20.08.2017

Electromagnetism, not force of impact, does the damage

HOW is a speck of dust like an atom bomb? It sounds like a child’s riddle. But the answer may explain the fate of many satellites that have failed prematurely in orbit over the years. For the riddle to work, the speck must be travelling at 70km a second, or thereabouts. If it is, the riddle’s solution is that both can generate an electromagnetic pulse capable of knocking out unprotected electronic equipment.

That, at least, is the hypothesis now being investigated by Sigrid Close of Stanford University. Dr Close came up with it in 2010, when she was shooting small particles at various types of spacecraft material placed inside a vacuum chamber at the Max Planck Institute in Heidelberg, Germany. These experiments suggested that when a micrometeoroid, to give such dust its technical name, collides with a satellite, it will not just do a small amount of mechanical damage. If travelling fast enough, it will also create a shock wave that vaporises part of the spacecraft’s metallic skin.

This will, in turn, create a plasma—a gas so energetic that its atoms have parted company with some of their electrons, thus becoming ions. The plasma will then expand from its point of origin into the vacuum of space. An atom bomb similarly creates an expanding plasma, albeit on a far grander scale. And, in both cases, the difference in velocity within the expanding plasma between slow-moving (because heavy), positively charged ions and fast-moving (because light), negatively charged electrons creates an electrical imbalance that causes the emission of a pulse of electromagnetic energy powerful enough to disable nearby electrical equipment.

The definition of “nearby” is different in the two cases, of course. For a bomb it is many kilometres; for a speck of dust mere centimetres. But even that short range, Dr Close reckons, is enough to do a satellite serious harm.

Working out the details is tricky. But, in a paper published recently in the Physics of Plasmas, she and her collaborator, Alex Fletcher of the Massachusetts Institute of Technology, use a type of modelling called particle-in-cell simulation to do so. This technique attempts to follow the zillions of particles in a plasma by treating groups of them as “superparticles”. These groups can be handled, statistically, in a way which apes reality closely enough to be useful while remaining simple enough to be processed by a computer. Their conclusion was that the pulse’s radiation will have a higher frequency than was previously believed, and thus be more powerful than current satellites are designed to withstand.

Dr Close and her colleagues are now conducting more tests on the ground, to see if their calculations are correct. If those calculations do stand up, that will have implications for satellite design. More shielding of vulnerable parts may be needed, which will add to weight, and therefore to cost. That extra cost may be worthwhile, though. Losing a satellite halfway through its expected working life is pretty expensive, too.

Quelle:The Economist