The NASA Extreme Environment Mission Operations (NEEMO) 21 mission began on July 21 as an international crew of aquanauts splashed down to the undersea Aquarius Reef Base, located 62 feet below the surface of the Atlantic Ocean in the Florida Keys National Marine Sanctuary. The NEEMO 21 crew will perform research both inside and outside the habitat during a 16-day simulated space mission. During simulated spacewalks carried out underwater, they will evaluate tools and mission operation techniques that could be used in future space missions, including journeys to Mars. (Photo by Karl Shreeves/NASA)

A tourist stands on the shore of a private beach in Naama Bay, Sharm el-Sheikh, south Sinai, Egypt, Sunday, November 8, 2015. Britain's foreign secretary says airport security in many cities will need to be overhauled if it is confirmed the Russian plane crash in the Sinai was caused by a bomb planted by the Islamic State group or someone inspired by the militants. (Photo by Vinciane Jacquet/AP Photo)

Police react to a suspicious vehicle near La Carillon restaurant following a series of deadly attacks in Paris, November 15, 2015. Thousands of French troops deployed around Paris on Sunday and tourist sites stood shuttered in one of the most visited cities on Earth while investigators questioned the relatives of a suspected suicide bomber involved in the country's deadliest violence since World War II. (Photo by Pascal Rossignol/Reuters)

Fan Bing Bing, Concha de Plata (Silver Shell) award winnerfor best actress for the feature film “I Am Not Madame Bovary” speaks on her telephone during the awards ceremony at the San Sebastian Film Festival, September 24, 2016, in San Sebastian, northern Spain. (Photo by Vincent West/Reuters)

Members of a cheerleading team warm- up during the Russian Cheerleading Championship of students in Moscow on November 20, 2016. (Photo by Natalia Kolesnikova/AFP Photo)

Russian artist Maria Gasanova adjusts her make-up as she finishes “The Siberian Beauty” body art work from her “The Alive Painting” series before the opening ceremony of the Cultural Universiade as part of preparation for the 2019 Winter Universiade Krasnoyarsk, in the Siberian city of Krasnoyarsk, Russia, December 9, 2016. (Photo by Ilya Naymushin/Reuters)

Women wearing traditional dresses with pins of pictures of former North Korean leaders Kim Il Sung and Kim Jong Il welcome foreign reporters on a government organised visit to the Kim Jong Suk Pyongyang textile mill in Pyongyang, North Korea May 9, 2016. (Photo by Damir Sagolj/Reuters)

(L-R) Playmates Summer Altice, Monica Sims and Heather Rae Young attend the Playboy party with TAO at Spire Nightclub on February 4, 2017 in Houston, Texas. (Photo by Christopher Polk/Getty Images for Playboy)