Hubble, shuttle, humans and future exploration
The telescope's achievements are immense. Few instruments in the history of science have had quite the same "wow factor"; and anyone who looks at its iconic pictures cannot fail to question - just a little bit - their place and significance in the grand scheme of things.
As we've come to expect on such occasions, Nasa has released a suitably stunning image to celebrate the birthday.
The photo shows just a small portion of the Carina Nebula, a colossal birthing cloud for new stars in our galaxy. The pillars of dust and gas which dominate the scene are about three light-years long. Amazing.
The version rendered on this page uses data acquired by Hubble's Wide Field Camera 3, which was installed on the last shuttle servicing mission and promises a raft of further discoveries as the telescope moves into its twilight years.
How appropriate that Hubble was launched on the "Discovery" shuttle. Its commander for that mission was Loren Shriver, with whom I managed to exchange a few words this week.
The deployment mission sticks out in my mind because of the issue Loren and his crew encountered with one of the solar arrays.
These panels were supplied to Nasa as part of the 15% European contribution to Hubble and were manufactured in my home town, Bristol, at BAe.
So as you can imagine, there was much pride in that fact... but also a little consternation when the roll-out of one of the arrays didn't go quite as planned.
As Loren recalls, the problem had nothing to do with the arrays themselves, but rather an error in the software managing the unfurling process:
"It all came down to a sensor and a software routine. If the sensor sensed that there was too much tension being placed on the array when it was being unrolled then it would cut off the motors to try to prevent any damage to the solar array. It turned out the software routine was the culprit. It was getting triggered somehow and cutting off the motors when indeed there was no excess tension. Someone worked it out in the end, and a command was sent to the computer from the ground to bypass that part of the software; and then the solar array rolled out to its full extension.
"But you know, we had Bruce McCandless and Kathy Sullivan suited up in the air-lock ready to go out and do the job manually. They were almost at vacuum in the air-lock and another five minutes they would have been outside. As a consequence, of course, they missed the release of the telescope because we did it almost immediately."
I promised I would occasionally dig around in the BBC archive for items of interest, and on this special occasion I've pulled up the former BBC science correspondent James Wilkinson's report on this solar array roll-out incident. James catches perfectly the drama of the moment.
The shuttle is inextricably linked to the story of Hubble. Without the shuttle, Hubble wouldn't have enjoyed the longevity it has; and there's no doubt the observatory has been one of the best advertisements for the reusable spaceplane's capabilities.
But both will soon become history. Hubble's last servicing mission should give it at least four years of further operational life. For shuttle, however, the end is even nearer, with the Discovery obiter itself due to make the last flight later this year.
Both space-borne astronomy and human space exploration are on the cusp of major change - although perhaps not on totally separate paths.
The great space observatories of the future will increasingly be sent a long distance from Earth, to the so-called Lagrangian Points - gravitational "sweetspots" in space where craft can hold station with relatively little effort.
The advantage of putting telescopes at these locations is that they enjoy very stable viewing conditions - none of the big swings in temperature and light endured by space telescopes positioned much closer to Earth, for example.
Indeed, in Hubble's early years, it had a job keeping still enough to take its snaps because those Bristol solar panels would wobble as they warmed and cooled (BAe Bristol made an improved set of arrays for the first shuttle servicing mission in 1993).
Europe's Gaia telescope will follow them in 2012; and then in 2014, Hubble's "successor" - the James Webb Space Telescope, a colossal machine about the size of a tennis court - will also make its way out to L2.
Just like Hubble, all these telescopes will experience wear and tear, and because they are not close to Earth and have not been designed for astronaut servicing - their missions are highly unlikely to last 20 years.
In the case of the far-infrared Herschel telescope, it's very probable that the observatory will go blind even before Hubble because its special detectors need to be cooled by liquid helium and this cryogen is rapidly boiling off.
All this raises the interesting question: if you gave future L2 telescopes a plug-and-play design like Hubble so they too could be serviced, would you ever consider sending astronauts that far to do the job?
It's a question I put this week to John Grunsfeld, the astronaut they call "the Hubble repairman". John flew three Hubble servicing missions, including the fifth and final mission last year.
He broadly supports President Obama's new space exploration policy. One of its key features is the so-called "flexible path" idea, which envisages robots and humans going to ever more distant and challenging locations in space.
This might require special stations being set up in space where craft can re-fuel before moving off to their next destination.
John believes this sort of thinking could make L2 servicing missions achievable.
"The fuel depot concept, although I'm not very fond of it, it does require that you have a kind of servicing architecture, perhaps with space tugs that would allow you to go out to L2, robotically at first, and refuel the cryogens to cool the detectors, or fuel the spacecraft themselves to extend their lives.
"With Hubble, that's been the huge enabler. The hundred or a thousand-factor increase in the return of exciting science has been the ability to upgrade and repair the telescope. So I think in the future, we will incorporate these features into new observatories but only if the space infrastructure allows it. [The Obama plan] is our opportunity as we redefine that infrastructure for low-Earth orbit and beyond."
If you haven't yet caught our Hubble 20th anniversary audio-slideshow with Professor Alec Boksenberg, you can find it here. Alec is one of the UK's most distinguished astronomers, and was part of the team which designed the Hubble Faint Object Camera, one of the telescope's first instruments (parts also made in Bristol).
At the time of Hubble's launch I had a radio show in Cambridge, and Alec was a regular contributor to the programme. He would update us on the telescope's early performance and how Nasa proposed to deal with its flawed mirror.
We've also asked English Astronomer Royal, Sir Martin Rees, to pen a few thoughts on Hubble. You can read them here. In addition, I've attached another item from the BBC archive - a radio feature produced by my former colleague David Whitehouse. Broadcast on the eve of the launch, the feature looks at what Hubble might achieve.
I think it's probably true to say the old telescope has exceeded everyone's expectations. Raise a glass.