Friday 15 April 2011 - Comets: Dirty Snowballs in space

EXAM 3 =-=-=-= 22 April 2011 &*^%$ READING list is on web for Wednesday 13 April

Gene Shoemaker 1928-1997 Gene Shoemaker did more than anyone else to show us that cosmic impacts have helped shape the bodies of the Solar System. His dream was to go to the Moon and "bang on it with my own hammer". He didn't make it to the Moon while he lived, but 1 ounce of his cremated remains were aboard a spacecraft that crashed into the Moon in 1999. Gene is the only human whose remains are on a body besides the Earth. For a brief description of this and to read the inscription on the capsule that held his remains click here.

Parts of a comet.. (NOTE: Images NOT TO SCALE- the coma is on order of a million times size of nucleus!) When a comet is cold and far from the Sun, the ice-rock nucleus is bare (the "dirty snowball"). When a comet nucleus comes in close to the Sun (within 2 or 3 AU), the ices start to sublimate, and the sublimated gases form a temporary atmosphere called a coma. The size of the coma can be up to a million times the diameter of the nucleus! Even more extended is the comet tail, which can extend up to 100 million km from the nucleus! See comet tails image for further details. The material that leads to meteor showers (see below) is composed of particles larger than the dust particles in the tails, and are much less affected by radiation pressure.

(1) Tails of a comet (2) Hale-Bopp (1997) Comets have 2 tails- a dust tail and an ion tail. The dust tail is composed of very fine dust particles, which are pushed away from the Sun by radiation pressure. The ion tail is composed of ionized (charged) molecules, which are pushed away from the nucleus by the Suns magnetic field. Note that the tails point away from the Sun whether the comet is approaching or receding from the Sun. These tails are NOT stuff "left behind" along the orbit of the comet.

The image of Hale-Bopp, a comet which was seen in 1997, shows a good example of the 2 tails. The upper (bluish) tail is the ion tail, which glows due to (collisonal) excitation and (radiative) de-excitation of individual atoms and molecules. The bottom (whitish) tail is the dust tail. The dust in the tail simply reflects sunlight. Some comets do not show two distinct tails. This is often due to a viewing angle affect- the two tails overlap as seen by us, and so look like a single tail.

(0) Halley in 1910 (1) Halley as seen by Incas (2) Halley on Bayeux Tapestry (1066) (3) Halley viewing (1910) (4) Halley Rag (1910) (5) Comet Pills (1910) Just a few examples of historical record of Halleys Comet. Visits of Halleys comet to the inner solar system have been recorded for 2 millennium.

The apparition in 1910 (first picture) was a very favorable one, meaning the Earth was in a good position vis a vis the comet (unlike the poor apparition in 1986- see next slides). Coupled with much darker skies in 1910 (few electric lights) the Comet was observed to stretch across half the sky!

(1) Halley's orbit (2) Halley's 1986 perihelion (2) Halley in 1986 from Arizona Halley was last at perihelion (closest approach to Sun) in February 1986. This was not a good apparition (as you can see, when the comet was nearest Sun- 9 Feb 1986- hence brightest- we (the circle with cross in it shows position of Earth) were on the other side of the Sun, so Halley was not visible in the nighttime sky). Many people were disappointed with their view of Halley in 1985/86, mostly due to the poor viewing geometry (and also light pollution). I saw Halley from Kitt Peak, a very good dark site, and thought it was a good show.

The 2061 showing of Halley should be much better than the 1986 showing. The orbital lineup of Halley and Earth in 2135 promises a TRULY SPECTACULAR show- thats only 124 years from now! Eat right, don't drink and drive, don't text and drive, exercise regularly and you might make it til 2135! (I will be 183 years old).

(1) The Halley Armada (2) Halley's nucleus dimensions (3) Halley nucleus image A number of spacecraft were sent near Halley in 1986. The most ambitious was Giotto, a European spacecraft. Because of the comets retrograde (backwards relative to Earth) orbit, the encounters were are extremely high speeds. The most spectacular result was the closeup images of the actual nucleus taken by Giotto.

Mark Twain and Halley Just an interesting "factoid" about Twain.

