 18th century catalog of clusters, galaxies and nebulae – hence their “M” designations.

To find M46, M47 and Hartley 2, start with Orion's Belt and shoot down to Sirius. The clusters are about one fist to the left of Sirius and easily picked up in binoculars from outer suburbs and rural locales. The comet will be in the same binocular field of view as the clusters for the next few nights. Created with Stellarium

Comet Hartley 2 has been fading since earlier this fall and will require binoculars with an aperture or lens diameter of at least 50 mm to see. You can find your binocular’s aperture by looking on the barrel or near the focusing mechanism.  For example, 7×50 indicates a magnification of seven times and an aperture of 50 mm or about 2-inches. The comet is now around 7th magnitude and looks like a dimly glowing patch in binoculars. Telescopes will still show its bright center or nucleus blazing away inside the larger, fainter coma. I’ve included a finding chart belwo suitable for both binocular and telescope users.

This chart shows the comet tonight through December 9 as it travels south in Puppis near M46 and M47. Other star clusters in the neighborhood invite intrepid telescopic observer to expand their exploration of the region. Created with Chris Marriott's SkyMap software

Blizzard rages on Comet Hartley 2 – You gotta see this

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This Nov. 4 image, taken by the Deep Impact spacecraft during its closest approach, shows part of the nucleus of comet Hartley 2. The sun is off to the right. A distinct cloud of individual ice and snow chunks surrounds the comet. All images credit: NASA/JPL-Caltech/UMD

NASA just released several spectacular new photos of Comet Hartley 2 taken during the November 4 flyby. Living in Duluth, Minn. and having a fondness for snowstorms, I was tickled to see that the photos showed the comet in the process of creating its own blizzard. Hartley 2′s porous and fluffy snow chunks measure between an inch and a foot across. They’re composed of water ice similar to snow on Earth, but their method of delivery is decidedly different. Instead of falling down from the sky, Hartley 2′s snow falls up. It’s shot up from beneath the surface when sunlight causes dry ice (frozen carbon dioxide) to vaporize. The gas rises and breaks through the surface in the form of jets or mini-geysers, carrying pieces of water ice and comet dirt with it. This is the first time scientists have seen individual hunks of snow and ice around a comet.

Tons of fluffy snowballs the size of pennies to basketballs sparkle in the sunlight to the left of Comet Hartley's nucleus. The sun's heat in the vacuum of space will vaporize much of the material and form the fuzzy glow around the nucleus called the coma.

“When we first saw all the specks surrounding the nucleus, our mouths dropped,” said Pete Schultz, EPOXI mission co-investigator at Brown University. “Stereo images reveal there are snowballs in front and behind the nucleus, making it look like a scene in one of those crystal snow globes.”

Sunlight heats the Hartley 2's nucleus and converts solid dry ice beneath the surface into gas. As the gas breaks through the surface in the form of jets, it carries along water and ice and snow picked up along the way.

Remember that smooth middle section of Hartley 2? Well, it appears a different mechanism is at work there. Instead of getting blasted out, water ice turns to vapor and then percolates through the loose surface material.

Got a pair of those red-blue 3D glasses? You'll enjoy seeing the entire nucleus of Hartley 2 with jets and its icy particle cloud. Circles have been added to highlight the location of individual particles.

During the flyby, the Deep Impact probe was struck at least nine times by particles with a mass slightly less than that of a typical snowflake. No damage was done, but the potential was there considering the craft buzzed by at more than 27,000 miles per hour. Hey, it’s bad enough during a storm in Duluth, when you can feel the sting of ice crystals and snow on your face in a “wimpy” 30 mph wind. At least our storms come and go, but you wonder how long Hartley 2′s blizzard has been raging. Has it been snowing there for a matter of months when the comet is near the sun, or do at least a few carbon dioxide jets remain active throughout the comet’s orbit? If that’s the case, Hartley 2 might be a world of endless snow storms. For more information and photos, please see NASA’s Mission News. You can also watch a short movie of Hartley 2′s icy particle sprays in action.

