OneWeb, the London-headquartered tech company trying to develop a global internet service delivered from space, reaches a key milestone on Thursday.
The firm is launching another 36 satellites, taking its in-orbit mega-constellation to 254.
Although many more are needed to complete the network, this number is enough to start offering a commercial service to a great swathe of the Northern Hemisphere.
This should start at the year's end.
"These things don't happen overnight; there's been a tremendous amount of hard work taking place over the last few months. But this launch is special," said OneWeb CEO Neil Masterson.
"This is the one that gives us connectivity from 50 degrees North latitude to the North Pole, and covers Northern Europe, the UK, Greenland, Canada, Alaska and Iceland," he told BBC News.
The latest batch of satellites is due to go up from the Vostochny Cosmodrome in the Far East of Russia.
The Soyuz carrier rocket is timed to leave the launch pad at precisely 09:48 local time (12:48 GMT; 13:48 BST). Confirmation of the successful release of all the spacecraft, together with the acquisition of their signals, is not expected to be achieved until several hours later.
It's little more than a year since the British government and the Indian conglomerate Bharti Global bid to buy OneWeb out of bankruptcy, each putting $500m (£400m) into the struggling firm.
Since then other investors have joined the project, convinced of its future viability. Chief among these is the major French satellite telecommunications operator, Eutelsat.
And Bharti's decision this week to pump a further $500m into the venture essentially completes the necessary financing ($2.4bn) to roll out the remainder of the mega-constellation - an orbiting fleet of some 650 spacecraft. This will require a further 10 launches beyond Thursday's mission, but should be done by the middle of 2022.
Of great significance to the UK market was the announcement on Sunday of a memorandum of understanding signed between OneWeb and British Telecom. The pair are to explore how they can work together, both in Britain and across the globe.