So from that I would guess that the Order of Terra was the Knightly order given in the Client State of the Old Earth Union as this was being intergrated into the Imperium.
From the "prestigous" and "downgraded" I would guess it's likely to have been either after Vland or Gateway, so 76 or 100 sounds about right.
If it used to be more prestigious than the Order of Antares, one of the domain orders, it seems that it must itself have been a domain order. So does that mean that at that time the domain was called the Domain of Terra?
76 or 100 does not sound right for an Order of Terra, not even as an order for a mere client state (since the Old Earth Union would not have been an Imperial client state in 76 or 100); nor does it seem likely that the domain would have been named the Domain of Terra in 76.
However, the domain (and its order) could have been renamed after 588, presumably as a result of the political strife between the Solomani and the Vilani factions at court. The order seems to have become moribund by 900. That suggests a marginalization of the Domain of Terra following the creation of the Solomani Autonomous Region in 704.
GURPS Rim of Fire has the Old Domain Capital in Dispora Sector so my guess would be the Sector Capital of Dispora.
Could you provide a page reference? Is the capital of Diaspora mentioned by name?
The capital of Diaspora Sector is Libert (1109 Diaspora).
By the Classic Era, but capitals can move. The old headquarters of the Order of Sol is on Exeter (2729 Diaspora). That suggests that Exeter could have been the capital at one point.
The chronology could be something along these lines:
76 -- Domain of Diaspora formed along with the other domains (Feel free to suggest other names (Domain of Rim? Rim Domain? :file_28

?? -- Order of Diaspora formed.
588 -- Terra and the other worlds in the Old Earth Union joins the Imperium.
??? -- Domain of Diaspora renamed the Domain of Terra and its order renamed accordingly. (This would be some time before Zhakirov married Antiama in 679 and broke the power of the Solomani faction at court).
704 -- Solomani Autonomous Region formed.
900 -- Order of Terra becomes moribund.
1002 -- Domain of Terra and the Order of Terra renamed.
Comments? Suggestions?
Hans