Today, we await an incoming missile. The University of Houston's College of Engineering presents this series about the machines that make our civilization run, and the people whose ingenuity created them.

In 1908 a meteorite exploded over Siberia's barely populated Tunguska River region with a destructive force of 10 to 50 megatons of TNT. It flattened forests and killed reindeer herds, but only a few people died. It took years to diagnose what'd happened. Once we knew, we shrugged it off as too rare to worry about, even though that same explosion over a big city could kill ten million people.

Now a disturbing incident comes to light. In 1930, inhabitants along the Curuça  River in Western Brazil near Peru saw a blood-red sky and a rain of red dust followed by an eerie whistling sound and fireballs in the sky. Another meteorite in a remote place! This explosion was only 1/10 as powerful as the Tunguska explosion. Yet that's still 50 times as powerful as the Hiroshima bomb.

A Jesuit missionary arrived five days later. He talked with locals, who were still terribly alarmed. He told their story, seven months after the fact, in a Vatican newspaper.

We've only recently diagnosed the Curuça  event. It was probably a cluster of three meteorites. That's why dust from the first impact was seen before the other two landed.

There's a grim message here. We once thought the Tunguska Meteorite was a once-in-300-years event. But Curuça  was only 22 years later. Within our solar system, and just outside it, some ten trillion meteorites, comets, and asteroids move in their shaky orbits. Cosmic junk reaches us all the time.

The Tunguska meteor was 90 feet across. The meteorite that we believe killed off the dinosaurs and 3/4 of Earth's living creatures, 65 million years ago, had to've been around 10 miles in diameter. That one completely altered the Earth.

Today, astronomers are building complete records of orbiting objects. By the year 2008, they hope to know just what's headed our way. Then the question will be, "How much warning do we have?" Five years is too little. The closer the object is, the harder it'll be to deflect its orbit. Fifty years warning should give us time to launch an explosive device that could deflect a small asteroid. Of course anything as little as the Tunguska or Curuça  meteorite will be very hard to locate in time to intercept.

As a postcript to Curuça, It came to light in that Jesuit priest's obituary that he reached the Curuça  River in the nick of time. The people had seen the meteor as the wrath of their God and were preparing to commit mass suicide. He talked them out of it.

If that sounds silly, ask how you or I would react to a piece of iron, large enough to exterminate us, headed this way. Somewhere out there, there is one. We have to be cool-headed problem-solvers today, if we don't want to face that inexorable force of nature -- without the means for averting it.

I'm John Lienhard, at the University of Houston, where we're interested in the way inventive minds work.

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Huyghe, P., Incident at Curuça. The Sciences, March/April, 1996, pp. 14-17.

Gehrels, T., Collisions with Comets and Asteroids. Scientific American, Vol. 274, No. 3., March, 1996, pp. 54-59.

For more on the Tunguska meteorite, see Engines Episode 670.