Herbert Mehnert a Cline Scholar at the Pisgah Astronomical Research Institute spent his summer researching Comet Photometry and Morphology. Herbert was introduced to PARI by one of his college professors and jumped at the opportunity to work at the former NASA research institute.

"People don't look up anymore," explains Herbert Mehnert.

Herbert spent the summer of 2011 working at the Pisgah Astronomical Research Institute as a Cline Scholar student comet photometry and morphology.

"I think it's partially because the exposure to space and astronomy is much less than it used to be, with government programs being cut and all. When you take someone out here to a dark sky sight and tell them you can see the milky way, they get excited."

When Herbert was introduced to PARI by his college professor, Don Smith, who took them on a field trip to the remote research institute in Rosman, NC, Herbert was excited to know there was a community and research institute full of people interested in the same topics as him, particularly optical astronomy.

Herbert studies Comet Photometry and Morphology. Comet Photometry uses telescopes and cameras to measure the brightness of a comet, which provides scientists with information about its surface, craters, pits, valleys and mountains. The brightness of comets are more difficult to map than stars because the data involves using the nuclear condensation, surrounding cloud or coma and one or more tails extending outward from the comet. Comet Morphology studies the projected velocity and direction of a comet, based on its orbit, trail and size.

What's the difference between a comet and a meteorite? A comet is a structure composed of ice, dust, and elements such as ammonia, carbon dioxide and methane, that orbits around the sun. As it comes close to the sun, the nucleus begins to melt and turn into gas, forming a coma, or cloud. The radiation from the sun pushes this cloud away from the center of the comet, forming a dust tail. The most famous comet, comet Halley, travels around the sun every 76 years, and will reappear in the year 2062. Meteorites on the other hand are solid rock formations found in space. When meteorites enter the earth's atmosphere they heat up and turn into a fire, and appear as a shooting star.

Visit the PARI Sky center for more information and up-to-date celestial news. Also recommended are NASA's page on comets or the Comet Photometry website.

Artist concept of Stardust-NExT approaching comet Tempel 1.
Credit: NASA/JPL-Caltech/LMSS

The human race is certainly leaving its marks on the Solar System. On Valentine's Day, NASA revisited the locale of one of those marks—comet Tempel 1—and far from being a simple "I was here" (like the three scratch marks left behind by Jules Verne's intrepid explorer in Journey to the Center of the Earth), this one was a crater 200 meters across created to see what makes the comet tick….

Cast your mind's eye back five years. Remember NASA's Deep Impact spacecraft, the one that lobbed a heavy metal projectile at Tempel 1 in hopes of seeing what came flying out of the blast, and more excitingly what the hole it made looked like afterward? The idea was to get a better grip on how the comet is put together (is it crunchy, powdery, ice-hard; is it light like Styrofoam, or weightier like block-ice, or concrete?).

Well, as it turned out back in 2006, Deep Impact successfully bullseyed the little 4.5-mile long ice potato—and the impact was so effective that the spectacular blast, as good as anything from the ILM special effects department, completely obscured the spacecraft’s camera-eye view, hiding the would-be crater from sight. The mission was a success, I should add; Deep Impact got plenty of good data and images of the comet and the blast plume—just not the man-made-object-made crater it made….

Fast forward to February 14, 2011. Exercising a good reduce-reuse-recycle ethic, NASA sent the "NExT" spacecraft past Tempel 1, visiting the same comet twice for the first time in history, and doing so with an existing spacecraft that had completed its initial mission years before.

Cast your mind's eye back to the Stardust mission, which flew through the tale of comet Wilde 2, collecting particles from the plume in a block of comet dust "fly paper" made of aerogel (basically glass spun up so light and fluffy as to barely register any weight or substance, a piece of which looks like a slab of solid smoke), and dropping the collector pod back on Earth for the first ever comet sample return mission. Stardust flew on, circling the Sun for several years, and was finally re-tasked as NExT—the "New Exploration of Tempel 1" mission.

It's as if NASA is playing a celestial shell game: Where's the spacecraft now? What is it named? Which comet is it going to this time?

NExT flew within 112 miles of the Tempel 1 nucleus and captured over 70 images. Yes, scientists hoped to capture an image of the crater left behind by Deep Impact—and they did; see if you can spot it too. But, there is plenty more to interest us than a hole in the snow. The fact that this comet had been visited five years prior means that we can compare images from then and now to see any changes that may have taken place. As it turns out, five years is not only the interval between the visits by Deep Impact and Stardust/NExT, it's also the orbital period of comet Tempel 1—so, the comet had made exactly one swing around the Sun between visits, providing the opportunity to study Sun-induced changes.

