Philip McBride said the Â£6m investment is a large one for the company.
Astronaut, Tim Peake, was the star guest at the announcement made by First Minister Arlene Foster on Tuesday.
Invest NI are understood to be grant-aiding the expansion.
Tim Peake told a press conference it was "an absolute pleasure and honour" to be in Belfast.
He said Thales' Northern Ireland factory "is one of those locations in the UK space industry that is at the forefront of cutting edge technologies and it makes me proud to say that."
Susannah Streeter, BBC News business reporter
Thales hopes the electric propulsion technology, which will be developed in Belfast, will be a game changer for the satellite industry.
It gives engines a lot more thrust enabling the payload spent on fuel to be cut by a factor of five, which ultimately should enable satellites to be put into orbit more cheaply and be kept there for a longer period.
Thales looked at sites across Europe before deciding to set up the facility in Belfast. It's part of its expansion in the space technology sector in the UK from just 10 staff in 2014 to a projected 350 by 2018.
So is this a vote of confidence in a post-Brexit vote Britain?
CEO Victor Chavez told me it's a vote of confidence in the investment environment created by the UK government in recent years to foster the space and aerospace sector.
"The most important issue is less about any tariffs that might appear but just how committed the government is to take practical steps to change the way industry invests in technologies that are at the cutting edges of physics and engineering."
Thales, a French company, currently makes missiles in Northern Ireland for the defence industry.
Their Belfast site will be the first in the UK to produce  electric-powered rocket booster technology.
The company currently employs 20 people on their space projects but Mr McBride expects that number to grow as the ESA orders more satellites.
He said: "We are at the very start of an adventure.
"The more work the ESA places with Thales, the more work that's coming to Belfast."
Mr McBride added that the space sector is a very competitive one but that Thales now had a unique facility in the UK and and that "being first to market is always an important thing".
The head of Thales in Belfast has said the city's engineering heritage was a factor in the company choosing its Castlereagh site to manufacture electric-powered rocket boosters for the European Space Agency (ESA).