An asteroid that once was seen as a danger to the Earth may soon be a once-in-a-century opportunity to get a close look at one - and learn more about the ones that really are a hazard.

The asteroid is called Apophis. It's a near-Earth asteroid that is a type called a chondrite, essentially a stony body that has a high silicate content and few metals. It is about 330 meters across, and it's due to pass the Earth in 2029.

Apophis was first found in 2004. At first it was a cause of concern because it looked like it was might hit the Earth on the 2029 pass. An asteroid that big is large enough to do a lot of damage if it hits. The crater would be on the order of 3 kilometers wide, twice as large as Meteor Crater in Arizona. Destruction would be widespread. The blast would be equivalent to a large nuclear bomb.

But later observations showed that it was not likely to hit the Earth. It would pass near what is called a keyhole, a region of space that would be just the right distance from Earth that its orbit would change just enough that Apophis would hit some seven years later in 2036. But in 2009 calculations showed that the odds of it passing through that very narrow space were only one in 135,000.

Dan Durda, principal scientist in the department of Space Studies at the Southwest Research Institute in Colorado, noted that the chances of Apophis hitting the Earth are actually smaller than the odds of an asteroid we don't know about yet doing so.

But the fact that the asteroid was coming so close got Durda thinking. Apophis will come no less than about 29,450 kilometers (18,300 miles) away. It will be a lot easier to see the asteroid, of course, and train various instruments on it. But even better would be to actually get a spacecraft there. To that end, he sent a proposal to NASA, which is considering a probe to chase Apophis down and do a seismic study as it swings by.

As Apophis comes close to Earth, it will undergo huge tidal stresses. Those stresses occur because one side of Apophis is closer to Earth than the other side, and thus feels a stronger gravitational pull. This is the same reason the ocean's tides happen on Earth - in that case the moon pulls on the water and makes it bulge out, which creates tides.

Apophis is small and rigid, so as the Earth raises tides on it the material of the asteroid will resist. It's basically going to be popping and creaking as it passes, said Durda. Nature will be doing a seismic sounding for us.

If a probe could get there and measure the stresses, 'listening' to the asteroid, it can tell how solid it is, and what it is made of. Asteroids that are solid and dense are a much bigger problem than those that are loose piles of rubble. Loose piles of rubble don't deliver as much impact energy and tend to break up before they hit, creating spectacular meteor showers rather than gigantic craters.

Durda said any mission to investigate Apophis has to launch around 2021. That's because Apophis is in an orbit that is tilted relative to Earth's, so even though it crosses Earth's orbit, getting a spacecraft there is energy-intensive and takes some time.

There is a lot of good science, he said, that can be done without a probe. Radar measurements can give observers an idea of the Apophis' speed and distance, allowing for more exact orbital calculations. Seeing its spectrum can offer insight as to its rotation. But there is no substitute for actually being there.

Even if a mission doesn't make it through the review process at NASA, Apophis' flyby will be a once in a lifetime event, well worth seeing. As it passes, anyone who is in northwestern Europe will be able to watch it move across the sky. Most asteroids aren't visible to the naked eye at all.

It's a real chance to study this kind of object, Durda said.

NASA Asteroid NASA Arizona