Evaporating Comets

Seth Jarvis

This week’s Cosmic Quiz winner is Mary Newland, who asked, “Every year I like to watch the Perseids meteor shower. If the meteors are continually being shed, why does the comet not cease to exist?”

That is a great question!

As a matter of fact, comets do cease to exist over time.

Let’s start with some background information on comets:

What is the life expectancy of a comet?

What is the life expectancy of a comet?

Comets can be thought of as large dirty snowballs. The core of a comet, known as the nucleus, is typically a several mile-wide lump of frozen water that also contains various amounts of frozen gasses such as carbon dioxide, carbon monoxide, methane, and traces of ammonia. Mixed into all these ices are small amounts of rock and dust. An assortment of tar-like hydrocarbon compounds covers the surface of the nucleus.

Nearly all comets follow huge looping orbits around the sun, spending almost all of their time in the dim, cold regions of space far beyond the outer planets. For many comets, the time between successive close approaches to the sun is measured in thousands of years.

Comets spend most of their lives far from the sun.

Comets spend most of their lives far from the sun.

Even a “Short Period” comet like Halley’s Comet (above) with its 76 year orbit spends most of its time far from the sun.

A comet’s orbit will periodically carry it, briefly, to the inner solar system. In the toasty-warm inner solar system the heat from the sun causes some of the ices near the surface of the comet’s nucleus to sublimate, transforming the ices directly from a frozen solid into a gas.

The expanding ball of gas and dust forms the comet’s head, also known as the coma, which grows to become hundreds of thousands of miles in diameter.

As these ices turn themselves into gas they release into space the trapped dust and bits of rock that are embedded in the ice.

Comet Halley's nucleus, as imaged by the Giotto spacecraft in 1986.

Comet Halley's 10 mile-long nucleus as imaged by the Giotto spacecraft in 1986.

Pressure from the sun’s light and magnetic field pushes the coma away from the sun, creating the comet’s tail, which itself can become tens of millions of miles long. All this from mountain-sized lump of dirty ice!

While a comet is in the inner solar system it is warmed by the sun and constantly sheds bits of dust and tiny pieces of rock. These drift through the solar system following roughly the same orbit as the comet.

comet-animation

Comet 73P/Schwassmann-Wachmann losing matter as it warms.

It is not uncommon for a comet’s orbit to intersect with Earth’s orbit, and our planet finds itself several times each year plowing through a region of space that’s been recently dirtied-up by a comet. For the few days during which earth passes through a comet’s orbit our world encounters an above-average number of meteors.

It’s sort of like driving your car through the countryside. There are always a few unfortunate bugs that will cross paths with your windshield, but occasionally you’ll encounter a hapless swarm of gnats. Splat!

That’s the way it is with meteor showers. The sky overhead in the wee-small hours of the morning is the “front windshield” of our planet as we zip through space at 60,000 miles per hour, and meteor showers are ill-fated collections of dust and gravel, shed by comets, whose orbits around the sun place them in front of us, where they meet their brief, fiery demise.

Astronomers estimate that a comet nucleus can survive anywhere from a few dozen to a few hundred passages around the sun before it has lost so much material that it no longer creates a coma and tail and is doomed to spend the next few billion years as an unremarkable asteroid.

Nucleus of nearly-dead Comet 9P/Tempel, from the 2005 "Deep Impact" mission.

Nucleus of nearly-dead Comet 9P/Tempel, imaged by the 2005 Deep Impact spacecraft.

Comet Swift-Tuttle, with an orbital period of 133 years, is the comet that is the source of next week’s Perseid Meteor Shower.  This comet’s life expectancy is thought to be only 50,000 years or so, which in astronomical terms is an eyeblink.

Now that we know that comets can indeed “cease to exist,” the really interesting question becomes, “If comets don’t last very long, then what replenishes the supply of comets to our solar system?”

That, dear reader, needs a whole ‘nuther blog post.

But I’ll give you a hint: “Oort Cloud.”

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2 thoughts on “Evaporating Comets

  1. An Oort Cloud? You are kidding, right? The Oort Cloud idea has been debunked for a long time based on the calculations used to propose that ridiculous notion was found to be just very bad mathematics and so the Oort cloud cannot exist and is just a myth.

  2. John, there is a lot of evidence, mainly derived from observations of the orbits of long period comets, to support the Oort Cloud hypothesis. Moreover, Kuiper Belts have been observed around other stars, and since Kuiper Belts are compatible with Oort Clouds, it does not seem unreasonable to conclude that Oort Clouds are common features of solar systems.

    I’m unaware of any serious, objective “debunking” of the Oort Cloud. Can you cite the work that you believe debunks this model? Also, if _not_ from an Oort Cloud, from where do long period comets originate?

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