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Transcript for Science in Paradise

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Science in Paradise: Big Dish

Nestled in the hills of Puerto Rico, the Arecibo Observatory tunes in to the universe, searching for signals from distant space, using the world's largest radio telescope. Arecibo searches for pulsars and distant galaxies, listening for a signal that may indicate the presence of intelligent life elsewhere. The telescope also monitors the presence of asteroids, especially any that may be headed toward Earth.

Curriculum Links
National Science Education Standards
Related Frontiers Shows and Activities
Activity 1: Model a Big Dish
Activity 2: Risky Business
More on Cosmic Collisions



asteroids, astronomy, meteors, space


geometric shapes, probability


radio waves, sound


risk assessment




5-8: Transfer of Energy
9-12: Interactions of Energy and Matter
5-8: Earth in the Solar System
9-12: Origin and Evolution of the Earth System, Origin and Evolution of the Universe
Abilities of Technological Design, Understandings about Science and Technology
5-8: Natural Hazards
9-12: Natural and Human-induced Hazards
5-8: Nature of Science, History of Science
9-12: Nature of Scientific Knowledge, Historical Perspectives



Arecibo is a huge dish 305 meters in diameter, shaped like part of a sphere and set in an ancient limestone crater. Most of Arecibo's work is radio astronomy -- listening to and amplifying radio waves, enabling astronomers to discover stars and galaxies at the edges of the universe. A recent upgrade makes Arecibo ten times more sensitive than it used to be. In addition, its new, more powerful radar-emitting device can detect and create images of asteroids -- especially those that might be headed toward Earth.

Arecibo and other radio telescopes are reflecting dishes that use curved surfaces to collect and reflect radio waves. If you have a satellite dish on your house or in your backyard, you are using a smaller version of a reflecting dish. Arecibo's "antenna dish" (the curved surface) collects and reflects signals. When radio waves strike the surface, they are reflected back through the focus (the dome), which concentrates the signals.

In this activity, you'll experiment with a curved reflecting dish that will work like the Arecibo detector. Then you'll use your model to detect electromagnetic waves.

  • oradio (not a Walkman)
  • appliance that produces radio static ("noise")
  • cooking wok
  • aluminum foil
NOTE: Appliances that produce static include motorized toys, blenders, small electrical motors, grinders, microwaves, etc. Experiment with different receptors and static producers. If using a blender, mixer or microwave, remember to boil or blend water; do not run the appliance empty. You can use a digital or analog radio or a small TV and do not necessarily need a radio with an antenna.


  1. Line the bowl of the wok with several layers of heavy duty aluminum foil. Smooth the foil. Carefully remove the foil so it keeps the shape of the wok. This is your model of the Arecibo dish.

  2. If your radio has an external antenna, push it in to its shortest length.

  3. Switch on the radio. The instructor or another student should switch on a nearby appliance that produces radio static ("noise").

  4. While the noisy appliance is running, move the radio next to the appliance and tune the radio to the static. (Note the difference between the background static and the static produced by the appliance; background static is more random, but the appliance static usually has a "pulse.")

  5. When you have tuned in the static so it's distinct, slowly separate the devices until the static created by the appliance is no longer broadcast by the radio. You'll have to walk about 15 to 20 feet away from the appliance.

  6. Hold the reflecting foil dish behind the radio so its bowl side faces the contracted antenna and aims directly at the appliance. How does the dish affect the static? (The foil dish should create static again.) Aim the bowl in different directions. How does the direction of the bowl effect reception? Does the distance to the antenna affect the static?

    illustration of radio/wok experiment

  7. The wok is also spherical and can be used as a reflecting dish. Its smooth surface will concentrate the waves better than the foil, which, if it's crinkled, might reflect the waves in random directions. Try using the wok instead of the foil dish. How does the change affect reception?

