The vessel that went missing Sunday in the North Atlantic while exploring the Titanic’s wreckage is a submersible not a submarine, and there is a key difference.

The Titan, with five people on board, remained missing Tuesday even as an international search and rescue effort was underway.

The National Oceanic and Atmospheric Administration explains the difference. A submarine has enough power to leave port and come back to port under its own power.

But a submersible has more limited power and range. It needs a mother ship from which launch, to return to, and for support and communications.

The Titan’s mother ship is the Polar Prince, a former Canadian Coast Guard icebreaker.