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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Planet Jupiter. Credit: NASA, ESA, A. Simon-Miller (GSFC),
I. de Pater, M. Wong (UC Berkeley)

Opposition is the best time to see a planet like Jupiter: it's at its largest visual size, it's nowhere near the glare of the Sun, and it's in the sky all night long. Score on three counts—how many things in life work out so that the best of all worlds occur at the same time?

The Opposition of Jupiter occurs about once each year, the timing governed mostly by Earth's relatively speedier orbit around the Sun. Earth takes one year to revolve, while Jupiter takes closer to 12 years. So, opposition actually happens every year plus one month; in the time Earth has taken to come back around to the previous point of opposition, Jupiter has moved along one twelfth of its orbit, so it takes Earth an extra month to "catch up."

This year, the date of the Opposition of Jupiter is September 21, at which time the planet will be only 3.95 Astronomical Units away—about 591 million kilometers (369 million miles). Sounds faraway—and it is—but Jupiter's size helps make up for this. With a diameter of 142,984 kilometers (over 11 times that of Earth), even at this distance there's a lot to see, even through a small telescope.

I remember observing Jupiter through my first telescope—a 4-inch reflector. Jupiter's four "Galilean" moons are easy to see even through a telescope that small, but I could also make out, just barely, the parallel streaks across Jupiter's face that are formed by its cloud belts…ah, my young eyes then! At Chabot, we'll be observing Jupiter in the weeks ahead—come up and take a look through some very large telescopes, Friday and Saturday nights!

Right now, Jupiter rises after 9:00 PM, but by mid-September it will rise closer to 7:30 PM. We should be picking it up toward the end of September, and then onward into the Autumn.

Current Event from Jupiter: Another "big whack" observed, on August 20th 2010. Any planet is potentially subject to the occasional impact by a large object—an asteroid or a comet. Even Earth. But, it appears, Jupiter has been hit by comets and/or asteroids rather frequently of late (three times in the last 13 months), leading some to suggest that this kind of thing may go on more often that at once thought.

So as the recent clustering of the planets Venus, Mars, and Saturn ride off into the sunset to our west, we can now turn the other way and welcome the return of Jupiter, and maybe give our thanks to this huge debris-vacuuming gravity trap of our solar system that may have taken some cosmic bullets for us….

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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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Artwork from Jules Verne’s 1865 novel, From the Earth to the Moon

Sounds like something out of a Jules Verne novel… but that's exactly what NASA's up to this year with the upcoming LCROSS (Lunar Crater Observation and Sensing Satellite) mission, scheduled for launch on June 2nd and impact sometime in October– exact date TBA.

And it's not unprecedented, either: the Lunar Prospector spacecraft back in 1998/1999, whose instruments detected possible signs of water ice in craters around the Moon's poles, was crashed into the Moon's South Pole at the end of its mission. The aim was to blast up a cloud of material from the lunar surface and spectroscopically analyze the plume in search of water vapor. None was detected then, but that's where LCROSS comes in.

LCROSS will seek to verify the presence or absence of water ice and related hydrated materials buried at the bottom of a permanently shadowed crater floor on the Moon's South Pole. Water ice cannot persist on any part of the Moon's surface that is subjected to sunlight, but because of the Moon's low axial tilt with respect to the ecliptic (the Sun's apparent annual path in the sky)– only about 1.5 degrees– there are craters at the Moon's poles whose floors never see the light of day, all month long and year round. Water ice could persist near the surface in these places.

LCROSS consists of two pieces: a "Shepherding Spacecraft" that will guide the whole affair to the proper location on the Moon's South Pole, and the Centaur rocket stage that propelled the spacecraft to the Moon. The pair will separate, and the Centaur rocket will become the primary impactor, striking ground and producing a crater and plume of ejected material. Viewing the event from above, the Shepherding Spacecraft will use cameras and other instruments to analyze the plume from a distance, and will then follow the same course as the Centaur, descending four minutes after impact through the ejected plume and analyzing material samples as it falls.

Then, the Shepherding Spacecraft, too, will impact the Moon– and the plume it kicks up may well be visible through modest sized telescopes on Earth. We're planning to watch the explosion live through our telescopes at Chabot, weather permitting. Keep an eye on our website for details.

Now, back to Jules Verne for a moment. The launching of a projectile with the intent of striking the Moon was indeed the subject of one of his novels, From the Earth to the Moon, published in 1865. Fired from an enormous cannon, the goal of that post Civil War mission was to catch the attention of anyone living on the Moon, to open up a line of communication with their civilization.

My wife asked me if crashing a probe into the Moon would have any harmful effects, particularly if in fact there is any form of life (subsurface microbes or such) living there. Well, certainly, if you happen to be a lifeform living at ground zero of the impact… but the fact is the Moon is frequently struck by meteorites much larger than the LCROSS impactor anyway. To paraphrase Douglas Adams, "that kind of thing goes on all the time."

One last fun tidbit about the Jules Verne novel: the launch site for his cannon-fired projectile was a place in Florida, 50 miles south of Tampa Bay, and only about 135 miles from the Kennedy Space Center, from which LCROSS will be launched…

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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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