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The Kelly Astronaut Twins' Spaceflight History, by the Numbers

NASA Astronaut Twins: Scott and Mark Kelly
Scott Kelly (left) and his identical twin brother Mark at the Cosmonaut Hotel in Baikonur, Kazakhstan on March 26, 2015, shortly before Scott launched on a one-year mission to the International Space Station. (Image credit: NASA/Bill Ingalls)

Former NASA astronauts Mark and Scott Kelly will be honored by their hometown of West Orange, New Jersey, today (May 19). You can watch a livestream of the dedication ceremony on the West Orange Public Schools' Facebook page.

The Kelly brothers — the only set of twins ever to fly in space — will receive "Mayoral Medals," and the elementary school they attended (currently called Pleasantdale Elementary) will be renamed after them during a ceremony in West Orange. Scott Kelly recently returned to Earth in March, ending a nearly yearlong mission to the International Space Station. Mark, who retired from NASA in 2011 (Scott retired last month), helped scientists on Earth during the mission by participating in NASA's Twins Study, an unprecedented project that meticulously monitored the identical twins to see exactly how the human body changes during ultralong space missions.

Here's a brief rundown of the identical twins' spaceflight history, along with a biographical tidbit or two.

Date of birth: Feb. 21, 1964

Who is older, and by how much: Mark (6 minutes)

Year selected to NASA astronaut corps: 1996

Year retired: 2011 (Mark), 2016 (Scott)

Who flew to space first? Scott (1999)

Total time in space (both twins): Approximately 574 days

Individual time in space: Approximately 54 days (Mark) and 520 days (Scott; this is the current American record)

Total spaceships flown: 4. Space shuttle Discovery (Mark 2 times and Scott 1 time); space shuttle Endeavour (Mark 2 times and Scott 1 time); Russian Soyuz spacecraft (Scott 2 times); International Space Station (Scott 2 times).

Total spaceflights: 8. Mark: STS-108 (2001), STS-121 (2006), STS-124 (2008), STS-134 (2011). Scott: STS-103 (1999), STS-118 (2007), International Space Station (ISS) Expedition 25/26 (2010-11), ISS Expeditions 43-46 (2015-16).

Missions commanded: 5. Mark: STS-124, STS-134. Scott: ISS Expedition 26, ISS Expedition 45, ISS Expedition 46.

Spacewalks performed: 3 (all by Scott)

"Return to Flight" mission flown, after loss of space shuttle Columbia: 1 (Mark, STS-121)

Space toilets replaced: 1 (Mark, STS-124)

Telescopes repaired: 1 (Hubble Space Telescope, Scott, STS-103)

Christmases spent in space: 3 (all by Scott)

Editor's note: and its partner are media supporters of the Pleasantdale Elementary School renaming ceremony to honor the Kelly astronaut brothers.

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Elizabeth Howell
Elizabeth Howell is a contributing writer for who is one of the few Canadian journalists to report regularly on space exploration. She is pursuing a Ph.D. part-time in aerospace sciences (University of North Dakota) after completing an M.Sc. (space studies) at the same institution. She also holds a bachelor of journalism degree from Carleton University. Besides writing, Elizabeth teaches communications at the university and community college level. To see her latest projects, follow Elizabeth on Twitter at @HowellSpace.