After tracking asteroid 99942 Apophis with NASA's giant Goldstone radar dish, astronomers are now certain that the threatening asteroid has essentially no chance of striking Earth in 2036.

Astronomers surely enjoy dramatic stories as much as the rest of us. But today they played spoilers with the welcome announcement that the sizable Earth-crossing asteroid 99942 Apophis will pose no threat when it comes near our planet in 2036.

Right now Apophis is in the midst of a rather distant yet much-awaited pass in Earth's vicinity, coming within 9 million miles (14½ million km) earlier today. It's been tracked for about 2½ weeks by NASA's 230-foot (70-m) Goldstone radio/radar dish in California, and those observations have given astronomers the confidence to issue an "all clear" for the foreseeable future.

"Goldstone single-pixel observations of Apophis have ruled out the potential 2036 Earth impact," says Jon Giorgini, a dynamicist at NASA's Jet Propulsion Laboratory. Based on revised orbit calculations, he says Apophis will then come no closer than about 14 million miles — and more likely miss us by something closer to 35 million miles. Moreover, the radar data have improved the asteroid's positional uncertainty so much that dynamicists can now accurately predict its trajectory decades into the future.

"We're observing it at 75-meter resolution, which is better than we expected," notes Lance Benner (Jet Propulsion Laboratory), who's leading the radar effort. "The signal-to-noise ratios [of the radar echoes] are a bit stronger than we thought they'd be, so the radar astrometry is more precise than we expected."

Apophis was discovered in 2004 by observers Roy Tucker, David Tholen, and Fabrizio Bernardi. At first, orbital computations suggested that this near-Earth asteroid, initially designated 2004 MN 4 , had a 3% chance of striking our planet in 2029. About a year later, it was named Apophis, for the Egyptian god of evil and destruction. (An apt name, don't you think?) Fortunately, by then prediscovery observations had led to a revised orbit, which ruled out an impact in 2029.

But we weren't out of danger yet. A collision remained possible in 2036, and the chance of that hinged on the near-miss flyby in 2029, when Apophis will zip by just 20,000 miles (32,000 km) away. Were that to occur at a particular spot in space, what dynamicists call a keyhole, an impact would become very likely on the return visit in 2036. The problem is that the orbital specs of Apophis weren't known accurately enough to predict exactly where it would fly past in 2029.

Adding to the uncertainty is the extent to which a subtle force, known as the Yarkovsky effect, might be altering the asteroid's orbit. This effect is caused by the uneven way that a spinning body absorbs sunlight and then reradiates it back to space. Ground-based observers determined that Apophis rotates in 30½ hours, but it likely has more than one period involving multiple spin axes.

The object's shape and spin orientation are unknown — and might remain open questions until 2029. "We might get coarse-resolution images that barely resolve the object and indicate its orientation," explains Benner, "but even that could be optimistic."

Conceivably, gentle but persistent nudging from the Yarkovsky effect might have pushed Apophis straight through the 2029 keyhole. But again, says Giorgini, there's no longer any chance of that. The Goldstone observations have "shrunk the orbital uncertainties so much that, regardless of what the still-unknown physical parameters of Apophis might be, radiation pressure can't be enough to move the measurement uncertainty region enough to encounter the Earth in 2036."

Were this asteroid to hit us, very bad things would happen. Apophis is an estimated 900 feet (270 m) across, and it would strike with the kinetic-energy equivalent of roughly 500 million tons of TNT.

Just-released infrared observations from the European Space Agency's Herschel spacecraft suggest that the diameter of Apophis might be some 20% larger. "“The 20% increase in diameter, from 270 to 325 m, translates into a 75% increase in our estimates of the asteroid’s volume or mass,” says Thomas Müller (Max Planck Institute for Extraterrestrial Physics), who is coordinating the Herschel observations. However, his team's modeling assumes that Apophis is spherical — and the actual shape is thought to be elongated.

We haven't heard the last word on this little interplanetary demon. Goldstone radar observations of Apophis will continue through January 17th, and additional tracking is planned next month with the giant Arecibo radio dish in Puerto Rico. All that pinging should yield super-accurate positional data and, perhaps, reveal the asteroid's shape and spin state.

But the worry about Apophis has only been postponed, not eliminated. Its orbit is not all that different from Earth's, and some day in the distant future the two bodies will either have a catastrophic collision — or an encounter so close that Earth's gravity will yank Apophis onto a new and significantly different interplanetary path.

In fact, "The 2068 impact probability for Apophis is now one in 189,000," notes Rusty Schweickart (cofounder of the asteroid-monitoring B612 Foundation), "which is higher than the 2036 impact probability was."