Asteroid Watch

						December 10, 2001

		Meteoric Decline It's astonishing to realize that Earth cruises through space at the rapid speed of 18.5 miles per second — or 66,600 miles per hour. Because space is vast and sparsely populated, our planet passes largely unimpeded along its routine, elliptical path around the Sun. Earth regularly comes into contact with comet trails and showers of meteors (the Geminids are coming our way this week), but encounters with larger objects are rare. Some 65 million years ago, Earth encountered some rare traffic: a six-mile-wide asteroid. The planet was rocked by the collision, with popular theory holding that the impact brought an end to the age of the dinosaurs, as the fallout from the collision made Earth uninhabitable. Could this happen again? Are there other asteroids, large or small, coming our way? A report published in last month's Astronomical Journal tells us that the risk is not as great as we once figured; in fact, it is about one-third of previous estimates. Using data from the Sloan Digital Sky Survey (a project that aims to map one-fourth of the sky), astronomers concluded that the solar system contains about 700,000 asteroids big enough to destroy civilization (such asteroids are six-tenths of a mile in diameter or greater). The odds of Earth colliding with one of these rocks over the next century are about one in 5,000 (compare this with previous estimates of one in 1,500). This should keep the scenarios of movies such as Deep Impact and Armageddon safely in the realm of fiction, but even so, scientists and governments are not complacent about the threat. An asteroid collision of sufficient magnitude would prove as disastrous for us as it did for the dinosaurs. Impact by an asteroid of about a mile wide could result in a groundburst explosion similar to a million megatons of TNT. The heat and pressure caused by the impact could instantly wipe out life in the surrounding region. The huge masses of dust and gases that would be blasted into the atmosphere could block out sunlight for a period of time long enough to kill significant portions of life on Earth. Students can read a recent story from CNN which describes how the Sun plays a role in putting rogue asteroids in Earth's path. In 1998 NASA established the Near-Earth Object (NEO) Program to detect, track, and profile potentially hazardous asteroids and comets that could collide with Earth. Students can read the NEO Program's interesting account of how asteroids may have assisted the development of life on Earth.
						Destination: Asteroid In 1996, NASA's NEAR Shoemaker spacecraft was launched on a mission to the asteroid Eros, a 21-mile long rock that travels mainly inside the orbit of Mars. NEAR orbited Eros for a year, and then managed the amazing feat of the first-ever landing on an asteroid. That was in February, and two weeks later, communications with the spacecraft ceased. Data sent back from the spacecraft has been invaluable to scientists who are trying to understand asteroids, and, perhaps, how they may have influenced the development of life on Earth. Learn more about the NEAR mission at NASA's own Web site.
		Teaching the Problem (The following SimLibrary activities require Logal Express. Sign up for a free trial subscription.) 1. Risk of Collision Scientists use the Torino Scale to evaluate the risk of a collision between an approaching asteroid and Earth. The scale maps the asteroid's estimated kinetic energy against the probability that a collision will occur. Where the object falls on this map determines on a scale of 0 to 10 how serious a risk an NEO poses. White indicates events having no likely consequences. Green indicates events that require careful monitoring. Yellow indicates events that merits concern. Orange indicates threatening events. Red denotes certain collisions. Kinetic energy is one of the major factors in determining the possible risk in case of collision. Students are introduced to kinetic energy in the Middle School Gateways activity, Potential and Kinetic Energy. Students can learn more about kinetic energy during collisions in the Physics Explorer activity, Collisions.
		2. Asteroid Risk Hazards a. Dust Clouds A large asteroid crashing into Earth would send a significant amount of dust and gases into the atmosphere surrounding the entire planet. Scientists believe that it could take up to two years for the dust to settle. During this time, the dust would reflect sunlight back into space, preventing it from reaching Earth. Darkness would cover the planet and the global temperature would drop. Organisms that could not adapt to these changes in light and temperature and would therefore die out. Species up the food chain would starve as food sources vanished. With the death of much of Earth's phytoplankton and plants, atmospheric oxygen levels would also drop, and many species would suffocate. Some scientists believe that a large asteroid flaming through Earth's atmosphere would burn off much of the planet's protective ozone layer. The dust cloud would reflect ultraviolet radiation from the Sun for the initial post-impact period. When the dust finally settled, the ultraviolet radiation would be at least double what it was before the collision, due to the ozone depletion. The high radiation levels could cause mutations and cancer. Students can learn about reflection in the Physics Explorer activity, Reflections in a Plane Mirror. Help them make the comparison between the dust in the atmosphere and the silver coating on the back of a glass mirror. Both reflect light instead of transmitting it. Biology students can learn how sunlight is a necessary ingredient for plant energy in the Biology Gateways activity, The Role of Light in Photosynthesis. Then they can see how oxygen is a by-product of photosynthesis in the Biology Gateways activity, An Overview of Photosynthesis. Finally, students can see how the whole food chain would be affected in the Biology Gateways activity, Exploring Food Chains and Food Webs. Students can learn more about the ozone layer in the Riverdeep article "An Olympic-Sized Ozone Hole." They can also look at mywave's "Life Without Ozone", part of the "Explorations" section. b. Tsunamis An immediate hazard resulting from an asteroid collision is a tsunami, a shallow-water wave that hits the coast with a very high front. Carrying tremendous amounts of energy, tsunamis can crush buildings, strip away vegetation, and cause flooding hundreds of meters inland from the coast. Tsunamis are characterized in deep water by extremely large wavelengths (the distances between wave crests) and small amplitudes (the heights of the waves). As the waves approach the shore, the friction from the increasingly shallow ocean floor reduces the velocity of the wave. As the velocity decreases, the wavelengths become shorter and the amplitude increases significantly. Students can explore these wave relationships in the Middle School Gateways activity, Amplitude, Wavelength, and Frequency of Waves.

