International Space Station (ISS) Mission Commander Jeff Williams today slid quietly into the record books as he became the most experienced NASA astronaut to date. Now on his fourth space mission, the former US Army Colonel racked up 521 days cumulative days in orbit – beating former astronaut Scott Kelly's record of 520 days. When Expedition 48 comes to an end and Williams returns to Earth on September 6, that total will increase to 534 days.

Spaceflight has come a long way from the days of Yuri Gagarin, who on his historic first manned space mission in 1961 spent a total of 1 hour and 48 minutes in flight. Today, missions of several months aboard the ISS are routine with new records being set on a regular basis.

It also means that record keeping has become a bit more complicated. While Williams is now the most experienced US astronaut, the single-longest flight by NASA is still held by Kelly, who spent 340 days in space on a single ISS visit.

However, neither of these men come close to the Russian single-flight record set by Valeri Polyakov aboard the Mir space station of a 437-day visit in 1995, or the all-time cumulative space endurance record of cosmonaut Gennady Padalka, who clocked in at 879 days over five missions to Mir and the ISS.

According to NASA, before William's return to Earth along with cosmonauts Oleg Skripochka and Alexey Ovchinin aboard the Soyuz TMA-20M spacecraft, the ISS crew is scheduled to complete loading to the unmanned Dragon cargo ship for its departure on Friday as well as carrying out a schedule of scientific experiments.

Former astronaut Scott Kelly congratulates Williams in the video below.

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