Scientific teams accompanying Lecomte, including NASA, will collect more than 1,000 water samples and study plastic pollution, mammal migration and the effect of extreme endurance events on the human body.
The mapped-out route will take Lecomte through the centre of the infamous Great Pacific garbage patch, or Pacific trash vortex, a giant gyre of marine debris the size of Texas.
Much of the research will focus on plastic pollution in the Pacific Ocean, specifically the build-up of "plastic smog" containing billions of pieces of microplastic.
"In reality the truth is much worse - the ocean is now filled with microplastics ..."
In particular, they are anxious about microplastic particles, bits measuring no more than two-tenths of an inch (5 mm), which come from large plastic trash that has fragmented into smaller pieces or microbeads in products like facial soap, body wash and toothpaste.
A crowd of around a few dozen people gathered at the beach in Choshi, a couple of hours from Tokyo, to see the swimmer off. Lecomte, wearing a wetsuit and flippers, drew cheers as he set off doing a brisk crawl. His journey is expected to last at least six months.
He plans to swim eight hours at a time, eating 8,000 calories a day and resting each evening on a support boat. He will then return to the water in the same spot the following day. I remember times when we would go on the beach and walk and never see any plastic.
For Lecomte, raising awareness of these issues is more important than setting any record.
"You have to make sure you always think about something positive", he says.
"When you don't have anything to occupy your mind, it goes into kind of a spiral, and that's when trouble starts", he said.
And he would know. When he finally reached dry land in France, his first words were "never again", but he was soon looking for a new challenge.
"I'm not an Olympic swimmer, but I am an adventurer in the way that I push my limits", said Lecomte, who became the first person to swim across the Atlantic in 1998, in just 73 days.