First closeup photos of Comet Hartley 2 are in!

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One of the very first closeup photos of the center or nucleus of Comet Hartley 2 from the Epoxi Mission flyby earlier this morning. The bright geysers are jets of icy vapor and dust shooting from cracks in the surface. Comet are fluffy, friable objects; heat from the sun causes some of a comet's ice to vaporize. All images credit: NASA/JPL-CalTech/UMD

Even as NASA’s Deep Impact probe flew by Comet Hartley 2 this morning, a new comet was discovered overnight by Japanese amateur astronomers Kaoru Ikeya
and Shigeki Murakami in the constellation Virgo. Located not far from Saturn’s position in the sky, the new object is called C/2010 V1, and at 8th magnitude, bright enough to see in a small telescope. Once an orbit is established, I’ll post a map to help you find it. For intrepid amateurs who’d like to give it go right now, the comet will be approximately two degrees west of Saturn tomorrow morning at the start of dawn.

Around 9 a.m. Central time today, the spacecraft successfully shot through Hartley 2′s coma and photographed the comet’s nucleus. It’s risky business sending a machine into a comet because of the risk of impact from dust and grit flying all over the place. This is particularly true when you consider the spacecraft’s encounter speed of 27,500 miles per hour.

The Deep Impact probe zooms by Comet Hartley 2 in this artist view of today's flyby. Credit: NASA

Shortly before the flyby, Deep Impact was placed in AutoNav or autopilot mode, where its instruments were programmed to focus on the brightest thing in the field of view other than the sun.  Because it takes a radio signal traveling at the speed of light 75 seconds to cross the 23 million mile gulf between Earth and Deep Impact, the spacecraft has to be able to follow Hartley 2 on its own in real time. Sunlit jets of dust and vaporized ice should provide the light needed to for the cameras to stay centered on the target.

All images and data were stored on Deep Impact’s computers and are currently being downloaded to Mission controllers at the Jet Propulsion Lab in Pasadena, California. Hundreds of photos were taken and images should continue to flow all day long. To stay abreast, you can follow the coverage LIVE on the Web. Check out the latest images HERE or see all five closeup photos together in one collage.

Here are a couple more photos of the 1.4 mile-long peanut-shaped nucleus taken during Deep Impact’s closest approach to the comet. Fantastic! Just relish the detail.

Hartley 2 is the fifth comet nucleus visited by a spacecraft. Previous missions included comets Halley, Borrelly, Wild 2 and Tempel 1.

How exciting to peer into the heart of the comet that we've seen all fall long through binoculars right here on Earth. The smooth area between the bouldery ends looks especially interesting - what causes the difference in texture?

Will Comet Hartley 2 spawn a new meteor shower?

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Comet Hartley sports a short tail extending to the lower right in this photo taken earlier this month. Credit: Michael Jaeger

It’s very unlikely, but two intriguing pieces of evidence point to Hartley 2, the comet of the moment,  as a possible source of two recent bright fireballs seen over Canada and the southeastern U.S. On Oct. 16, two NASA all-sky cameras photographed a bright, slow fireball over Alabama and Georgia. What was odd was that this fireball was very similar to one photographed five hours earlier by the all-sky cameras network of the University of Western Ontario. Because the fireballs were seen by multiple cameras – each with their own perspective on the events – they were able to triangulate the fireballs’ positions and determine where in space they originated.

Guess what? “The orbits of the two fireballs were very similar,” according to Bill Cooke of NASA’s Meteoroid Environment Office. “It’s as if they came from a common parent.” The orbits of the two fireballs were not only similar to one another, but also roughly similar to the orbit of the comet. Since comets spew dust and small chocolate-chip sized gravel as they go, and since Hartley is close enough to Earth right now for our planet to potentially graze its dust trail, the fireballs could have originated there.