		Analyzing the Problem Ask students: How do each of the following factors affect an asteroid's rank on the Torino Scale? mass of asteroid size of asteroid velocity of asteroid external temperature of asteroid asteroid's distance from Earth asteroid's angle of approach toward Earth Consider a scenario in which an asteroid 10 km in width collides with Earth. Have students rank the order in which the following organisms are likely to go extinct. Ask them to justify their rankings. Emphasize that these are generalizations and that actual events would not fall into such neat categories. organisms within the general region of the impact deep-sea dwelling plants deep-sea dwelling fish plants on the opposite side of the planet from the impact small herbivores on the opposite side of the planet small carnivores on the opposite side of the planet large herbivores on the opposite side of the planet cave-dwelling organisms on the opposite side of the planet Ask students: Why are tsunamis difficult to detect while still out at sea? Extending the Problem NASA offers several Web sites for students who want to learn more about Near-Earth Objects (NEO): Near-Earth Object Program Asteroid and Comet Impact Hazards Discovery is NEAR (Near Earth Asteroid Rendezvous Mission) Additional educational sites dealing with asteroids include: Discovery's Planet Earth: Asteroids Solarview's Asteroid Introduction Dinosaurs have long been a popular subject with people of all ages. Students can obtain a wealth of information on the mighty creatures at the following Web sites: BBC's Walking with Dinosaurs National Geographic's Dinorama Discovery's Tracking Dinosaurs...In Our Parks Zoom's Zoom Dinosaurs Ask students to research different theories of why dinosaurs became extinct. A meteorite is any particle or chunk of stone or metallic material that survives its passage through Earth's atmosphere and hits the ground. Students can read the interesting story of a meteorite that landed in the Yukon Territory, Canada, early last year: CNN.com's article, "Study: Carbon-rich meteorite may give new clues to origin of life." This summer, people along the U.S. eastern seaboard were intrigued by the brief appearance of a bright fireball in the sky. Find out more in this CNN.com article. Bringing things down to a smaller scale, this week, students can expect a colorful meteor shower as the Geminids arrive in Earth's neighborhood. Read this feature from our news partner, CNNfyi.com, "Meteors, sun, moon to star in double feature." Students can also find out how to collect and examine micrometeorites.