ESA astronaut Paolo Nespoli looks on after landing back on Earth from their Soyuz TMA-20 spacecraft on May 24, 2011 in Zhezkazgan, Kazakhstan. The mission of the Soyuz TMA-20 was launched last December to the International Space Station with the flight engineers Catherine Coleman from the United States, Russian cosmonaut Dmitry Kondratyev and Italian astronaut Paolo Nespoli. The next expedition is expected to launch this June. (Photo by Stephane Corvaja/ESA via Getty Images)

Sailors stand on the deck as the USS New York as it passes the Statue of Liberty during the start of Fleet Week May 25, 2011 in New York City. Fleet week, which has been held in New York City since 1984, celebrates the U.S. Navy and Marines Corps with a week of ship visitations and military demonstrations. Fleet Week concludes on Memorial Day with a military flyover to honor those killed while serving in the military. (Photo by Spencer Platt/Getty Images)

A gallery assistant at the Saatchi Gallery admires a sculpture by artist Dirk Skreber entitled “Untitled (Crash 1)” on May 26, 2011 in London, England. The sculpture features in the Saatchi Gallery's exhibition, “The Shape of Things to Come: New Sculpture” which showcases work from 20 international artists. (Photo by Oli Scarff/Getty Images)

Adult gentoo penguins keep an eye on their newly born chick at Edinburgh Zoo on June 10, 2011 in Edinburgh, Scotland. The gentoo chicks which started hatching in early May are looked after by both parents who regurgitate part digested food. When the chicks are hungry they peck the beaks of their parents to signal feeding time. (Photo by Jeff J Mitchell/Getty Images)

“Contrails (short for “condensation trails”) or vapour trails are artificial clouds that are the visible trails of condensed water vapour made by the exhaust of aircraft engines. As the hot exhaust gases cool in the surrounding air they may precipitate a cloud of microscopic water droplets or, if the air is cold enough, tiny ice crystals”. – Wikipedia

Photo: An airplane leaves a vapor trail as it flies in front of he moon on January 17, 2007 above Hanover, Germany. (Photo by Alexander Hassenstein/Getty Images)

Female Chimpanzee “Fifi” eats carrots as she sits in her enclosure at Taronga Zoo on her 60th birthday May 21, 2007 in Sydney, Australia. Fifi is the oldest of the Zoo's chimp population, which consists of 19 individuals. (Photo by Ian Waldie/Getty Images)

Mary McHugh mourns her slain fiance Sgt. James Regan at “Section 60” of the Arlington National Cemetery May 27, 2007. Regan, a US Army Ranger, was killed by an IED explosion in Iraq. (Photo by John Moore/Getty Images)

People walk past a downed tree in Central Park on October 31, 2011 in New York City. Central Park may lose 1,000 trees following a freak October snowstorm while the park only lost 125 trees due to Hurricane Irene. (Photo by Mario Tama/Getty Images)