The instruments were integrated onto a Delta II rocket for the satellite's launch, from Vandenberg AFB, CA, into a polar orbit in the early morning on February 23, 1999. ARGOS, which has a 3-year planned operational life, will carry nine primary experiments that contain 31 different sensors and sub-experiments.

The Unconventional Stellar Aspect (USA) experiment, one of the primary experiments, was designed to observe bright X-ray sources to further our understanding of these exotic objects, which are mostly binary systems in our Galaxy containing black holes, neutron stars or white dwarf stars. Studying these systems allows particle astrophysicists to glimpse matter in its most extreme states where densities can be higher than in an atomic nucleus, and extraordinarily strong and relativistic gravitational forces and enormous magnetic fields are acting in concert to produce dramatic phenomena not observable in earth based laboratories.

USA will make significant contributions to applied science, environmental science, and engineering research as well. It will use these same X-ray sources to test new approaches to satellite navigation, conduct the first X-ray tomographic survey of the Earth's atmosphere, and will test new concepts for fault-tolerant computing in space.

"This is the first time that SLAC has been directly involved in a space based experiment. USA gives us the chance to do in-depth particle astrophysics research on many different black hole and neutron star systems in our Galaxy. The collaboration between particle physicists at SLAC and astrophysicists at the Naval Research Laboratory, which began in 1991, gives us a unique approach to studying these systems" said Prof. Elliott Bloom, spokesperson for the SLAC collaboration who helped to build the USA hardware.

The USA sensors are sensitive to X-ray wavelengths of about 1 to 10 Angstroms. USA observations of celestial black hole and neutron star binaries will provide information about the behavior of relativistic gravity near black holes where the fields are very strong. USA can also test the standard model of particle physics under extreme conditions of temperature and density and perhaps find evidence for new types of matter. To probe new regions of physics, USA can observe time variable phenomena in these sources with a time resolution of less than 100 microseconds.

USA is an X-ray telescope consisting of two large area X-ray sensors and a gimbaled mounting. With a field-of-view of about 1.5 degrees, and an area of about 2000 cm^2, the detectors will measure the time-varying X-ray output of celestial sources with excellent timing information and reasonable energy resolution. To accomplish its mission, USA will observe a small number of targets, selected prior to launch, and re-measure the X-rays from these targets repeatedly. The scientists anticipate that each of approximately 30 bright sources will be observed several times during the first month of operation. Two to four observations are planned for each orbit, depending on particle background and source location.

As part of its engineering research goals the USA Experimental team will test new concepts for fault-tolerant computing in space. To achieve this goal, USA carries a two-computer testbed that consists of a military radiation-hardened processor side-by-side with a commercial off-the-shelf (COTS) processor. This testbed will allow scientists to demonstrate the capability of using advanced fault tolerant software algorithms to enable the COTS processor to be used for high-performance space-based computing tasks.

The Principal Investigator for the USA experiment is Dr. Kent S. Wood from the US Naval Research Laboratory (NRL). Dr. Michael N. Lovellette, also from NRL, is the Project Scientist. Dr. Michael Wolff and Dr. Paul Ray of NRL have also been prominent in the construction phase of USA. Professor Elliott Bloom, Stanford Linear Accelerator Center (Stanford University), is the lead Co-investigator for the Stanford part of the collaboration, which also includes Dr. Gary Godfrey of SLAC, Stanford Physics Professor Peter Michelson, Visiting Professor Lynn Cominsky (Sonoma State University), and a growing number of Stanford graduate students. There is also a broader membership in the USA collaboration poised to analyze the prolific data soon to be beamed down from the spacecraft. Besides those at NRL, Stanford and Sonoma State, these include scientists at Calgary University, MIT, and NASA AMES. The Department of Defense and the Department of Energy have provided funds for USA.