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ACCESS is a cosmic ray detector that will be launched and attached to the International Space Station in about 2007 to help us understand the origin, variety, distribution and life span of elementary particles in our Galaxy.
NASA's ACCESS mission hopes to get us up to date, at least within a few million years, anyway, by studying galactic cosmic rays (GCRs). GCRs are most likely generated and accelerated by supernovae, or star explosions, and they come in many forms. They are the nuclei of elements (that is, the protons and neutrons), as well as electrons and positrons, moving through space at close to the speed of light. Like meteorites, cosmic rays carry great histories. Most of the heavy elements in the universe are thought to be created and dispersed by supernovae. The iron in our blood, for example, was once contained in a star, and blown into space when the star exploded. Therefore, knowing where cosmic rays originated, what they are made of, what sent them flying, and where they are going can help us piece together the structure and evolution of the universe. We are only beginning to understand the gross properties of the bulk of cosmic rays in our Galaxy as well as their sources. We know roughly the amount of energy needed to produce cosmic rays. We have good theoretical models of how they accelerate to near light speed. We know much about solar cosmic rays, those from our Sun. And we have the technology to carry out the measurements needed to answer our lingering questions. With X-ray and gamma ray telescopes, we have explored supernova remnants, the probable source of galactic cosmic rays -- that is, cosmic rays originating from outside our solar system. High-altitude balloon-borne experiments, such as ISOMAX, CAPRICE, HEAT, IMAX, JACEE, and BESS, and ground-based observations have filled in some GCR information. With ACE, SAMPEX, Geotail, and Wind, we are observing those cosmic rays produced by our Sun. ACCESS will help close the loop, working in conjunction with other cosmic ray detectors (ACE, for example, also studies GCRs) to aid in our understanding of the galactic component of cosmic rays. ACCESS is uniquely suited to study those cosmic rays of very high energy, about 1015 eV. Aboard the International Space Station, ACCESS will be in a prime position to collect cosmic rays, with a four-year exposure in outer space, unimpeded by the Earth's atmosphere.
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The information we have about such particles in the Galaxy is a bit outdated -- by about 5 billion years, actually. This is because much of what we know about the chemical composition of our Galaxy comes from meteorites, which formed several billion years ago. Meteorites tell us how much of a given element -- carbon, gold, zinc, etc. -- was around at the time of its formation long ago. This process works quite well, and we have learned much about the distribution of elements in our early Galaxy from meteorite studies. Yet, you can imagine that neighborhoods tend to change over a few billion years. Stars die; new ones are born; and before you know it, things start to look different.