New York City policemen tangled with demonstrators at a subway station on the opening day of the New York World's Fair, April 22, 1964. Youths attempted to stall the train, which was headed from the city to the fairgrounds, as a form of protest on behalf of civil rights for blacks. (Photo by Charles Gorry/AP Photo)

Pakistani students look out from a vehicle on their way to school near the Army Public School which was targeted by Taliban militants last year, in Peshawar, Pakistan, Monday, January 12, 2015. (Photo by B.K. Bangash/AP Photo)

Actress Victoria Justice arrives at the 2011 MTV Video Music Awards at Nokia Theatre L.A. LIVE on August 28, 2011 in Los Angeles, California. (Photo by Christopher Polk/Getty Images)

A Pakistani farmer guides his bulls as he competes in a traditional bull race in Khunda village in the northwestern district of Attock, some 80 kilometers from Islamabad on January 3, 2015. Tens of thousands of people gather whenever there is a bull race held in Pakistan which are usually the highlight of festivals organized in rural areas of the Asian country, and attract lots of spectators due to their thrilling nature. The traditional competition attracts landlords and farmers from all around the province where the race is held, and they all bring their fastest and strongest bulls in hopes of gaining a reputation. (Photo by Aamir Qureshi/AFP Photo)

Astronauts Harrison Schmitt (left) and Eugene Andrew Cernan practice taking geological samples at the Kennedy Space Center in Florida, in preparation for NASA's scheduled Apollo 17 lunar landing mission, on August 28, 1972. Schmitt is the Lunar Module Pilot and Cernan is the mission's Commander. They are training for their period of EVA (extravehicular activity) on the Moon. (Photo by Space Frontiers/Getty Images)

Two stags fight in a wildlife park in Aurach near Kitzbuehel, in the Austrian province of Tyrol. (Photo by Kerstin Joensson/Associated Press)

Fishermen prepare marlin and sailfish, destined for Jakarta, for auction at a fish market in Tegal, Central Java November 26, 2014 in this photo taken by Antara Foto. (Photo by Oky Lukmansyah/Reuters/Antara Foto)

The Hubble Space Telescope is shown following its release from the space shuttle Discovery Wednesday, February 19, 1997. The Hubble Space Telescope, one of NASA'S crowning glories, marks its 25th anniversary on Friday, April 24, 2015. With more than 1 million observations, including those of the farthest and oldest galaxies ever beholden by humanity, no man-made satellite has touched as many minds or hearts as Hubble. (Photo by AP Photo/NASA)