Science
Demonstrations and Discussion
To understand comets and Near-Earth-Objects, students created comets from dry ice, corn starch, and many other ingredients (see the bottom of the lecture transcript). Then, CSS members dropped the icy balls from atop the Robert Andrews Millikan Memorial Library, the tallest building in Pasadena.
Top: Student-made comets impact on the "Earth." Middle left: a comet that has missed the mark. Middle right: a comet ready for its voyage. Bottom: the views from the tenth floor of Millikan.
And a QuickTime movie of a falling comet: click here.
Lecture Transcript
I. INTRODUCTION
Basic Terminology
1. Asteroids
Asteroids are small bodies of rock and/or metal in orbit around the sun. The largest asteroid, and the first one to be discovered, Ceres, is 910 km in diameter (compare this to the approximate diameter of the smallest planet, Pluto, which is 2300 km). Most asteroids in the solar system are located in the asteroid belt, between the orbits of Mars and Jupiter. It is widely believed that asteroids are the remnants of a planet which tried to form between Mars and Jupiter, but was disrupted by Jupiter's strong gravity. The perturbing influence of the planets especially Jupiter and Saturn, have scattered asteroids across the solar system, including some into orbits which can threaten the Earth.
2. Comets
Comets are small bodies of ice and rock which orbit the Sun in highly elliptical orbits. A typical comet will come in closer to the Sun than the Earth, but may go well into the outer solar system. When comets enter the inner solar system, some of the ice on their surfaces are vaporized by the Sun's heat, carrying away dust and gas to create the familiar tail of the comet. Comets are typically much smaller than asteroids, usually only a few kilometers in diameter. However, since they travel through the inner solar system at high velocity on their elliptical orbits, they too pose a threat to the Earth.
3. Meteors and Meteorites
Meteors are any objects (asteroids or comets) that enter the Earth's atmosphere. the tremendous heat generated by friction as the high velocity objects hit the atmosphere causes small objects to bum up long before reaching the ground: the familiar (if incorrectly named) shooting star. Larger objects may not entirely burn up before reaching the ground: any remnants of these objects after they hit the ground are called meteorites. Much of our knowledge of the composition of the asteroid belt is based on studying meteorites and comparing the results of those tests with our telescopic observations of the asteroids.
B. Origins
Asteroids and comets are the "leftovers" of the solar system. As the gas and dust which made up the solar nebula five billion years ago cooled, different materials condensed out of the nebula at different distances from the Sun, based on temperature. Near our Sun, only matter with high melting points - metals and silicates - froze out of the nebula. Farther from the Sun, more volatile compounds, like water, carbon dioxide, methane, and ammonia, were able to freeze out.
These materials condensed into small fragments, which grew by accretion as fragments collided into each other at low enough velocities to stick together rather than break apart. This led to the formation of the inner planets: Mercury, Venus, Earth and Mars. However, the rocky and metallic fragments which existed beyond the orbit of Mars were unable to accrete into a larger body, due to the influence of Jupiter. Jupiter' s strong gravity caused enough perturbations on the orbits of the fragments to keep them from coming together, hence the formation of the asteroid belt.
In the far outer solar system, small fragments of ices coalesced together. However, given the great distances between fragments far from the Sun, they were unable to come together into a planet like in the inner solar system. Instead, these objects were consigned to remain small icy bodies in the outer solar system, forever to remain out there unless the gravitation perturbation from a passing star could fling some of them inward, where they would be called comets.
II. NEAR EARTH ASTEROIDS (NEAS)
A. Types of NEAs
Near-Earth asteroids (NEAs, for short) are asteroids whose orbits take them near the Earth, in some cases crossing the Earth's orbit. NEAs are divided into three classes, based on the characteristics of their orbits:
1. Apollos
Apollo asteroids are those asteroids whose orbits cross the orbit of the Earth. There are over sixty known Apollo asteroids, and scientists estimate there are between 500 and 1000 Apollo asteroids that are at least I km in diameter (the largest known Apollo asteroid. 1978 SB is about 8 km in diameter). Such an object would wreak havoc on Earth if it were to collide with the planet.
