A jet airliner leaves a vapor trail as the planet Mercury is seen, lower left quadrant, transiting across the face of the sun in Las Vegas, Nevada, May 9, 2016. Mercury passes between Earth and the sun only about 13 times a century, with the previous transit taking place in 2006. REUTERS/David Becker - RTX2DIB8

Undated NASA Messenger probe images obtained on September 29, 2011, shows a large crater with a floor partially covered by large numbers of Coalesced Hollows. Mercury may have a lot in common with Earth, but close-up images and data captured by NASA'S MESSENGER probe this year show it's still a bit of a planetary weirdo. REUTERS/NASA/Handout

An American flag is silhouetted as the planet Mercury is seen, lower left quadrant, transiting across the face of the sun in Las Vegas, Nevada, May 9, 2016. Mercury passes between Earth and the sun only about 13 times a century, with the previous transit taking place in 2006. REUTERS/David Becker - RTX2DIB9

A handout picture shows NASA's Messenger spacecraft approaching Mercury on January 14, 2008, capturing this view of the planet's rugged, cratered landscape illuminated obliquely by the sun. This image was taken from a distance of approximately 11,000 miles, about 56 minutes before the spacecraft's closest encounter with Mercury. It shows a region 300 miles across, including craters less than a mile wide. REUTERS/NASA/Handout

The top of the Stratosphere tower is silhouetted as the planet Mercury is seen, lower left quadrant, transiting across the face of the sun in Las Vegas, Nevada. Monday, May 9, 2016. Mercury passes between Earth and the sun only about 13 times a century, with the previous transit taking place in 2006. REUTERS/David Becker - RTX2DIB7