Martin recently posted that TA1 has just come been shipped to the printers. A typical print run based on most d20 publisher updates, from being shipped off to the printer to arrival at the warehouse for distribution and sale seems to be three to six weeks, depending on the publisher and the size of the manuscript. I'd venture to guess that TA1 will be available by May 20th, if I had to name the earliest time it could be available.
Shortly after that, the Gateway Domain sourcebook gets sent off to the printer. Assuming the high end of six weeks for the print run, Gateway won't even be available until July 7th, at the earliest. (Gateway should be the big T20 book for GenCon, then.)
The Starfall Cluster comes immediately after that, since it too is near ready for print. So six weeks later, around August 18th or so, should be the shipping date for Starfall Cluster, assuming that other TAs don't slip in there between print runs and push it back.
That's my best guess. At the beginning of the year, I also made the guesstimation that the Gateway Domain book would be ready sometime in April, so understand that, while these dates are the earliest I would guess we'd see these books, delays happen and can push the schedule even further back.
However, when they arrive, I will be eagerly awaiting them, because T20 has allowed me to return to Traveller gaming. That is a Good Thing(tm).
Until then, Happy Travellin',
Flynn