03/05/2013 12:21 pm ET Updated May 05, 2013

Waiting for ISON

Living our 2-D lives on planet Earth, arguing over Sequestration and who has the remote, we tend to forget that the solar system is a cosmic shooting gallery. The point returns on rare occasions like last month's Russian meteorite fall, but we are generally oblivious to the number of rocks and dirty iceballs floating around our cosmic territory that could see planet Earth as an opportunistic target.

Rarely, a wave of activity takes place from what astronomers like to call small solar system bodies -- several events stacking up in a year -- and we are reminded that humans are not masters of the universe, but simply idle viewers of a much larger canvas than we can ever hope to affect.

The universe has set up such a year for 2013, and the finale will be Comet ISON (C/2012 S1), an icy visitor from the outer edges of our solar system that could thrill us this coming November as it grows as bright as the planet Venus in the early morning sky.

The fact that our Sun has most of the mass in the solar system and pulls in a variety of comets, asteroids and smaller debris scientists like to call meteoroids, is nothing new. Roll back the clock four billion years and we experienced the so-called Late Heavy Bombardment, a period when Earth, the Moon, Mercury and all the other inner solar system bodies were being pelted relentlessly by a vastly larger number of impacts than we see today.

During the Late Heavy Bombardment, a cosmic rain of ruin changed the inner planets forever, leaving bodies like the Moon and Mercury heavily pockmarked with impact craters. Earth, with our erosion, plate tectonics and resurfacing, was the exception, able to hide the deep scars of the past. But two minutes gazing at the Moon with a pair of binoculars drives the point home pretty well.

And most planetary scientists believe the biggest impact of our early history, 4.53 billion years ago, saw a now-gone, Mars-sized body slamming into Earth, creating a ring of debris around our planet that eventually accreted into the Moon.

The history of giant impacts on Earth is also pretty well known. The most celebrated one, which shows up in grade school textbooks the world over, is of course the K-T Impact (now referred to as the K-Pg Impact, for the Cretaceous-Palogene Boundary, an homage to changing geological terminology). The K-Pg Impact occurred when a 10-kilometer asteroid plunged into the Yucatán Peninsula 66 million years ago, and the resulting global firestorms, ignition of free atmospheric oxygen, and other catastrophic mayhem not only killed off the dinosaurs but 75 percent of all species on the planet. Several other major extinction events are known in Earth's history, as are other major impacts. And the fact that we now know about these periodic big impacts doesn't mean they're going to simply stop to honor our increased awareness.

Planetary scientists have now catalogued nearly 10,000 so-called Near Earth Objects (NEOs). These are small solar system bodies, nearly all asteroids but also 93 comets, that balance gravitational dominance by the Sun but also tugs from Jupiter and other bodies, and pass quite close to Earth. Some of them ultimately will intersect Earth's orbit, and if they do so when we're at the right (wrong!) spot in our orbit, we're going to have an interesting day.

At least the accounting of these celestial threats is improving. A generation ago you used to be able to say that the number of scientists working on keeping track of NEOs was smaller than the number of day shift employees at a McDonald's restaurant. Now, automated telescopes like those of the Catalina Sky Survey are cataloguing nearby space rocks to identify potential threats.

The historical record of major impacts leaves us much to think about. There was the K-Pg Impact. In 1908 a small body -- perhaps a cometary nucleus -- exploded in an airburst event over the Podkamennaya Tunguska River, flattening 80 million trees over an area of the Siberian forest spanning 2,150 square km. In 2002 a space rock just 10 meters in diameter exploded in an airburst over the Mediterranean Sea. Six years later a small NEO, asteroid 2008 TC3, broke up over the Sudan.

And last month we had the amazing coincidence of the Russian meteorite event in Chelyabinsk Oblast', the first time that a solar system body caused numerous injuries, 1,500 altogether. These were from flying glass shattered by the superheated air and the resulting shock wave, not from the impact itself. The airburst happened only 16 hours before the close passage of a small asteroid, 2012 DA14, within 27,700 km from Earth's surface -- 13 times closer to us than the Moon.

Of course a small object like the Chelyabinsk meteoroid (a few meters) or asteroid 2012 DA14 (30 meters) would not cause global catastrophic damage like the K-Pg Impact. The doomsday scenario requires an asteroid or comet of about 10 km diameter and then it can be lights out for civilization.

The cosmic rain mostly comes in silence. Every day, some 400 tons of material falls into Earth, nearly all of it harmlessly burning up high in the atmosphere. Some of it, small grains of solar system dust, floats gracefully down to Earth's surface. Most of the dust in your house consists of dead skin particles; nearly one percent of it, however, settled into your house from space. But the unprecedented number of people injured in Russia and the close shave of what could have flattened most of a city are stark reminders of the ambivalence of the cosmos.

The majority of asteroids live in reasonably close quarters between the orbits of Mars and Jupiter. But other bodies, rocky and icy, lie scattered in other areas of the solar system. The icy disk called the Kuiper Belt past the orbit of Neptune serves as a storehouse for bodies like the former planet Pluto. Perhaps 50,000 such icy bodies reside in that region. But far, far out -- on the edge of our solar system and perhaps extending one-quarter of the way to the nearest star, is the mysterious Oort Cloud. This huge shell contains as many as 2 trillion comets, blocks of ice with dust particles locked in their interiors.

Comets serve as windows into the solar system's primitive past. When on rare occasions comets are kicked in toward the Sun, they swoop past our star, warm enough to transform their solid ices into gases, and grow magnificent clouds called comas and long tails of gas and dust, reflecting sunlight and sometimes even shining by fluorescence.

We have such an icy treat in this year of 2013 -- in fact, two. Right now Comet PANSTARRS (C/2011 L4) is a bright visitor in the evening sky, appearing like a 2d magnitude star with a fuzzy tail. And late this year Comet ISON promises to be magnificently bright -- a rare treat that will astonish us.

For now, we need to watch our skies, enjoy PANSTARRS, and wait for ISON. These cosmic events remind us how incredibly large the universe is, and how relatively small we are. Sometimes that can be a very useful thing to be reminded of. Whether it becomes historically bright or simply another typical bright comet, ISON will tell us once again that although the universe takes shots at us, fortunately it usually misses.

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