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….

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.

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.

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