An organization devoted to tracking asteroids representing a risk to the Earth says the potential threat of a catastrophic impact may be ten times more than previously believed.

The B612 Foundation, in an Earth Day presentation Tuesday, released a video representation of the locations where asteroids have hit the Earth in the past, each exploding with energy equivalent to an atomic bomb.

Their visualization is based on data from a nuclear test monitoring system operated by the Comprehensive Nuclear-Test-Ban-Treaty Organization.

Twenty-six explosions on or above the Earth from 2000 up to last year -- releasing energies ranging up to 600 kilotons -- were all down to asteroid impacts, foundation head Ed Lu, former space shuttle astronaut, reported in the presentation at the Museum of Flight in Seattle.

"Our previous estimates of the impact odds for small asteroids (such as city killers) were low by about a factor of 3 to 10," the foundation said in a release accompanying the video. "This new data suggests that Earth is hit in a random location by a multi-megaton asteroid impact (large enough to destroy a major city) about every hundred years."

Very few of the asteroid strikes presented in the video visualization had been detected before their impacts the foundation reported, and even in cases where they were seen there little opportunity of advance warning.

The B612 organization has as a primary goal the construction and launch of a space telescope to be called "Sentinel," mean to identify and track unknown asteroids and other near-Earth objects that might be on a possible collision course with our planet.

"While most large asteroids with the potential to destroy an entire country or continent have been detected, less than 10,000 of the more than a million dangerous asteroids with the potential to destroy an entire major metropolitan area have been found by all existing space or terrestrially-operated observatories," Lu says. "Because we don't know where or when the next major impact will occur, the only thing preventing a catastrophe from a 'city-killer' sized asteroid has been blind luck."

Sentinel, planned for a 2018 launch, could identify and then track as many as 200,000 Earth-crossing asteroids in its first year of operation alone, the foundation said.