How three Gravitas barges are building a 2.4-kilometre colony ark

The Meridian Colony Ark is 2,400 metres long. No drydock in the solar system can hold it, including our own Kepler Yards at L5, so when the program was approved we had to answer a basic question: where do you build something that big? The answer is nowhere and everywhere. The ark is assembled in open orbit above the Olympus Ring, and the shipyard moves around it.

That moving shipyard is a trio of Gravitas Dock Barges. Each barge is 340 metres long, flown by three pilots with a crew of 26 riggers, welders and inspectors. Gravitas is slow, with a top speed of just 45 km/s and a range of 6 AU, but speed is not the point. Its xenon ion thrusters give it extraordinarily fine control, and each barge can manoeuvre 18,000 tonnes of structure with centimetre precision.

The ark is built in 120-metre segments. Spine sections and ring frames are fabricated at our Mars yard and lifted to orbit by Cascade Dropships, where a Gravitas captures them, aligns them and holds them against the growing hull while welding drones work. A second barge works the opposite end, and the third shuttles between the stockpile and the work face. On a good day the team adds about 4 metres of finished spine.

The fuel figure surprises people. A Gravitas uses 8.0 tonnes of xenon per 100 AU, which sounds high for an ion ship, but barges spend most of their time holding station under load, not travelling. Across the full ark program we expect the three barges to use about 60 tonnes of xenon combined, a rounding error against the ark's 8,950 Mc price.

Building in open orbit creates problems a dock does not. Thermal cycling every 2 hours as the work face passes in and out of Mars's shadow means steel expands and contracts by several millimetres per segment. We schedule critical welds during the shadow pass, when temperatures are stable, and the Gravitas crews have become very good at working in the dark.

Safety is the other big one. With 78 people on the barges plus EVA teams, we run a dedicated Seraph Medevac on standby near the work site, and every rigger carries a personal recovery beacon tied into the barge autonomy system.

The first Meridian Colony Ark flew in May 2186 for drive trials while still partly unfinished. The second hull is now about 40 percent complete. When it departs for Titan with 4,800 settlers aboard, it will carry the fingerprints of every Gravitas crew who worked on it.