Two fireballs with "Hartley-esque" orbits observed on Oct. 16th by cameras in western Ontario (left) and the southeastern USA (right). Orbits of each are shown at top. Credit: UWO/NASA/Bill Cooke

It also could be pure coincidence. Cooke points out that on any given night at least a few Hartley-esque meteors hit the atmosphere. But you never know. If more meteors are on their way from Hartley, we ought to find out soon. A possible shower of Hartley dust might occur on the nights of November 2 and 3. If it happens, they’ll appear to emanate from the constellation Cygnus, which is nearly overhead when night begins. Since meteor showers are named after the constellation from which they originate, this one would be called the Cygnids. Will they show? Probably not but who knows. I plan to be out there just in case, and you might want to, too.  Click HERE for more information.

I wanted to share two excellent photos of the full or nearly full moon submitted by readers this past week. Thank you Jane and Andrew! Very nice work guys. Enjoy.

The full moon rises along with the Earth's shadow at Brighton Beach in Duluth last weekend. Credit: Jane Gilley

 I like how the sunlit peaks guide your eye to the moon in this photo taken in the Sierra Nevadas. Credit: Andrew Kirk

Let the night bring what it may

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The Andromeda Galaxy, the brightest visible from the northern hemisphere, is one of the highlights of October skies. Photo: Bob King

Stand out in your front yard and photons from distant galaxies cover the grass like falling leaves. If you choose to sample some of these, point a telescope skyward and a few drop into your instrument where mirrors and lenses cup and then refract them into your eyeball. It sounds miraculous, but this photon rain from all corners of the universe drizzles down every day and night.

Jupiter and contrail in the moonlit sky last night. Details: 24mm lens at f/2.8, 25-second exposure at ISO 800. Photo: Bob King

Last night, sunlight reflected from the barren, dusty moon lit the gravel road my dog and I walked. A contrail from a transcontinental jet caught the moonlight and glowed like a white snake across the south. Then an interesting thing happened.  Winds aloft bent the trail in whole – without breaking it into pieces – until it stood straight up and down near Jupiter like a very skinny tornado. It was a weird sight well worth slapping the camera onto a tripod in a hurry.

While Comet Hartley 2 was visible through binoculars and telescope Saturday night, two things are happening that are changing the best times to see it. If you’ve been following the comet either from the armchair or at scopeside, you’ve noticed that it’s been zipping eastward and dropping lower in the sky night after night. Remember when it was in the W of Cassiopeia? That was just over a week ago. Now Hartley 2′s not far from the winter star Capella in Auriga the Charioteer. Auriga’s low in the northeast, so you have to wait until around 10 o’clock for a good view. One more thing. The moon’s out now, and its light brightens the sky, washing out the faint, diffuse comet.

Comet Hartley 2 passed between the star cluster NGC 1528 (left) and the nebula NGC 1491 last Thursday. The time exposure shows the pinkish dust tail extending from the comet's nucleus. Credit: Michael Jaeger

What to do? Set the alarm for the early morning hours when the moon has set and the comet is high in the sky. That’s exactly what I did this morning. At 4 a.m. the comet was almost overhead. When the sleep cleared from my eyes, I sprawled out on the driveway and tried to see it with the naked eye. Yep! There it was. Not bright by any stretch mind you, but it was more obvious than any other time I’ve tried to see it with my original equipment. Through a 15-inch reflecting telescope at low magnification, the big blob glowed a pale emerald green. Tomorrow I’ll post a fresh chart of Hartley 2′s tour dates through Auriga and beyond.

Orion (at right) and other bright winter stars decorate the branches of bare poplar trees in my backyard this morning. Photo: Bob King

I’ve always been an admirer of stars seen sparkling through bare branches, and I’m guessing you’ve noticed this beautiful juxtaposition, too. I’m reminded of Christmas trees and ornaments, or maybe it’s just the bold contrast between tiny, flickering lights and black branches. Whichever or both, I always feel compelled to stop a moment and soak in this vision of Earth and sky touching. When I walked into the backyard to photograph the zodiacal light before dawn, winter’s bright stars were busting through the branches.

