Pennies in heaven?

Why slam a copper impactor into Comet Tempel 1 ("A Grand Slam: In a winning move, NASA probe burrows into a comet," SN: 7/9/05, p. 22)? Wouldn't copper vapor contaminate the spray? Why not a high-temperature ceramic?

P.M. deLaubenfels
Corvallis, Ore.

According to Casey Lisse of the Deep Impact team, copper was chosen because its density put a lot of mass into a small package, its relative softness reduced bounce at impact, and its large atomic number and density contrast with those of elements that were expected to spew from the comet at impact (see "Deep Impact," in this week's issue).—R. Cowen

Time for testing

The information in "Cancer Switch: Good gene is shut off in various malignancies" (SN: 7/16/05, p. 35) represents the first indication of a potential test for the onset of esophageal cancer (EC), the fastest-growing cancer in the United States. EC kills 94.5 percent of patients diagnosed. Unfortunately, until EC has reached stage IV, no symptoms present themselves to the patient. A routine screening procedure that might be developed from testing for a methylated Reprimo gene would be vital.

Les Coleman
Westerly, R.I.

Relatively straightforward

In "Realistic Time Machine? New design could forgo exotic ingredient" (SN: 7/16/05, p. 38), you refer to a standard concept of a person speeding "in a rocket traveling slightly less than the speed of light" and say that "motion at such enormous speeds drastically slows the clock for the traveler." That reasoning, which is common, troubles me. If the traveler is traveling speedily with respect to an observer, then, clearly, the observer is traveling speedily with respect to the traveler (who is observing the observer). Since all motion is relative, why shouldn't the observer's clock slow down as observed by the traveler?

Arra Avkain
Fresno, Calif.

The difference is subtle, but it's because the traveler changes direction—turning around is a form of acceleration—that less time passes for that voyager than for the observer.—P. Weiss

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