Stanford Linear Accelerator Center

March 4, 1999



ARGOS Mission Seeks New Information about Black Holes and Neutron Stars

. An experiment to study black holes, neutron stars and other exotic celestial objects was launched Feb. 23 from Vandenberg Air Force Base in California, with the goal of learning more about matter in its most extreme states. 

A collaboration of astrophysicists the Department of Defense's Naval Research Laboratory (NRL) and particle physicists from SLAC and have built an x-ray detector that will be used in one of nine primary experiments on board the Air Force Space Test Program's Advanced Research and Global Observation Satellite (ARGOS). The detector is called the Unconventional Stellar Aspect (USA) experiment. The Department of Defense and the Department of Energy have provided funds for USA. 
The ARGOS Satellite in orbit with USA pointing at the viewer. In the lower corner of the picture is former SLAC/Stanford graduate student, now Dr. John Hanson, sitting in the USA support structure, which he helped design and build as part of his Ph.D thesis.

ARGOS was launched by a Delta II rocket that placed the satellite into a polar orbit where it is expected to operate for at least three years. 

The USA experiment was designed to observe bright x-ray sources, mostly binary star systems, including a black hole, a neutron star, or a white dwarf, orbiting with a more typical star. In neutron stars, gravity has compressed matter down to densities larger than those found in the nucleus of an atom. In all of these types of binary systems, extraordinarily strong, relativistic gravitational forces and enormous magnetic fields act in concert to produce dramatic phenomena not observable from Earth-based laboratories. 

In addition to providing valuable new information for astrophysicists and particle physicists, USA has been designed to make significant contributions to applied science, environmental science, and engineering research. It will use x-ray sources to test new approaches to satellite navigation and to conduct the first tomographic survey of Earth's atmosphere. It will also test new concepts for making spacecraft computers more reliable, an approach called fault-tolerant computing. 

Liftoff of the Boeing Delta II rocket with the ARGOS satellite on board, in the early morning, on February 23, 1999 from Vandenberg Air Force Base. (See also the QuickTime movie.)
is the first time that SLAC has been directly involved in a space-based experiment," said SLAC Professor Elliott Bloom. "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." The instrument's observations of binary star systems that contain a black hole or a neutron star should provide new information about the behavior of relativistic gravity near these compact objects. The instrument will also probe regions of extreme temperature and density in order to test the standard model of particle physics and, perhaps, find evidence for new types of matter. 

To search for interesting new physics, USA can observe variations in x-ray intensity with a time resolution of less than 100 microseconds. The experiment consists of two large area x-ray sensors on a gimbaled mounting. The sensors are collimated proportional counters, sensitive to X-rays between 1 and 10 angstroms, and with a field-of-view of 1.5 degrees. SLAC's main role in the experiment was to build and test the collimators, and construct the mounting. SLAC scientists and graduate students also helped to debug and test the readout electronics. They are currently busy trying to understand the calibration of the instrument, and preparing to analyze the large quantities of scientific data that are expected shortly. USA has an average telemetry rate of about 40 Kbytes per second - small by particle physics standards - but equal to that of the largest space-based experiments currently in orbit.
The Delta II Rocket shortly after liftoff, prior to separation. (See also the QuickTime movie.)
To accomplish its mission, USA will observe a small number of preselected targets 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, which should begin about May 1, 1999.

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 processor. The testbed will allow scientists to determine the effectiveness of advanced fault-tolerant software algorithms designed to allow the processor to continue operating even when damaged.
Assembly of the ARGOS satellite onto the Delta II rocket inside the Mobile Launch Support Complex, located on the launch pad at Vandenberg Air Force base near Santa Maria, California. (See larger photo for more detail.)
The principal investigator for the USA experiment is Kent S. Wood from NRL. Wood is a Stanford University Alumnus. Michael N. Lovellette, also from NRL, is the project scientist. Michael Wolff and Paul Ray of NRL have also been prominent in the construction phase of USA. Bloom is the lead co-investigator for the Stanford part of the collaboration and the group leader of SLAC's Particle Astrophysics group. Other Stanford members of the collaboration include Gary Godfrey of SLAC, Stanford physics Professor Peter Michelson, Visiting Professor Lynn Cominsky (Sonoma State University) and a growing number of SLAC/Stanford graduate students (Ganya Shabad, Daniel Engovatov, Pablo Saz-Parkinson, and Kaice Reilly). There is also a broader membership in the USA collaboration poised to analyze the prolific data soon to be beamed down from the spacecraft. In addition to those at NRL, Stanford and Sonoma State, these include scientists at University of Calgary, Massachusetts Institute of Technology, and NASA Ames Research Center. 

The USA, installed on top of the ARGOS satellite. Michael Lovellette, USA Project Scientist from the Naval Research Lab, 2nd from right.

More Resources

For website information on ARGOS and its payloads, please refer to and

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