A friend of one of the passengers on board the submersible vessel that went missing on a trip to explore the wreckage of the Titanic said that "every single minute" felt like hours as time ticked away to find them alive.

Speaking from Svalbard in Norway on Tuesday, Jannicke Mikkelsen said she was not sleeping and just hoping for good news, as rescuers scoured thousands of square miles in the remote North Atlantic to find her friend Hamish Harding, 58, and four others.

One pilot and four passengers were inside the submersible early on Sunday when it lost communication with a ship on the surface about an hour and 45 minutes into its dive to see the wreckage of the Titanic in deep waters off Canada's coast. The submersible was the highlight of a tourist expedition that cost $250,000 per person.

Mikkelsen said Harding was "being branded as a UK billionaire going for a sightseeing trip to the Titanic."

But, she said, expeditions were also for scientific research.

"These types of expeditions are very expensive and we need people like Hamish who can pay and sponsor such an expedition, but also take the risk of joining such an expedition with his expertise," Mikkelsen said.

Mikkelsen said she met Harding in 2017 while working at NASA's Kennedy Space Center. Together they took part in 2019's "One More Orbit" flight mission that set a record for the fastest circumnavigation of Earth by aircraft flying over both geographic poles.

The Titanic expeditions start in St. John's, Newfoundland, before heading out approximately 400 miles (640 km) into the Atlantic to the wreckage site, according to the website of Oceangate, the private company that operates the submersible vessel.

In order to visit the wreck, passengers climb inside Titan, the five-person submersible, which takes two hours to descend approximately 12,500 feet (3,800 m) to the Titanic.

Mikkelsen said she last spoke to Harding right before his dive to the Titanic and wished him "godspeed."

"I didn't consider that this type of expedition would be as dangerous as it's turned out to be," she said.

"As explorers, we are pessimistic and objective. And as it stands right now, it would be a miracle if they are recovered alive."