Close-ups of asteroids captured by robotic spacecraft.

When you hear the word "asteroid," what image is evoked in your mind? A confusion of tumbling, skittering mountains in space suitable for a breath-defying spaceship chase? Blue-white digital outlines of big rocks flying around a video screen, ready to be blasted into pixel dust by your torpedo cannon? Or perhaps, as some science fiction stories would have it, a tiny barren world complete with a breathable atmosphere and giant space worms living in deep crater caverns….

Ah, gotta love science fiction.

As Sci-Fi is often more colorful than reality, are the facts about asteroids a bit more down to Earth (so to speak)? Big gray rocks, perhaps quite large, but otherwise less interesting even than the granite mountains of an Earthly place like Yosemite? Well…try scooping up a handful of sand; at first glance you may see only a pile of tawny-gray grains. Look more closely—with a magnifying glass or microscope—and a veritable treasure trove of colorful jewels is revealed.

NASA's Spitzer Space Telescope is just such an instrument, and has revealed that asteroids may have more variety than once imagined.

Spitzer recently ran out of the supply of coolant that kept its infrared cameras cold—at least, cold enough to sense the subtle heat radiation emitted by distant celestial objects. Now that Spitzer's instruments have "warmed up" to a balmy negative 406 degrees F, observers have shifted their attention to other objectives, including asteroids.

In particular, Spitzer has observed the infrared emanations from a hundred or so Near Earth Asteroids: those that cross Earth's orbit in the course of their own elliptical routines. There are plans to observe 600 or more NEAs in the future—out of about 7000 currently known Near Earth Objects (which includes comets and meteoroids).

The observing program aims to give us a clearer focus on the individual characteristics of these flying mountains that Earth shares space with: the sizes, the composition, and even the age and origins of what is proving to be a diverse and rich population. Spitzer's infrared cameras see past the façade of mere visible light, which, at the distance of most NEOs, doesn't tell us much more than the amount of light they reflect. A big, dark asteroid and a small, light-colored one may reflect the same amount of light, giving those distant specks in the telescopic image the same appearance.

But throwing in an infrared measurement of an asteroid's heat emissions can reveal much more: the temperature of an asteroid is governed by the amount of sunlight it absorbs, which is in turn governed by size, color, composition, and its distance from the Sun.

Already Spitzer's initial 100 asteroid observations have revealed a wide variety of characteristics—maybe not unlike that handful of sand grains scrutinized under a microscope.

Meteorites, having fallen to Earth and been examined up close, have long revealed that their "source mountains" (their parent asteroids, from which they were broken away by collisions) are diverse in composition. We find stone meteorites, and meteorites of solid nickel-iron, sometimes embedded with crystalline gemstones of great beauty.

Combining the compositional data on NEOs gleaned from Spitzer with their orbital characteristics determined through ground-based observatories–such as Chabot's own Asteroid Search program—we are learning a great deal about the NEO neighborhood, and what exactly may be passing quietly in the night. It's all good information, giving us a better handle on how to protect our planet from possible impacts, and laying the groundwork for future missions of exploration to Earth-passing asteroids.

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Apophis is about the same size as the asteroid that blasted
the mile-wide Barringer Crater in Arizona.
Credit: David Roddy, USGS

Fortunately, scientists have already predicted, 20 years in advance, that this is our lucky day: Apophis won't hit the Earth at that time. Rest assured (pretty much).

Discovered in 2004, Apophis is an asteroid about 270 meters across that orbits the Sun at distances ranging from about one astronomical unit (1 AU; the distance between Earth and the Sun) and about three quarters of an AU. Apophis orbits the Sun once every 323 days.

After its initial discovery, before our knowledge of its orbital trajectory had been refined, astronomers had predicted that there was a small chance it could hit the Earth on April 13, 2029, but as we got a clearer picture of its orbit the probability dwindled to practically nothing. Instead, Apophis will pass by Earth no closer than about 18,000 miles. Whew! Disaster averted, and we didn't even have to send Bruce Willis to deal with it.

But wait–that's not all. Though Apophis almost certainly won't hit us in 2029, there's a chance that this close encounter will set the asteroid up for an impact with Earth in 2036–something like 1 in 45,000.

So, if we know there won't be an impact in 2029, why don't we know whether or not there will be one in 2036? Why all the suspense?

Here's where I pull out my pinball analogy. Think of a pinball machine. The play zone around your flippers represents near-Earth space, the various bumpers up in the field represent all the planets, the Sun, and other large asteroids of the Solar System, and the pinball represents a Near Earth Asteroid, like Apophis.

When the pinball inevitably comes into the play zone, there are two possibilities: either it will hit (or be hit by) one of your flippers and thus be deflected back into the field where it will bounce around some more between bumpers, or it will sail right through that dreaded "window" between the tips of the flippers and fall into the end pocket–which represents Terra Firma and a catastrophe if a NEA falls there. As any pinball player knows, it's nearly impossible to predict exactly what path the pinball will follow into the play zone until it gets close.

It's a lot like that with a NEA in the Solar System: as it orbits around the Sun, its course is influenced by the gravitational pull of planets, large asteroids, and potentially smaller asteroids that it might pass close to. A very small deviation in a NEA's direction or speed can, over time, "amplify" into a very large difference in position much farther down the road.

Given the 2029 close encounter with Earth, though we're reasonably confident Apophis won't hit us on that pass, we don't know precisely how that encounter will alter Apophis' orbit. The gravitational interaction between Earth and a NEA passing close by is a complex one, with many variables, not the least of which is Earth's non-uniform gravitational field.

If Apophis passes Earth through precisely the right "window" in 2029–say, right between the flipper tips–then it could be set up for an impact at its 2036 encounter. That window, called a gravitational keyhole, is only about 600 meters across for the 2029 encounter.

