Rick Robinson

Dec 26th 2022

The Elusive Unicorn Meteor Shower


Late last year, an unusual news story flashed through the media sky like a shooting star, come and gone in the blink of an eye. As EarthSky reported, a rare unicorn meteor shower, the alpha Monocerotid meteor shower, was predicted for the night of Nov. 21, 2019.

As it turned out, while observers reported the occasional stray meteor that night, none saw the dazzling burst of meteors that they were hoping to see. But the true mystery of the alpha Monocerotids remains, along with the likelihood that on some upcoming November night this celestial “unicorn” will indeed put on a spectacular sky show.

What Goes Around Comes Around

The central puzzle of the alpha Monocerotid meteor shower is that while it’s almost certainly produced by a comet, we don’t know which one. The meteor showers that periodically put on nighttime shows are usually associated with periodic comets. These are comets that make regular visits to Earth’s neighborhood every few years or decades.

According to Sky & Telescope, summer’s rich and bright Perseid meteor shower, an annual favorite, is linked to Comet 109P/Swift-Tuttle. Another annual meteor shower, the Eta Aquariids, has a far more famous source: Halley’s Comet.

The meteors that light up the sky during a meteor shower originate as dusty or gravel-like fragments of the parent comet. They break loose as the comet passes near the sun, vaporizing the icy materials that hold the comet together. This process creates the spectacular tails that some comets grow as they pass through the inner solar system — and it leaves bits of comet spread out all along the parent comet’s orbit.

When Earth passes near the comet’s orbit, some of the comet fragments hurtle into Earth’s atmosphere and burn up, producing a meteor shower.

A Long Time Coming

What sets the alpha Monocerotids apart from other annual meteor showers is that its parent comet remains undetected and unknown. It is believed to be a “long-period” comet, one that hasn’t made more than one complete orbit in the last 200 years. Indeed, for all we know, the parent comet of the alpha Monocerotids could take a million years between return visits to the inner solar system.

This uncertainty about the alpha Monocerotids extends to how bright (or not) its annual visits will be. If we knew its parent body’s exact orbit we would be able to reliably predict how bright it will be from year to year. But uncertainty about its orbit means that while we can count on it to show up each year in late November, we can’t be sure how many meteor showers will appear in a given year.

We know that the unicorn meteor shower has sometimes put on impressive shows, most recently, says Live Science, in 1985 and 1995. Some researchers thought that signs pointed to 2019 as another spectacular year, but that time it fooled us.

Elusive Unicorn

One final surprise from the unicorn meteor shower is its name. The unicorn in the name is not the celestial equivalent of a black swan, a reference to the meteor shower being unusual or mysterious. Instead, as with other meteor showers, the name refers to the region in the sky — the radiant — where the meteors seem to be coming from, due to perspective effects.

In the case of the unicorn meteor shower this region is the far-northern constellation Monoceros, the celestial Unicorn. A final little mystery here is that most people aren’t even aware that there is a unicorn constellation in the night sky. As Constellation Guide explains, in spite of its prominent location in the northern sky, near the Big and Little Dippers, Monoceros has few bright stars, and forms no easily recognized figure.

Somehow it seems appropriate that this elusive unicorn of a constellation should be linked to the most unusual and little-understood of annual meteor showers.