Like most people who suffer from Acute Protestant Work Ethic Syndrome, I keep a calendar to remind me of upcoming events. I made an entry recently for 2028. It said: "Oct. 26, 12:30 p.m. (CST), duck."

"Duck" here was an instruction. It did not refer to an item on a menu, say, or a hunting pursuit. Writing it down was a way of reminding me to duck so I wouldn't get hit by a mile-wide asteroid that some astronomers, whose job it is to keep us perpetually mystified and anxious, thought might slam into Earth that day. They recently announced exactly that possibility.The next day, of course, other astronomers said they'd studied this more thoroughly and were convinced the asteroid would miss Earth by 600,000 miles or so. That was the same day other scientists said low-salt diets probably were bad for you. All of which left me with a certain lack of confidence.

If this theoretically impossible asteroid accident actually happens, ducking would be a fine, if inadequate, response, though in 2028, if I'm still here, I'll be 83 years old and may need a little help bending over. One of my rules of life is that any time you can avoid getting hit by a mile-wide rock traveling 17,000 mph, you should do it. I normally don't give people advice of any kind, but that's a recommendation you can take to the bank.

The asteroid either threatening us or not in 2028 has a harmless name - 1997 XF11. The designation sounds like one of the fighter planes the Pentagon seems fond of paying too much for. Just so you know, 1997 XF11 is one of 108 such "potentially hazardous objects" (PHAs), at least according to a statement e-mailed to me from the International Astronomical Union (AEIOU).

(By the way, because my cause is accuracy and my job is to annoy people into it, I was quick to send the AEIOU a reply pointing out that "objects" in "potentially hazardous objects" does not start with "A.")

Anyway, Harvard University keeps a Web site on which it lists the PHAs - minor planets, asteroids, comets and similar fly-by-night objects - expected to pass within 0.2 Astronomical Units (UOs) of Earth in the next 33 years.

Putting the distance from Earth in Astronomical Units is especially helpful to mathematically challenged journalists like me, because it requires us to convert distances into miles by multiplying huge figures. That, of course, is a sure way to improve our souls.

An AU , by the way, is some 93 million miles, equal to the mean radius of the Earth's orbit around the sun.

So when I saw on the Harvard list that 1997 XF11 was expected to pass about 0.0003 AUs (UOs) from Earth (a figure changed the next day to .0064), I multiplied 0.0003 times 93 million and came up with four city blocks. But on second - and more accurate try - I got 27,900 miles, the minimum distance Hillary Rodham Clinton wants her husband to keep from Monica Lewinsky.

Indeed, that would put 1997 XF11 closer to us than the moon, although the later calculations promised it would fly by harmlessly instead of slamming into Earth and turning Cleveland into turnip soup. Which is good, because if Cleveland buys the farm, non-Clevelanders will be in a pile of trouble, too.

Astronomers who originally predicted the possible disaster were careful to say that there's "still some uncertainty to the computation." And I'm sure the later 600,000-mile guess contains uncertainty, too.

Which shows you the sorry state of astronomy these days. Its practitioners can't even predict a precise target for a chunk of rock 30 years away.