Tempel 1 is about 4.5 by 2.9 miles in size, and has an average density of about 0.62 grams per cubic centimeter—about five times denser than the densest Styrofoam, two-thirds as dense as ice, and 600 times more dense than aerogel…. Its elliptical orbit carries it between the orbits of Mars and Jupiter—and it is that range of exposure to solar radiation that has researchers looking for physical changes in the comet.

By the way, the Deep Impact spacecraft was also reused to make a second comet flyby. Five years after bombing Tempel 1, Deep Impact, renamed EPOXI, flew by the peanut-shaped comet Hartley 2, back in November. Between Stardust and Deep Impact, there were a lot of firsts: first sample return mission from a comet; first time a single spacecraft has visited two comets; first time two spacecraft have visited the same comet….

It's doubtful there is enough fuel left on either of these veterans for another opportunistic encounter, but talk about bang for the buck….

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Comet Hartley 2 during the November 4, 2010 NASA/EPOXI
flyby mission

If you were with us early that morning, you were one of a lucky and intrepid few to experience live the flight of NASA's EPOXI mission (aka the Deep Impact spacecraft) past the nucleus of tiny Comet Hartley 2—a comet that we had been watching through our telescopes for a few weeks. Live space exploration as a public experience is a rare and extraordinary opportunity, and we strive to share it with you whenever we can. Kudos to those who got up before 6:00 AM to join us.

EPOXI is the name for the extended mission of the re-purposed Deep Impact spacecraft, which you may remember lobbed a projectile at Comet Tempel 1 back in 2005. Since that encounter, the spacecraft has been orbiting the Sun, keeping itself busy with other observational activities, including looking for extra-solar planets. But the time had come, the orbital trajectories matched, for an encounter with Hartley 2, a smallish comet circling the Sun between the orbits of Earth and Jupiter. The EPOXI flyby was the fifth time in history that we have captured close-up images of a comet nucleus, and the first time a single spacecraft has bagged two.

What are scientists looking for by sending spacecraft to these distant space-bergs that they can't get with big ground-based telescopes, or the Hubble? In a word, details. From afar, we have observed comets for a long time, and can point telescopes at their nuclei, their comas (the shroud of gas they develop when the comets' ices are warmed in a pass by the Sun), and their long tails—yes, plural: comets typically develop two tails, one a trail of dust and the other a plume of gas, blown by the ever-present breeze of plasma from the Sun called the solar wind.

But comets are tiny objects, generally–even famous comet Halley is less than ten miles across—and usually relatively far away from Earth (a fact that we can give thanks for). Comet Hartley 2 is only about a mile across. This makes seeing details of comet structure and surface difficult even with the biggest ground-based telescopes. And though EPOXI's sweep is considered a close encounter, even that situation would be like using a modest-sized telescope to examine Emeryville, California from Los Angeles.

When EPOXI flew past the comet nucleus that morning, coming to within 435 miles, the comet was quite active, spewing out jets of carbon dioxide gas and traces of cyanide (!) in several directions. With its cameras trained on the nucleus during the flyby, EPOXI captured stunning detailed images that revealed which parts of the comet's surface were the sources of the gas plumes—two dots that had never before been connected in our observations of comets. Adding to the excitement of this encounter, an unexpected discovery was made after the flyby, when the data collected by EPOXI was analyzed: unlike the other comet nuclei that we have probed up close (Halley, Borrelly, Wilde 2, and Tempel 1), Hartley 2 was surrounded by a cocoon of snow!

Seeing the structure of the comet (which turns out to be quite peanut-shaped), the surface details across different parts of it, and the nature of the regions spewing out the gases, scientists can gain insight into the overall nature of this comet. Stay tuned for more as EPOXI data is further analyzed….

Back to our own flyby experience: sitting in our Megadome Theater watching 30-foot images from NASA/JPL Mission Control and waiting for our first up-close glimpse of the comet nucleus would have had me sitting on the edge of my seat—if I could have sat down. It was that exciting….

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Comet Wild 2, as imaged by NASA's Stardust spacecraft.

Well, at Chabot, we like to bring things down to Earth a bit—not to diminish their wonder and awe-inspiring beauty, but rather to give us a chance to connect with pieces of the Universe in a personal way that—we hope—will only enhance their wonder.

