Halffull Or Halfempty

Much to the chagrin of her crew, thousands of satisfied scientists and Shuttle director Bob Crippen, Columbia's 13th mission would be forever saddled with the age-old question: 'Is the cup half-full or half-empty?' It has often been posed in situations which, at least in some quarters, are perceived as unworthy of the risks involved. To be fair, the cargo trucked into orbit by the STS-52 crew on 22 October 1992 was a mix of different payloads: an Italian satellite, three experiments on a cross-bay bridge and a myriad of other investigations; even a quilt affixed to Columbia's RMS mechanical arm.

The major payload - if it could be called that - for Commander Jim Wetherbee and his crew of five was the second Laser Geodynamics Satellite (LAGEOS-2), part of a collaborative venture between NASA and the Italian Space Agency (ASI). It was essentially a large aluminium ball, 60 cm in diameter, with a dense brass core which gave it a heavier-than-it-looked mass of 410 kg. It had been built in Italy, at a cost of $160 million, using blueprints from NASA's Goddard Space Flight Center, which had launched an identical satellite almost two decades earlier.

Like LAGEOS-1, it was a passive spacecraft dedicated to 'laser ranging'. In other words, laser beams transmitted from Earth would impinge on 426 nearly-equally-spaced, cube-corner retroreflectors, or prisms, embedded in LAGEOS-2's shell. By measuring the round-trip travel time of the beams, it was expected to very precisely determine the distances between ground stations with an accuracy of just a few centimetres. Data from LAGEOS-1 had already revealed that the Pacific tectonic plate, upon which lies the Hawaiian island chain, is slowly drifting, at a rate of a few centimetres per year, northwestwards in the direction of Japan. Additionally, it helped to confirm that the East Pacific Rise - an 'axis' of sea-floor spreading - is separating the Hawaiian islands from South America at approximately the same rate. Movement, or lack of movement, on this scale, is important as it enables geophysicists to understand what goes on beneath our feet, providing insights that may allow them to predict the occurrence of earthquakes or volcanic activity. ''The satellite may be small, but the data returned is bigtime science,'' said LAGEOS Project Scientist Miriam Bartuck, adding that it also had applications in understanding our planet's 'wobble' and determining more accurately its size and shape.

The solid, outermost layer of Earth - measuring up to 100 km thick in places - is known as the 'lithosphere' and is composed of vast, rigid 'plates' which float on a semi-liquid region called the 'mantle'. These plates move with respect to one another, but generally at speeds no higher than a few centimetres per year. This motion, dubbed 'plate tectonics', is the means by which North and South America are presently moving away from Europe and Africa, and although these motions are incredibly slow, they can be catastrophic when collisions occur.

Occasionally, tectonic plates bump into one another, spread apart or scrape horizontally past each other; many of the world's great mountain chains, earthquake-prone areas and volcanic hotspots are the consequence of this activity. One tectonic model which, it was hoped, LAGEOS-2 would address was the theory

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