A C-17 Globemaster III crew prepares an air refueling mission September 27, 2012, at Joint Base Charleston, S.C. (Photo by Airman 1st Class Matthew J. Bruch/U.S. Air Force photo)

Newly commissioned 2nd lieutenants throw their caps in the air as a team of F-16 Air Force Thunderbirds fly over the 2011 graduating class of the U.S. Air Force Academy on May 25, 2011 in Colorado Springs, Colorado. A total of 1,021 graduates received their diplomas in front of their families and dignitaries at the Academy's Falcon Stadium. (Photo by John Moore/Getty Images)

Maria Ines Banegas portraying “La Pirata” (The Pirate) takes part in a human living statues contest in Buenos Aires, Argentina, September 24, 2016. (Photo by Enrique Marcarian/Reuters)

Valentin Arias Delgado and Diana Franco Durango, from Colombia, perform their routine at the Stage style Tango World Championship, in Buenos Aires, Argentina, August 31, 2016. (Photo by Enrique Marcarian/Reuters)

Hot air balloons are prepared for a flight as others take off during an international hot air balloon festival at Maayan Harod National park in northern Israel September 30, 2015. (Photo by Baz Ratner/Reuters)

A man photographs the main engines of the space shuttle orbiter Discovery on display at the Udvar-Hazy Smithsonian National Air and Space Annex Museum in Chantilly, Virginia August 28, 2015. Shuttle Discovery had 27 years of service and flew 39 times before being retired in 2011. (Photo by Gary Cameron/Reuters)

A policewoman wipes her tears as she stands in front of metal caskets containing the bodies of Special Action Force (SAF) police who were killed in Sunday's clash with Muslim rebels, at Villamor Air Base in Pasay city, metro Manila January 29, 2015. Philippine President Benigno Aquino urged legislators on Wednesday not to abandon a plan for autonomy for Muslims to end a decades-old insurgency after the clash in which dozens of people were killed, saying doing so would dash hopes for peace. (Photo by Romeo Ranoco/Reuters)

Employees work at a sorting centre of Zhongtong (ZTO) Express ahead of the Singles Day shopping festival, Chaoyang District, Beijing, November 8, 2015. On China's giant Singles Day internet shopping festival, the country's delivery firms are stretched so thin that they are looking for tie-ups, listings and new investors to husband their resources. (Photo by Jason Lee/Reuters)