Head of the Gravitational Biology Section Ruth Hemmersbach (L-R), DLR-CEO Pascale Ehrenfreundhe, ESA-president Johann-Dietrich Woerner, German Chancellor Angela Merkel and the German astronaut Alexander Gerst view a moving centrifuge in the DLR-Laboratory for Life Science Experiments during visits the European Astronaut Centre (EAC) in Cologne, western Germnay on May 18, 2016. / AFP PHOTO / POOL / SASCHA SCHUERMANNSASCHA SCHUERMANN/AFP/Getty Images

Head of the Gravitational Biology Section Ruth Hemmersbach (L-R), DLR-CEO Pascale Ehrenfreundhe, ESA-president Johann-Dietrich Woerner, German Chancellor Angela Merkel and the German astronaut Alexander Gerst view a moving centrifuge in the DLR-Laboratory for Life Science Experiments during visits the European Astronaut Centre (EAC) in Cologne, western Germnay on May 18, 2016. / AFP PHOTO / POOL / SASCHA SCHUERMANNSASCHA SCHUERMANN/AFP/Getty Images

COLOGNE, GERMANY - MAY 18: German Chancellor Angela Merkel meets the astronauts Pedro Duque, Alexander Gerst and Samantha Cristoforetti (L-R) after visiting a centrifuge in the DLR-Laboratory for Life Science Experiments during a visit of the European Astronauts Center (EAC) of the European Space Ageny (ESA) on May 18, 2016 in Cologne, Germany. Merkel is visiting the facilities, as well as the nearby German Center for Air and Space Travel (das Deutsche Zentrum fuer Luft- und Raumfahrt, or DLR) in order to promote Germany's participation in ESA's projects, which it sees is important to climate change, energy and agricultural sector research. (Photo by Volker Hartmann/Getty Images)

(FILES) This file photo taken on February 21, 2016 shows Chairman, chief executive, and co-founder of the social networking website Facebook Mark Zuckerberg speaks during a press conference presenting Samsung's new Galaxy 7 mobile device, on the eve of the official opening of the Mobile World Congress in Barcelona. Zuckerberg meets May 18, 2016 with leading US conservatives to try to head off a storm over allegations that the social network suppressed right-leaning political viewpoints. Zuckerberg proposed the meeting after technology news outlet Gizmodo last week reported allegations that Facebook was deliberately omitting articles with conservative viewpoints from a sidebar that lists popular stories. Facebook has denied the allegations, reportedly made by a former employee. / AFP PHOTO / LLUIS GENELLUIS GENE/AFP/Getty Images

Zhang Dejiang (2nd L), who chairs China's communist-controlled legislature, and Hong Kong Chief Executive Leung Chun-ying (3rd L) are shown a specialised wheelchair as they take a tour of an innovation booth at Science Park as part of an event surrounding the Belt and Road Summit being held in Hong Kong on May 18, 2016. The three-day trip to Hong Kong by Zhang Dejiang, who chairs China's communist-controlled legislature, is the first by such a senior official for four years. His visit comes as concerns grow that freedoms are under threat in semi-autonomous Hong Kong as Beijing tightens its grip. / AFP PHOTO / POOL / ISAAC LAWRENCEISAAC LAWRENCE/AFP/Getty Images

Zhang Dejiang (L), who chairs China's communist-controlled legislature, and Hong Kong Chief Executive Leung Chun-ying attend a presentation on innovation at Science Park as part of an event surrounding the Belt and Road Summit being held in Hong Kong on May 18, 2016. The three-day trip to Hong Kong by Zhang Dejiang, who chairs China's communist-controlled legislature, is the first by such a senior official for four years. His visit comes as concerns grow that freedoms are under threat in semi-autonomous Hong Kong as Beijing tightens its grip. / AFP PHOTO / POOL / ISAAC LAWRENCEISAAC LAWRENCE/AFP/Getty Images

Zhang Dejiang (2nd L), who chairs China's communist-controlled legislature, and Hong Kong Chief Executive Leung Chun-ying (C) are shown a specialised wheelchair as they take a tour of an innovation booth at Science Park as part of an event surrounding the Belt and Road Summit being held in Hong Kong on May 18, 2016. The three-day trip to Hong Kong by Zhang Dejiang, who chairs China's communist-controlled legislature, is the first by such a senior official for four years. His visit comes as concerns grow that freedoms are under threat in semi-autonomous Hong Kong as Beijing tightens its grip. / AFP PHOTO / POOL / ISAAC LAWRENCEISAAC LAWRENCE/AFP/Getty Images

Zhang Dejiang (2nd R, front), who chairs China's communist-controlled legislature, takes a tour of an innovation booth at Science Park as part of an event surrounding the Belt and Road Summit being held in Hong Kong on May 18, 2016. The three-day trip to Hong Kong by Zhang Dejiang, who chairs China's communist-controlled legislature, is the first by such a senior official for four years. His visit comes as concerns grow that freedoms are under threat in semi-autonomous Hong Kong as Beijing tightens its grip. / AFP PHOTO / POOL / ISAAC LAWRENCEISAAC LAWRENCE/AFP/Getty Images