Such it is with the Personal Comet. We make them in our classrooms, and our teen volunteers—Galaxy Explorers—sometimes set up a table in our exhibits to make comets for our visitors.

Last week I was teaching a lively group of 3rd graders our class, "Shooting Stars" (I'm not the usual teacher for this class, but love teaching this one especially).

"Let's make a comet," I announced, receiving a classroomful of suddenly puzzled 8-year-old faces. "Follow me." I led the group to the back of the room and had them sit in front of our rolling comet kitchen, tying on an apron and donning the "Comet Chef" hat.

"What's in a comet?" I quizzed. Hands went up, and answers were plucked out. "Water." "Ice." "Very small pebbles."

"Good answers. Let's start cookin'…."

One by one, and two at a time, I called up volunteers (no lack of these at all) to add ingredients into the mixing bowl.

Water first—two cupfuls. What else? The Chef helped out his students a bit: pebbles are fine, but what are they?

Silicon, calcium, to name a couple—so we add a source of these: sand. Then, iron, and magnesium—two more known comet constituents. Source? Dirt! Dirt can contain these, among other things.

For the carbon in our comet, we added—carbon: black charcoal dust; just a dusting.

There is nitrogen mixed up in comets too, so we had to add some to our concoction. Our nitrogen source (other than all the gaseous nitrogen floating about us in the air) is ammonia—NH₃ (what's a little hydrogen in the compound, more or less? In fact, ammonia itself has been detected in comets, so our recipe is true).

Organic compounds—carbon based organic molecules—have also been detected in comets…so we add a couple glugs of corn syrup. Okay, that's cheating a bit, because comet organics aren't known to come from agricultural products….

And lastly, but not leastly, is the two-in-one ingredient: contained in a plastic bag, I had the class use mallets to pulverize a few cupfuls of dry ice pellets into a fine powder. This adds not only the carbon dioxide to the material makeup of the Personal Comet, but that which really makes the difference between a bowlful of slightly sweet mud and the astral nugget of a comet: COLD! Space is cold, and so is dry ice.
In goes the frigid, dry frost, and out erupts clouds of billowing vapor—very exciting stuff. Now the Chef had to work fast, stirring and mixing and shaking the brew, and then squeezing the convulsive mixture in its plastic bag, with gloved hands, into a hard, solid lump. It takes a lot of squeezing, as it turns out….

Is it done yet?

I pulled out of the bag our Personal Comet, holding it out for the kids to marvel at—and they did. The double-fist-sized, dirty whitish, somewhat gritty hard blob is covered with pits and knobs and spouting plumes vapor. And, magically, it looks almost identical to a photograph of the nucleus of comet Wild 2, taken by NASA's Stardust spacecraft some years ago.

Everyone got to touch the comet—quickly, because it was quite cold—and make a personal connection with a tiny piece of the heavens. Now, I think, these kids are armed with an experience that will make their first comet-observing experience (yet to come for many of them!) a bit deeper, as I hope when they do see the vastly awesome sight in the sky, or through the eyepiece of a telescope, they'll also remember what it's like to touch a comet, or smell a comet, or see one spouting vapor right before them, in the palm of their hand….

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Hot spot created by impact on Jupiter, taken by NASA's Infrared Telescope Facility in Hawaii. Picture credit, NASA.

Evidently, the aftermath of some kind of collision on Jupiter was spotted by an amateur astronomer in Australia that Monday morning. He spotted a dark marking near the planet's South Pole, and alerted NASA. NASA in turn turned its large infrared telescope in Hawaii onto the scene of the crash.

There glowed the thermal footprint of the likely impact, the affected area roughly the size of the Earth. Had this impact taken place on Earth instead, the results would have been catastrophic. Fortunately this was Jupiter, half a billion miles away and large enough to absorb the impact without lasting effects. (And, owing to the fact that Jupiter is a gaseous planet with no solid surface, it would quickly heal from the trauma, not unlike that liquid-metal Terminator from the second movie of the same name.)

A significant event? Yes, in fact. But that's not all…

Rewind 15 years to July 20th, 1994, the middle of the week during which twenty-something fragments of the broken comet Shoemaker-Levy 9 were in fact colliding with Jupiter… An amazing coincidence? Yes; the two events likely have nothing to do with each other. So, then, a common event, if we're seeing two of them in the span of only 15 years? Well… not really.

