Earth has been hit and is constantly at risk of attack by interlopers from space. These are called "near earth objects" (NEOs). Major players are asteroids. Most burn harmlessly during their trip through the atmosphere. However, just as in the intensely mediocre films, Armageddon and Deep Impact, there is more than a zero chance that a large one will threaten earth in the near future.

Quietly in the background of our already nervous world, scientists have been making plans for how to prevent a catastrophe whether or not Bruce Willis is available. NASA recently presented its report to Congress. More recently Rusty (Russell) Schweickart, lashed out at NASA for a recent study on the threat from NEOs (Near Earth Objects) impacting our planet. Schweickart was the lunar module pilot for Apollo 9. He is now Chairman of the B612 Foundation and a member of the Association of Space Explorers (ASE) where he is on the Committee on Near Earth Objects.

B612 is a group of astronomers, astronauts and scientific specialists who have dedicated time to working on methods to deflect the orbit of asteroids in order to prevent another catastrophic "event" like the 1908 Tunguska explosion which has been shown to have been caused by a 45-50 meter diameter asteroid exploding in Siberia. It destroyed 2000 square kilometers of Siberian forest "... and maybe a few reindeer." Schweickart noted that, "Had it hit a couple of hours later it might have wiped out London or Moscow..."

Both groups call for early warning detection systems, "deflection capability", and an ability to coordinate the responses internationally. The possibilities of such an impact are becoming less as time goes on. Partly because we are now cataloging the objects that present possibly dangerous trajectories.

By 2019, he said, there will be more than 10,000 objects "...with a non-zero probability of impacting Earth." "A non-zero probability." What great euphemisms scientists can invent!

At this time, we were warned, there is no one and no agency of the U.S. government or of any other on the planet that is responsible for dealing with the potential threat nor for developing "Mission Rules" for the deflection of NEOs.

In true astronaut-geek speak Schweickart warned that there is "... the possibility-in an evolutionary sense-of a Control-Alt-Delete; a reboot of the evolutionary system that has already occurred many times on Earth."

"If we do our homework right, never again should an asteroid that can do damage on the ground impact the Earth," Schweickart suggested. "We're living at a time - with our technology - we have the capability to eliminate this major shaper of evolution - the evolution of life on this planet."

The "Tunguska Event" was the catastrophe in Siberia that is now accepted as the explosion of an asteroid above Tunguska in deserted Siberia in June, 1908. The 100th anniversary is next year. Start planning your Chicken Little parties early.

It was early morning 30 June, 1908. Witnesses, of which there were few, recalled in recently translated testimony that they saw a fireball falling from the sky as far as 110 miles away. Seismic recordings were made 600 miles away and 40 miles from the event people were knocked down or knocked into unconsciousness. The closest witnesses were "reindeer herders" about 20 miles away who were blown out of their tents into the air. "Everything around was shrouded in smoke and fog from the burning fallen trees," said one witness. Another man was blown into a tree and died later according to a report by the Planetary Science Institute . Russian scientists interviewed people from the Vanavara Trading Post. One translated account included, "I saw the sky in the north open to the ground and fire poured out. The fire was brighter than the sun. We were terrified, but the sky closed again and immediately afterward, bangs like gunshots were heard. We thought stones were falling... I ran with my head down and covered, because I was afraid stones may fall on it."

Since the object, now believed to have been a meteorite of about 30 meters (98 feet) traveling at 15 km per second (9.3 miles per second), exploded before impact; there is no crater. A scientific group in 1993 studied the records and were later corroborated when Russian scientists found rocks of the same composition as "common stone meteorites" blasted into trees at the site. It was the kind of Earth-altering event that is thought to happen relatively often in planetary time.

(The photo of the Tungaska site is from the 1927 Kulik expedition)

He noted that the force of the blast was about that of 10-15 million tons of TNT and that an atmospheric shock wave circled the globe twice. Fine dust permeated the atmosphere sufficiently that "...for two days afterwards, there was so much fine dust in the atmosphere that newspapers could be read at night by scattered light in the streets of London, 10,000 km (6,213 miles) away."

A Russian scientist believes that the Tunguska Event was responsible for global warming rather than man-made gases. Vladimir Shaidurov, of the Russian Academy of Sciences, theorizes that water vapor thrown into the earth's meteorological system is the cause of present climatological change. "Andrew E. Dessler of the Texas A & M University, writing in The Science and Politics of Global Climate Change, claims that: "Human activities do not control all greenhouse gases, as the most powerful greenhouse gas in the atmosphere is water vapor. Human activities have little direct control over its atmospheric abundance, which is controlled instead by the worldwide balance between evaporation from the oceans and precipitation."

The English version of Pravda on-line recently offered the theory again that it was caused by a UFO. It is not a very compelling theory but the fact that there is an English version of Pravda and that it reads like a super-market tabloid was a fascinating aside in this research.

The NASA/JPL photo of Asteroid 243, Ida and Dactyl (Asteroid and her satellite) was shot by the Galileo spacecraft in 1993 on its way to Jupiter at 10,500 km (6500 miles) from the pair.

In 2002 Earth had a "close shave." Asteroid 2002MN became one of only 6 recorded asteroids to penetrate within the orbit of the moon. Astronomically that is surprisingly close. Especially since it was only discovered 3 hours after its closest shave with a defenseless planet. It came within 12,000 km (74,575 miles) which is 0.0008 astronomical units (distance from Earth to Sun). If it had hit it would have been as powerful as Tunguska - equal to a few H-bombs and it was too small to be in the group that we are to be planning defenses against.

The March, 2007 report NASA gave Congress (of which astronaut Schweickart complained) is available as a pdf download from NASA .

"The objectives of the George E. Brown, Jr. NEO Survey Program are to detect, track, catalogue, and characterize the physical characteristics of NEOs equal to or larger than 140 meters in diameter with a perihelion distance of less than 1.3 AU (Astronomical Units) from the Sun, achieving 90 percent completion of the survey within 15 years after enactment of the NASA Authorization Act of 2005. The Act was signed into law by President Bush on December 30, 2005."

Whichever group of astronomers, scientists and assorted world-shakers is correct, they seem to agree on the danger and the need to plan for a response. As if we didn't have enough to worry about already. It is time to listen to Chicken Little: the sky might fall. We now have or could have methods to protect the current version of Earth without needing an evolutionary reboot - "restart" for Mac-lovers.