1. Visitors from the Unknown
Comets
Visitors from the Unknown
Where do comets come from? Where do they go? These questions have haunted humanity from remotest antiquity. In this presentation, we will explore what is known today of these mysterious visitors in our night skies.
Mr. K., NASA/GRC/LTP
Edited: Ruth Petersen

2. Preliminary Activities
1. Word Activity: Look up the word “COMET” in a dictionary. What is the origin of the word? Who were the people who made up this name? What did they believe about their world?
2. Science Activity: Use the www or other references to learn about the structure and composition of comets. What is the primary ingredient of most comets?
How were comets viewed long ago?
What can you learn from the word “Comet”?
How are comets understood today?

3. 3. History Activity: In 1910, the earth actually passed through the tail of Halley’s comet. Use the www or other references to learn about what happened. What did people think was going to happen? How did they prepare? What really happened? (Hint: What are comet pills?)
In 1910, people responded in the most uncanny ways to a heretofore unheard of astronomical event. Expectations ranged from the subtle to the ridiculous. In the end, the great night came and passed. Even the most sensitive scientific instruments failed to register anything at all.

4. Formation of the Solar System
It began with a Nebular Gas Cloud
From which developed Proto-Sun & Planets
From which came the Sun, Planets, & Debris
Which finally cleared out to leave just the Sun & Planets (and their moons, etc.)
Debris
Rocky Matter
(Source of craters)
Icy Matter (Source of comets)
In modern astrophysics, the prevailing theory is that rocky matter and icy matter were left over in abundance after the formation of our solar system. We still see evidence of this matter today: rocky matter in meteors that streak through our skies each and every night, and icy matter in the ghostly comets that mysteriously appear, then pass through our skies from time to time.

5. What do you see happening here?
Rocky Matter ...
What do you see happening here?
What do you think that the end result will be?
Can we see evidence of this kind of bombardment today?
Left-over matter, from microscopic grains of dust to mountain-size chunks of rock and iron, continuously pummels the planets and their moons. This pummeling was most severe in the earliest stages of solar system history when matter literally rained down out of the skies of the newly formed worlds, leaving a brutal wealth of craters on every solid surface. Of course, the intensity of this post-accretional bombardment decreased eons ago, but occasionally a major impact event still occurs. Here is a crater formed within the last thousand years on earth. Can you identify the site?

6. Jan Oort’s Cometary Cloud
Icy Matter ...
Jan Oort ( ), a Dutch astronomer, speculated that a great reservoir of icy/gritty debris orbits our sun at enormous distances compared even to the orbits of remote worlds like Uranus and Pluto. Occasional perturbations or collisions within this cloud send pieces of loosely bound ice and debris into interstellar space or hurtling toward the sun. These sunward travelers appear in our night skies as comets.
Jan Oort’s Cometary Cloud

7. Why do you suppose they leave the cloud in the first place?
Most comets that we see today are believed to have come from the Oort cloud.
Comets are heated by the sun as they approach. They evaporate, forming long, wispy tails, and often leave behind a trail of sand and rocks in their wake.
Why do you suppose they leave the cloud in the first place?

8. Could our sun have a dark companion?. . . . . . . . . .
Could our sun have a dark companion? . . . . . . . .
Some scientists who think “Yes” have actually named the star “Nemesis”! . . . .
Arguments are abundant about the disturbances that might actually occur in the Oort cloud to send these cosmic travelers on their way in the first place. One particularly fascinating theory is that our sun is a binary star with a dark companion whose millennia-long orbit passes into and out of the Oort cloud. . . . . . . . . . . .

9. “Life History" of a Comet
From the Oort Cloud
Why do you suppose the tail always points away from the sun?
What are the head and tail made of?
The transformations that occur in the loosely bound chunk of snow, ice, and debris as it approaches the sun are absolutely fascinating. The ancients were terrified by the appearance of these bodies, thinking them harbingers of war, pestilence, and disaster. The Greeks sedately named them, Cometes, the Bearded Ones. (Incidentally, the word disaster derives from the Latin Dis, god of the underworld, and astro, star.)
“Life History" of a Comet

10. SOHO (NASA’s Solar Orbiting High-energy Observatory) actually photographed the passage of two comets whose orbit brought them so close to the sun that they were pulled to their destruction by the immense solar gravity. The partial disc in the upper left is part of the observing hardware on the spacecraft: it serves to block out the intense light of the sun so that nearby solar phenomena, such as the mass ejection shown (bright streamer), may be studied.
The SOHO Twins

11. Structure of a Comet Toward Sun Tail.
Can you find the head and tail of this comet? Which direction is the sun? Which direction is the solar pressure?
The head and tail are made of materials that “evaporate” from the tiny solid nucleus.
Solar Pressure
OK…Let’s review. Can you find the various directions in space associated with this comet?
Head

12. Nucleus - Few 10’s of km Coma - May be 1000’s of km
The head of the comet contains the ‘tiny’ nucleus of loosely massed snow, ice, and dust, and the vast cloud of evaporated material, the coma, that attends it. The coma may be as much as a million miles or more in diameter – twice the size of the moon’s orbit around the earth! For all of its brightness, the gasses in the coma are many times more sparse than those in the best vacuums achievable in laboratories on earth today.
The Head of a Comet

13. The Tail may be Millions of km
The tail is material from the coma that is pushed outward by the pressure of sunlight. Light is now understood to be a stream of particles – photons – and, as such, is able to exert a miniscule but measurable pressure. The comet’s tail always points away from the sun.
The Tail of a Comet