2. Amors
Amor asteroids are those asteroids which come close to, but stay outside of, the orbit of the Earth. Many of these objects cross the orbit of Mars. These objects are often asteroids in transition from the asteroid belt (where they may have been knocked out by a collision or by a gravitational resonance with Jupiter) to the inner solar system (Apollos and Atens). There are estimated to be 1000 2000 Amors at least I km in diameter, with the largest 30 km across.
3. Atens
Aten asteroids are those asteroids which come close to, but stay inside of, the orbit of the Earth. On occasion these objects, at the farthest point from the Sun in their orbits cross the inner portion of the Earth's orbit. Aten asteroids are the rarest of the NEAs: only about a dozen Atens largest than I km in diameter are known to exist.
B. Populations of NEAs
As mentioned, there are believed to be several thousand large asteroids that either cross or come near the orbit of the Earth. However, we have catalogued only about one hundred of the largest NEAs. However, we can estimate the total population of these Earth-crossers with some certainty. By observing the largest asteroids (of which we believe to have observed all that currently exist) we note a relationship between the size of the object and the number of objects of that size. This size relationship usually follows an inverse power law: the smaller the object, the more of that size that exist. Depending on the modeling, the exponent ranges from -2.5 to -3.5: decreasing the size of an object in half increases the population by a factor of 5.5 to 11.
C. Discovering NEAs
The potentially large number of objects that could strike the Earth makes it vital to find these objects and catalog their orbits, so we know which ones pose the greatest threat to the Earth. Several small projects are currently underway to discover NEAs. In Arizona, the Spacewatch telescope on Kin Peak southwest of Tucson has a dedicated search program to find these objects using an 61-inch telescope. On Palomar Mountain in California, astronomers Eleanor "Glo" Helin and Carolyn and Eugene Shoemaker use an 18-inch telescope in a similar program. Last year, Congress directed NASA to study the feasibility of setting up a worldwide system of telescopes dedicated to cataloging all the NEAs that could threaten the Earth.
III. COMETS
A. Types of Comet Orbits
Comets are almost always found in very elliptical orbits, compared to the planets, but even among comets there is a wide range of orbits.
1. Short Period Orbits
These are comets that complete their orbits in under 200 years. Their orbits usually keep them with the orbits of the nine planets of the solar system. The most famous example of a short period (or periodic) comet is comet Halley. It completes one orbit around the Sun in 76 years traveling to within 0.53 AU of the Sun (closer than Venus) and traveling as far out as 35.2 AU (farther than Neptune). Other examples of short period comets include Encke, Swift-Tuttle, and Tempel II.
2. Long Period Orbits
These are comets that take more than 200 years. in some cases much more than 200 years, to complete an orbit. These comets may travel up to 40,000 to 50,000 AU from the Sun - about one sixth of the way to Alpha Centauri - and take a million years or more to complete their orbits. They can swoop down into the inner solar system very quickly, though, traveling at high velocity, and are capable of doing considerable damage of one struck the Earth. The most famous example of a long period comet is comet Kohoutek, which came into the inner solar system in 1973.
3. Hyperbolic Orbits
These are comets whose orbits have been perturbed by a large object - usually Jupiter and have been thrown into a new trajectory at a high enough velocity that they can escape from the solar system entirely, never to return. It is difficult to verify whether a comet has been ejected from the solar system or simply is in a very long period orbit: a small error in an observation of the comet can make a difference in the determination of its orbit, and no one has been around long enough to see a long period comet return! Like long-period comets, they travel fast enough through the inner solar system to cause considerable damage if one hit the Earth.
B. Comet Discoveries
Comets are discovered at a far slower rate than asteroids: perhaps a dozen or two a year, as opposed to hundreds of new asteroids a year. However, we have learned a fair amount about comets based on ground-based observations as well as close encounters with comet Halley by an intentional armada of spacecraft in 1986. While comets do contain some rock, they are mostly composed of ice, usually a combination of water, carbon dioxide (dry ice), ammonia, and methane. Although they appear very bright and white, the nuclei of comets are covered with dust that is as dark as charcoal. A typical comet nucleus is about 5-10 km in diameter.
