On August 12th, 1883, Mexican astronomer José Bonilla was preparing to study the Sun at the recently opened Zacatecas Observatory. However, the Sun’s surface was marred by numerous objects quickly travelling across its disk. Over the course of the day and the next, Bonilla exposed several wet plates to take images of the 447 objects he would observe. They weren’t released publicly until January 1st, 1886 when they were published in the magazine L’Astronomie. Since then, UFOlogists have crowed these photographs as the first photographic evidence of UFOs. The chief editor of L’Astronomie passed the observations off as migrating animals, but a new study proposes the observation was due to the breakup of a comet that nearly hit us.
The only piece of evidence the authors, led by Hector Manterola at the Universidad Nacional Autónoma de México, use to suggest that this was a comet in the process of breaking up, was the descriptions of the objects as being “fuzzy” in nature and leaving dark trails behind them. Assuming this were the case, the authors consider how close the object would have been. Since astronomers at observatories in Mexico City, or Puebla had not reported the objects, this would imply that they did not cross the disc of the Sun from these locations due to parallax. As such, the maximum distance the object could have been is roughly 80,000 km, roughly 1/5th the distance to the moon.
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But the team suggests the fragments may have passed even closer. By the time comets reach the inner solar system, they have a significant velocity of some tens of kilometers per second. In such a case, to transverse the disc of the Sun in the time reported by Bonilla (a third to a full second), the object would have been, at most, at a distance of ~8,000km.
At such distances, the overall size of the fragments would be in rough agreement of sizes of other fragmented comets such as 73P/Schwassmann-Wachmann 3, which gave off several fragments in 2006. Based on the number of fragments, estimated sizes, and density of an average comet, the authors estimate that the mass may be anywhere between 2 x 1012 and 8 x 1015 kg. While this is a very large range (three orders of magnitude), it roughly brackets the range of known comets, again making it plausible. The upper range of this mass estimate is on par with Mars’ moon Deimos, which is generally held to be similar in mass to the progenitor of the impact that killed the dinosaurs.
One oddity is that one would likely expect such a close breakup to result in a meteor storm. The timing of these events is just before the annual Perseid meteor shower, but reports for that year, such as this one, do not depict it as being exceptional, or having a different radiant than should be expected. Instead, it notes that 157 of the 186 meteors observed on the 11th were definitively Perseids, and that the “year’s display cannot be reckoned as a fine one by any means.” Meanwhile, the Leonid meteor shower (peaking in November), was exceptional that year, generating an estimated 1,000 meteors an hour, but again, no records seem to indicate an unusual origin.
In total, I find the characterization of Bonilla’s observation as a comet plausible, but generally unconvincing. However, if it were a fragmented comet, we’re very lucky it wasn’t any closer.