Direct determination of metastable phase diagram by synchrotron radiation experiments on undercooled metallic melts.

The phase selection process during the crystallization of undercooled metallic melts is studied in situ by combining the electromagnetic levitation technique with energy dispersive x-ray diffraction of synchrotron radiation. The crystallization of metastable bcc phase in binary Ni-V alloys was identified. A metastable phase diagram of Ni-V alloy is constructed, which shows the primarily solidifying phase as a function of composition and undercooling. The analysis within nucleation theory emphasizes the important role of metal oxide as a heterogeneous nucleation site controlling the phase selection.

Growth and form of planetary seedlings: results from a microgravity aggregation experiment.

The outcome of the first stage of planetary formation, which is characterized by ballistic agglomeration of preplanetary dust grains due to Brownian motion in the free molecular flow regime of the solar nebula, is still somewhat speculative. We performed a microgravity experiment flown onboard the space shuttle in which we simulated, for the first time, the onset of free preplanetary dust accumulation and revealed the structures and growth rates of the first dust agglomerates in the young solar system. We find that a thermally aggregating swarm of dust particles evolves very rapidly and forms unexpected open-structured agglomerates.

Spacelab-1: an early space station for science and technology.

The first flight of Spacelab is primarily a system verification and test flight, but it will also carry a payload of scientific and technological experiments to demonstrate the capability of performing multidisciplinary research in space. The payload covers the disciplines atmospheric physics, plasma physics, solar observations, astronomy, Earth observations, and material and life sciences. In this paper we will give for all represented disciplines short discription of their scientific objectives and experimental techniques. Particular emphasis is placed on how different disciplines utilize features characteristic for manned space stations, such as weight and power capabilities the availability of a human operator, the microgravity environment, the possibility to return samples or recordings and the recoverability of hardware together with a reflight capability. These advantages will be reviewed against the background of the relatively short mission duration, safety requirements and environmental influences caused by the presence of men in space. We will identify a research scenario for which Spacelab offers distinct advantages compared to conventional free-flying satellites, one which might also be pursued from larger space stations in the future.

Measurement of the polarization of extreme ultraviolet synchrotron radiation with a reflecting polarimeter.

The polarization of extreme uv light provided by a normal incidence monochromator used with synchrotron radiation from the 6-GeV electron synchrotron DESY has been measured. The measured polarization increases from 82% at 500 A to 87% at 1000 A, and is in excellent agreement with the calculated polarization of the light incident upon the monochromator grating. A reflecting polarimeter consisting of a polarizer and an analyzer each made of four gold mirrors has been used in the measurement.