(Inside Science) -- On June 30, 1908, an object believed by many to be a stony asteroid with a diameter roughly the width of a football field exploded high in the sky near the Podkamennaya Tunguska River in Siberia, after blazing a fiery path through Earth's atmosphere. The explosion's power was roughly equivalent to that of a 3-5 megaton nuclear bomb. This event devastated about 800 square miles of largely uninhabited forest.

On June 30, organizers around the world will commemorate the 107th anniversary of that event by holding the first Asteroid Day.

In addition to asteroids, the event will focus on other threats to Earth, such as comets and large meteors.

Asteroids are usually well-behaved objects that travel around the Sun within the asteroid belt, which lies between the orbits of the planets Mars and Jupiter.

But individual asteroids can occasionally break away. Some, like the Tunguska object, which could also have been a comet or other body, can head towards Earth's neighborhood -- where they could threaten an Earth encounter.

Astronomers say that a collision with an object the size of the Tunguska object is unlikely.

But even a much less powerful hit could cause havoc in a metropolitan area. So Asteroid Day, according to its sponsors, is not only a commemoration but also a call to action.

"We need to find out the potential of an impact in the future and what that will mean for our species and our civilization," said Ryan Wyatt, director of the California Academy of Sciences' Morrison Planetarium and Science Visualization.

The day will feature more than 50 public events in 17 countries. These will focus mainly on the possibilities and implications of future collisions of asteroids – and other "near-Earth objects" such as comets – with our home planet.

"If we can track the trajectories of asteroids and monitor their movement in our solar system, then we can know if they are on a path to impact Earth," former Apollo astronaut Rusty Schweickart told the organizers of Asteroid Day in a statement. "If we find them early enough, we can move them out of Earth's orbit, thus preventing any kind of major natural disaster."

Ed Lu, a Ph.D. physicist and former Space Shuttle astronaut, sees the issue as qualitatively different from such threats as hunger, health, and economic difficulties. "I don't know how to solve those problems," he said. "This is a problem we face that does have solutions."

Organizers are calling on the public to sign what they call the 100X Declaration.

"We are trying to ramp up the rate of detection by 100 times," explained Brian May, an astrophysics Ph.D. and the lead guitarist of the British rock band Queen, in a statement about the declaration. "Signing the 100X Declaration is a way for the public to contribute to bringing about an awareness that we can protect humanity now and for future generations."

May scored the music for the movie 51 Degrees North, which depicts an asteroid impact. It was made by German filmmaker -- and cofounder of Asteroid Day -- Grigorij Richters. The movie will premiere tomorrow at Britain's Science Museum in London.

Planetary astronomers agree that asteroids can threaten the Earth. However, they warn that the risks of calamitous collisions should not be overplayed.

"Anything that can keep asteroids in the public mind is useful, if it's done in a way that's not sensational and not done in a negative way," said Gareth Williams, associate director of the Minor Planet Center at the Harvard Smithsonian Center for Astrophysics in Cambridge, Massachusetts.



Williams' center acts as clearinghouse for data on asteroids and other space rocks. Astronomers inform the center about new observations and their calculations of objects' paths.

"We are currently tracking about 600,000 asteroids," Williams explained. "But the total number is up in the billions. Some objects are 1,000 kilometers across, and others are the size of pebbles."

That tracking ability stems mainly from increasingly sophisticated telescopes and light detectors developed over the past three decades. Space missions have even landed on asteroids.

"Asteroids have gone from points of light on detectors to worlds that we've actually visited," Williams said.

Astronomers identify wayward asteroids by studying their movements against the background of the stars.

"Most near-Earth objects have unusual motions, distinct from those of the main asteroid belters," Williams said.

About three days of observation usually serve to confirm whether or not a newly detected asteroid could eventually threaten our planet.

Williams' center looks for asteroids and other objects that could potentially impact Earth within six days. NASA's Near-Earth Object Program at the Jet Propulsion Laboratory in Pasadena, California and astronomers at the University of Pisa in Italy take a longer view; they calculate potential paths about a century into the future.

Not surprisingly, the paths become less certain the further the calculations move into the future. But the good news is that observations and calculations so far predict no really serious Earth impact over the next century.

"One that we're really concerned about will come in the 29th century," Williams said. So eight centuries from now, our descendants might regard Asteroid Day in an entirely different way from tomorrow's participants: as the reason to grieve for the impending end of human civilization or to celebrate technology that detected and deterred a dangerous impact.