This week marks 15 years since a Space Shuttle last returned from orbit and the end of NASA's Space Shuttle program.

Space Shuttle Atlantis landed at the Shuttle Landing Facility (SLF) at Kennedy Space Center 15 years ago this week, marking the final mission for the program.

STS-135 was originally designated STS-335, the Launch On Need (LON) mission for the previous STS-134 flight of Space Shuttle Endeavour. LON missions had been a feature of the Space Shuttle program following the Columbia disaster.

If a Space Shuttle was damaged, another could be launched at short notice to rescue the crew. NASA assigned Launch On Need flights separate STS-3xx designations, so the rescue mission for STS-116 was STS-317, STS-117 was STS-318, and so on. There were exceptions (a notable one was the STS-125 Hubble servicing mission, which had STS-400 ready to go if anything went wrong), but the Space Shuttle program should have ended with STS-134.

However, lawmakers and NASA managers opted to make STS-135 the final Space Shuttle mission. There could be no Launch On Need mission waiting in the wings this time around – the Space Shuttle program was being wound down – so the crew would have had to come down from the International Space Station (ISS) aboard Soyuz capsules if Atlantis could not return to Earth safely. There were also only four crew members, a figure not seen since the STS-6 mission of Challenger in 1983.

The crew was commanded by Christopher Ferguson, who would later join Boeing's Commercial Crew Program and was assigned to (although did not fly) Boeing's calamity capsule, the Starliner. The pilot for STS-135 was Douglas Hurley, who went on to fly the first crewed test flight of SpaceX's Crew Dragon in 2020. Rounding out the crew were mission specialists Sandra Magnus and Rex Walheim.

The launch occurred on July 8, 2011, and Atlantis returned from orbit on July 21, bringing the Space Shuttle program to a conclusion.

It wasn't, however, the end of crewed spaceflight for the US, although the gap that followed was considerably greater than expected or hoped. After the Space Shuttle retired, the US was dependent on Russia for getting its astronauts to and from the ISS. NASA later awarded Commercial Crew contracts to SpaceX and Boeing to restore launches from US soil, but it took SpaceX until 2020 to get the first humans to the outpost, and Boeing until 2024.

The latter's mission was marked by ignominy due to failures and faults during the mission that meant managers opted to send the capsule back to Earth empty from the ISS and have the Starliner flight test crew return in 2025 aboard a SpaceX vehicle. NASA and Boeing have yet to set a definitive date for when a crew might venture to the ISS once again aboard Starliner.

All of which, 15 years after a Space Shuttle returned from orbit for the last time, has left the US space agency in a bit of a pickle. Instead of depending on the Space Shuttle, the plan was to reduce risk and add redundancy to US capabilities with two providers, but things haven't worked out that way. To make matters worse, there is uncertainty about how much longer SpaceX will continue to fly the Crew Dragon beyond its contracted missions. The vehicle is reusable, but no more are planned to be manufactured, and SpaceX reckons that each should be good for 15 flights, with some refurbishment.

This should be enough to get through to the end of the ISS program and beyond, but in the longer term an alternative will be required. For SpaceX, this will likely be Starship.

The fate of the Space Shuttles themselves is also not completely set. Atlantis has been staged in a facility at Kennedy Space Center as if in orbit, and the California Science Center has mounted Space Shuttle Endeavour on an external tank and solid rocket boosters, as though ready for launch. The fate of Space Shuttle Discovery is, however, uncertain. Certain lawmakers would very much like to transport the retired orbiter from the Smithsonian's Steven F. Udvar-Hazy Center in Virginia to Houston, Texas.

For others, well, there's always a used Orion capsule. ®