Where do Comets Come From?

The first comet I saw was Halley in 1982, though I tried in 1973 to spot Kohoutek but couldn’t see it. Kohoutek was a bust, literally, it broke up. Halley was a disappointment, so dim. In 1996 comet Hyakutake arrived and made my toes curl! Hale-Bopp in arrived in 1997, sweet! Since then, a series of OK comets have graced our skies, but most were barely visible with the naked eye, or you needed binoculars or a telescope to see them. Comet McNaught (C/2006 P1) was spectacular in the southern hemisphere and visible in daytime worldwide. I didn’t look. Last year we had a pretty nice comet. Comet NEOWISE (C/2020 F3) was naked eye visible and looked pretty good, especially in binoculars.

Comets have a reputation, they seem to elicit a visceral response in people. The Greek and Roman names describe a head with long hair. The English derived the word cometa from Roman (Latin), and we now have the word comet. Who wouldn’t be creeped out by a head with long hair flying through the sky?

During the period 7 to 1 BCE, a couple of comets appeared, and some historians think it plausible a comet represented the star of Bethlehem, as prophesied, heralding the coming of the messiah.

Where do these hairy heads come from? Usually far, far away.

From centuries of observation and measurement two primary sources of comets have been identified. The Kuiper belt and the Oort cloud. Far, far away, but still part of our solar system.

Funny how our solar system arranges itself. We have two belts, the Asteroid belt and the Kuiper belt, and one cloud, the Oort cloud. The asteroid belt is thin, the Kuiper belt is more like a doughnut, and the Oort cloud is closer to a thick-walled bubble in shape.

The Kuiper belt lies just outside the orbit of Neptune. Our once planet, now dwarf planet Pluto resides at the inner edge of the Kuiper belt. This belt is a mishmash of larger (Pluto) and smaller (comets) icy objects. The Kuiper belt is our source for most periodic comets, those we see every so many years (usually 200 years or less).

The Oort cloud is way out, way past the Kuiper belt, nearly a light year out there. It is thought to contain millions of objects, comets for sure. It is the primary source of non-periodic or long period (thousands to millions of years) comets. It’s so far away we really do not know what variety of objects exist there, other than comets.

What’s in the Sky?

Comet Leonard (C/2021 A1)

  • December 11 – 12; pre-dawn; east: scan up and down with binoculars, up to 10 degrees.
  • December 14 – 18; after sunset; west-southwest: scan above and along the horizon below Venus. It moves southerly day by day and will be directly below Venus on the 18th.

December 11; 6:10pm; start northwest: Watch the ISS pass between Saturn and Venus