We have also learned that comets do pose a potential threat to the Earth. In the early 1990s, scientists tracing the future orbit of Comet Swift-Tuttle noted that their was a chance the comet would strike the Earth early in the 22nd century. This is a very uncertain prediction, and it almost certain the comet will not come close to the Earth (its orbit having been ever-so-slightly altered by the gravitational influences of the planets as well as being shifted about in its orbit slightly by the jetting action of gasses that are released from the nucleus). But this potential future near-impact, along with other close encounters that have taken place in the last few hundred years, have made it clear that comets do post a significant threat to the Earth.
C. Shoemaker-Levy 9
Comet P/Shoemaker-Levy 9 was discovered by Carolyn and Eugene Shoemaker and David Levy in March 1993 from the 18-inch telescope on Palomar Mountain, California. It had a unique appearance to astronomers: instead of a single point with a tail, it appeared as a bar which, when observed by more powerful telescopes, turned into a line of 21 small nuclei that had once been a single comet. Astronomers tracing its orbit found that it passed very close to Jupiter in July 1992, close enough that the planet's powerful gravity was enough to tear the comet nucleus apart into numerous chunks. When astronomers plotted its orbit into the future, they found that the fragments would collide with the planet in July 1994.
Although the fragments would hit the planet on the far side of the planet as seen from the Earth, the impact sites would rotate into view from the Earth in less than a half-hour after each impact. An event like this had never been witnessed before by humans, and with no experience in this area, predictions ranged wildly from no impact effects visible to the creation of gigantic storms on Jupiter. As it turned out, the impacts did create large dark spots on the planet, some of which were as large or larger than the Earth. The planet's strong winds slowly tore apart these spots, but even today, over half a year after the impacts, a dark band is still visible on Jupiter where the impacts took place.
It is believed that if these fragments had struck the Earth, they would have been powerful enough to kill millions, if not billions, of people, and countless other living things.
IV. IMPACTS
A. Rate of Impacts
Objects encounter the Earth every day: Nearly all of these objects, though, never reach the Earth's surface. The tremendous heating caused by the friction me object encounters when it hits the Earth's atmosphere at over 10 km/s disintegrates small objects. These are the "shooting stars" one sees in the night sky mat flash into and out of existence in a second or two.
Larger objects, large enough to make it though me Earth's atmosphere unscathed, encounter the Earth less frequently. One average, about one object around half a meter in diameter and weighing a couple hundred kilograms strikes the Earth every year. These objects have the explosive force of a small non-nuclear bomb. Nearly all of these objects either hit the ocean or in unpopulated areas of the continents and are never noticed.
Still larger objects hit the earth even less frequently, but with certainty. An asteroid with a diameter of 20 meters, capable of causing an explosion equivalent to a 200-600 kiloton atomic bomb, takes place approximately every 25 years. A NEA with a diameter of 100 meters hits the Earth every 3000 years on average. The impact of such a body would be equivalent to a powerful atomic bomb (10-30 megatons), carving out a I kilometer diameter crater and throwing dust high into the stratosphere. The largest objects, 5 to 10 kilometers across, strike the Earth every 30-100 million years, with the equivalent power of megatons of TNT, enough to wipe out much of the life on Earth.
B. Craters
As discussed above, not all objects that encounter the Earth strike the surface. Most burn up in me Earth's atmosphere, some explode in me atmosphere and cause damage without hitting the surface of the planet (as was the case with the Tunguska explosion in 190). However, those objects that do make it to the surface of the Earth will form craters. Craters are the natural byproduct of the collision of a high-velocity projectile (such as an asteroid or a meteor) with a surface (such as the surface of the Earth).
1. Scaling Laws
Scientists study the formation of craters in the laboratory by shooting high velocity projectiles into targets made of rock, sand and other materials, to simulate a meteor impact. Scientists also use larger events, like underground nuclear tests, to understand how cratering works. Though these tests as well as through me use of advanced physics, we have learned some general rules about the relationship among the different physical quantities of a crater. For example, in general, the diameter of a crater is about ten times its depth. There are also relationships that determine the diameter and depth of a crater based on the size and velocity of the impacting body, but they are fairly complex and won't be presented here.
