COMING UP:

The 3 Most Flown Space Shuttles of NASA's Fleet

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SPACE.com is counting down to the launch of NASA's last space shuttle mission, STS-135 aboard Atlantis. Coming Wednesday: The Only 2 Female Space Shuttle Commanders.

Over the course of 30 years, NASA's space shuttles have played a critical role in constructing the International Space Station, launching and servicing satellites and observatories in orbit, and delivering astronauts and many tons of cargo into space.

As the agency prepares to launch Atlantis this Friday (July 8) on the very last mission of the program, here's a look at NASA's three most flown space shuttles: Discovery, Atlantis and Endeavour.

Currently, through 134 flights, NASA's five orbiters have combined for a staggering total of 537,114,016 miles (864,401,218 kilometers) traveled and have logged 1,320 days in space. [8 Surprising Space Shuttle Facts]

The agency tragically lost two of its shuttles – Challenger and Columbia – in accidents that stunned the nation. Since 2003, NASA has maintained a three-orbiter fleet.

Challenger broke apart 73 seconds after launch on Jan. 28, 1986, at the beginning of the orbiter's 10th flight. Seven astronauts died in a horrific accident caused by a faulty seal on one of the shuttle's twin solid rocket boosters, which was aggravated by exceptionally cold weather.

NASA lost the shuttle Columbia and its seven-astronaut crew on Feb. 1, 2003, as the orbiter attempted to return home at the end of its 28th flight – the STS-107 mission. An investigation later revealed that a piece of foam insulation from Columbia's fuel tank broke off during launch and impacted the orbiter's left wing. The shuttle's damaged heat shield failed to protect the vehicle as it reentered Earth's atmosphere, and Columbia broke apart as it flew over Texas.

Both catastrophes brought NASA's space shuttle program to a temporary halt as operations and safety procedures were modified. Yet, the accidents helped shape the agency's future, and the shuttle program would go on to persevere through the misfortunes. [Space Planes: Evolution of the Winged Spaceship ]

NASA's fleet leader: Discovery

Discovery, the veteran of the fleet, is NASA's oldest and most traveled shuttle. After 27 years and 39 flights, Discovery has logged 365 days — a full year — in space, and journeyed 148,221,675 miles (238,539,663 km). Over its career, Discovery also made 5,830 orbits of the Earth.

Discovery made its maiden launch on the STS-41D mission in August 1984. The workhorse orbiter lifted off on its final flight, STS-133, on Feb. 24, 2011. The busy 13-day mission to the International Space Station capped off Discovery's prolific career, which includes the deployment of the Hubble Space Telescope in April 1990.

Discovery was the first shuttle to visit the International Space Station in May 1999, and would return to the outpost another 12 times. NASA's fleet leader also performed both return-to-flight missions after the shuttle program was grounded following the losses of Challenger and Columbia.

"I think the legacy that this shuttle has made for herself is just nothing short than cause for celebration," STS-133 mission specialist Michael Barratt said during Discovery's final flight. "Just something that our country should be very, very proud of."

Discovery was the first of NASA's space shuttles to be retired. The orbiter landed at Kennedy Space Center in Florida for the final time on March 9. Following a months-long decommissioning process, Discovery will eventually be placed on public display at the SmithsonianNational Air and Space Museum's Udvar-Hazy Center near Washington, D.C.

Rounding out the shuttle program legacy - Atlantis

Atlantis is NASA's second most flown orbiter, and is the vehicle that will bring the agency's iconic shuttle program to a close at the end of its STS-135 mission. Atlantis was first launched on Oct. 3, 1985 on the STS-51J mission. [Counting Down to NASA's Last Shuttle Flight]

Up to and including its most recent flight, STS-132, Atlantis has traveled 120,650,907 miles (194,168,813 km). The orbiter has made 4,648 orbits of the Earth and to date, has spent 293 days, 18 hours, 29 minutes in space.

Atlantis' STS-135 mission will be the orbiter's 33rd flight, and will tack on 12 days and many more miles to these current figures.

Among Atlantis' many accomplishments, the shuttle made the first docking to the Russian Mir space station in June 1995, delivered the space station's Destiny laboratory, Columbus laboratory and Quest airlock and portions of the station's backbone-like truss.

Atlantis' STS-125 mission, which launched on May 11, 2009, was also the last flight to service the Hubble Space Telescope.

"I looked a little bit into Atlantis' history and it's very interesting," Atlantis' STS-135 commander Chris Ferguson said in a preflight interview. "It was actually the most frequent visitor to Mir; I didn’t know that we had a Phase 1 program in the ’90s where we visited the Russian Mir space station and Atlantis made seven of the 11 trips back and forth, so Atlantis really became the cornerstone of docking operations with an orbiting space vehicle, and of course it factored heavily into the construction of the International Space Station; last Hubble servicing mission; there's a lot of claims to fame for Atlantis, and we’ll try to send her off in good fashion."

Following its STS-135 mission, Atlantis will be showcased at the visitor center at Kennedy Space Center in Cape Canaveral, Fla.

NASA's good ship Endeavour

Endeavour is the youngest shuttle in NASA's orbiter fleet, and was built as a replacement for Challenger. The shuttle flew 25 missions over the course of its spaceflying career, logging a total of 122,883,151 miles (197,761,261 km) on its odometer.

Endeavour launched on its maiden flight, STS-49, on May 7, 1992. Its most recent mission, STS-134, was the agency's second-to-last shuttle flight, making Endeavour the second orbiter in NASA's fleet to be retired.

NASA's youngest shuttle launched on its 25th and final mission on May 16, 2011 to deliver extra supplies and a $2 billion astrophysics experiment to the International Space Station. Endeavour landed at Kennedy Space Center for the final time on June 1.

Over the course of 25 missions, Endeavour made 12 visits to the space station and delivered and attached the station's first U.S. segment, the Unity node, in December 1998.

"It's very bittersweet," pilot Greg Johnson said in an in-flight interview during Endeavour's STS-134 flight. "I just love this vehicle. I fell in love with this vehicle the first time I got to fly, three years ago on STS-123. This vehicle is a wonderful machine and it's an honor and a privilege for each one of us to be a part of her final flight."

At the end of its career, Endeavour recorded 299 days in space, and made 4,671 orbits of the Earth. Endeavour is now being prepared for its future on display at a museum. The orbiter has been promised to the California Science Center in Los Angeles.

You can follow SPACE.com Staff Writer Denise Chow on Twitter @denisechow. Visit SPACE.com for complete coverage of Atlantis' final mission STS-135 or follow us @Spacedotcom and on Facebook.

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