A few days ago, three astronomers from Mexico posted a paper online (PDF) claiming that an observation from 1883 indicates a small comet passed within a few thousand kilometers of the Earth’s surface, and perhaps as close as 500km! Had this hit us, we would’ve been hammered by thousands of explosions as powerful as the largest nuclear explosions ever detonated.

The thing is, I’m not buying it. While superficially the interpretation fits the observations, there are way too many problems with it.

Here’s the deal. During the days of August 12 – 13, 1883, a Mexican astronomer named Jose A. y Bonilla reported seeing hundreds of objects passing directly in front of the Sun. They were small, appeared fuzzy, and left behind a misty appearance. In total, Bonilla says he saw 447 such objects!

The authors of this new work claim that what Bonilla may have seen was the remnants of a small comet that had previously fragmented. We’ve seen comets do this, and in fact it’s somewhat common. In 2006, Hubble took the picture shown above of the comet 73P/Schwassmann-Wachmann 3, which had recently disintegrated. So that part isn’t too far-fetched. However, once you make that assumption, things get pretty dicey.

The authors use the observations by Bonilla to estimate the distance and size of the comet fragments. Bonilla observed these objects at an observatory in Zacatecas, Mexico, but they were not seen transiting the Sun by any other observatories anywhere else. This can be used to narrow down their location; it means they must have been close to Earth. Had they been far away then other observatories would have seen them moving across the Sun. It’s like a bird flying by just outside your window; someone looking out a different window wouldn’t have seen it, but a bird a few hundred meters away would be visible to both.

Doing some simple math, the authors calculate the comet fragments were no closer than about 500 km (300 miles) from the Earth’s surface, and no farther than about 65,000 km (40,000 miles).

This right there is enough for me to be extremely skeptical of this idea. When a comet breaks up, it spreads out. Even when intact, the material surrounding a comet can be tens or even hundreds of thousands of kilometers across! Claiming that a comet broke apart, yet managed to constrain its pieces to volume of space less than a few thousand kilometers across strains credulity.

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The Leonid meteor shower peaks tonight!

This is a relatively nice shower to watch. There are usually 20 – 30 meteors per hour, so you’ll see one every few minutes. Sometimes — though rarely — it can peak at much higher rates, but I don’t think anything like that is predicted this year.

Watching a meteor shower is actually pretty easy. All you need is a big view of the open, dark sky — the fewer trees, buildings and lights the better — and something comfortable to relax on like a beach chair (the kind that opens up so you can lie on it). The best time to watch is after midnight; that’s when the Earth is facing into the oncoming bits of gravel and ice, and you see more meteors (like seeing more bugs hitting your car’s front windshield than the rear one). The Moon is not quite full, and should be low in the sky after local midnight. Not ideal observing conditions, but not too terrible.

If you want details, I wrote an article on 12 things you need to watch a meteor shower. It was written for the 2007 Perseids in August, but it still applies. Just dress warmly!

Also, the folks at the UK’s Sky at Night magazine put together a nice informational video about the Leonids and meteor showers (it’s from last year, but still relevant):

That’s it! Happy meteor hunting!

Leonids over Uluru image courtesy Vic and Jen Winter at ICSTARS.

The annual Leonid meteor shower peaks this year on or around Tuesday night. It’s a slow-peaking shower, so even if you go out tonight, or later than Tuesday, you’ll probably see a few meteors streaking across the sky.

I’ve written about them many times in the past; a review is on my Bad Astronomy site, and I wrote a guide to watching the Perseids which is still apropos of the Leonids. NASA’s science news page has lots of info, and the International Meteor Organization has technical aspects, too.

Leonids over Uluru image courtesy Vic and Jen Winter at ICSTARS.