After Jupiter, is Earth next?


Palo Alto, Calif. -- TOMORROW, fragments of the comet Shoemaker-Levy 9 are expected to begin smashing into Jupiter at about 130,000 miles an hour.

The largest piece is likely to strike with explosive energy exceeding the potential of all the nuclear weapons ever made.

It may even be as forceful as the object that hit Earth at the end of the Cretaceous era 65 million years ago and apparently led to the extinction of most living species, including the dinosaurs.

Shoemaker-Levy will give scientists an unprecedented opportunity to advance our knowledge about the kinds of cosmic crashes that threaten Earth.

Regrettably, the federal government has been less than enthusiastic about financing scientific observation of the event.

After Shoemaker-Levy 9 was discovered in March 1993, the National Science Foundation and NASA asked scientists for observation and investigation proposals.

The organizations supported only a fraction of the ideas offered, and no new government money was authorized. It all had to be diverted from other budgets.

The impact sites will be on Jupiter's far side but will rotate into view in less than an hour, so lasting effects can be studied by observatories on Earth and in orbit. The crippled Galileo spacecraft will have a direct view of the far side and the explosions. The spacecraft Voyager 2 also will look directly at the impact site, but it will be some 6 billion kilometers away.

Still, additional resources could have been put to good use for new types of observational equipment.

Why should we be so interested?

On a typical day, the Earth collides with more than 100 tons of space debris in pieces so small that they pose little threat.

But much larger collisions also occur. On June 30, 1908, a comet fragment the size of a 15-story building slammed into the atmosphere over the Tunguska River in Siberia.

Statistically speaking, another collision with an object that large is likely to happen over the next several hundred years.

Fortunately, a planetary defense against all serious collisions may be feasible. Some initial steps were taken by astronomers and other scientists who published the Spaceguard Survey for NASA in 1992.

The report recommended an observation program with a relatively modest start-up cost of around $15 million that would catalog and track asteroids that cross and come close to Earth's orbit.

There are thousands of asteroids with which we could conceivably collide. But because their paths are relatively fixed, once an asteroid is identified its potential for colliding with Earth can be calculated.

A much more difficult problem lurks in the outer fringes of the solar system, where trillions of comets dwell undetected.

As many as 10 new comets randomly enter the inner solar system each year. Because of their high speed and unpredictable paths, the time available to detect a collision-course comet would be much less than for an asteroid.

How can we defend ourselves from these lethal cosmic objects? The required efforts vary from the straightforward -- a greatly expanded comet and asteroid watch -- to the formidable -- developing a new "star wars" type of technology to deflect collision-bound asteroids and comets.

The U.S. need not pay the entire bill for this cosmic alarm system. All of Earth's inhabitants have a stake in a planetary defense.

Von R. Eshleman is professor emeritus of electrical engineering at Stanford University.

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