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Within eighteen months it became obvious that the Proxima hostilities were going to be considerably more than a brief trial of strength between neighbouring solar systems.

Although Proxima Centauri had no equivalent to our warp drive or the Alpha Potential Mass Drive it was recognised that it would be only a matter of time before they either developed a similar system or duplicated one of ours. The defence of our own solar system had therefore to be considered, and this raised a major question, as it had been agreed that the greatest allocation of resources had to be made to the development and supply of offensive craft. The enormous cost of producing a single warship meant that building and maintaining an effective battle fleet would leave very little over for home defence, itself requiring massive investment.

The solution finally agreed was a three tiered defensive system. The Perimeter from Pluto's orbit outwards was seeded with static nuclear mines which were cheaply and easily manufactured and would be a serious threat to craft emerging from a warp jump. They were designed to home in on mass of the order of a warp generator and would take advantage of the short period that any such vessel required to replenish its power banks after jump.

The second line of defence was provided by the Sentinels. The best known of these craft are undoubtedly the Sentinel Majors, whose massive, squat shape and huge main venturi tube are unmistakable.

Only one in five of these ships was manned, the others being no more than powered weapon packs fitted with sophisticated detection and response gear. The fifth vessel of each flight was the manned command centre, which co-ordinated the operations of the weapon carriers.

The Sentinel was fitted with a very large conventional nuclear propulsion system based on liquid hydrogen and oxygen, producing about 5 million lbs potential thrust. This facility allowed it a high degree of tactical mobility, but its drawback was rapid fuel consumption, almost I ton per second. Against this it was reasoned that the armament would be effective enough to ensure that combat would be resolved speedily and would protect the ship when mobility was reduced.
The armament itself consisted of a variety of weapons from nuclear general purpose rockets and particle accelerator beams to high intensity lasers. The Sentinel Major was a formidable obstacle, and proved its worth during the Proxima attack on Mars in 2052 when over one third of the attacking force was destroyed before ever reaching target.
The third line of defence was of course the planet based interceptor squadrons, which maintained constant patrols around each defence area.

There is still a large number of Sentinels in active service on the Perimeter, while others have been converted into navigation beacons and research data collectors.