2. Crater Morphologies
Craters take on different forms based on the amount of energy used to create them. As a rule, the more energy used to create a crater, the more complex it is. The simplest crater is called, logically, a simple crater. is nothing more than a bowl-shaped depression in the surface caused by the impact scooping material away from the impact site. As more energy is used, craters become larger, and their sides become steeper. The sides of the crater become steep enough that gravity causes the sides to slump down to the crater floor and partially fill it in, creating what' s known as a slumped crater. r even more energetic impacts, a small mountain, called a central peak, forms in the middle of the crater. This is caused by the surface, in a molten state, rebounding after the impact and then freezing solid. This is very similar to the rebound you see when you see a drop of water fall into a pond. More energetic impacts cause the creation of a natural ring cratering record on Earth and elsewhere.
The Earth is a poor place to look for impact craters. Most of the planet is covered with water, and the rest of the surface is subject to erosion, volcanic eruptions, earthquakes, and other effects which wear away at impact craters. Even so, there are a number of impact craters visible on the Earth. Perhaps the most famous is Meteor Crater in Arizona, a small, simple crater that is the best preserved one on the planet, in part because it is only about 25,000-50,000 years old. Other, larger craters can be found throughout the world, from Iowa to Quebec to South Africa, although they are much less recognizable as such. On other worlds, though, there are far more craters to be studied. The Moon has no atmosphere and has been geologically dead for billions of years, making it a prefect laboratory to study the various classes of craters. Everything from tiny simple craters to large multiring impact basins (Mare Orientale) have been observed on its surface. Mercury, Mars, and many of the satellites of the outer planets are also covered with craters.
We can use these craters, along with a basic understanding of the geology of these worlds, to understand the cratering records on these planets. We now know that many of these craters date back 4 billion years and more, to the earliest days of the solar system, when small planetisimals still filled the system and often struck other bodies. The rate of impacts decreased as most of these planetisimals either collided with planets or were ejected from the solar system, and the impact rate has been fairly steady over the last 2-3 billion years.
V. EFFECTS OF AN IMPACT
A. Blast and Thermal Effects
The impact of an asteroid or comet has much in common with the explosion of an atomic bomb, with the significant exception that impacts do not generate any radiation. As discussed above, an object 20 meters in diameter has the explosive power of 15 - 40 Hiroshimas, and larger objects are even more powerful. These impacts can create powerful blast waves that can knock down reinforced buildings up to miles away, and generate firestorms for many miles. The tremendous heat generated by a large impact would burn some of the nitrogen in the Earth's atmosphere, forming nitrogen oxides that, when mixed with water, form acid rain, strong enough to kill plant and animal life in lakes, rivers, and shallow parts of the ocean.
B. Atmospheric Dust and Climatological Effects
A large impact would also throw millions of tons of dust into the Earth's stratosphere, more than 10 kilometers above the surface. High above the jet stream and other weaker effects limited to the troposphere, the dust could remain in the atmosphere for years. The dust would block sunlight from reaching the Earth, killing plant life and eventually animal life on the surface.
The dust could also cool the surface by keeping the sun's energy from reaching it causing the onset of a "nuclear winter" originally predicted to occur after a nuclear war. This "impact winter" could causes temperatures worldwide to drop by 10 degrees Celsius or more. If this took place during me growing season in either hemisphere, the effects on the crops would be disastrous, with severe consequences for the worldwide food supply.
If the impact took place in me ocean, a large amount of water would be vaporized and go into the atmosphere, where it could clean out any dust fairly quickly. However, the water vapor would remain in the atmosphere for some time, and since water vapor is a greenhouse gas, it would cause worldwide temperatures to increase by several degrees Celsius. The increased water vapor would also attack the ozone layer, allowing more harmful ultraviolet radiation to reach the surface.
