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Saturn from Cassini Astronomy 161:
An Introduction to Solar System Astronomy
Prof. Richard Pogge, MTWThF 2:30

Lecture 44: Comets

Key Ideas:

Structure of Comets

Nucleus is a "dirty snowball" of ices & rock

Comets have long elliptical orbits


Comets are small bodies consisting of aggregates of ices mixed with rock & dust.

As they approach the Sun, they heat up and the ices sublimate (go from solid to gas):

Their sudden appearances have made them objects of wonder and fear throughout human history.

Halley's Comet

In 1705, Edmund Halley computed the orbit of the great comet of 1682 using Newton's laws for describing planetary orbits.

It was seen again on Christmas day 1758, 12 years after Halley's death.

Orbital properties:

Historical Appearances of Comet Halley

Comet Halley is one of the most observed comets in history. The earliest recorded appearance is a Chinese observation of the apparition of 240 BC.

It has been seen every 76 years or so since 240 BC:

Great Comets

Every decade or so, a particularly bright comet appears in the sky that is visible to the naked eye.

If they put on a particularly spectacular appearance, they are termed "Great Comets".

A number of such great comets have been influential, for example Halley's Comet, and the great comet of 1577 that inspired Tycho Brahe.

The most recent great comet was Comet McNaught, which put on a great show in the southern hemisphere in early 2007, but was not much to look at while visible from the northern hemisphere. It was even visible in daylight!

Long- and Short-Period Orbits

Comet orbits naturally divide into two types:

Long-Period Comets: (P>200 years)

Short-Period Comets: (P<200 years)

Origin of Comets

The division of comets into long- and short-period comets reflects their origins.

Short-period comets are from the Kuiper Belt:

Long-period comets are from the Oort Cloud:

Structure of Comets



Comet Tails

Comets have two tails:

Dust Tail:

Ion Tail:

Properties of Halley's Nucleus

Small and irregular in shape:

Low density:

Very dark & cratered surface:

"Dirty Snowball" Model

The favored model for comet nuclei is the "icy conglomerate" model of Whipple, usually called the "dirty snowball model". In this model a comet nucleus is an aggregate of:

The interior may be porous, giving it a density lower than that of normal ices.

Why Study Comets

The comets are small, undifferentiated objects that are leftover icy planetesimals from the formation of the Solar System.

Instead of having to travel out into deep space to study the icy bodies of the outer Solar System, comets conveniently bring in material to us as they pass by the Sun. Future space missions to comets will involve chemical analysis and sample return.

Recent & Upcoming Spacecraft Visits to Comets

There were many more questions raised than answered by the spacecraft that flew past Comet Halley in 1986. There are a number of current and planned missions to study comets. Of particular current interest are:

Stardust (US): Launched Feb 1999

Deep Impact (US): Launched 2005 January 12, Fly-by of Tempel 1 2005 July 4:

Rosetta (ESA): Launched 2004 March 2

Comet Model

At the end of class, I constructed a simple model of a comet nucleus using water, dry ice (frozen CO2), carbon, anhydrous ammonia (a stand in for NH3), and silicaceous sand:
Comet Nucleus Model
Comet Nucleus model, with the instructor.
Comet Nucleus Model
Comet Nucleus model close-up, showing the porous ice-matrix structure.
Photos by Katie Schlesinger - click on the thumbnail to view full size.

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Updated: 2007 November 25
Copyright Richard W. Pogge, All Rights Reserved.