When the string of fragments of Shoemaker-Levy 9 hailed down on Jupiter, it was the first time in history that humans had observed actual impacts on a Solar System body (other than perhaps the Sun–but as it turns out comets hitting that huge target are not uncommon). The Shoemaker-Levy 9 impacts, and the one on July 20th this year, left highly visible marks that lasted for days. The amateur astronomer who discovered the recent scar did so with a relatively small 14.5" backyard telescope! So, if this sort of impact were a common event, even if the impacting comets or asteroids were never seen, the gashes they leave in Jupiter's atmosphere ought to be spotted from time to time.

Impacts—on Jupiter, Earth, and all the bodies of the Solar System—do occur, and the smaller the impacting object, the more frequently they happen. For a planet like Earth, on average a chunk of rock a few meters across enters our atmosphere about once a year, and often burns up completely or explodes before hitting the ground. A 50 meter object, again on average, is likely to strike Earth once in a century. A one-kilometer object impact averages every few hundred thousand years, and a multi-kilometer sized asteroid or comet similar to the one that wiped out the dinosaurs and which would cause global catastrophe—well, the last one of that size struck ground 65 million years ago.

As for Jupiter, being a larger target than Earth, having a much stronger gravitational pull, and being close to the asteroid belt—well, Jupiter's impact statistics should probably involve higher frequencies than Earth.
In fact, impacts like the one on July 20th are happy events for us; every time Jupiter is hit by a large object, that's one less object in the Solar System that could potentially hit the Earth in the future. So, on July 20th, Jupiter took another bullet for us.

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A few weeks ago, this asteroid came really close to hitting Earth.

Called “2009 DD45”, the asteroid was estimated to be around the same size as the one that exploded in the atmosphere near the Podkamennaya Tunguska River in remote Siberia on June 30th, 1908, flattening 80 million trees across eight hundred square miles of remote forest. Of course, if an asteroid of this size were to hit a city or in an ocean offshore from a populated area, tens of thousands of people would likely die.

Then, just as the last of the night sky observers were completing their collective sighs of relief, on March 17th, 2009 another Tunguska-class asteroid, 2009 FH, passed by about 53,000 miles from Earth. Thankfully, neither of these asteroids actually hit us. But astronomers didn’t even observe 2009 DD45 until 4 days before its closest approach. It's orbit was calculated and it was determined that it would miss the Earth. But it's likely that asteroids of this size are fairly frequently buzzing by the Earth. And until recently, most of them have been undetected.

In 1998, NASA started the Spaceguard Survey which set out to discover 90% of those Near Earth Asteroids (NEAs) 1 km in diameter and larger. An impact by an asteroid this size would likely cause global destruction and an end to much of life as we know it so it’s definitely reassuring that 10 years after its inception, the Spaceguard Survey had found about 80% (CK) of them. But unfortunately, once we’ve found them, there’s still no international concensus or infrastructure in place in how to deflect or destroy them. But the Survey is limited by its mandate to find those mass extinction-sized asteroids as well as by the size and sophistication of the telescopes that are dedicated to searching the skies.

As former Apollo 9 astronaut, Rusty Schweickart said in a recent phone conversation, "in the process of finding the big ones, you also find a bunch of small ones, and the smaller ones are obviously far more numerous than the large ones." But it will take many more resources and new telescopes to continue searching for and tracking the smaller ones. And unfortunately, once we’ve found them, there's still no international consensus or infrastructure in place in how to deflect or destroy them. Raising awareness and building alliances amongst governments and space agencies is Schweikart's current "mission". He founded the B612 Foundation and Association of Space Explorers to tackle these goals on different fronts.

The message that I hope is conveyed with the Asteroid Hunters TV segment is that we are not immune from asteroid impacts here on Earth. Rusty Schweikart puts it best in a portion of his interview that didn’t make it into the final program:

"Well, asteroids and comets are good news and bad news, you know? But for them we wouldn’t be here, and on the other hand, if we don't actually take some action now, at some point we won’t be here anymore, because there's no question that we will be hit by asteroids, and we’ll probably be hit by, we would be hit by comets as well. Unless, we use the technology that we have and the brains that we have in order to protect the Earth from asteroid impacts, and we can do that. We can basically now, with current technology, assure that no asteroid ever hits the Earth again. That can do any serious damage."
-Rusty Schweikart

Here's a little exercise from Rusty that you can do to get a sense of what we know today about exactly what's out there:

