ture of the Milky Way, and extragalactic radio sources. Obser- 

 vations of continuum and spectra] line sources are conducted 

 with a wide variety of instruments ranging from single anten- 

 nas to interferometers such as the Very Large Array (VLA). 



The Division is currently in the process of building a major 

 astronomical facility: a six-element submillimeter wavelength 

 telescope array (SMA) to be located near the summit of 

 Mauna Kea, Hawaii. The SMA is intended to make observa- 

 tions with unprecedented resolution in the still largely un- 

 explored submillimeter band between radio and infrared 

 wavelengths. 



Other research in the Division includes the application of 

 radio frequency techniques to the study of geophysical prob- 

 lems, tests of general relativity, and the development of 

 atomic clocks, and satellite-borne optical intetferometers. 



Research on space-based applications of tethered satellites 

 was advanced considerably by the launch March 9, 1994, of 

 the SAO-conceived and -designed Small Expendable Deploy- 

 able System (SEDS-2) as a secondary pay load aboard a Delta II 

 rocket. With a tether length of nearly 20 kilometers, it was 

 the longest object ever placed in space. The 23-kilogram mini- 

 satellite reached final deployment in just I hour and 48 min- 

 utes, achieving the primary goal of the experiment: to 

 suspend such a payload from a long line in space with a mini- 

 mum ot swing. The SEDS-2 remained in space to determine 

 its resistance of its braided polyethylene tether to 

 micrometeoids, space debris, and atomic oxygen. The tether 

 was apparently severed by a micrometeoid (or debris) on 

 March 15. While the SEDS-2 payload and part of the tether re- 

 entered the upper atmosphere and burned, about 10 to 12 kilo- 

 meters of the space string remained in orbit until May 

 18 — delighting thousands of amateur astronomers who were 

 able to observe its passage over many parts of the world. The 

 SEDS-2 experiment marked the third SAO-developed system 

 launched within a year. 



Solar and Stellar Physics 



John Raymond, Associate Director 



Scientists in the Solar and Stellar Physics Division carry out a 

 wide range of research projects using data from x-ray and 

 ultraviolet satellite observatories and telescopes on the 

 ground. Major themes include the study of stellar coronae 

 based on ultraviolet emission, investigation of the formation 

 and early development of stars, and analysis of the surface lay- 

 ers of the Sun. 



The atmosphere of the Sun, our nearest star, contains com- 

 plex magnetic structures that influence both solar radiation 

 and the solar wind — the stream of energetic particles ejected 

 at high speeds to bathe the bodies of the solar system, includ- 

 ing Earth. By designing and building increasingly sophisti- 

 cated experiments to study the Sun's hot outer atmosphere, 

 SAO scientists were the among first to identify the domi- 

 nance of magnetic fields. In addition, the SAO-inspired inter- 

 disciplinary study of Sun and stars has applied detailed 



knowledge of the solar corona to understand those processes 

 that govern the cotonae of distant stars. SAO also continues to 

 study cycles and activity in stars like the Sun, with the goal of 

 identifying and predicting the behavior of our own star. 



Research highlights in 1994 included: 



The second flight of the Spartan 201 satellite carrying an 

 SAO-designed Ultraviolet Coronal Spectrometer to make mea- 

 surements of the solar corona was successfully accomplished as 

 part of the space shuttle Discovery mission in September 

 1994. Observations of a polar hole and a polar plume structure 

 within a coronal hole were made; and, velocity distributions 

 and temperatures of a number of regions and structures in the 

 extended solar atmosphere were obtained for the first time. In 

 fact, the second Spartan flight was timed to coincide with the 

 passage of the Ulysses spacecraft beneath the solar south pole. 

 The combination of observations will provide the first oppor- 

 tunity to compare conditions in a coronal hole, where the 

 high-speed solar wind originates, with conditions far from the 

 Sun. 



Theoretical Astrophysics Division 

 A.G.W. Cameron, Associate Director 



An underlying goal of astrophysical research is to understand 

 as much of the Universe as possible within the current state of 

 technological development. Data come from astronomical ob- 

 servations carried out with equipment collectively sensitive to 

 the entire range of the electromagnetic spectrum — from long- 

 wavelength radio waves to very short-wavelength gamma 

 rays. In the last thtee decades, access to space and the tech- 

 nological development of sensitive new detectors have rev- 

 olutionized the gathering of data about the universe. 

 Transforming that data into understanding is the role of 

 theory. 



The increasing use of supercomputers to construct analyti- 

 cal models of objects, or simulations, has transformed the the- 

 oretical approach to such an extent that computational 

 astrophysics is now considered on par with observational and 

 theoretical astrophysics. Computational astrophysics requires 

 that the simulation programs incorporate enough detailed 

 physics for the results to bear a close resemblance to reality. 

 This Division attempts to apply such an analytical and com- 

 putational approach to understanding the universe. 



One area of research is the theoretical detetmination of the 

 rates of reactions involving atoms and molecules, an import- 

 ant factor in studies of the interstellar medium, stellar atmo- 

 spheres, and planetary atmospheres, including our own. 

 Another area is the study of conditions in the very Early Uni- 

 verse. Here, the theory involved lies at the frontier of high- 

 energy particle physics and there is a considerable interaction 

 with particle physics and the application of general relativity 

 to cosmology. 



Other research using simulation include studies of the late 

 stages of stellar evolution, in particular supernova explosions, 

 and the associated production of chemical elements. Also of 



49 



