NASA science writer D. C. Agle likes to describe them as the celestial equivalent of sonograms. They are radar echoes that are bounced off asteroids passing close to the Earth. And astronomers love em. Coordinated by NASAs Jet Propulsion Laboratory in Pasadena, Calif., these microwave images from the outer limits have stories to tell us about near-Earth objects or NEOs. It is believed that massive NEOs have hit the Earths surface in the past causing mass extinctions and global climate change. The standard ground-based tools for asteroid science require a nights sky, and what you come away with in the end is a photographic image of a dot, said JPL radar astronomer Steve Ostro. But with radar astronomy, the sky at high noon is just as inviting as that at midnight, and without launching a full-blown space mission we can actually get valuable information about the physical makeup of these objects. Enter the World War II-era technology of radar short for ra(dio) d(etection) a(nd) r(anging). The obscure field of radar astronomy uses tools that are similar to your kitchen microwave oven. Those readers old enough may remember that the first microwave kitchen ovens manufactured by Raytheon for $5,000 in 1947 were called radar ovens. Modern microwave ovens are no different you are cooking your food with radar waves. Radar antennas on Earth emit a radio beam of directed microwave signals toward a passing NEO. These microwaves dont cook the asteroid, but when the radar pulses bounce off the asteroid they produce whats called an echo. You can make out surface features (on the echo images), said Ostro. A good echo can give us a spatial resolution finer than 10 meters." More than 190 near-Earth asteroids have been discovered using radar. The objects are somewhat like snowflakes since no two are exactly alike. NEOs can be both rocky and metallic; some even have their own orbiting mini moons. While theres plenty of evidence of ancient terrestrial impacts by NEOs, what are the chances of being whacked by one today? In the winter of 2004, a big asteroid named Apophis got the full attention of JPL astronomers. Astronomers figured out a rough orbit for Apophis, a 1,300-foot-diameter cosmic mountain tumbling end- over-end through space. The astronomers realized that, based on preliminary calculations, the NEOs orbit took it mighty close to terra firma. Scribbling a few numbers, Steve Ostro made a sobering prediction Apophis might slam into the Earth in the year 2029. Alarm bells went off in the minds of Ostro and his JPL team. It was time to get more accurate information about Apophiss orbital mechanics. Ostro and three other radar astronomers used the Arecibo Observatory in Puerto Rico to ping Apophis. The radar data significantly improved the asteroids orbital estimate. We were able to rule out a potential Earth collision in 2029, Ostro said, looking very relieved. Earth was lucky this time, according to Ostro. Apophis was removed from NASAs 10 Most Wanted list. Whats in the Sky: If you look for the constellation Sagittarius low in the south before dawn, you will be looking toward the core of our galaxy over 27,000 light-years away. Former NASA science writer Lou Varricchio, M.Sc., is Vermont's NASA/JPL solar system ambassador. You can order his book about the Moon, titled Inconstant Moon, through Amazon.com or BarnesandNoble.com.