Astronomers discovered the asteroid in 2004. At the time, it looked as if it had a 2.7 percent chance of hitting us in 2029. Additional tracking enabled scientists to refine their calculations of the asteroid's orbit. Those calculations ruled out a smack-down in 2029, but left Apophis with a 1 in 45,000 chance of connecting with Earth in 2036.
New numbers released yesterday, however, now put the odds for an impact in 2036 at 1 in 250,000. As astronomers continue to track the asteroid, they say they expect the odds to shrink further.
At some 0.27 kilometers (about 0.2 miles) across, the object would be capable of widespread destruction on a regional scale, according to calculations made by scientists at the Sandia National Laboratory in Albuquerque, N.M.
"The refined orbital determination further reinforces that Apophis is an asteroid we can look to as an opportunity for exciting science and not something that should be feared," notes Don Yeomans, who heads the Near-Earth Object Program Office at NASA's Jet Propulsion Laboratory in Pasadena, Calif.
What about 2068? Astronomers are now keeping a close eye on the asteroid for any potential 2068 encounter. At the moment, they're giving it one chance in 3 million of nicking the third rock from the sun.
To understand that difference, journey with me if you will to William Tell's archery range. We have an apple, and in lieu of a youngster's head to mount it on, we'll imagine it sitting somewhere in front of a target with its colorful concentric rings.
The likelihood that an arrow will hit the bulls-eye and split the apple depends on a couple of things: the accuracy of the archer's aim, and where the apple is located in relation to the bulls-eye.
In astronomical terms, the target and its rings represent what researchers call the uncertainty region. It's the circle in space where the asteroid is calculated to pass on a certain date and at a certain time, plus or minus a few target rings. That bit of slop-over exists because scientist may not have enough observations of the asteroid's travels across the sky to be able to shrink the uncertainty region.
If each new batch of observations and calculations put the Earth father and farther outside this region of uncertainty, the asteroid becomes a great night-time show for anyone with a telescope or set of binoculars.
But if additional calculations go the other way, pushing Earth into the uncertainty region, the probability of an unpleasant encounter starts to rise.
Based on what astronomers know now, "Earth is more nearly centered in the 2068 uncertainty region," Dr. Tholen explained in an email exchange. "So if further observations improve the orbit without changing it significantly, then the uncertainty region will shrink with Earth still inside it." If that's true, it could spell bad news for our planet's inhabitants.
When Apophis swings by in 2029, the earth's gravitational pull will significantly change the asteroid's orbit, he continues. "A tiny error in the orbit will get magnified a lot after 2029," he writes. "To know where it will be with reasonable accuracy in 2068, we need to know where it is now to incredibly high accuracy, and that's tough to do with only four years of observations."
For the record, Dr. Tholen and his colleagues made the observations on which Apophis's newest collision probabilities are based. And you can bet he and others will be doing more of the same. The next opportunity for observations comes next year, but only for a week because the asteroid is moving behind the sun. Conditions for observers get a bit easier in 2011, he writes.
In the end, many astronomers expect new observations to further reduce the likelihood of a 2068 encounter.