It is 15 years since the last Hubble servicing mission

By the time its final servicing mission, STS-125, arrived, Hubble Space Telescope (HST) had already been in orbit for 19 years. Following the 13-day mission, the HST continued to generate prodigious amounts of science while also suffering the occasional failure.

Space Shuttle Atlantis launched on May 11, 2009, from Kennedy Space Center's Launch Complex 39A. The HST was grappled by the Shuttle's arm on May 13, 2009, setting the scene for a mission requiring five spacewalks that scientists hoped would give the old thing another five years of operational life.

By the time Atlantis launched, the HST was ailing. The plan had been to launch in 2008, but a failure of Hubble's Science Instrument Command and Data Handling (SIC&DH) unit meant a swift rejig of the payload and a shift to 2009 to give engineers time to get a replacement ready for the mission. It would, after all, be the last time astronauts laid gloves on the observatory.

The mission was hectic. The first spacewalk replaced the failed SIC&DH unit, the Wide Field Camera, and added a Low Impact Docking System just in case a future spacecraft might need to dock with the Hubble. Subsequent spacewalks replaced the gyroscopes, batteries, and other instruments. Slightly alarmingly, one astronaut, Mike Massimino, had to use brute force to remove a handrail during a spacewalk to repair an instrument that had never been intended to be fixed on orbit.

The final servicing mission left Hubble in excellent health. So much so that it has far exceeded the initial estimates for the remainder of its operational life. However, nothing is forever, and Hubble is showing its age. It is down to three gyros, one of which is showing signs of failure and the observatory's payload computer has suffered the odd glitch or two.

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While the recent solar activity might have delighted earthbound observers with an impromptu light show in the night sky, it is also a stark reminder that, without a reboost, Hubble's orbital lifetime is limited. Hubble's orbit is decaying naturally, but the process can be accelerated by additional atmospheric drag.

Thanks to the final servicing mission, the HST is equipped with hardware to permit a rendezvous. However, despite proposals from SpaceX and other commercial operators, there is nothing in the schedule for reboosting Hubble or having the spacecraft re-enter the Earth's atmosphere in a controlled fashion.

The mission that almost never happened

Initially scheduled for around the end of 2005, the Hubble repair mission was canceled in 2004 by then-NASA Administrator Sean O'Keefe in the wake of the Columbia disaster.

The justification for the cancellation was simple. Following the Columbia accident, there needed to be a way to rescue a Space Shuttle crew if their vehicle could not make a safe return to Earth. For International Space Station (ISS) assembly and resupply missions, it wasn't a problem - but a mission to the HST afforded no such safe haven.

Yet following an outcry over the decision, NASA thought again. STS-125 was back on, although now scheduled for 2008, before being delayed to 2009, and a rescue mission, STS-400 (later STS-401 and then back to STS-400 once more,) was drawn up. Just in case.

STS-400 was thankfully never needed and would have required another Space Shuttle to be launched to rescue the HST servicing crew. It would also have resulted in an abrupt end to the Space Shuttle program.

As it was, just over two more years remained before the program came to an end in 2011. ®