1965 comet

2007 comet

2007 comet, fireworks and storm Picture (actually 3 shots with a little Photoshopping done to get the colors right) showing Comet McNaught in the evening sky. Taken on a beach in Australia.

Comets visible to the naked eye (at least from dark sky) come by every decade or so. Bright comets come by once or twice a century. These are 2 of the brightest comets of the past 60 years. I can still remember like it was yesterday seeing the 1965 comet while freezing in the pre-dawn chill of a clear January Pensylvania night (I was 13 years old).

I did NOT see the 2007 comet. When it was visible from the northern hemisphere, it was faint and we had a run of cloudy weather in Oklahoma. Only when the comet moved into the southern sky and was no longer visible from the northern hemisphere did it become the spectacular comet seen here.


In 1994, Comet Shoemaker-Levy 9 broke into fragments and the pieces slammed into Jupiter. The explosions produced by the impacts had millions of times the energy of all the A-bombs and H-bombs ever made.

Shoemaker-Levy 9 slams into Jupiter in 1994

SL9 impact sites on Jupiter


Comet orbit and origin of meteor showers.. Small pieces of material are shed from a comet as it orbits the Sun. (These pieces are the size of BBs to peanuts- MUCH larger than the micron-sized dust particles that produce the dust tail.) This debris trail is concentrated along the orbital path of the comet. If the Earth's orbit happens to intersect this trail, we can get a meteor shower, a time when there is an enhanced number of meteors seen in the Earth's sky. As the Earth orbits the Sun, it intersects a number of debris trails on about the same day each year, leading to well- know meteor showers, such as the Leonids in mid-November, the Geminids in mid-December, and the Perseids in mid-August. Very rarely, the Earth hits a particularly dense clump of debris, and there can be a spectacular meteor storm, with thousands of meteors visible for a relatively brief period.

There have been no meteorites known to be associated with meteor showers, indicating that the cometary debris particles are probably small enough to burn up completely in the Earth's atmosphere. The rocks that become meteorites that land on Earth come from asteroids, not comets.

Leonids from above Image of Leonid meteors from ABOVE! (Looking down from Space Shuttle.)

Leonids using "fisheye" lens Nice picture of Milky Way and some Leonid meteors, taken using a "fisheye" lens, which shows the entire sky.

Legendary 1966 Leonid Meteor Storm In 1966, there was a meteor storm associated with the Leonids. Photographs just don't do this event justice. Quotes about this event from January 1967 issue of Sky and Telescope: "A rate of 150,000 meteors per hour was seen for about 20 minutes" "Meteors coming too rapidly to count". The rate of 150,000 / hour is equal to 40 per second!! WOW!

More spacecraft missions to comets:

(1) Deep Impact (artists conception). (2) Deep Impact encounter sequence (3) Best overall image of Tempel 1 nucleus NASA's Deep Impact mission (2005) was a smashing success! A copper impactor, with mass of about 3 moderate-mass football players, separated from its "mother ship" and smashed into Comet Tempel 1 at a speed of about 10 km/sec. The impact left a crater about the size of a football field (but unfortunately all the dust kicked up by the impact made it difficult for the mother ship to image the crater). Material heated by the impact continued to spew from the crater for several days (remember the gravity of the nucleus is very low). The comet was found to be dustier (and less icy) than expected. Spectroscopic signs of lots of organic (carbon containing) (NOT biological!) molecules were found in the ejecta. The nucleus (like that of Halley) was found to have a low density- about half that of water, perhaps indicating a "fluffly" composition and/or voids and cracks.

The nucleus of Temple 1 is about 8x5 km in size, thus it is somewhat smaller than Halley.

Deep Impact - The Movies!. 2nd movie The first movie shows the impact of the copper "bullet" as imaged from the mother ship. The second movie shows the comet as imaged by a camera on the bullet.

Deep Impact (the original movie) There was a 1998 disaster movie (asteroid hits Earth) called DEEP IMPACT. Oh wow, I was just looking at wikipedia to find the date of the movie and see that Elijah Wood was the nerdy young amateur astronomer in the movie- he was of course Frodo Baggins in the Lord of the Rings movies! (I promise this movie trivia will not be on the exam) I learn such interesting stuff on wikipedia - not that I believe all of it.