A fuzzily defined pressure range of vacuum. The fundamental way most people seem to use the word is as arbitrarily vacuous - having a low enough density for any phenomenon relying on low pressure. Density in vacuum can be measured in either particles per cubic meter or in a pressure measure, such as atmospheres, Pascals, or millimeters of mercury.

For reference, here are some quantitative measures of vacuum:

low vacuum 105 - 103 Pascals

medium vacuum 103 - 10-1 Pascals

high vacuum 10-1 - 10-4 Pascals

very high vacuum 10-4 - 10-7 Pascals

ultra-high vacuum (UHV) 10-7 - 10-10 Pascals

extreme-ultrahigh vacuum (EHV or XHV) < 10-10 Pascals

space-vacuum 10-15 Pascals

highest man-made vacuum (circa 2002) 10-10 Pascals

"Hard vacuum" is a commonly used (and abused) phrase in science fiction. Babylon 5 once described a character as having "sucked hard vacuum 'til he died"; this in turn inspired the title of an an extremely ingenious but highly obscure board game created by Darrell Hayhurst. I feel like writing about the boardgame this morning, because the sight of the phrase has triggered an attack of nostalgia.

Hard Vacuum was published by Fat Messiah Games, and was set in an alternate history in which Pulp Science allowed World War II to extend into space. All the major participants had a different improbable technology, each owing a lot to 1940s SF serials and comics, fuelling their ships.

The idiosyncratic background doesn't attempt to pass a reality check, but its assumptions make for a fun and characterful (if somewhat abstract) set of rules. Many of the principles of the era's air combat are extended into space: players' ships are tiny, vulnerable shells of riveted metal with very limited fuel and ammunition. There are no computers, no targeting screens, no guided missiles. As a result, the rules only need to model a very limited number of features, and they do it in a very elegant way that mixes "arcade" simplicity with a "simulation"-style affection for physics. Ships are subject to momentum and shooting is affected by the relative speeds and angles of the participants, but no serious calculations or nasty tables are required - quite a feat of game design.

The game scales very well to accomodate different player numbers. All moves are decided simultaneously and in secret, then applied simultaneously, then shooting occurs simultaneously - so two opposing players could control an entire squadron each, but ten players could each take control of a single craft and divide into two teams. This actually speeds up the game, and is the most entertaining way to play. As there are no encrypted radios in the game background, any co-ordination with your wingmen has to be spoken aloud so the enemy can hear - but the rules make a predictable move certain death, so cooperation is an interesting challenge.

Lest I sound too much like the raging fanboy I probably am, I should point out that there are a few notable flaws. The core rules are of excellent quality, but the individual sides' special abilities and ship characteristics have some balance problems. The ship design rules are coarse-grained, somewhat open to abuse, and favour certain design types unreasonably strongly. There's an odd bug involving extremely difficult shots and the likelihood of hitting a vital system, but it's easily player-patched by agreement. The number of fiddly little tokens is perhaps a little high. But the greatest flaw is simply that few people ever played it and now almost no-one does. Life is tough for small-press DTP games, no matter how clever they are.

It's a trivial thing, I know, but I miss this game.

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