The asteroid (99942) Apophis will not hit Earth. New measurements announced by the IAU last week (5/16/06) confirm that the 2029 encounter (earlier rated as a 1-in-37 chance) will be a miss. More important, the new observations show that a collision in 2036 can also be ruled out (the 300 mile course correction in the new 2029 estimate extrapolates to millions of miles of difference by 2036). Once rated at 4 on the Torino impact hazard scale (the only object ever rated that high), its rating is now back to 1. (Note: 1 is still slightly elevated risk, since 0 is normal, but the risk is very, very low.)
Apophis is 300 meters across. By comparison, the 1908 Tunguska event in Siberia is believed to have been caused by a bolide only 60 meters across, largely made of ice, which exploded in the atmosphere before it could reach the ground. Were Apophis to collide with Earth the impact would likely be a 900 Megaton explosion, equivalent to sixty-thousand Hiroshima-type bombs. Apophis will pass Earth at a distance of under 20,000 miles--closer than our own geosyncrhonous communications satellites.
More good news: we will have another near miss by the asteroid 2004 VD17 in 2102. The only other asteroid to have been rated over 1 on the Torino impact hazard scale, new measurements this week (5/20/06) have reduced the chance of collision with 2004 VD17 to 1-in-7500. Although it should pass Earth even closer than Apophis, scientists are confident enough of its orbit that its Torino rating now has been reduced to back 1. 2004 VD17 is about 600 meters across. Were it to collide with Earth, the potential impact would be 14 Gigatons.
Then there's (4179) Toutatis. It is 5 kilometers long (but relatively oblong). It passes by the Earth-Moon system every four years, and has been observed to fly by as close as two Lunar radii. An impact would be equivalent to several hundred Gigatons (more than all the world's nuclear arsenals exploded simultaneously). And Toutatis is only half as big as the monster that wiped out the dinosaurs 65 million years ago. Fortunately, Toutatis' Torino rating remains 0 as there is no chance of actual impact in the forseeable future. But Toutatis is a warning: there are others like it we have not yet discovered. There are estimated to be about 1100 or 1300 NEO asteroids of Toutatis' size or larger, of which (thanks largely to NASA) we know about 70%-80% now.
While NASA and other groups--mostly volunteers--continue to scan the skies for future Near Earth Object (NEO) threats, the European Space Agency (ESA) has decided to test a possible solution: ESA is planning a NEO deflection mission, aptly named Don Quijote. The larger half of the mission, 1-ton Hidalgo, will smash into a 300-600 meter asteroid at 20,000 mph (current candidate asteroids are 2002 AT4 and  1989 ML). Its companion spacecaft Sancho will arrive at the asteroid a few months prior, place plenty of seismometers, and then in the words of ESA it will, "retreat to a safe distance to observe the impact without taking unnecessary risk (with an attitude appropriate to its name)." Contractor selection and final target selection will occur by next year, but no date has been set for the mission.
So this has been a good week for the asteroid hunters. If Hurricane Katrina has shown us anything, it is that--even when forseeable--mitigation measures for low-probability, high-risk events are notoriously hard to plan and budget. And perhaps most worrisome now is the eruption of Mt. Ranier in Washington State. Last eruption was in the early 1800s, last big eruption 1000 years ago, and the potential victims are the approximately 200,000 people who live where the lava flowed during the last big eruption. Another eruption is a dead certainty... just like major earthquakes in California.
But on the issue of impacts, I am pleased to say Congress has begun to step up to the plate. In December of last year, Congress finally fully funded the $7 Million NEO survey program to track asteroids 100 meters or larger. If they maintain this funding, the goal of 90% detection of all such asteroids believed to exist in the solar system should be achieved in 15 years. If the NEO survey program is successful, the risk of impact will either be reduced by a factor of 1000, or we'll find out we'd better do something fast.