A bizarre way to find space junk

July 14, 2004

IN A bid to predict the threat to satellites from space debris, the European Space Agency is teaming up with a pair of physicists who spend their time smashing eggshells. This improbable collaboration is founded on the idea that eggshells and discarded pieces of space rockets will break up in a similar way. After a rocket has lifted its payload into space, the final stage and its fuel tanks are often abandoned. If a micrometeoroid or piece of space debris penetrates the tank, the dregs can ignite, causing an explosion that shatters the rocket body and creates a cloud of debris.

Since the space age began in the 1950s, there have been almost 200 explosions in orbit, just under half of which involved old rocket bodies. The resulting debris poses a threat to orbiting craft. Pieces as small as 1 centimetre across could destroy a satellite, according to Heiner Klinkrad, an expert in space debris at ESA in Darmstadt, Germany. But no one knows how many fragments there are, and what orbit they are in. Klinkrad was wondering how ESA could improve its estimates of the whereabouts of this debris when serendipity struck. On a TV show last Easter, he saw a couple of physicists exploding empty eggshells and explaining how the way they fragmented could mimic what happens to brittle structures like planes when they blow up. It occurred to Klinkrad that rocket bodies should behave similarly. In their experiments the two physicists, Falk Wittel from the University of Stuttgart in Germany and Ferenc Kun from the University of Debrecen in Hungary, first poke small holes in each end of a raw hen's egg, blow out its contents, and fill the empty shell with hydrogen. They then suspend it upright inside a plastic bag and ignite the hydrogen escaping from the hole at the top of the egg. As air is drawn in at the bottom of the eggshell, it mixes with the hydrogen inside the egg, which eventually explodes. The pair then collect the shards from the bag and measure their size. Though there are obvious differences in shape and size, an eggshell and an old rocket stage are both little more than a thin, brittle shell enclosing a large volume of potentially explosive gas.

The frigid cold in space increases the brittleness of the rocket body, so like an eggshell it breaks by cracking into pieces rather than deforming. More than 8500 pieces of space debris bigger than 10 centimetres across are being tracked by the world's space agencies. In a bid to estimate the danger from fragments in the 1 to 10-centimetre range, ESA ran experiments in the early 1990s in which rocket fuel was used to blow up metal cylinders in an underground bunker. They found the distribution of the size of the fragments followed a power law: most of the material ended up in intermediate-size shards, with many small pieces and few large ones. The mass distribution of the eggshell fragments in Wittel and Kun's experiments followed a similar power law, Klinkrad says, indicating that exploding eggs really do behave like shattered rocket bodies. But the size of the fragments of space debris is only half the story.

ESA also needs to know in what orbits they are likely to lie, and here Wittel and Kun's latest work, which they will report in a forthcoming edition of Physical Review Letters, should help. They have developed a predictive computer model based on their egg fragmentation data. The model represents the brittle shell as a dense network of nodes connected by elastic springs. An explosion inside the shell sets the springs vibrating, and if a spring is stretched too much it breaks. This starts a crack, which then grows and spreads as more springs break, until the whole shell has shattered into pieces. Using the model, the pair are able to predict not only the number and size of the fragments, but, crucially, how fast they fly out. This, Klinkrad says, will help ESA pin down what orbit the debris from exploding space junk is likely to have ended up in.

This article appears in New Scientist issue: 17 JULY 2004


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