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Treebeard's Stumper Answer
15 November 2002

Catch a Falling Star

This is a challenge, with a stumper. The Leonid meteor shower will peak on Monday night and Tuesday morning (Nov. 18-19), with maybe 2,000+ meteors per hour. You might see meteors at 8:00 P.M. (PST), but our best viewing will be from 2:15 to 2:45 A.M., despite the almost full moon. Set your alarm and stumble out to a dark place and look to the northeast! You can see a few meteors on any dark night if you're lucky, but they are always unpredictable. So how can this 2002 Leonid meteor shower be predicted almost to the minute? Why two peaks? Should we worry?

This chart is extracted (and modified) from an animation by Francis Reddy and Greg Walz-Chojnacki. NASA has more charts comparing predictions for the 2002 Leonids in different cities, and links to the sources. I like this one because it shows both peaks for the shower. "UT" is Universal Time based on the time in Greenwich, England. Subtract 8 hours to get Pacific Standard Time (PST) here in California. The two peaks are on different sides of local midnight, which is why there has been some confusion about the date. Get outside on Monday night / Tuesday morning to see the action!

I managed to get this photo from my porch last year. I set up my Olympus 2040 digital camera on my tripod pointed at Orion, with manual exposure set to f/1.8 @ 16 seconds, and I took a hundred random photos hoping to catch something. I got lucky. This year I'll try again using "Black Frame" noise reduction. There are actually two parallel meteor streaks in this photo. See my Winter Stars (11 Jan 02) and Shooting Stars (4 Dec 98) for more info.

The November Leonids have been the outstanding meteor shower since 1998 when we saw them in Joshua Tree on a DMS school trip. This year is predicted to be the best yet, and we might have to wait a century for a repeat. (Sorry...)

This stumper isn't really about predicting. Even I can make an "exact prediction" that's wrong! But why are the actual peak times of this meteor shower so well defined? Aren't falling stars chance events? I'll get up early in case the predictions are wrong!

There's good Leonid 2002 info and (soon) reports at NASA, Celestial Delights,,, and Sky and Telescope. NASA TV will stream live video coverage on the Web.

The Leonid meteors are the remnants of Comet 55P/Tempel-Tuttle that orbits the sun every 33.2 years in a known path. Comets are "dirty snowballs" that partly vaporize near the sun and shed long icy debris trails that follow along the orbit. This week we passed through material from the 1767 and 1866 passes around the sun. Since the comet's orbit is precisely known, these meteor showers were predicted almost to the minute. I saw many meteors despite the bright moon. Solitary rocky asteroids are potentially more dangerous, so NASA is now tracking them too.


We had an exhausting week of all-day conferences at school, so I figured it was ok to borrow from my earlier stumpers about the Leonid meteor shower in Winter Stars (11 Jan 02) and Shooting Stars (4 Dec 98). But I did my homework, and so did 20+ Dunn Middle School kids! I set my alarm for 2:15 A.M. and put my Olympus 2040Z digital camera on a tripod in the shadow on the northeast side of our house away from the moon. The full moon was very bright, but I did see many meteors including a few fireballs that left a greenish afterglow.

I set my camera in manual exposure mode to the max at F1.8 for 16 seconds at ISO 400, and started taking pictures. I only caught one falling star in about 80 random photos, but its a good one that shows the characteristic green color, with Jupiter and the backwards question mark of Leo all in the same frame. Compare the stars with the chart from the great freeware astronomy program Cartes du Ciel (below right) to get your bearings. The red X shows the radiant point that all Leonid meteors seem to come from. So the meteor in my photo is moving away to the right, changing from greenish to reddish as it burns to nothing.

Digital camera CCD sensors have a problem with random pixels (and JPEG artifacts) during long exposures, so I tried using "Black Frame" noise reduction. Before going back to bed, I took another long exposure with the lens covered, and then used this free software to "subtract" the two images. The software complained that my "black frame" was too bright, but it still helped. A bit of the raw image is below left (also inverse black and white). It works, but there's some loss of detail in the meteor trail.

There's lots of info about the Leonids on the Web, so I don't have to say much here. Comet 55P/Tempel-Tuttle swings around the sun every 33 years, shedding some material each time which follows along the orbit. The comet last circled the sun in 1998, and it's long past us now, heading away. This year, we passed through tag-along debris dating back to the comet's earlier 1767 and 1866 passes around the sun. I figure (1998-1866) / 32 = 4 orbits and (1998-1767) / 32 = 7 orbits.

The plot below from R.H. McNaught and D.J. Asher shows our (predicted) path among the debris trails from different comet passes around the sun. We know the debris trails are long because we still passed through them four years after the comet passed. We know they are narrow because the meteor peak activity only lasts for 1/2 hour or less.

Think of the colored ellipses on the chart as slices of long spaghetti-like trails coming up at an angle towards you through the chart. Predictions are that we won't have another Leonid meteor storm for 100+ years, so that was worth getting up for! The Geminid Meteor Shower will peak on the night of Friday the 13th in December, but it's not the same.

There's another good stumper here. Comet 55P/Tempel-Tuttle went around the sun in 1998, so the particles that we encounter as meteors are following after the comet as it returns outbound from the sun. But the tail of a comet always points away from the sun, coming or going, pushed away by the force of the solar wind. If the comet's tail is ahead of the comet on the outward trip, then why are the meteor debris trails behind? (See this International Meteor Organization (IMO) page for a hint, but think first!)

Leonid meteors are tiny bits of comet fluff that are moving very fast for a good (but safe) show. The International Space Station went right through the storm without incident, though NASA turned the Hubble Space Telescope lens away as a precaution. Powerful meteor strikes like the 10-megaton 1908 Tunguska, Siberia event are always possible. A recent report in Nature says that "Every ten years an explosion equivalent to three Hiroshima atom bombs rips through the Earth's upper atmosphere" and a 10-megaton event might "happen every two or three centuries". Yipes, does that fall under "homeland security"? Think of Dr. Strangelove, the reaction could be worse than the action.

Here are some Web links for your own research on my stumper. You'll find many more links with your own Web search.

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