Nearly 400 miles off the Massachusetts coast, a self-sailing, solar-powered, boat is bobbing along all alone. Looking like a very lonely, very miniature cargo ship, it's at the start of a voyage that will hopefully take it more than 3,000 miles across the Atlantic and into the record books.

Solar Voyager launched from Gloucester, Massachusetts, at the beginning of the month, and is headed, very slowly, toward Portugal. If it survives, it will be the world’s first autonomous surface vessel to cross the ocean, and the first to do it on solar power. It’s not the first to attempt the crossing, however, and the others have not fared well.

“Several people have tried, and they didn’t make it,” says Isaac Penny, one of the boat's builders. “A lot of things could go wrong.”

Unlike Bertrand Piccard’s upcoming transatlantic flight on the sun-powered Solar Impulse 2, the Solar Voyager has no human navigator. The computer in control is following pre-programmed GPS waypoints. Every 15 minutes, it reports its position online for everyone to see, along with data like speed, solar power generated, battery level, and local temperature.

At 18 feet long, Solar Voyager is roughly the size of an ocean kayak, and looks reasonably robust until you see it pictured next to another ship. The aluminum shell is just 2.5 feet across. Early prototypes built from plastic proved too fragile for the ocean conditions in the Atlantic, where waves can easily reach 30 feet high in a storm, and cause trouble even for cruise ships. “It’s pretty rough out there,” says Penny.

Almost all of the available upper surface of the wee vessel is given over to solar panels, 280 Watts worth. Below deck are 2.4-kWh batteries to run at night. A Go-Pro is set up to take pictures and short videos which will (hopefully) be retrieved when the boat next encounters a human. That may take a while. Solar Voyager's two propellers provide a max speed under five mph, so Penny expects the crossing to take around four months, weather dependent.

Penny and his fellow engineer Christopher Sam Soon have day jobs working on medical surgery robots. They built Solar Voyager in their free time, undertaking this voyage simply for the challenge. They kept the boat deliberately simple—less complexity means fewer parts that can fail. They skipped the sophisticated charging algorithms to maximize battery storage and allow for overnight sailing, as that would require extra sensors. As it is, the boat just charges as much as it can, when it can, and sails as far as possible overnight. Once the battery's tapped, it drifts along until the sun comes up, hopefully not too far off course.

“We have a lot of redundancy in the system,” says Penny. The solar panels are split, so if one part fails the other will still generate electricity. Thanks to dual propellers and rudders, the journey won't be skunked if any one part gets tangled or fouled. “It means that it doesn’t go as fast as it could, but it’s more likely that it will get there,” Penny says.

In 2013, a similarly autonomous and solar-powered boat dubbed Scout made it 1,300 miles from its starting point in Rhode Island, before losing all contact with its team of builders near where the Titanic went down. That's the best any autonomous vehicle's done so far, but manned crossings have been more successful. In 2007, the catamaran Sun21 made it across the Atlantic completely under solar power. In 2012, the giant MS Tûranor travelled 37,286 miles around the world powered by the Sun.

Penny welcomes the challenge, inspired by the tales of other explorers. “There was a time when I was looking for work, and I may have read too many adventure books,” he jokes.

If Solar Voyager makes history, Penny and Sam plan to fly to Lisbon to witness landfall. Penny says they're not interested in going for a round-the-world trip, but because Solar Voyager is powered by the sun, it is theoretically capable of sailing forever, at at least until something breaks or it gets swallowed by a whale. But for now, the team will settle for a little European vacation.