During the early morning hours of April 13, 2029, observers in Asia and North Africa will have a chance to witness a rare celestial event as an asteroid, 99942 Apophis, passes within 20,000 miles of Earth.

"It's not gonna knock your socks off, and it certainly won't be the brightest object in the sky, but it'll be easily observable with the naked eye," said Don Yeomans, manager of NASA's Near Earth Object (NEO) Program.

The approach of an asteroid this large -- Apophis is more than 1,000 feet in diameter -- and this close to Earth occurs only about once every 1,500 years.

Scientists are awaiting the close flyby with mixed emotions: excitement at a unique scientific opportunity and uneasiness that it might be a sign of more ominous things to come.

Inside look

A team of researchers headed by Daniel Scheeres, an aerospace engineer from the University of Michigan, hopes to take advantage of Apophis' close approach to learn more about how asteroids are assembled and to gather information about seismic activity inside the rock.

The beauty of this event is that it is a kind of natural experiment that scientists would never be able to recreate, Scheeres told Space.com in an email interview.

Tidal forces from Earth's gravity will twist and churn the asteroid's insides and deform its exterior as it passes by the planet. Scheeres said that currently, the plan is to use ground-based radar to monitor the asteroids movements and telescopes to observe changes in its surface features and rotation.

But even the most sophisticated ground-based observations won't be sufficient for gathering detailed information about the interior of the asteroid, Scheeres said.

That kind of detail would require that a network of probes capable of measuring acceleration and seismic activity be embedded in the asteroid's surface. Another possibility would be to place a probe in orbit around the asteroid in order to keep tabs on it and to map its surface. No such space missions are currently in the works, however.

Apophis was discovered last year and is named after a snakelike Egyptian god of darkness and chaos. The name is appropriate. For a brief period of time last winter, scientists had given Apophis, then known as 2004 MN4, a 1-in-40 chance of colliding with Earth in 2029.

Additional observations ruled out the 2029 impact, and scientists now predict there is about a 1-in-10,000 chance that the asteroid will hit Earth in 2036, on yet another of its trips around the Sun on a course that crosses the orbit of Earth.

A large part of the uncertainty surrounding Apophis' movements is due something called the Yarkovsky Effect. When rotating bodies like asteroids pass through our solar system, they absorb solar radiation from the Sun that they then re-radiate.

The miniscule but persistent pressure from this re-radiation can cause a rock to speed up or slow down and change its flight path.

In many ways, the hubbub surrounding Apophis stems from an unusual confluence of events as the detection of near-Earth objects coincides roughly with humanity's demonstrated ability to meet them. Emboldened by the success of recent missions like Stardust and Deep Impact, some scientists think it prudent to launch a space mission to determine whether Apophis poses a significant threat.

Let's go!

Astronomers know that in 2029, Apophis' path will be bent significantly by Earth's gravity. They don't know the exact outcome.

In May, former Apollo astronaut Rusty Schweickart sent a letter to NASA administrator Mike Griffin urging the agency to investigate whether in 2029 Apophis might enter certain gravitational "keyholes" near Earth that would alter the asteroid's flight path in a manner that could put it on a more certain collision course with our planet in 2036.

In order to more accurately track its movements, Schweickart also proposed launching a space mission to place a radio transponder on Apophis. An official to response to Schweickart's letter is expected from NASA within the next few weeks.

As demonstrated by the Deep Impact mission, in which NASA smacked a comet with a small probe, it is possible to strike a fast moving body in space using current technologies.

"You don't have to change the course of the comet very much to miss the keyhole if you do it a number of years in advance," said Clark Chapman, an astronomer at the Southwest Research Institute in Colorado who has served on a number of committees concerning near-Earth objects.

Chapman urges caution, however, and said that scientists shouldn't rush to action. "You don't want to nudge it until you know what the nudge is going to do," Chapman said. The worst thing that could happen, of course, would be to nudge the asteroid in the wrong direction, based on the incomplete data now in hand, and actually cause a future collision.

Sooner rather than later

Most scientists agree that 2029 is the absolute deadline if an intervention mission is to be launched. After 2029, the distance Apophis would need to be moved in order to avoid an impact would be too great given current technologies.

In his letter, Schweickart said plans for a space mission to place a transponder on the Apophis should be in place by 2014 and that an intervention mission, should it prove necessary, be launched prior to 2029.

However, Apophis will veer within an observable distance of Earth twice more before 2029 -- once in 2013 and again in 2021. Based on data collected from those two flybys, Yeomans said scientists should be able to conclude with 99.8 percent accuracy whether a future impact scenario can be ruled out and he believes we should therefore wait before launching a mission that could cost hundreds of millions of dollars.

Until then, Yeomans says he won't be losing sleep over Apophis.

"It's an interesting object and it's raised some interesting issues, but a worrisome threat? No," said Yeomans. "We've got plenty of time."

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