By the end of this year, the National Aeronautics and Space Administration (NASA) hopes to find about 90% of the largest asteroids that, were one to strike Earth, would have global consequence ranging from its blast to dust thrown into the atmosphere, firestorms and acid rain. These are asteroids that can be as large as mountains but are at least one kilometer (3,280.8 feet) in diameter. NASA estimates that there are 900 of these objects in potentially hazardous range of Earth.
But the more immediate threat is from much smaller asteroids, such as the asteroid that has a 1-in-25 chance of hitting Mars Jan. 30. This asteroid, which has unglamorous name of 2007 WD5, is only 50 meters (164 feet) and is barely a chip off the massive, 10 kilometer- (6.2 miles) wide asteroid that wiped out the dinosaurs some 65 million years ago. Small, yes, but such an asteroid has the explosive force of a 10-megaton nuclear weapon.
"There are thought to be about 75,000 potentially hazardous asteroids larger than 50 meters, and the vast majority remains undiscovered," said Donald Yeomans, manager of the NASA Near Earth Object Program Office at the Jet Propulsion Laboratory in Pasadena, in an e-mail response to questions. "Hence, at the moment, we would not have much warning time prior to a collision. That's the bad news," he said.
But the "good news is that an object of this size would only cause local damage if it hit, or exploded above, a populated area, which seems unlikely," said Yeomans. That's because two-thirds of the Earth is ocean and one-third is not densely populated, he said.
A 50-meter asteroid, similar to the one inbound on Mars, will hit Earth once every 500 to 1000 years, according to Yeomans. This is same size of the object that struck Tunguska, Siberia, one hundred years ago. The Tunguska object disintegrated in the Earth's atmosphere but its blast flattened and scorched trees over an area of some 800 square miles. The Mars asteroid is traveling at 30,000 miles an hour and a strike may create a more than a half-a-mile wide crater.
But the U.S. isn't searching for those smaller potentially hazardous asteroids, even though in 2005 Congress directed NASA to find by 2020 potentially hazardous objects of 140 meters and larger.
A mid-size object of 140 meters (459.3 feet) and larger, with an impact energy of 100 megatons or more, can be expected to hit earth once every 5,000 years -- a 1% probability of impact every 50 years. By contrast, 1 kilometer or larger asteroids have a mean impact frequency of about once every 500,000 years, according to testimony by Yeomans in November on near earth objects before the U.S. House Committee on Science and Technology.
Congress didn't set aside money for this expanded asteroid hunt, and out of NASA's annual budget of about US$17 billion, it spends just $4.1 million to find potentially hazardous near earth objects.
Russell "Rusty" Schweickart, the former astronaut, is now chairman of the B612 Foundation in Sonoma, Calif., which has been pushing NASA and Congress since 2001 to develop a comprehensive plans for dealing with asteroid "with our name on it" that includes a deflection plan.
"The reality is we have the knowledge to be able to protect life on Earth from this happening," said Schweickart. "If we were really responsible, if we really set about his process ... we could essentially preclude any substantial asteroid from ever hitting the Earth again."
Schweickart believes that the Mars asteroid "will cause thoughtful people to realize that this happens."
The nearest known risk to Earth the asteroid 99942 Apophis, a 400-meter (1300 feet) asteroid that has an impact probability of 1 in 45,000 in 2036.