Block Island—a half-ton meteorite found on Mars by NASA's Opportunity rover.Image credit, NASA/MER OpportunityEver been driving down a lonely desert highway when you suddenly glimpse something in the corner of your eye that makes you think, "What was that?!" You brake, tires screech, you spin the wheel and make a wild U-turn, cutting into the shoulder and leaving a rooster-tail of dust as you floor the gas to get back to what you thought you saw….

Okay, dramatic desert car scene ended. That would be the Hollywood movie version of what NASA's Mars Exploration Rover Opportunity did recently, on the lonely desert highway that it's scouting on Mars.

On its determined long trek from Victoria Crater to the larger Endeavour Crater (a 12-mile span that Opportunity has completed about one fifth of over the past year), the rover passed by an X-box-sized block of iron that presented the appearance of a meteorite. It snapped a picture in passing, which was eventually transmitted to Earth and examined. By this time, Opportunity had already traveled about 180 meters beyond the block (dubbed "Block Island"). This is when the rover was commanded to backtrack all the way to the find (though it's doubtful it worked up a rooster tail).

Upon returning to Block Island—quite obviously an iron-nickel meteorite by appearance alone, but whose composition was confirmed by the rover's alpha particle X-ray spectrometer instrument—Opportunity took more pictures, including extreme close-ups with its microscope camera, which revealed surface patterns similar to those found on Earth iron-nickel meteorites that have been exposed to long-term weathering by wind and sand.

As interesting as stumbling upon a half-ton meteorite on the dusty plains of Mars' Meridiani Planum is, what this particular chunk of weathered iron is telling scientists sparks the imagination. In a nutshell, given the thinness of Mars' current atmosphere, scientists wouldn't expect a meteorite of this size to survive impact intact, at the speed it would be going. One of the possible explanations for Block Island's rock-houndable state is that when it fell to Mars, Mars' atmosphere was substantially thicker than it is now.

Further examination of the meteorite may reveal clues as to how long ago it fell through Martian skies. Evidence that Mars' atmosphere was warmer and thicker in the distant past, as well as the possibility that there was liquid water on the surface, has been mounting over the years. The age of this meteorite-fall could shed more light on the history of Mars' environment. If it fell billions of years ago, Block Island would weigh in as more evidence to support our current suspicions. If, however, we find that it fell more recently, then this could indicate that the atmosphere was more substantial later in Mars' history than we thought.

Imagine, if you will, a Mars that looks even more Earthlike than it does now: seas of water with waves rolling into shorelines, great clouds sending downpours of rain and snow onto mountains and plains, streams and rivers snaking through the landscape. Maybe, maybe, even some form of life?

All that from a rock? Yes, rocks talk, if we listen.

A few weeks ago, this asteroid came really close to hitting Earth.On March 3rd, 2009 at 1:40PM GMT, just a mere month after we'd finished the Asteroid Hunters segment, an asteroid of up to 165 feet in diameter snuck up on us, coming within approximately 37,000 miles from a direct impact with Earth. That's almost seven times closer than the distance to the Moon and about twice the distance of some communications satellites that orbit the 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.

The 1992 'Peekskill' meteorite and its point of
impact in Peekskill, New York. Credit: "Pierre ThomasWhy is it that meteorites are brought to me for identification in clusters? I don't mean that people bring clusters of meteorites-but it seems I get calls and visits from possessors of unknown rock samples, hopeful that they are of extraterrestrial origin, in bursts. This time I got two inquiries in two days!

The first thing I tell people is that I'm not a meteorite expert, but that I have a contact who is. This rarely discourages them from wanting to bring their rocks in for a look.

The first sample was brought in by a family who said they collected the chunk of iron from Lake Tahoe. This one actually looked promising to my mostly untrained eye: a fist-sized chuck of magnetic metal, with pits and holes and an overall melted look. I took some pictures to send off to our regional expert and told the family I'd call them to let them know what he said. The response to the pictures was pretty certain: it wasn't a meteorite, but a chunk of metallic slag. I was told that this is a common mistake; that often bits of slag from old foundries or other sources are taken for meteorites.

The second sample brought to me didn't really strike me as a meteorite, by appearance. It was metallic, but not magnetic; it was pretty heavy for its size; it didn't have any obvious signs of melting, and no real pits or holes-other than one, deep, tunnel-like hole the width of a finger. It didn't appear jagged or shrapnel-like, as fragments from an exploding metallic meteorite often do. Finally, it had wide, flat facets that looked much more like the result of natural rock cleaving as pieces of Earth's crust break apart.

I went ahead and performed a density measurement on the sample. It was pretty heavy, so our sensitive balance scales wouldn't handle the load. Instead, I resorted to our "learn your weight on other planets" scale-the one that tells you how much you would weigh on the Moon, Mars, and other planets, in addition to your Earth weight. (I found this scale useful when I had a package to mail and needed to know the weight; by selecting the Moon weight of the package, I would pay only one-sixth the normal Earth rate!)

The double-fist-sized sample was 11.3 pounds, which converted to 5126 grams. Then, I selected a graduated beaker from our lab, filled it with water and submerged the sample. Reading the difference in water level with and without the sample, I measured a volume displacement of 750 cc. So, the density-mass divided by volume-turned out to be about 6.83 grams/cc. That's twice the typical density of silicate-type rocks (stone), and fairly close to that of pure iron.

I sent the owner off with my appraisal that the rock didn't present the appearance of a meteorite, and though the density was in neighborhood of that of iron, the appearance (black, inside and out) and non-magnetic nature suggested some other metal or metal-stone mixture. As always, I encouraged him to seek an expert appraisal.

Let's face it, all rocks found on Earth are ultimately of extraterrestrial origin-though what we regard as Earth rock has been on Earth for many billions of years, and shaped, reshaped, and metamorphosed by eons of weathering and geological activity. Meteorites, then, are only the newcomers….