“Explorers” is episode twenty-two of season three of Star Trek: Deep Space Nine.
Commander Sisko has returned from Bajor with a new beard as well as the blueprints to a solar sail powered vessel. The Bajorans were apparently building these 800 years ago, and there are rumours that they may have actually used them to visit Cardassia. Given that Cardassia is several light years away, though, at the very least, the likelihood of an extremely sub-lightspeed solar powered vessel managing to make the journey seems unlikely. At least, not without its crew being cryogenically frozen or simply arriving very, very dead.
Sisko has decided to build one of these vessels, using only the same materials and equipment as the Bajorans did, so that he can test it out to see how it works. The finished vessel is extremely low tech for a space vessel (one wonders just how the Bajorans got them into orbit originally) and he and Jake set sail.
Dr. Bashir, meanwhile, is looking forward to/dreading the arrival of the USS Lexington. The medical officer on board was in his year at the academy, and was the only one who beat him, graduating valedictorian, due to Julian making a stupid mistake, as mentioned in “Distant Voices” (although an impression was given that it may have been deliberate, not accidental).