NEW YORK CITY Kennedy. The Challenger. 9/11.

You may remember where you were when you heard about, witnessed on television, or maybe in person, the Nov. 23, 1963, assassination of President John F. Kennedy; the Jan. 28, 1986, explosion of the space shuttle Challenger; and the Sept. 11, 2001, terrorist attacks.A visit to Sept. 11 memorial is a gut-wrenching, emotionally disorienting and profound reminder.It is also essential.The Sept. 11 memorial was dedicated May 15, 2014. It opened to the public May 21, 2014.A visit last month to the 9/11 Museum was part of a CityPASS field trip that included the Intrepid Sea, Air & Space Museum, Top of the Rock Observation Deck at Rockefeller Plaza, Empire State Building and Metropolitan Museum of Art, with a stay at the Residence Inn by Marriott-New York Manhattan-World Trade Center.On April 17, the tip of One World Trade Center, aka Freedom Tower, was shrouded in fog. Two reflecting pools, each the size of the footprint of one of the towers, silently speak the names of those who perished when two hijacked commercial airliners were crashed into the 110-story towers; a third plane crashed into the Pentagon in Arlington, Virginia; and a fourth plane crashed in a field in Shanksville, Somerset County. The museum, which has 110,000 square feet of exhibition space, also honors those killed in the Feb. 26, 1993, World Trade Center bombing.Upon entering the museum, you're plunged into a subterranean world of somber horror. Twisted pieces of beams face you as you descend a staircase. Projected images and recordings recount the day's events. There are projections of handmade "Missing Persons" posters. "Have you seen this man?" states one. Descending further, you pass the Vesey Street stair remnant, the so-called "Survivors' Stairs."A beam struck by the plane that hit the South Tower is displayed. The twisted, rusted, bizarre shape is a modern sculpture to modern terror.The "In Memoriam" exhibition honors 2,983 victims with biographies and profiles, portraits, spoken remembrances and mementos from family members. In a room, photos of the victims are shown accompanied by reminiscences of family and friends. Photos, many of them portrait-style college or employee ID photos, line two other walls, with touch-screen tables in between for visitors to locate photos of victims.A quote by Virgil says it all: "No day shall erase you from the memory of time."The history of the World Trade Center, built 1966-70, is told. Steel trident exterior walls thrust one-quarter mile into the sky, built on Manhattan Schist. Said World Trade Center architect Minoru Yamasaki of his design in 1964: "Paramount in importance is the relation of world trade to world peace."In a gallery are timelines of all four hijacked planes' crashes (8:46 a.m. North Tower hit), television news coverage, artifacts from the scene (fire and emergency vehicles wrecked when the towers fell), personal belongings (a woman's pair of high heels, a man's glasses, wallets, children's clothes), mementos of lives lived and lost (a softball and glove, ballet slippers, needlepoint) and bizarre juxtapositions: a Pentagon sign split in two states: "Office of the Deputy Under Secretary of the Army International Affairs."The Sept. 11 memorial has more than 10,313 artifacts, including 2,136 archival documents and 37 large artifacts such as first-responder vehicles and steel from the buildings.The collection includes photographs, audio and video, personal effects and memorabilia, tributes and remembrances, recorded testimonies and digital files and websites. More than 2,900 artifact donors gave items to the museum. More than 1,970 oral histories have been recorded.The timeline gallery concludes with an analysis of the response to the 9/11 terror attacks, al-Qaida and Middle East petrol politics.Museum docents explain certain displays.You may be overcome, too. And, yet, after visiting the 9/11 museum, you may want to, and need to, visit again.The Sept. 11 memorial is a shrine, not unlike the Gettysburg National Military Park, Adams County; the Vietnam Veterans Memorial, Washington, D.C.; and so many more national landmarks that one can, and should, visit again and again.It takes courage. It's the least we can do for those whose courage was put to the ultimate test.Next week: New York City, Part 2: Intrepid Sea, Air & Space MuseumIn two weeks: New York City, Part 3: Top of the Rock Observation Deck at Rockefeller Plaza, Empire State Building, Metropolitan Museum of Art, Residence Inn by Marriott-New York Manhattan-World Trade Center, and CityPASS