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. Mind you, Bonilla claimed to have seen these objects over the course of two days. That means they would've been stretched out along a path that was a million km long at least, yet so narrow that only one observatory on Earth saw them transit the Sun. That is highly unlikely. Worse, the very fact that no one else saw anything makes this claim even less tenable. A comet, even one that's broken up, can be very bright, and the closer it is to us the brighter it can be. I have personally seen a comet in broad daylight! Even if this purported comet couldn't be seen during the day for some reason, at some point in the days or weeks before or after these observations, the comet should have been visible in the evening sky (or at least at dusk or dawn). Yet no one saw anything. Worse than that, there were no meteor showers reported that day or night. Comets are basically giant snowballs peppered with dust and gravel. Every time they get near the Sun, some of that ice sublimates (turns from a solid into a gas), forming the fuzzy ball we associate with comets, but also releasing some of the embedded rocky material. When the Earth passes through this stuff, we get meteor showers as it burns up in our atmosphere. A comet passing a few thousand klicks from the Earth would have generated a lot of meteors. It's practically impossible for me to believe that one could get that close to us and not even be noticed, and not create a meteor shower that would have been practically biblical in size*. It's not like there wouldn't have been ammo for such a meteor shower. The authors of the new study calculate the sizes of the fragments given their distance and the size Bonilla reported. They find the fragments would have been a few dozen meters across to as large as a kilometer. If there were hundreds of objects this size, there would've been millions as small a few centimeters across. Objects that size make brilliant fireballs as they burn up in our atmosphere, and would've been visible during the day, even with the Sun shining. Again, no reports of any meteor storms, despite a comet being a few thousand kilometers away and a million kilometers long. Also, the Earth is moving, and covers a lot of ground (OK, space) in a day. Having the Earth move at least 2.5 million km during that time, and never getting closer or farther than 500 - 65,000 km is too much to ask. Let me be clear: I'm not casting doubt on Bonilla himself; it's perfectly reasonable to think he saw something. But there are a lot of other things he might've seen; flocks of birds for example, or an odd atmospheric phenomenon (there are a few volcanoes in that area, which is interesting). While on the surface the explanation of a fragmented comet appears to fit the observations, in reality there are just too many basic problems with this interpretation. It's clever, and interesting, but there are simply too many flaws in the idea for me to think it's correct. Image credit: NASA, ESA, H. Weaver (APL/JHU), M. Mutchler and Z. Levay (STScI)
^* I'll note that later that year, in November, there was a huge meteor shower: the annual Leonids shower, which usually generates a few dozen to a hundred meteors per hour, stormed down thousands of meteors per hour! But that is a well-known and now well-understood phenomenon, and is known to be associated with an intact comet named Tempel-Tuttle. This purported fragmenting comet event was a full three months earlier, when the Earth was millions of kilometers away from the orbit of that comet. They are certainly unrelated.