On Mir's Loading Dock Sits a Russian/American Cosmic Ray Experiment
Destined for Berkeley
by Robert Sanders
When the Space Shuttle Atlantis docked with the Russian space station Mir Nov. 15, one of its missions was to collect important baggage destined for Berkeley that had been waiting on Mir's loading dock nearly four months.
The shipment consisted of hundreds of glass plates that sat outside Mir for four years collecting tracks of cosmic rays--naked atoms that zip through space from points outside our solar system.
The plates, part of a joint American and Russian experiment called TREK, were lofted to Mir in the summer of 1991 and remained mounted outside the space station until July, when the last panels were taken inside and stored until the next shuttle docking.
The scheduled launch date of Saturday, Nov. 11, slipped to Sunday, so the pickup date was Wednesday, when Atlantis docked with Mir for a planned three days. The shuttle carrying five astronauts docked for only the second time with Mir, delivering mail, food, water and supplies, plus a Russian docking module and two solar arrays. Atlantis should return to Earth with TREK on Nov. 20.
Other glass plates returned in 1992 have provided important data on the abundances of various chemical elements in cosmic rays, says Andrew Westphal, a postdoctoral physicist here. They have shown, for example, that the composition of iron and nickel in cosmic rays is identical to their composition in the sun.
"Our detectors worked beautifully and showed that the sun and the solar system are not unusual in their composition of iron and nickel," Westphal says. Some earlier experiments suggested there was a significant difference between solar abundances and abundances outside the solar system where cosmic rays originate.
Westphal, team leader P. Buford Price, professor of physics, and their Russian colleagues recently submitted their initial analysis of the data to the Astrophysical Journal.
The remainder of the TREK detectors coming down with the space shuttle will provide information on the abundances of the heavier isotopes in cosmic rays--platinum, gold, lead, mercury and osmium, plus even heavier elements like thorium and uranium.
"There's a real mystery about the composition of these ultraheavy elements," Westphal says. From earlier measurements "they seem to be coming directly from distant supernovas. But why do the ultraheavy cosmic rays have abundances like we see in supernovas while the lighter ones don't? Does that mean they have different origins?"
The cosmic ray tracks in the TREK detectors may help answer these questions.
Cosmic rays are bare nuclei--atoms stripped of their electrons--that speed through the solar system and interstellar space and are believed to be remnants of exploding stars. Because they come directly from outside the solar system, they offer a rare glimpse into the composition of our galaxy and the source of cosmic rays.
Of particular interest to Price, Westphal and their Russian colleagues are the very heavy nuclei in cosmic rays--nuclei of atoms ranging from nickel to uranium and perhaps beyond.
These heavy nuclei are extremely rare, yet accurate knowledge of their abundances could tell astrophysicists how old cosmic rays are, the mechanisms by which they were produced within exploding stars and how they were accelerated to such high speeds.
All the cosmic rays except protons--the nucleus of the lightest element hydrogen--disintegrate high in the earth's atmosphere. Thus detectors for the heavy elements must be lofted above the atmosphere. At a height of 220 miles, Mir is far above most of the atmosphere, so the experiments should havedetected an undistorted sample of cosmic ray particles.
Berkeley physicists designed and built the experiment with the help of Russian physicists. They now are collaborating in the data analysis with Russian astrophysicists Vladimir G. Afanasyev and Valeri V. Akimov of Moscow's Space Research Institute of the Academy of Sciences in Russia. TREK was the first U.S.-designed instrument to fly on a Soviet spacecraft following the two countries' civil space agreement in 1987. The U.S. part of TREK is funded by the National Aeronautics and Space Administration.
The TREK detectors capture the wakes of cosmic ray particles as they pass through a special glass. Once back on earth the glass is etched with highly corrosive hydrofluoric acid to reveal these tracks, which are then analyzed by automated scanning and measuring instruments to determine speed, mass and atomic number (the number of protons packed into the nucleus of the atom).