• Go to: neo.jpl.nasa.gov/risk
• See two tables, the first table says "Recently Observed Objects" and the table below says "Objects not recently observed." You’ll notice in the bottom table that Apophis is the 4th one listed.
• Click on "Apophis". At the top you see a bunch of boxes, like the diameter at .27 km, or 270 meters.
• Down below that you see 3 lines, those are the 3 potential impacts. The first one is April 13, 2036. Go over to the right on that line you'll see the column "Impact Probability" is 2.3 x 10-5 – click on that. So there is the probability, 1 in 43,000 of that particular impact.
• Now if you go back to the main table you can do the same thing with every single one of those lines.
• Now go to the very top of the page and hit "Discovery Statistics." Scroll down to a blue and red graph "Known Near-Earth Asteroids". This shows the discovery rate beginning back in 1980 going up to almost current time. Notice the knee in that curve in 1998 – that’s when the Spaceguard Survey began.
• Scroll down to table just below the graph and look across that table to the far right side, to see that a a total of 6166 NEOs (of ALL sizes) have been discovered.

Rusty concludes that, "…what we really care about is not only the things that large, we care about things that can hurt us. Things that can hurt us go down to 40 to 45 meters or so. Instead of there being 940 of them, there are more like 600,000 of them. So the new charge for NASA, which they have so far ignored, is to find 90% of the objects 140 meters and larger by 2020. You can't reasonably set a goal to find everything down to 40 meters because it's just beyond the capability of telescopes and the money available. So NASA, working with Congress, set the goal at 140 meters. Now nevertheless, when you are looking for 140 meter objects, it’s going to take bigger telescopes than the ones to find a kilometer. Therefore we are going to find many many smaller objects as well. So 10 to 15 years from now, instead of that number on the far right hand column being 6000, it will be 1 million."

Watch the Asteroid Hunters television story online.

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Comet Holmes, photographed on October 24, 2007, shortly after its
unexpected outburst.
Credit: Conrad Jung

British astronomer, Edwin Holmes, discovered Comet Holmes in 1892. The comet orbits the Sun once about every seven years, bopping between the endpoints of its elliptical orbit from a point a bit beyond Mars' orbit (at its closest distance to the Sun) all the way out to the distance of Jupiter's orbit. Ordinarily, this comet doesn't become an unaided-eye apparition, usually cruising by well below the necessary brightness for this type of visibility. In fact, it's usually not easily visible in telescopes.

However, on October 24th something happened that surprised us all: Comet Holmes experienced an "outburst," suddenly exuding gas that expanded to form a bubble-like cloud around the comet nucleus.

The bubble — called the "coma" — is much larger than the icy nucleus; in fact, a typical comet coma fills a volume of space greater than a planet, even if the nucleus itself is a modest object only tens of miles across. Most likely induced by the comet's closest passage to the Sun last May, and all the solar heating of its ices that passage entails, the outburst was so quick and so pronounced that the comet went from an extremely faint 18th magnitude object to about a million times brighter, becoming easily visible to the unaided eye.

This is why I've dubbed Holmes the "Popcorn Comet." One moment it was a small, dark, dense kernel of ice and gravel, speeding along it's path farther and farther away from the Sun, the next moment (literally hours later), POP!, it was surrounded by a cloud many times its size and suddenly reflecting a million times more sunlight. By October 27th, the shell of the coma had expanded to a size quite a bit larger than the planet Jupiter.

I've had a number of calls and emails asking about this comet. One person, in fact, emailed from San Lorenzo asking what this fuzzy thing was that he could see in the northeastern sky. "It looked like a small moon," he commented.

At the moment (November 6th) the comet is still visible, appearing as a slightly fuzzy "star" in the constellation Perseus, which is somewhat high in the northeast sky during the evening hours. For a time, Comet Holmes was the third brightest object in Perseus, but has since tapered a bit. It's difficult to say how long it will remain visible, as comets are notoriously unpredictable in their appearances and disappearances.

For myself, it was a real treat, as unaided-eye comets are fairly rare — and though Holmes, making its way around the Sun every 7 years, isn't like a Haley that only passes by once (or, if you're lucky, twice) in a lifetime, Holmes also doesn't normally put in an appearance. So, while I have a chance of seeing Haley once more, when I'm an old blogger, I may never see Holmes again.

I was delighted to wake up my daughter at 10:30 PM so that she could get a glimpse of Holmes through my spotting scope. Now, she can take the memory with her through her life.

Benjamin Burress is a staff astronomer at The Chabot Space & Science Center in Oakland, CA.

latitude: 37.8148, longitude: -122.178