Spacewar!, and Mapping Gravity

The blog War is Boring has a brief shout-out to Spacewar!, the first video game. Programmed by a inventive group of graduate students to run on MIT’s Program Data Processor-1 (PDP-1), Spacewar! was a real-time two-player game in which spaceships swooped around the gravity-well created by a collapsed star, shooting missiles at each other against the backdrop of an expensive-looking starfield. Created in 1961, it was, as War is Boring puts it, “the Pentagon-funded video game” that created an entire form of entertainment. If you want to try it out yourself, there’s a JavaScript emulator of the game, as well as lots of historical articles and versions of the original code, at Norbert Landsteiner’s Spacewar! website.

War is Boring misses a neat irony about the Spacewar! story though. What made the game hideously hard and an obsession for physicists and engineers was its realistic physics, complete with the “real” gravity of a star. Yet, at the same time as the students at MIT were programming Spacewar! the US Navy was discovering that gravity in the real world was not as smooth as it was in Spacewar!‘s featureless sphere – and that the difference could be the Achilles heel of the US nuclear deterrent.

Though the force of gravity is the same all around the world, variation in the mass and shape of the earth mean that gravity varies, subtly, from place to place around the world. The fact itself was not terribly new – the first gravity surveys began in the seventeenth century and the US Navy was making gravity measurements from the submarine S-21 in the 1920s – but it acquired much greater significance after the Second World War.

Suddenly, the fact that differences in local gravity would have a small but measurable affect on a ballistic trajectory became important – especially when the object on that trajectory was a missile that had to travel several thousand kilometers, like, coincidentally a submarine-launched ballistic missile. Combine that with the fact that the launching submarine relied on an inertial guidance system whose measurements of motion could be skewed by those same gravitational anomalies. (This was the same navigation problem that led to the Transit satellites and then GPS.) In the early 1960s, at the same time as Spacewar! was being birthed at MIT, it turned out that the inconsistency of gravity could be vitally military important.

Understandably, this situation put the US military’s gravimetry program into high gear. The Army Map Service had 125 gravity survey crews in the field. The Air Force had a Geodetic Survey Squadron (the 1381st) and a vast library of gravity survey data at Aeronautical Chart and Information Center in St Louis, Missouri, but the most substantial contribution probably came from “The Triplets,” three converted merchantmen equipped to calibrate the Ships Inertial Navigation System (SINS) that would be used by the Navy’s ballistic missile submarines. The shortest-lived of the three, USNS Michaelson (T-AGS 23), served for seventeen years. The longest-lived, USNS Dutton (T-AGS 22), lasted thirty-one years after making the equivalent of one-hundred circumnavigations of the Earth.

By then, the state of the art for gravity measurement had advanced by leaps and bounds, helped in part by the computer revolution that began with the PSP-1 and its counterparts. NASA and the Hopkins Applied Physics Laboratory (APL) discovered that sea surface height, measured by the sensitive radar altimeter on the Seasat satellite, could be used to infer the differences in local gravity. It was an impressive enough achievement that it led to a brand-new Navy satellite – GEOSAT – whose altimeter could measure the ocean height with an accuracy of three centimeters. Sea surface topography, it would turn out, had all sorts of valuable military uses. That, though, is another story or two.


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