Astronomy Picture of the Day 06-22-04

Explanation: How did unusual spires form on comet Wild 2? Close inspection of images taken of Comet Wild 2 by the passing Stardust spacecraft in January show numerous strange pinnacles as long as 100 meters long jutting off the surface. The pinnacles were unexpected - close-ups of other comets and asteroids show no such features. In fact, no other Solar System object besides Earth is known to show such unusual structures. Other unusual Wild 2 surface features including long cliffs, deep pits and craters, and numerous active jets. Together with the spires, these features are hypothesized to be indicative of a very rigid surface sculpted by impacts and explosive sublimation. Initially, Wild 2 was expected by many to be held together only quite loosely. Stardust is scheduled to fly by the Earth in 2006 and drop off a sample of Wild 2 for detailed scrutiny.

Comet's dust clouds hit NASA probe 'like thunderbolt'
UNIVERSITY OF CHICAGO NEWS RELEASE
Posted: June 17, 2004

Two swarms of microscopic cometary dust blasted NASA's Stardust spacecraft in short but intense bursts as it approached within 150 miles of Comet Wild 2 last January, data from a University of Chicago instrument flying aboard the spacecraft has revealed.

"These things were like a thunderbolt," said Anthony Tuzzolino, a Senior Scientist at the University of Chicago's Enrico Fermi Institute. "I didn't anticipate running into this kind of show." Tuzzolino and Thanasis Economou, also a Senior Scientist at the Fermi Institute, will report their findings in the June 17 issue of the journal Science.

The materials streaming from a comet range in size from particles that could fit on the head of a pin to boulders the size of a truck. Stardust mission planners correctly estimated that their spacecraft could safely avoid the hazardous larger objects by passing the comet at a distance of approximately 150 miles and using very effective dust particle shields.

Based on the data collected by the Dust Flux Monitor Instrument, Tuzzolino and Economou estimate that NASA achieved its goal of collecting at least 1,000 samples measuring at least one-third the width of a human hair or larger during the flyby.

The Stardust spacecraft is scheduled to return the samples to Earth in January 2006. Scientists will study the samples, the first ever returned to Earth from a comet, for insights into the early history of the solar system.

The Dust Flux Monitor Instrument collected data for 30 minutes when the spacecraft passed closest to the comet last Jan. 2. Stardust encountered the first swarm of dust particles when the spacecraft passed within 146.5 miles of the comet's nucleus. The monitor detected a second intense swarm after passing the comet when the spacecraft was approximately 2,350 miles from the nucleus.

"We believe that we see fragmentation of large dust lumps into swarms of small particles after they are coming out from the nucleus," Economou said.

In between the particle swarms, the impact of which lasted just a few seconds each, the dust monitor went for periods of several minutes before it detected another particle.

This isn't Tuzzolino's first encounter with a comet, though it is by far the closest. He helped design, build and test the Dust Counter and Mass Analyzer instrument that passed Comet Halley at a distance of 5,000 miles or more in 1986 aboard two Soviet Vega spacecraft. Halley had emitted a spray of dust "much smoother" than that of Wild 2, Tuzzolino recalled.

"In general, one thinks of a comet as emitting gas and dust in a nice, uniform steady state, sort of like a hose," he said. Halley did show fluctuations, "but not to this extent."

The dust monitor detected its first impact when Stardust was 1,010 miles from the cometary nucleus. The last impact was recorded at a distance of 3,500 miles as the spacecraft sped away. During one intense event, the dust monitor detected more than 1,100 impacts in one second. The largest particle measured during the cometary flyby measured an estimated 500ths of an inch in diameter.

A similar instrument to the University of Chicago Dust Flux Monitor Instrument is a component on NASA's Cassini mission to Saturn. Cassini's High-Rate Detector, which Tuzzolino also built, is part of a larger instrument, Germany's Cosmic Dust Analyzer, which will study the ice and dust particles that form the major components of Saturn's ring system. Cassini is scheduled to become the first spacecraft ever to orbit Saturn on June 30.

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