August 5 – Comet Chameleon

Today’s factismal: The word comet means “long-haired”.

Back in the days of the ancient Romans, when a comet would appear in the sky it was always an evil omen; they considered it to be a “bad star” or disaster. Even though the comet signaled death, destruction, and the start of the primary season, the Romans and Greeks kept their sense of humor. Because the comet had two long tails streaming out like hair in a sea breeze, they called it “long haired” or komētēs.

Comet Lovejoy as seen from the ISS (Image courtesy Dan Burbank, NASA)

The two tails of Comet Lovejoy as seen from the ISS
(Image courtesy Dan Burbank, NASA)

Nowadays we don’t think of comets as being evil omens but we are still fascinated by the tails that they trail, starting with the number of tails – two.  A comet has two tails because it is made of two types of stuff. Thanks to spectroscopy, we know that the nucleus is mainly made up of ices (water ice, ammonia ice, and even methane and carbon dioxide ice) with pieces of dust for texture; this mixture of rock and ice is why comets are often called “dirty snowballs”.

The two tails of Hale-Bopp (Image courtesy NASA)

The two tails of Hale-Bopp
(Image courtesy NASA)

As the comet gets closer to the Sun the outermost ice heats up and spews out gasses that form a globe called the coma (which means “hair” – yep, those Greeks had a thing). The gasses in the coma then become ionized and get dragged out by the solar wind forming the long glowing tail that is characteristic of comets; this gas tail always points straight away from the Sun. Little flakes of rock dust can also be lost. Because the dust is denser than the gas and isn’t ionized, it can form a second tail that curves away from the comet. (So straight tail=gas, curvy tail=dust. Now go impress your friends.) That dust is left behind in orbits that sometimes lead it to fall on Earth as fireballs.

Comet Hyakutake passes the Sun (Image from SoHo)

Comet Hyakutake passes the Sun (Image from SoHo)

The interesting thing about the tails is that they do more than expand. Thanks to all that heat from the Sun, they also glow. And scientists can use that glow to tell us what the comet is made of; things like amino acids and phosphorus – the building blocks of life. Scientists have been doing this for an amazingly long time; on August 5, 1864, Giovanni Batista Donati did the first spectroscopic analysis of a comet and discovered that they had carbon in them. The other interesting thing about comet tails is that they can help us know where the comet formed. Because different things turn solid at different temperatures, by looking for these things in a comet’s tail, we can learn how far away from the Sun it was when it was born. But in order to do that, we need more information on comets.

Comet Sealy in the Texas night sky (My camera)

Comet ISON in the Texas night sky
(My camera)

And that’s where you come in. Comet Hunters is looking for comets that have become trapped in the asteroid belt by Jupiter. If you look through the images from Hawai’is Subaru telescope on Mauna Kea, you might spot one hiding in among the rocks. To learn more (and chase some tails), head over to:

One thought on “August 5 – Comet Chameleon

  1. Pingback: December 31 – Hairy Situation | Little facts about science

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