C. Biological Effects
A large impact would cause tremendous damage to the Earth's biosphere. The dust raised by an impact would block out the Sun's light, which, combined with the decrease in temperature caused by an "impact winter" would kill off many types of plants. This effect would work its way up the food chain, killing off the animals that feed of the plans, and the animals that feed off those animals, etc. This was the likely course of events 65 million years ago, when a large asteroid or comet hit the Earth.
VI. EXAMPLES
A. Meteor Crater
Meteor Crater is perhaps the best known impact crater on the Earth. Located in northern Arizona about 40 miles east of Flagstaff, it is the best preserved impact crater on the planet, and the first to be recognized to be caused by an asteroid impact and not volcanic activity. It was created about 25,000 to 50,000 years ago when an iron asteroid about 30-50 meters in diameter stuck with the force of 30 megatons of TNT. The blast scooped out a crater nearly a mile in diameter and about 500 feet deep. It likely destroyed every living thing within several dozen miles of the impact site.
B. Tunguska
On June 30,1908, a mysterious explosion rocked an area of Siberia along the banks of the Tunguska River near the town of Kirensk. Although there was no of official investigation of the event for nearly twenty years (due to World War I and the Russian Revolution), the site was still impressive when investigators finally reached it in the mid 1920s. Trees up to fifteen miles from the center of the explosion has been burned, and trees up to forty miles away had been knocked down (the large number of trees knocked down made it easy to find the center of the explosion: all the fallen trees pointed away from it). No impact crater was found from this event, and the explosion remained a mystery for some decades until the physics of impacting objects was better understood. It is no believed that a small comet of weak asteroid about 30 meters in diameter disintegrated about 8.5 kilometers above Tunguska, causing the explosion without leaving a crater caused by an impact.
C. Chicxulub
The impact theory for the demise of the dinosaurs 65 million years ago was first proposed in 1980 by Luis and Walter Alvarez. However, at that time no impact crater large enough and the right age to be associated with a "dinosaur-killer" impact was known to exist on the Earth. However, in the early 1990s, geologists discovered a large impact crater most of it submerged under the Gulf of Mexico, near the Yucatan Peninsula of Mexico. Using radiometric dating techniques, the crater, Chicxulub, was found to be about 65 million years old, just the right age to be associated with a dinosaur-killing impact. Chicxulub was likely caused by an asteroid or comet 10 kilometers in diameter striking the Earth, leaving behind the 180-kilometer diameter crater.
VII EXERCISES
A. Make a Comet
It's fairly easy to understand what an asteroid looks like: they are not dissimilar to many types of terrestrial rocks, and many samples of asteroids, in the form of meteorites, are found in schools and museums. However, getting an understanding of what a comet is like is more difficult. You can try to think of them as big snowballs, but that picture isn't totally accurate. This experiment, performed as a demonstration, gives students a better picture of what a comet is like. It's icky and smelly, which means kids will love it.
Required Materials-
- Dry Ice (5 lbs) available from ice companies or ice cream parlors; CAUTION: Dry ice is -79 degrees C (-110 degrees F). Any more than brief exposure will cause burns. Be careful when handling it.
- Water (around half a gallon) in pitcher;
- Ammonia (a few drops or sprays of window cleaner);
- Dirt (fine grained, one handful);
- Corn Starch, or Worcester Sauce (a couple of pinches or drops);
- Trash Bags (2);
- Large Bowl or Small Pot;
- Water Proof Gloves (the better insulated the warmer your hands will remain)
- Cloth Towel;
- Paper or Cloth Towels;
- Hammer;
- Mixing Spoon or Stick.
These ingredients are either actual components or handy analogous ones. The dry ice is frozen carbon dioxide. Water, ammonia, organic (carbon based) molecules, and silicates are all present on comet nuclei. They have been identified through spectral measurements of comet tails and the collection of tiny ice particles by very high flying research aircraft.
Here is the recipe:
Line me bowl with a trash bag. Place the other trash bag on the floor. Pour about a pint of water into the bowl. Add the corn starch or Worcester sauce, ammonia, and some of the dirt; mix a bit.
