Nickel and X-ray satellites

Burning its way into space on a column of fire last December, the world’s most powerful X-ray satellite took just 29 minutes to travel from the European Space Agency’s spaceport in Kourou, French Guiana, to its home in Earth’s orbit. The satellite, known as XMM-Newton, will be able to detect the spectral features of astrophysical objects such as the corona of stars, supernova remnants, binary star systems, clusters of galaxies and active galactic nuclei.

XMM-Newton will detect the density and temperature of these objects, as well as their ionization state, mass motions, red shift and gamma-ray bursts. Using X-ray spectography, the unit can detect the temperature and the relative abundance of elements such as iron, oxygen and silicon.

The optical heat of the satellite, which contains three telescopes, is an array of three mirror modules, developed by Media Lario in Como, northern Italy. These mirrors focus the X-rays on to instruments behind the modules, which record, analyze and transmit the data to earth.

The modules are the most powerful ever developed and enable astronomers to discover more X-ray sources than was previously possible using space observatories.

Each 410-kg module consists of 58 nested, slightly conical nickel mirrors with a conicity of just one degree or less. Think of 58 tubes within tubes, each slightly large than the one inside it and separated from its neighbour by a few millimetres. The mirrors are 0.6 metre long and range in diameter from 0.3 to 0.7 metre.

Each mirror is made on a mandrel, which is polished to an accuracy of three Angstroms. First, 2,000 Angstroms of gold, which constitute the X-ray reflecting surface, are evaporated on to the mandrel; then, 99.99% pure nickel is electroplated on to the gold. The nickel, which forms the structural part of the mirror, ranges in thickness from 0.6 mm for the smallest- diameter mirrors to 1.2 mm for the largest.

Nickel has several desirable properties that make it the material of choice in the manufacture of the XMM mirrors, explains project manager Robert Lain. Besides adhering well to gold, it is reasonably homogenous and has good mechanical properties, compared with other metals. Copper, for example, is too soft to withstand the force exerted during launch. Nickel also has a co- efficient of thermal expansion close to that of the spoke-like mounting structure that is made of the high-nickel alloy N06600.

Several tonnes of nickel were used in the construction of the mirror modules that were sent into space.

— The preceding is an excerpt from Nickel, a publication of the Toronto-based Nickel Development Institute.