As we gather more data on Apophis, we'll get a better prediction for what may happen in 2036–but right now the odds are that it will ultimately miss us at that time. That's a good thing, too, because at that time Bruce Willis will be 81 years old… and even John Glenn was only 80 when he returned to space…

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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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The Hoba meteorite in Namibia, Africa, the largest known
meteorite found; approximately the size of 2008 TC3 before
it burned up in our atmosphere.

Wait a moment… an asteroid you say? Hitting the Earth? Isn't that supposed to spell some kind of disaster, such as Dino-slaughter? Isn't that something we send people like Bruce Willis and Clint Eastwood to deal with before it becomes a problem down here on Earth?

Okay, so Asteroid 2008 TC3 wasn't an Earth-killer, but rather a crowd-thriller. It wasn't miles across-not even tens of meters across. It was, perhaps, a few meters in size, similar in volume to mid-size car. In fact, it didn't even hit the Earth's surface, but vaporized in the atmosphere.

Sounds a bit anticlimactic-and that's not the half of it. It's not even a rare event! Objects of this size are believed (and sometimes observed) to enter Earth's atmosphere a few times each year. So what's the blog deal?

The blog deal is this: this is the first time that an object this size has been detected approaching the Earth a significant period of time before actually impacting-in this case, about a day. 2008 TC3 was detected by the Mount Lemmon telescope in Arizona on Monday. The detection was reported to the Minor Planet Center, which collects such observations from observatories large and small (including Chabot Space & Science Center) in order to track and predict possible Earth impactors. In turn, the MPC alerted NASA of the impending impact.

Observers on the ground reported the fireball lit up the skies with the intensity of the Full Moon. A nearby airliner (not in danger, as the fireball exploded tens of kilometers above the ground, well above the airliner's flight path) reported seeing a bright flash.

In a sense, this event was kind of a dress rehearsal for the international system of predicting, and possibly defending against, impacts on Earth by much larger asteroids and comets. We already know of thousands of Near Earth Objects (NEOs-asteroids and comets that cross Earth's orbit and are large enough to cause a catastrophe should they strike the Earth). It is also expected that there are many thousands more that we haven't yet detected, being small enough to "fly under the radar" of our NEO detecting network.

Early detection and sustained tracking of NEOs is key to the protection plan against impact disaster. If we can accurately predict an impact far enough in advance, we could potentially send a spacecraft to it and gently "nudge" it off course and deflect the eventual impact.

So ends the existence of another chunk of rock that had, up to that point, been serenely orbiting the Sun for billions of years…

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Asteroid 35107, captured on Chabot Space
& Science Center’s telescope.

Photo By Conrad Jung and Gerald McKeegan

You must be very quiet; we are hunting…asteroids!

On July 14th, 2008, an almost Hollywood-like drama took place in space nearby: a "double," or binary, asteroid whizzed past Earth, grazing by at a distance of only 1.4 million miles. One of the rocks is over 200 meters across, the other a whopping 600 meters-about half the size of Half Dome in Yosemite!

1.4 million miles may sound like a large distance, but by the standard of big rocks flying by the Earth, that's breathtakingly close. Discovered only last January, this pair of asteroids went from being completely unknown to blasting by Earth's doorstep in only months. Had they actually hit the Earth, they would have caused major devastation at and near the impact site, with very little warning.

Fortunately, there are programs to search for and track these flying mountains-also called "Near Earth Objects" (NEOs)-and I'm very pleased to announce that Chabot Space & Science Center (specifically our 36-inch reflecting telescope, "Nellie") has very recently become an official contributor to the NEO search program of the International Astronomical Union's Minor Planet Center (MPC)! Nellie is designated by the MPC as Observatory G58.

In this MPC program, observatories around the world contribute by searching for and tracking NEOs: asteroids, and comets, whose orbits can carry them close to Earth and which are large enough to cause catastrophic damage should they hit us.

In order to take part in the NEO program, Chabot observers Conrad Jung (on the Chabot staff) and Gerald McKeegan (of the Eastbay Astronomical Society) conducted a four-month program to develop and hone the necessary skills and data processing techniques, as well as to configure telescope equipment, to meet MPC qualifications.

To that end, they observed a set of known asteroids-some NEO's and some "Main Belt" asteroids. (One of these Main Belt asteroids, "Carter 10683," was named for former Chabot board member and president of the Eastbay Astronomical Society, Carter Roberts, who, sadly, passed away earlier this year.)

Chabot's asteroid hunters will begin their tenure of official asteroid observation by verifying the orbits of recently discovered NEOs and reporting the additional observations to the MPC, where it will be used to refine our knowledge of the NEOs' orbits. The next step in the program will ultimately be to hunt for currently undiscovered asteroids.

The process for finding, tracking, and reporting NEO observations goes something like this. With a digital (CCD) camera attached to the telescope, a section of the sky is imaged three or four times in a half-hour period. The images are processed and compared, and any star-like dots that are found to move between one image and the next become suspect asteroids. (The word "asteroid," by the way, literally means "star-like"-so named because through most telescopes asteroids are too far away and too small to appear as anything more than points of light.)

The coordinates of any moving dots are calculated for all of the images they are in, and this information is sent to the MPC to be added to the data from other NEO hunting observatories. From the combined observations of all the observatories, a precision database of the orbits of near-Earth rocks is maintained, and with it NEOs that may pose a threat to the Earth may be identified.

Hunting NEOs may be like searching for needles in a really big haystack-but in jobs like this, the more eyes on the problem the better. Nellie is now one more eye on lookout duty…

Click here for a closer view of the asteroid shown above.

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