There's a "bubbling of excitement" in the team preparing for the UK's first orbital rocket launch but they are "laser sharp" as final checks are carried out, says the mission's boss.
The Start Me Up mission sets out from Spaceport Cornwall at Newquay Airport, with the launch window now starting around 10.16pm on Monday.
It will be the first orbital launch from UK soil - or anywhere in western Europe - and comes after technical issues pushed it back from before Christmas.
"Space launch is a very serious business and we'll be looking at integrating all the information about the system, about the weather, about the range," said Virgin Orbit boss Dan Hart.
There won't be the fire and noise of a NASA launch however.
Instead, a modified Boeing 747 named Cosmic Girl will carry the 21-metre LaucherOne rocket - which contains a number of small satellites - to 35,000ft before it fires into space and eventually reaches 8,000mph.
It'll happen about an hour after take off and the event will be livestreamed online.
"Right now everything is green" for launch, Mr Hart said on Sunday afternoon.
However, he added they were "proceeding cautiously" and are prepared to delay if necessary as there are other launch opportunities in the next few weeks.
"The rocket was armed yesterday [Saturday] and we'll be loading fuel later today. So we're in full motion right now," he told reporters.
With the rocket attached below the plane's wing, he said crosswinds were something they needed to keep an eye on.
Among the satellites on board are a prototype orbiting factory for making high-value alloys and semiconductors, and one to join a constellation of satellites monitoring illegal fishing, smuggling, trafficking, piracy and terrorism.
Melissa Thorpe, head of Spaceport Cornwall, said the teams involved had "absolutely smashed it" and she was feeling confident ahead of the big day.
"I just feel so excited... I cannot wait for the UK to join that exclusive launch club because it's gonna feel good," she said.
Mr Hart said putting the satellites into orbit would be relatively quick once the rocket is in motion.
"We'll do a three-minute burn on the first stage, followed by about six minutes of second-stage burn. We'll coast around Antarctica, come up around near Australia, then we'll watch the final burn and the payloads deployed."
He said "everything needs to go right" for the mission to work.
The Cornwall spaceport is the first to get a licence in the UK, but more could follow in the coming years as demand from businesses to launch small satellites increases.
"There are seven other spaceports that have been identified in the UK for both vertical and horizontal launch," said Ian Annett, Deputy CEO of the UK Space Agency.
He said Snowdonia in North Wales and Glasgow Prestwick Airport were among them and that local authorities were looking at those opportunities.
Some "futureproofing" was done at Newquay Airport to make it capable of also serving as a spaceport, said Ms Thorpe, including strengthening the taxiways and turnpads to ensure the Virgin Orbit plane can manoeuvre easier.
"When Virgin are not here we can open it up to other companies," she said.
"We have a brand new facility that's opening that will have other space companies - it's full and we haven't even opened it yet. That's how excited people are."