The Day the Earth Stood Still

June 30, 2008

The year is 1958. It's a normal June morning in Denver with sunny skies and moderate temperatures. People are on their way to work.

Without a second’s warning the sky explodes in a blinding flash. Buildings and houses are ripped apart across the metropolis. The tallest buildings are reduced to windowless charred steel skeletons.  Commuters are incinerated as searing hot winds tumble their cars. People are tossed like rag dolls as their clothes burn off their backs in neighboring Boulder.

The President must decide whether to order strategic defense bombers to fly beyond their fail-safe points and toward the Soviet Union. This can’t be anything other than a Communist sneak nuclear attack unleashing an estimated yield of 10 megatons of TNT over Denver.

But perplexed Air Force generals report that no incoming ICBM was tracked, and no Soviet bombers are seen coming over the North Pole. There is panic and confusion at the White House.

The Kremlin is equally shocked. Some top generals argue that the Denver event has been faked as a pretext for a first strike against the Soviet Union. Soviet bombers carrying megatons of death loiter at their fail-safe points. Any moment they may receive orders to penetrate U.S. airspace.

This was never in the war game scenarios.

Without evidence of invasion what does the President do? The weapon certainly didn’t just fall out of the sky!

This is no excerpt from a lost script for the Cold War movie, but a “what-if” scenario that perhaps has played out in a parallel universe.

Exactly 100 years ago today just such a devastating explosion flattened 800 square miles of a desolate forest in the Tunguska river valley in Siberia. The swath of destruction was equal to the area of Washington D.C and its Maryland and Virginia suburbs.

Barometers in London measured an atmospheric shock wave that traversed the globe. Seismometers measured a magnitude 5 earthquake on the Richter scale.  Dust was blasted to such high altitude it glowed brilliantly at midnight over Europe and Asia. The eerie etherial light must have seemed like the end of the world to some people.

When a survey team reached the site in the 1920s they expected to find a gaping meteor impact crater, like the one outside Flagstaff, Arizona. But there was nothing. The only forensic evidence was a leveled forest of 80 million trees snapped off like toothpicks and spread out radially from a central blast point like the spokes on a bicycle wheel.

Today everyone agrees that the absence of a crater means that a small comet or fragile piece of a stony asteroid was the culprit for the second largest natural explosion in recorded human history.   Initial estimates place the blast at 10 to 15 megatons of TNT, roughly the equivalent of up to 1000 Hiroshima atomic bombs going off at once.

(The 1883 eruption of the Indonesian volcano Krakatoa is the world champ at an estimated 200 megatons. In prehistory, Arizona’s Meteor Crater is estimated to have been caused by a 20-40 megaton explosion. The largest planetary impact ever witnessed was the collision of 23 comet fragments with Jupiter in 1994. The biggest fragment unleashed and estimated 6 million megatons).

Slamming into the atmosphere at 35,000 miles per hour the 1908 invader exploded several miles above the ground and sent a dragon’s breath of superheated air to fry Tunguska. To wreak such devastation the impacting body had been estimated to be the length of a football field. Something that size shouldn't hit Earth very often, roughly once every 1,000 years.

Recent supercomputer simulations have changed this picture. They have allowed for a major step forward in understanding the Tunguska event after a century of speculation. Sandia National Laboratories physicist Mark Boslough and his team modeled the hydrodynamics of an incoming mass heading through our atmosphere at hypersonic speeds.

The results show that more of the blast wave can be directed downward like a blowtorch jet. This means that the same devastation could have been caused by a smaller asteroid piece measuring less than 100 feet across and unleashing the energy of no more than five megatons of TNT.

If Tunguska's impactor was something smaller than previously thought then Earth could take a blow like this much more frequently, maybe once every couple hundred years. Why? Because the number of asteroid fragments in space quickly increases the smaller they are -- just as there are more pebbles on the beach than rocks.

On March 18, 2004 a near Earth asteroid (NEA) of comparable size passed just 26,500 miles above Earth. That’s just 4,000 miles higher than the geosynchronous orbits of our communications satellites!

On average, NEAs about the minimum estimated size of the Tunguska intruder are estimated to pass within four Earth-diameters roughly once every two years. But most of these small objects zoom by undetected. This particular close approach is unusual only in that scientists knew about it.

The intruder was picked up by the Lincoln Near-Earth Asteroid Research (LINEAR) program funded by NASA and the Air Force to search for NEAs. They look for asteroids ten times bigger than the Tunguska impactor, but they routinely detect much smaller objects. Based on these odds it’s not inconceivable that we or our children could see another Tunguska-like event.

The potentially most exciting new news from Tunguska is that a team headed by physicist Giuseppe Longo of the University of Bologna is going back this summer to retrieve what they believe is a piece of the asteroid. The mystery object is buried at the bottom of a strange cone-shaped lake five miles from the blast epicenter.

The 150-foot deep Lake Cheko might have been carved out by an asteroid fragment that made it to the ground, though it is uncertain if the lake predates 1908. Sonar taken in 2007 shows the presence of a three-foot wide object buried 30 feet below the center of the lake. It also seems to be the source of a small magnetic anomaly.

It could be a 100-year old time capsule from the explosion, waiting to be unearthed. The fragment could at last yield invaluable information about the pedigree of the impactor. Knowing its composition and density would allow for a better estimate of the size and nature of the intruder. On the year of Tunguska’s centennial anniversary the mystery of what slammed into Earth on that June morning may at last be solved.

Imagine if the Tunguska impactor had instead collided with Earth 50 years later, in the middle of the Cold War.

Would my nuclear confrontation scenario have become real?

And, how might it have ended?

Photos, top to bottom: NASA; NASA; Sandia National Lboratories; NASA; G. Longo