Residents around the lower Chesapeake Bay should look to the sky to understand what's beneath their feet.
A massive meteorite smacked the Earth in that region some 35.5 million years ago. The impact prepared the landscape for later formation of the bay. Moreover, as C. Wylie Poag with the US Geological Survey at Woods Hole, Mass., points out, its effect is "still felt in some unexpected ways."
The impact blasted a 145-mile-wide hole through southeastern Virginia. It's the seventh-largest impact scar known on the planet, covering an area twice the size of Rhode Island. At 3/4 of a mile, it's nearly as deep as the Grand Canyon.
A porous material called breccia now fills the hole and holds salty water. You can't drill wells in the area without knowing how to avoid this salty anomaly. Compaction and sinking of the breccia has also formed a lot of faults that earthquakes might activate.
Dr. Poag and several colleagues recently explained in the journal Science how they discovered the impact crater using clues from seismic data, local gravity variations, and drill core samples. Poag explains that the sediment never completely filled the crater. This left a low-lying region that became the Chesapeake Bay estuary when post-glacial sea level rise flooded it. That's why you can't understand today's geography without knowing its impact history.
Poag also notes that the impact would have had severe effects over a region 1,000 kilometers (625 miles) wide. That's much of the present urbanized area in the northeastern United States. The evidence left "provides an example of effects to be expected" if a kilometer-sized comet or asteroid hit the continental shelf today, Poag says.
That's instructive. But there's more to our growing knowledge of these wayward bodies than a wise concern about possible but rare future impacts. Just as Chesapeake Bay residents can't understand local geography without thinking of cosmic missiles, so scientists can't fully understand the history of earth and the solar system without knowing more about asteroids and comets.
They appear to be debris left over from the solar system's formation. As such, they may contain primordial material that has changed little over the 5 billion years since that time. Studying that material in detail will help answer questions as to what the primordial material was like and how it formed into planets.
Scientists need more than long-distance telescope and radar scans of material deposited by meteorites. Comet and asteroid flybys, including those of the Galileo Jupiter probe, are a prelude to extensive on-site studies. The National Aeronautics and Space Administration's $122-million NEAR (Near Earth Asteroid Rendezvous) mission launched last month will begin this research with Eros. At 25 miles long by 9 miles wide, it's the largest of the nearby asteroids. The spacecraft will meet up with Eros about 130 million miles from earth early in 1999. That's 1.4 times the distance between earth and the sun. Meanwhile, the US Air Force is preparing for a 1998 launch of a mission to visit several near-earth asteroids and study the impacts of small missiles on them.
But scientists want more than close-up surveys. They need samples to study. They would like, eventually, to visit an asteroid themselves. Geologist Eugene Shoemaker in Flagstaff, Az, explains that NEAR won't see crucial details of surface structure and subtle differences in composition on Eros. "You have to get right down there and put your nose against it," he says.
Besides their thirst for knowledge, solar-system scientists also speculate that at least some asteroids might someday be mined for high-value metals and other materials. All told, near-earth asteroids are high-priority research targets that are easier to get to than Mars. They could well provide "stepping stones for humans into the solar system," to use the colorful phrase of the Planetary Society's executive director, Louis Friedman.