Few science instruments have enjoyed the star status of the Hubble Space Telescope (HST).
Hubble needs regular maintenance
Since its launch 15 years ago, it has captured some of the most profound - and widely distributed - images of the Universe.
Its ethereal shot of a shimmering Eagle Nebula is as likely to be found on the side of a bus as in the pages of an astronomy textbook.
And, like a celebrated icon, Hubble has now become the focus of controversy.
Its future is up for grabs. And as scientists discuss how - even whether - to service the ageing telescope and prolong its life, the debate over using a human or a robot to do so has grown contentious.
While the US space agency (Nasa) talks tentatively of sending robots to service Hubble, a panel of the National Research Council (NRC), the principal operating agency of the National Academy of Sciences (NAS), has recently recommended that astronauts make the repairs.
Now, another vote has been cast in favour of sending humans to do the job.
In a statement released on Tuesday, the American Astronomical Society (AAS), the largest professional astronomical organisation in North America, said it endorsed the NRC position.
Nasa, it stated, should pursue a manned mission to repair Hubble and launch it as early as possible after the space shuttle is ready to fly again.
The AAS called Hubble "a remarkable instrument for scientific discovery", and supported the NRC's conclusion that a shuttle mission presented the lowest risk.
Is it safe?
The NRC panel comprised a group of distinguished astronomers, engineers, former astronauts and Nobel laureates.
Their December report concluded that an unprecedented robotic mission might not be developed in time to save Hubble, would be more technologically risky, and could critically damage the telescope.
It also found that the differences between the risks faced by a crew to service Hubble and those of a single shuttle mission to the International Space Station (ISS) were small.
"[The NRC] came down very clearly saying that if you really care about the space telescope, then doing the shuttle mission is the best answer," Harvard University Astronomer and AAS President Robert Kirshner told the BBC News website.
Congress directed Nasa to request a study from the NRC to look into HST servicing options after Nasa cancelled what was to be the fifth, and final, shuttle-servicing mission to the telescope.
Nasa administrator Sean O'Keefe, citing safety concerns, announced the decision to cancel the shuttle flight shortly after President Bush announced his space initiative and said that shuttle flights would be used only to service the ISS, as a step to putting humans on the Moon and Mars.
Since then, the space agency has looked at robotic options to repair Hubble's fading gyroscopes and batteries. The telescope is also slated for a new camera and spectrograph. With these repairs, Hubble's observing life could extend to 2013.
Although it recently signed a contract with a Canadian firm, MacDonald Dettwiler (MD) Robotics, to develop a servicing robot, Nasa has not fully committed to a robotic mission.
At the AAS meeting in San Diego last week, in a session outlining the options for Hubble, Col Mark S Borkowski, Nasa Project Manager for Hubble Robotic Servicing, said that 2005 would be used to firm up a plan.
"We have probably as many questions as you have about what is the future of Hubble, what is the best way to maximise the future of Hubble and what are our options," he said.
The agency is investigating the feasibility and cost of developing a robotic mission, and also the alternative of leaving Hubble to its fate.
Hubble needs an urgent upgrade
While the robotic technology developed by MD Robotics was encouraging, said Col Borkowski, it might neither justify its estimated $175-$300m price tag, nor be viable in time to save the telescope.
A preliminary design review of the robot is scheduled for March, with a critical review to follow later in the year.
Ultimately, he said, Nasa would decide whether a robot mission was worth it.
Plan B is not to service Hubble and bring it down at the end of its observing life. In this case, a robot could also be developed to de-orbit the instrument.
While Dr Borkowski stressed that the NRC recommendation of a shuttle flight would be a "key input" into the ultimate decision about repair, he also reiterated that Nasa had no plan to conduct shuttle servicing.
A thorny subject
This prompted pointed responses from some astronomers in the audience. Rodger Thompson, a University of Arizona astronomer and principal investigator of the Near-Infrared Camera and Multi-Object Spectrometer, which astronauts placed on Hubble in 1997, reiterated the cogency of the NRC finding.
"I have to say that you - Nasa - have a solution in hand and you have not taken it.
"You've created your own problem by refusing to use the low-cost option with small incremental risk," Dr Thompson said. "It's really up to you to reverse that, and make the decision to do the shuttle mission."
He later told the BBC News website that Hubble was specifically designed for shuttle servicing, and that four prior astronaut missions to the telescope were "very successful".
Dr Kirshner also asked the panel why, given that the best bet for saving Hubble was a shuttle mission, according to the NRC, "there was no planning at Nasa for a shuttle mission".
"I didn't say there was no planning," said Col Borkowski, "I said there is no plan."
If Nasa is parsing its words to both pursue a course of action for Hubble, and deftly keep its options open, it does so at time of great uncertainty and restructuring for the space agency.
Nasa is shuffling priorities and funding to accommodate the President's back-to-the-Moon initiative and is in leadership limbo as Mr O'Keefe prepares to step down.
Nasa officials said that the decision not to pursue a shuttle mission to service Hubble ultimately rested with Mr O'Keefe.
"A new administrator will have final decision in where we go," said Nasa spokeswoman Susan Hendrix, of the Goddard Space Flight Center.
Two out of three
As the clock ticks for Hubble, the observing carries on.
The telescope's science instruments are working well, with the exception of the Space Telescope Imaging Spectrograph (STIS), which shut down in August, said Rodger Doxsey, head of the Hubble Mission Office at the Space Telescope Science Institute (STScI), which operates Hubble for Nasa.
While Hubble awaits its tune-up, Nasa scientists weigh options for battery and gyroscope conservation.
"Our goal is to get the longest lifetime that we can," said Dr Doxsey. "We don't want to lose science waiting for the servicing to take place."
The ultimate lifetime limiters for Hubble are its batteries, which, like those in a mobile phone, lose their ability to recharge over time. Without servicing, the batteries will give out in 2007-08.
The gyroscopes keep the telescope stable, which anyone who has tried to steady a telephoto lens can appreciate. Hubble has six gyroscopes. Two have failed, but it needs only three to control it.
Anticipating further failures, Nasa Goddard would test a control mode next month for science to proceed on two gyros, said Dr Doxsey.
Astronomers petitioning for time with Hubble must consider the two-gyro mode in their proposals.
Meanwhile, the plans for a robot mission are underway. The proposed candidate, designed by MD Robotics, is a slightly modified version of Dextre, the dexterous robot built to service the ISS.
Looking like a human stick figure with arms akimbo and a long grapple arm in place of legs, the robot is designed for complex manoeuvres in space and has been tested at Goddard on a mock-up of Hubble.
Its charge in space is to replace the telescope's batteries, change the connectors on the gyroscopes, and install two new instruments: the new Cosmic Origins Spectrograph, which is sensitive to very faint UV light, such as that from intergalactic gas in the early Universe, and the Wide Field Camera 3, optimised for infrared, ultraviolet and visible wavelengths.
Dr Thompson said that, although he was impressed with the robot's performance at Goddard, an unprecedented robotic repair of Hubble was too risky to be pursued single-mindedly.
Given that a return to flight was inevitable, he said, and that many flights were necessary to complete construction of the ISS, routing one to extend the life of a scientific instrument that has already proven so valuable was reasonable.
"One flight to the Hubble Space Telescope will produce far more science than 25 flights to the ISS," he said.
Great Hubble discoveries were still to come, agreed Dr Doxsey, and efforts should be made to encourage them.
"We want to think twice about turning off a telescope that is in its prime," he said.