What's going on here? The wok or foil dish is comparable to the reflecting dish at Arecibo. Signals that reach the Arecibo dish are collected and reflected back to the dome; signals reaching the tin foil dish are reflected and concentrated at the radio or its antenna.


  • How might an umbrella lined with aluminum foil affect the detection of radio static? Make a prediction. Construct a reflecting antenna dish with aluminum foil and an umbrella. Test your detector. Can you detect local stations? Make a guess. Then, check it out!

  • Ordinary radio telescopes are constructed more in the shape of a parabola. Look for other examples of parabolas -- from flashlights to car headlights. How might a parabola reflect radio waves?


All risk is relative. Some risks worry us; some don't. Risk analysts try to understand our concerns by comparing and contrasting the controllable nature and seriousness of a risk. Examine the grid below. It is called a risk space. The position of each hazard was placed by a risk analyst. How concerned should we be about a possible asteroid collision with Earth? How does this risk compare with other risks we may be exposed to?
observed risk chart

  1. Do you agree or disagree with the placements of the various "hazards"?

  2. Is a collision with an asteroid controllable?

  3. What are an individual's risks associated with an asteroid collision?

  4. Will all asteroid collisions produce the same risks? What factors determine the seriousness of the risk?

  5. Where would you place an asteroid collision on this grid?

  6. If the odds of a 1 km asteroid hitting Earth are 1 in 1,000 in the next century, what are the odds for each year of that century?

NOTE: Students will have different views of where these hazards should be placed. Encourage discussion. Accept all reasonable answers that integrate both the control and observable nature of the risk. For more discussions of current issues in science, consult Decisions Based on Science available through NSTA.


It's often said that Earth exists in a "cosmic shooting gallery," because our planet is surrounded by asteroids, comets and meteors. Asteroids are pieces of space rock and metal, ranging in size from a few tens of meters to almost 1,000 km (600 miles) in diameter. Most orbit a region between Mars and Jupiter. If they cross Earth's orbit, they're called "Earth-crossing asteroids." Here are some factoids about asteroids and other cosmic collisions:
  • The impact of an asteroid that collided with Earth 65 million years ago is believed to have killed more than 70% of all living species on the planet, including the dinosaurs. Scientists theorize that this asteroid, the K-T impact, was at least 10 km (6 miles) in diameter.

  • Scientists have found evidence of more than 100 impact craters on the surface of the planet. A more "recent" (in geological terms) impact occurred in Arizona about 50,000 years ago, when a 10,000-ton iron meteorite hit the ground and formed Meteor Crater. And scientists believe a small, rocky asteroid no bigger than 60 meters (200 feet) exploded above Siberia in 1908.

  • Hollywood movies about cosmic collisions are fun to watch, but do not present real science. Hundreds of years could go by before the next object of any significant size strikes the Earth. At the present, scientists are investigating ways to deal with an asteroid headed our way. One solution is launching a nuclear device to deflect the asteroid orbit off course.

  • Scientists' predictions are sometimes wrong -- as was the case in the false alarm created by Asteroid 1997 XF11 in March 1998 -- but such an event demonstrates the need for good science.

  • How likely is it that Earth will be hit by an asteroid? Scientists have offered differing odds. Astronomer Steven Ostro, who you see in this episode of Frontiers, says there is a 1 in 1,000 chance that an asteroid 1 km in diameter or larger would collide with Earth in the next century. In the May 1998 issue of Scientific American, Philip Yam writes, "The odds that an 'extinctor,' an object two to five kilometers wide (about twice that of Asteroid 1997 XF11) will strike the planet this century range from about 1 in 1,000 to 1 in 10,000."

  • "Millions of real asteroids are out there, most of them uncharted, and one of them will hit Earth sooner or later. Finding a killer asteroid -- or determining that one doesn't exist -- would cost about fifty million dollars, less than half the budget of 'Armageddon.'" - Timothy Ferris


Scientific American Frontiers
Fall 1990 to Spring 2000
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