A Comet We’ve Never Seen Has Been Lighting Up the Sky for 100 Years

On Thursday night, Earth might pass through the trail of a comet that humans have never directly observed, but which has lit up our skies with fantastic outbursts of hundreds of meteors for at least the past century.

The meteors are called the alpha Monocerotids, because they are located in the direction of the constellation Monoceros, or the Unicorn. For this reason, the meteors are sometimes called the “unicorn” shower.

The alpha Monocerotids are chunks of dust that broke off of an unobserved comet and then burn up in Earth’s atmosphere to create shooting stars. The comet’s long debris trail has illuminated the night skies in the past, with particularly bright meteor showers in 1925, 1935, 1985, and 1995.

Technically, the alpha Monocerotids occur every year, but they normally only produce a small smattering of shooting stars. This is likely because Earth usually passes through the edge of the stream, rather than getting smacked by the more clustered trail in the center.

Earlier this month, meteor shower experts Peter Jenniskens of the SETI Institute and NASA Ames Research Center and Esko Lyytinen of the Finnish Fireball Network predicted that Earth is likely to pass through or near this meteor-dense part of the alpha Monocerotids rubble stream again on Thursday night.

Because this stream is fairly thin, Jenniskens and Lyytinen said that the shower would be very short-lived—no longer than 40 minutes—but that it could produce hundreds of meteors during that outburst. The pair projected that this outburst would probably peak around 11:50 pm ET and would be visible to skywatchers in eastern North America, western Europe, and South America. Jenniskens had previously predicted the 1995 event before it occurred.

But before you get hyped to see this rare meteoric event, you might want to read this blog post by Bill Cooke, who leads NASA’s Meteoroid Environment Office. After revisiting the known data about the shower, Cooke became skeptical and came to the conclusion that “there is a pretty good chance there may be no outburst at all,” according to the post. If there is one, it might not be the spectacle that some stargazers imagined.

The uncertainty stems from the unknown amount of time it takes this mysterious comet to complete an orbit around the Sun. Scientists know the orbital periods of comets that produce famous meteor showers such as the Leonids and Perseids, but Jenniskens and Lyytinen had to calculate a rough estimate for the comet associated with the alpha Monocerotids, since it has not been directly observed.

Based on the timing of past outbursts, the pair suggests that this comet travels around the Sun once every 500 years or so.

Cooke thinks this estimate is too rough to clearly herald another brilliant outburst. “The intensity of the outburst is very dependent on the size of the parent comet’s orbit,” he pointed out. “If it is much smaller, or larger, the distance from the stream center will be bigger, and there will not be any sky show, just the normal [alpha Monocerotids], puttering along with their normal rate of three or so meteors per hour.”

“[S]ince we have not yet discovered this mysterious parent comet,” he adds, “who knows how close the estimate of the orbit is to the actual?”

That’s not to suggest that there won’t be a beautiful shower with hundreds of visible meteors tonight. In fact, Cooke said that if there is, the number of meteors will be provide some much-needed data for calculating a more exact orbit for the unidentified comet.

As Cooke concludes, it’s never a bad idea to set aside some time to gaze at the night sky. But if you are looking forward to a radiant light show, you may want to to temper your expectations, embrace the mystery, and evaluate these dueling meteor predictions in real time.