Like a tapering cone, the zodiacal light towers in the east this morning just before dawn. You have three mornings left to spot it yourself before moonlight interferes Thursday. Details: 16mm lens at f/2.8, 30-second exposure at ISO 1600. Photo: Bob King

I’ve described the zodiacal light in a previous blog. It’s a tongue-shaped, diffuse glow that extends from the eastern horizon to at least half-way up in the sky just before the start of dawn this month. And it’s BIG. You’ll need dark skies and an open view to the east to see it. The light is sunlight reflecting off comet tail dust and fine debris from asteroid collisions that collects in the plane of the solar system. When the zodiacal light cone is angled high above the horizon muck, as it on moonless fall mornings, it’s at least as bright as the Milky Way. My eyes couldn’t help but be drawn to it this morning – at 5:45 a.m., the light reached all the way from the bottom of Leo up to Gemini, a span of nearly 80 degrees. Yes, the sky was dark, very dark.

The International Space Station passes below Sirius at 6:23 this morning. Details: 35mm lens at f/2.8, ISO 640 and 30-second exposure. Photo: Bob King

The night wouldn’t have been complete without a space station pass. While I put away equipment before hoping to grab a couple hours sleep, up came a star that rivaled Sirius, the sky’s brightest. Even better, it passed not far below that star, so for a time, we had two Siriuses. One stood still, the other had errands to run.

For Duluth and region, there are only a  couple so-so opportunities left in the morning sky before the station re-enters the evening sky on October 26. Tomorrow you can watch it for under a minute in the southeast at 5:16 a.m. On Tuesday morning, we’ll see a similar very brief appearance in the south at 5:42 a.m.

Will the dragon breathe fire tonight?

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Keep an eye out tonight (October 5) for Draconid meteors. The map shows the sky as you face north around 8:30 p.m. Unlike many meteor showers, this one is most active in the early evening rather than after midnight. Created with Stellarium

We haven’t paid much attention to Draco the Dragon this fall, but perhaps we should tonight. This evening the eastern half of the U.S. and western Europe are in line for a potential meteor shower that occasionally reaches storm levels. Called the Draconids, the meteors will appear to radiate from the head of Draco which is nearly overhead during the peak viewing window of 8:30-10:30 p.m. Central Daylight Time. To watch for them, go out during that time, get comfortable in a lawn chair and face north. Other directions will work as well.

The bits of dust and rock that are responsible for the Draconids are debris left behind by Comet 21P/Giacobini-Zinner. This comet was discovered by Michel Giacobini in 1900 and recovered again 1913 by Ernst Zinner. You’ll sometimes hear the Draconids referred to as the Giacobinids after the original discoverer.

This year, Earth encounters a narrow filament of comet dust during the early evening hours for U.S. and Canadian observers. Before you get too excited, you should know this shower is a no-show most years. Will this year be different? Let’s just say it’s definitely worth a look. Given the convenient hour, no moonlight and a radiant as conveniently placed as possible, it’s worth a shot. You’ll know you’re seeing a Draconid if you can trace the meteor’s path all the way back to the radiant point in the head of Draco. Draconids are also known for their slow speed and yellow color.

The shower amazed sky watchers in 1933 and 1946. That’s when Earth crossed the debris-strewn comet’s orbit just weeks after its passage near the sun. When a comet is near the sun, it develops a dust tail that salts and peppers its orbit with lots of fresh material. In 1933, thousands of meteors were seen; one observer in Ireland reported that they fell as frequently as snowflakes. Conditions were excellent again in 1946, especially for the U.S., Canada and South America, with counts briefly as high as 6800 per hour. Though tonight’s shower won’t come close to these spectacles, just wait until next year. That’s when the next BIG Draconid outburst is expected to happen. Stay tuned! For more on the history of the shower, please click HERE.