14. 1. Eruptive jets may result in pieces of the nucleus - big and small - being broken off and thrust into space!
Eruptive
Jets
Solid Mass
2. As the icy matrix evaporates, other loosely bound rocky material is liberated.
3. So … what do you think happens to all these solid pieces - the “Cometary Debris?
This image shows the only cometary nucleus seen and photographed up close to date--comet Halley. It was taken by cameras aboard the European spacecraft called Giotto. The environment surrounding the nucleus is a violent mix of eruptive vapors and dust. Spacecraft operators were surprised at the intensity of the reception they received as they observed the craft’s approach into heretofore uncharted territory.
The Nucleus of a Comet

15. Have you ever watched a meteor shower?
Do you ever wonder why they recur every year at the same time?
Did you know that there is a connection between meteor showers and cometary debris?
The existence of a dark companion to the sun was prompted, in part, by a study of mass life extinctions throughout the earth’s past geological ages. Since these extinctions occurred periodically, and since one of the most recent (the dinosaurs) seems to have been associated with a major impact event on earth, scientists were led, through a long chain of logic, to propose the Nemesis theory. Can you formulate a chain of arguments that leads from periodic mass extinctions to a dark star hypothesis, given what you have learned from this lesson so far? Try!!
Can you describe the connection?

16. Cometary debris travels with the comet along its orbit.
The debris appears to fall out of the sky - similar to driving your car in the rain! . . . . . . . . .
Meteors come, in part, from the rocky debris left behind by the passage of comets. Cometary debris continues indefinitely in orbit around the sun once the parent comet has passed. If the earth should happen to cross an old cometary orbit in its annual course around the sun, it will collide with this debris and brilliant meteor showers will be seen on the surface. Most meteors burn up in the atmosphere. A few, however, manage to make it all the way down to the surface. Those that do are no longer called meteors but meteorites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Planets crossing the comet’s orbit collide with the debris. . .

17. Now...Can you suggest a connection between meteorites and dinosaurs?
Perhaps a major impactor from a comet caused the Alvarez event?
What can you say about mass extinctions in general?

18. Large objects are believed to rain down to Earth periodically
Large objects are believed to rain down to Earth periodically. Major impacts, such as this one, may cause drastic changes in life on Earth, including mass extinction of species.
The idea of mass extinction following a major impact on earth is similar to the nuclear winter hypothesis associated with thermonuclear war. In each case, so much debris is lofted into the earth’s atmosphere at every level, that major climatic changes are induced at the earth’s surface – specifically, temperatures plummet, and a long, dark nuclear winter follows. In this grossly revised climate, many species that cannot survive the intense cold and darkness will disappear forever. Could this have happened to the dinosaurs? Luis and Walter Alvarez think “Yes”!

19. Perhaps periodic disturbances in the Oort cloud - as by the passage in & out of a Nemesis star in an eccentric, long period orbit - result in periods of increased cometary activity throughout the solar system. Increased activity results in more large-scale collisions.
If a large object were on collision course with the earth today, it is possible that astronomers would detect it, but unlikely that humanity could do anything to protect itself. Sixty-five million years ago, such a day would have dawned completely without warning. To be cliché, the dinosaurs could not have known what had hit them…

20. Here are Luis and Walter Alvarez at the now famous KT boundary.
Evidence?
Here are Luis and Walter Alvarez at the now famous KT boundary.
Tertiary - No Dinosaurs
An iridium-rich clay layer, that occurs around the world, and its location between Cretaceous rocks (last great age of the dinosaurs) and Tertiary rocks (the very next geological age in which dinosaur fossils are completely lacking) is the clue that lead Luis and Walter Alvarez to postulate their impact theory. Iridium is a chemical element found in abundance in certain types of meteorites.
Clay layer with Iridium.
Cretaceous - Dinosaurs

21. Magnetic map showing location of buried crater.
Is this where the meteorite that changed dinosaur history struck the Earth?
Magnetic map showing location of buried crater.
The location of the Alvarez impact remained uncertain for many years after the father-son team had proposed their hypothesis, until an oil mining company did a magnetic scan of a region in Yucatan. Their objective was to seek out and find subsurface oil; instead, they found the hidden remnant of an enormous crater – you guessed it - about 65 million years old!

22. Remember: Watch the skies!!!
Comets come in all shapes and sizes. One might occur at any time. Remember to watch your local newspaper, check the www, or contact your local observatory or planetarium to find out when the next comet will pass close by the Earth.
Remember: Watch the skies!!!
Our study of comets has led us through time, space, and earth’s geological history. There is much more to the story of comets. To help you explore further, some www sites are included at the end of this presentation.

23. Follow-Up Activities
1. Now that you know about the dinosaurs, check out the key word, “TUNGUSKA,’ for an account of a 20th century impact in a little know region on Earth! May we still expect such impacts to happen today?
2. How would you prepare if you knew that a major comet or asteroid were going to hit the Earth?
The Tunguska event is a major impact event of the 20th century. While much has been learned, much more remains shrouded in mystery. Astronomers did not anticipate the event – it caught the world completely off guard!

24. Most important of all: Remember to be safe and happy!
3. Since comets are mostly water, how might we use them in the future to modify the climate of a planet such as Mars? (Hint: Use the key word, “TERRAFORMING,” to get started.)
Most important of all: Remember to be safe and happy!
Robert Zubrin is also a good source to check here!

25. Web Sites for Further Exploration
Here are some more sites for you to explore.

26.
And please feel free to contact me with questions or comments!!!  CIAO!!!