Despite steady advances in ground-based astronomy, the Hubble Space Telescope, with its famously flawed mirror, its equally famed redemption and more than two decades as an icon of world-class science, is more productive today than at any point in its history, scientists say.

Despite 23 years in the harsh environment of space, Hubble still stands at the forefront of optical astronomy, thanks to a string of dramatic shuttle servicing missions and a hard-earned understanding of the observatory's quirks and idiosyncrasies.

A dramatic photo of the famous Ring Nebula in the constellation Lyra, assembled from new observations by the Hubble Space Telescope and ground-based instruments, shows a more complex structure than previously known. The distinctive "ring" is made up of gas blown away from a massive star some 4,000 years ago.

During the last shuttle visit in 2009, two new instruments were installed and two others were repaired by spacewalking astronauts. The shuttle crew also installed a full set of gyroscopes, new batteries and a refurbished fine guidance sensor to help the observatory find and lock onto its targets.

While no one knows how long Hubble will remain scientifically productive -- and with the shuttle's retirement, there is no way to carry out any future repairs -- the observatory currently is working in near flawless fashion. Mission managers are hopeful the telescope will remain scientifically viable through the end of the decade, if not longer.

"I think we can argue, reliably, that it's the most scientifically productive observatory of any kind, ever, in history," said Ken Sembach, head of Hubble operations at the Space Telescope Science Institute at Johns Hopkins University in Baltimore.

"We're now at about 11,300 papers published in scientific journals based on Hubble data and there are nearly a half million citations of those papers. So it's a huge piece of the scientific literature and a huge reference for future work in astronomy."

To use a different metric, Sembach said research based on Hubble data results in "essentially a new Ph.D. every 10 days."

Yet another way of assessing Hubble's value to the astronomical community is to consider the number of proposals received for each new observing cycle. The latest batch included nearly 1,100 requests for Hubble observing time.

"We can typically honor maybe 180 or 200, so the over-subscription is at least five- or six-to-one in terms of proposals and it's more than that in terms of time," he said. "We typically have about 20,000 orbits requested and typically only about 3,000 or 3,500 to give out."

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Hubble stays relevant on astronomy's high frontier