Two very different programs explore outer space and underwater themes.

The fantasy spin-off “Stargate Atlantis” (8 p.m., Sci Fi) debuts with a two-hour episode. A new team of Stargate travelers discovers a new base in the Pegasus galaxy, a water-soaked outpost with decided similarities to the legendary lost city of Atlantis. Along the way, they discover a put-upon race of humanlike creatures and a vicious, predatory group who feed upon them. Look for Robert Patrick (“Terminator 2”) as a guest.

• It was 35 years ago today that NASA sent Apollo 11 on its way. While the eyes of the world watched three men venture to the moon, few knew about another vitally important scientific mission launched only two days earlier. Walter Cronkite appears in the documentary “Disappearance of the PX-15” (8 p.m., Science Channel), an account of a little-celebrated 1969 NASA mission to the bottom of the sea.

On July 14 of that year, eight aquanauts boarded the 50-foot submersible craft PX-15 Ben Franklin to begin a 31-day mission to explore the Gulf Stream. In addition to the valuable oceanographic discoveries made by the vessel, the PX-15 demonstrated that aquanauts could survive a monthlong assignment in cramped quarters under dangerous conditions, not unlike those envisioned by NASA planners then setting the stages for Skylab and other space-station missions.

Tonight’s other highlights

• Cosmetics class on “Joan of Arcadia” (7 p.m., CBS).
• Scheduled on “Dateline” (7 p.m., NBC): Three individuals, all raised in the 1970s, break from their extremely religious cultures.
• A crash-landing on a carrier may have been a suicide on “JAG” (8 p.m., CBS).
• Daniel Valdez, Edward James Olmos and Tyne Daly star in the 1981 musical “Zoot Suit” (8 p.m., SiTV), which recalls the anti-Chicano race riots that plagued L.A. during World War II.