Put on the gloves. Wrap the dry ice in a cloth towel, place it over the trash bag on the floor. Use me hammer to grind up the dry ice into a powder. Gradually pour the dry ice powder into the water, mixing as you pour. There will be lots of vapor formed. The dry ice, water and other ingredients should form a thickening slush. Keep stirring for a few seconds as it thickens.
Now, using me trash bag to lift me slush away from the sides of the bowl, use your gloved hands to pack the slush into a ball. Keep patching and forming until the ball solidifies as a big lump. Peel back the trash bag. Scatter some more dirt over the lump. Pour some of the remaining water over the lump, turning it as you do so, so that a layer of water ice forms over the entire lump.
Observe the behavior of your miniature comet nucleus. It can be handled without gloves if the water ice coating is intact. If a spot feels sticky, pour water on the spot. It hisses and pops as carbon dioxide sublimes (goes from me solid state directly into a gas) and forces its way through weak spots in me water ice crust. On real nuclei this results in slight jetting forces mat can cause me nucleus to spin, slightly alter its orbit, or split apart (or "calve").
Note: Get three or four pounds of dry ice for each nucleus you plan to make. You can purchase it the afternoon or evening prior to the demonstration and store it in a freezer or ice chest. Place an inch or so of newspaper below the dry ice to prevent scraching of the surface on which the dry ice rests. Try me demonstration first to get an idea of the correct amount of water to use.
It's fun, it's a mess and it's one of the most memorable and scientifically accurate demonstrations in astronomy!
B. Crater Exercise
Craters show a variety of features, based on the energy of the impact which created them. Low energy impacts create simple craters, digging a bowl into the ground. More energetic impacts can cause the sides of crater walls to slump, create a central peak in the middle of the crater, and form multiple rings for the most energetic impact. To give students a chance to explore this range of phenomena, a simple series of experiments can be run with basic materials.
Required Materials:
- Flour (several pounds)
- Tempera paint
- Marbles of various sizes
- Box with sides at least four inches high (the lid to photocopier paper boxes is perfect)
- Garbage bags
Place the flour in the box and smooth and pack it lightly (experiment with different firmnesses). Place a dusting of the paint powder over the flour (colored water in a spray bottle works, but not as well). Use the marbles to bombard the surface (one at a time). Look for classical cratering features: basin, raised rim, ejecta blanket (material excavated from the crater and dumped around it, visible as white flour on the colored powder), and rays (material shot out at high velocity forming lines pointing directly away from the impact site).
Students should keep careful records and can do top and profile drawings of the craters and compare craters formed by different size projectiles, different velocities, and different angles of impact. Different size projectiles can be dropped from measured heights so that they will have common velocities. They should also remember that the quality of their tests is more important than quantity. Record these measurements and use basic physics (potential and kinetic energy) to figure out how much energy is released in each impact. Look for any relationships between the energy of the impact and the diameter, depth, or type of crater formed.
After several craters, the flour and tempera can be mixed and re-smoothed without changing the white of the flour too much. Then a new layer of tempera can be applied and additional experiments conducted. In real impacts the impacting object is destroyed or broken up into small chunks. Of course the marble will not do this and will remain whole in the crater.
VII. BIBLIOGRAPHY
Arny, Thomas T. Explorations: An Introduction o Astronomy. St. Louis: Mosby), 1994.
Bisard, Walter. "Cratering in the Classroom." From The Teaching of Astronomy (IAU Colloquium 105), Jay Pasachoff and John Percy, eds.. (Cambridge: Cambridge University Press), 1990.
Hartmann, William K. Moons and Planets rd edition) (Belmont, CA: Wadsworth), 1993.
Levy, David. The Quest for Comets. New York: Plenum), 1994.
Melosh, H. J. Impact Cratering: A Geologic Process. Oxford: Oxford University Press), 1989.
Shoemaker, Eugene and Carolyn. "The Collision of Solid Bodies." From The New Solar System 3rd edition), J. Kelly Beatty and Andrew Chaikin, eds. (Cambridge: Cambridge University Press), 1990.
Contact: vanhoosr@cco.caltech.edu