- SCIENCE NEWS This Week

blasted from the impact site should con- tain some shocked minerals, says Simonson. He also notes that the South African and Australian sites could have been geo- graphically much closer to each other at the time of the purported impact, and the effects of any blast actually may have been regional, not worldwide. -s. PERKINS

Lost in Space Comet mission appears to have broken apart - Just 2 weeks ago, the CONTOUR probe, launched July 3, was beginning its journey to two comets, one of which had broken into pieces. It now appears that the $159 million spacecraft has itself broken apart.

“I feel like I lost a relative,” says planetary scientist Lucy-AM A. McFadden of the Uni- versity of Maryland in College Park.

Engineers haven’t heard from CON- TOUR, or Comet Nucleus Tour, since Aug. 15, when they commanded the craft to fire its solid-propellant rocket. The maneuver was intended to place CONTOURon apath toward its first target, Comet Encke (SN: 8/3/02, p. 77).

The most compelling evidence that something terrible happened came from a set of grainy images. Hours after CON- TOURfell silent., the Spacewatch Telescope near Tucson found two objects at CON- TOUR’S predicted position and moving at its predicted speed, strongly suggesting the craft had broken up.

Although mission director Robert Far- quhar of Johns Hopkins University Applied Physics Laboratory in Laurel, Md., is hop- ing that CONTOUR remains mostly intact and capable of functioning, “it’s not very encouraging,” he told reporters last week. As of press time, Fquhar and his colleagues hadn’t heard from the craft. If no signal comes by Aug. 25, he says, the best bet is a final search in December, when the probe’s antennas wouldbe more aligned with Earth.

Astronomers have studied the nuclei of only two comets close up, Halley and Bor- relly (SN: 9/29/01, p. 196). If CONTOUR has indeed stopped functioning, researchers will have lost the opportunity for close-up studies oftwo additional comets, Ehcke and Schwassmann-Wachmann 3.

McFadden and her colleagues have another reason to lament the loss of CON- TOUR Data on Encke would have served

FANTASY VERSUS REALITY How the CONTOUR spacecraft might have appeared visiting a comet In 2006. Inset: Motion of what appear to be two pieces of the CONTOUR spacecraft on Aug. 15. Yellow circles containing black lines indicate positions of pieces 20 minutes later than positions shown as white lines.

as a guide for their own comet mission, Deep Impact. That probe, set for launch in 2004, will fire a projectile into Comet Tempe1 1 andanal~etheresultingdebris.-R. C O W E N

in Numbers Physicists model the power of social sway - Aromas from a kitchen fill a house. A drop of dye colors an entire bucket of water. It’s all because molecules diffise; they roam around in all directions.

But what would happen if the mole- cules-instead of moving completely ran- domly-were influenced by peer pressure? If molecules or particles or people tended to follow the paths oftheir trendiest neigh- bors, they might aggregate, even in loca- tions far from their origins.

This type of peer-influenced behavior might explain many puzzling patterns that show up in everything from financial data to fluctuations in animal populations, says a team of physicists that has modeled the behavior mathematically.

“The equations are complex, but they have a rather simple physical meaning,” says Jayanth R Banavar of Pennsylvania State University in State College. He and his col- leagues describe their model in the Aug. 19 Physical Revim Letters.

The researchers used simple diffusion as

the starting point for their model. Simple diffusion is like a random walk, Banavar explains. In such a walk, the direction of each successive step is governed by chance. If a crowd of random walkers starts from the same point, the pattern of dispersal of the crowd is predictable. At first, the walk- ers cluster relatively near the starting point, with only a few farther away. As time goes on, this bell-curve distribution becomes flatter and flatter until the crowd of walk- ers is evenly dispersed from the starting point, like the dye in the water bucket.

To bring in peer pressure, Banavar and his colleagues added a so-called drift term to the classic equation for diffusion. When applied to walkers, the term dictates that walkers are more likely to move in whichever direction they spot more people.

In the long run, if the peer pressure is small-which happens if walkers can’t see very far or aren’t influenced much by their peers-normal diffusion takes over, Banavar says. But if peer pressure is strong enough, at a certain point, diffusion is over- come by an increasingly powerful tendency to aggregate. Statisticians refer to the result- ing distribution as fat-tailed, which means the aggregates of people or particles often end up quite far away from the origin.

Banavar’s group “came up with the same tailed distributions that we found empiri- cally,” says H. Eugene Stanley of Boston University. Stanley says that there are numerous examples of data that form fat- tailed distributions, including bird popu- lations, nations’ economic productivity, and stock prices (SN: 11/27/99,p. 344). No one

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