The stone coming in our direction is an oblong, lumpy hunk of rock a bit over a kilometer long. As yet nameless, the only designation it carries is (29075) 1950 DA. But, its orbit makes it a very interesting hunk of rock.

Carl Wirtanen, an astronomer at the Lick Observatory in central California, first spotted this object in May of 1950. It was lost until December 31, 2000 when another group found what they thought was a new object. Conrad Bardwell at the Minor Planet Center quickly determined that the new object and Wirtanen's object, 1950 DA, were one and the same.

From that conclusion it was possible to find other photos of the asteroid taken during the 1980s. This provided a large number of data points, spread over several orbits, making it possible to calculate a precise orbit and earning it a number, 29075, as a "minor planet."

That orbit, it was promptly discovered, intersects the orbit of the Earth and on March 16, 2880, both bodies will be at that intersection. The question is, will it be a hit or a miss? The initial calculations showed a 1 in 10,000 chance of an impact. Very low, but still not zero.

A few times a year an asteroid is discovered that has some chance of hitting the Earth in the coming days, weeks, months or years. The NASA "Current Impact Risks" web site risk lists the objects and their relative risk of hitting earth.

An early prediction of a possible impact prompts astronomers to make additional observations. Those observations improve our understanding of the orbit and in almost every case the chance of a collision is reduced to some number very close to zero.

Enter Dr. Jon Giorgini and a group of 13 other scientists from the Jet Propulsion Laboratory (JPL), Washington State University, the Arecibo Observatory in Puerto Rico, the Lick Observatory and the California Institute of Technology (Caltech).

Using the large Goldstone radio astronomy dish in California along with the huge Arecibo facility, they were able to get radar data of 1950 DA. This not only revealed information about the size and rotation of the asteroid, but also allowed the orbit to be determined to an even higher degree of precision.

This time around that greater precision didn't rule out the chance of a future impact. In fact, the original estimate was way too optimistic. The original 1:10,000 odds now stand at 0.0033, one-third of one percent, or 1 chance in 300. People have been known to spend money buying lottery tickets with far slimmer chances.

"Asteroid 1950 DA is a very interesting object," said Dr. Benny Peiser, a spokesman for Spaceguard UK. It's interesting, "because it is the first Near Earth Object that scores higher than zero on the Palermo Technical Impact Hazard Scale."

The Palermo Scale measures the impact probability, date and energy to assign a hazard rating against the statistical background of such events. The rating given to 1950 DA (+0.17) indicates that the risk of it hitting Earth is about 47.9% greater than the expected statistical average chance of being hit by an asteroid or comet.

Lest there be any confusion, let's be clear about what this number means.

There is a 99 and 2/3rds percent chance that (29075) 1950 DA is going to pass a couple hundred thousand miles from Earth and not, repeat, not, enter our atmosphere, crash into water or land, and cause havoc and disaster.

Should we be doing something right now to deal with the danger from (29075) 1950 DA? The consensus is no.

Dr. Alan Harris, an internationally recognized expert in asteroids and a senior scientist at JPL, gives three reasons for not doing anything more than studying the object for now.

First, we could still find out that it isn't going to hit us in 2880. We'll have two more close approaches in the 21st century. Those close approaches will give us chances to study it in more detail. "Leaving us with a generous 800 more years," according to Dr. Harris, "to deal with it if it is on a collision course."

Second, we could move it the wrong way. While we have a very good idea of where it's going, it isn't a perfect picture. There's still room for error and we could nudge it into a collision instead of out of one.

Third, our descendants may have better ways of dealing with it than we can imagine.

"Would you expect William the Conqueror, " Dr. Harris quipped, "to anticipate and solve any of the major problems of our modern society?"

Dr. Harris is pretty emphatic that 1950 DA isn't something we need to get excited about right now. "To further put this in perspective," he said, "there is zero chance of an impact from 1950 DA between now and 2880."

There's a story to be told just in being able to compute that degree of accuracy that far in advance.

Because there are so many objects in space and each one exerts a gravitational pull on all the others, computing the future location of any one of them requires calculating where each of them will be at some future time. Since this process has a known uncertainty, the farther out in time the orbits are projected, the greater the uncertainty becomes.

1950 DA is a different case, though. In computing its future position, the uncertainties expand for a time and then contract. Over the course of nine centuries this process repeats several times. This contraction of the uncertainty appears to be caused by some unique gravitational interactions (resonances) with other objects in space, including the Earth itself.

"Resonance is well known," said Dr. Giorgini, "but the effect of it on modulating the uncertainty region seems not to have been recognized before."

The researchers, whose results will be published in the journal Science on April 5, 2002, also investigated the precision loss that can occur when computers use numbers that have a large number of digits to either side of the decimal point.

When dealing with such numbers, results can sometimes be lopped off or values rounded up or down. Even with that considered, the positional uncertainty is less than the diameter of the Earth.

Some other, very small, very subtle and very obscure effects were also factored into the calculation of a possible impact in 2880. The shape of the sun, for example, is not perfectly round.

For an object such as 1950 DA that has a highly inclined orbit, that means that the sun's gravity has an almost negligibly different tug at different parts of the asteroid's orbit.

Also, the sun is constantly shedding matter and energy into space. This has two effects on objects in heliocentric orbit. The first is that it produces a solar wind that pushes objects away from the sun.

It's a tiny force, but over time the cumulative effect can make a big difference. The second is that as the sun losses mass, its gravitational tug becomes smaller. Again, the effect is tiny, but over time the effects can be measured.

When all these pieces, and a few other arcane factors, were all considered, the chance of an impact could not be eliminated.

One huge factor that can't be well determined is changes in the asteroid's orbit due to effect of thermal radiation on the daylight side of the asteroid. This is known as the Yarkovsky effect.

As sunlight heats up the asteroid, some of that heat is radiated back into space. Since heat is energy and energy is mass, that radiation has a "push" just like a rocket engine. It's small, but over nine centuries it may literally make all the difference in the world.

There are many things we don't know well about asteroids in general and 1950 DA in particular. We need to know more about its rotation (we aren't even sure yet where it's pole of rotation lies), mass, optical and thermal behaviors, and the depth of its regolith (the powdery "dirt" covering the rocky core). Our ability to better predict an impact depends on understanding the Yarkovsky effect and how that affects this asteroid.

"1950 DA," Dr. Giorgini's paper states, "is the only known asteroid whose predicted impact potential depends primarily on its physical properties, not on positional measurement uncertainties. A satisfactory assessment of the collision probability of 1950 DA may require direct physical analysis with a spacecraft mission."

All the researchers agree that the important thing about 1950 DA is that it demonstrates that what we don't know about asteroids is more dangerous than what we do know.

"We need to know more about the physical characteristics of the asteroid," Dr. Giorgini said. We're making progress, but we still have a lot to learn.

Mark Perew is a freelance science writer and can be contacted via perew @ addr.com - removes blanks space around @ sign.

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