Comet Hartley 2 is a fuzzball moving eastward through the W of Cassiopeia this week. The green color is from fluorescing cyanogen - related to cyanide - and carbon compounds. Details: 35mm at f/2.8, ISO 3200 and 30-second time exposure. Photo: Bob King

Last night under exceptionally dark skies, I succeeded in seeing Comet Hartley 2 with my eye alone. It was a challenging observation, and the comet looked like nothing more than the faintest bit of haze. To catch it, I used averted vision, a technique of looking off to one side instead of staring directly at an object. Using your peripheral vision exposes a more light-sensitive area of your retina to the target. The view in 10×50 binoculars was great, thanks to the comet having moved away from the confusion of stars it’s nested in the past few nights. Hartley 2′s closest approach to Earth is still two weeks away. On Thursday and Friday it will be near the spectacular Double Cluster of Perseus, one of the finest and brightest deep sky objects in the heavens. Tomorrow I’ll provide an updated chart for finding the comet.

The comet doth please the queen

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Comet Hartley 2Â zips through Cassiopeia the "W" this coming week. With binoculars, look for a dim cloudy patch at the nightly positions shown. Illustration created with Stellarium

The comet on September 20. The green color, from fluorescing gas, and is only faintly visible in a larger telescope. Credit: Michael Jaeger

As Comet 103P Hartley 2 cruises through the bright constellations of Cassiopeia and  Perseus this month, it will be relatively easy to see in binoculars from the outer suburbs and countryside. At present, it looks like a dim misty patch of light among the rich star fields Cassiopeia the Queen. Yesterday night I estimated its size at 2/3 that of the full moon and brightness at magnitude 8. That’s two levels below the faintest star visible to the naked eye for most of us, but don’t think that means you can’t see it. Once my eyes were adapted the darkness, I spotted it in both 8×40 and 10×50 binoculars without difficulty.

Comet Hartley 2's orbit (in blue) takes it near the Earth in October. Closest approach will occur on the 20th. Credit: Caltech/JPL

It helps that Cassiopeia is high in the sky for northern hemisphere sky watchers, clear of horizon haze and much of the typical city light pollution. Hartley 2 should brighten further in the coming weeks as it makes its closest approach to our planet on October 20. On that day, only 11 million miles will separate us, or about 1/8 the distance of Earth to the sun. The hazy glow visible in binoculars is called the comet’s coma (KOH-ma). Those with keen eyes and dark skies will also be able to pick out the brighter nuclear region at the center of the coma. It’s unclear if much of a tail will develop, but if one does, I’ll report it here. Telescopic observers will see the nuclear region with ease as well as be able to tell that the coma is not perfectly circular.

The heart of all cometary activity is the nucleus itself, in this case, a small body made of ice and dust only about 1/2 mile across. When a comet’s orbit takes it into the inner solar system, heat and light from the sun vaporize some of the material, which then expands outward to form a temporary “atmosphere” or cloud around the nucleus. This is the coma we see in our binoculars. The nucleus itself is deeply shrouded within its dusty outflow and invisible except to the eyes of space probes able to get close enough for a good view. Thanks to NASA’s excellent planning, that’s exactly what will happen when the re-purposed Deep Impact probe, now called the EPOXI Mission, flies by the comet with its shutter clicking in early November.

Trouble finding the W of Cassiopeia? Face north and look to the left to locate the Big Dipper. A line drawn from the two end stars in the Dipper's Bowl extended upward will take you first to Polaris, the North Star, and then on up to Cassiopeia. The map shows the sky around 8:30-10 p.m.

With the moon out of the evening sky and Cassiopeia well-placed, now’s the time to hitch your wagon – and eyes – to a comet. I encourage you to share your Hartley observations, seen or unseen, on the blog using the comments link. Thanks!