The largest interstellar dust track found in the
Stardust aerogel collectors was this 35 micron-long hole produced by a 3
picogram mote that was probably traveling so fast that it vaporized upon impact.
The other two likely interstellar dust grains were traveling more slowly and
remained intact after a soft landing in the aerogel.
Image Credit:
Andrew Westphal, UC Berkeley
The largest interstellar dust track found in the
Stardust aerogel collectors was this 35 micron-long hole produced by a 3
picogram speck of dust that was probably traveling so fast that it vaporized
upon impact. The other two likely interstellar dust grains were traveling more
slowly and remained in
Image Credit:
UC Berkeley/Andrew Westphal.
Seven rare, microscopic interstellar dust particles that date to the
beginnings of the solar system are among the samples collected by scientists who
have been studying the payload from NASA's Stardust spacecraft since its return
to Earth in 2006. If confirmed, these particles would be the first samples of
contemporary interstellar dust.
A team of scientists has been combing through the spacecraft's aerogel and
aluminum foil dust collectors since Stardust returned in 2006.The seven
particles probably came from outside our solar system, perhaps created in a
supernova explosion millions of years ago and altered by exposure to the extreme
space environment.
The research report appears in the Aug. 15 issue of the journal Science.
Twelve other papers about the particles will appear next week in the journal
Meteoritics & Planetary Science.
"These are the most challenging objects we will ever have in the lab for
study, and it is a triumph that we have made as much progress in their analysis
as we have," said Michael Zolensky, curator of the Stardust laboratory at NASA’s
Johnson Space Center in Houston and coauthor of the Science paper.
Stardust was launched in 1999 and returned to Earth on Jan. 15, 2006, at the
Utah Test and Training Range, 80 miles west of Salt Lake City. The Stardust
Sample Return Canister was transported to a curatorial facility at Johnson where
the Stardust collectors remain preserved and protected for scientific study.
Inside the canister, a tennis racket-like sample collector tray captured the
particles in silica aerogel as the spacecraft flew within 149 miles of a comet
in January 2004. An opposite side of the tray holds interstellar dust particles
captured by the spacecraft during its seven-year, three-billion-mile
journey.
Scientists caution that additional tests must be done before they can say
definitively that these are pieces of debris from interstellar space. But if
they are, the particles could help explain the origin and evolution of
interstellar dust.
The particles are much more diverse in terms of chemical composition and
structure than scientists expected. The smaller particles differ greatly from
the larger ones and appear to have varying histories. Many of the larger
particles have been described as having a fluffy structure, similar to a
snowflake.
Two particles, each only about two microns (thousandths of a millimeter) in
diameter, were isolated after their tracks were discovered by a group of citizen
scientists. These volunteers, who call themselves "Dusters," scanned more than a
million images as part of a University of California, Berkeley, citizen-science
project, which proved critical to finding these needles in a haystack.
A third track, following the direction of the wind during flight, was left by
a particle that apparently was moving so fast -- more than 10 miles per second
(15 kilometers per second) -- that it vaporized. Volunteers identified tracks
left by another 29 particles that were determined to have been kicked out of the
spacecraft into the collectors.
Four of the particles reported in Science were found in aluminum foils
between tiles on the collector tray. Although the foils were not originally
planned as dust collection surfaces, an international team led by physicist
Rhonda Stroud of the Naval Research Laboratory searched the foils and identified
four pits lined with material composed of elements that fit the profile of
interstellar dust particles.
Three of these four particles, just a few tenths of a micron across,
contained sulfur compounds, which some astronomers have argued do not occur in
interstellar dust. A preliminary examination team plans to continue analysis of
the remaining 95 percent of the foils to possibly find enough particles to
understand the variety and origins of interstellar dust.
Supernovas, red giants and other evolved stars produce interstellar dust and
generate heavy elements like carbon, nitrogen and oxygen necessary for life. Two
particles, dubbed Orion and Hylabrook, will undergo further tests to determine
their oxygen isotope quantities, which could provide even stronger evidence for
their extrasolar origin.
Scientists at Johnson have scanned half the panels at various depths and
turned these scans into movies, which were then posted online, where the Dusters
could access the footage to search for particle tracks.
Once several Dusters tag a likely track, Andrew Westphal, lead author of the
Science article, and his team verify the identifications. In the one million
frames scanned so far, each a half-millimeter square, Dusters have found 69
tracks, while Westphal has found two. Thirty-one of these were extracted along
with surrounding aerogel by scientists at Johnson and shipped to UC Berkeley to
be analyzed.
NASA's Jet Propulsion Laboratory, Pasadena, California, manages the Stardust
mission for NASA's Science Mission Directorate, Washington. Lockheed Martin
Space Systems, Denver, developed and operated the spacecraft.
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