A fifth and final servicing mission to NASA’s venerable Hubble Space Telescope will place new financial burdens on an agency that already has had to scale back its science and aeronautics programs. It also will entail risks, both to the astronauts who will do the repair work and perhaps to the schedule for fielding a replacement for the space shuttle, which is slated to be mothballed in 2010.

And yet the decision to go forward with the mission in 2008, announced Oct. 31 by NASA Administrator Mike Griffin, was the correct call. Assuming all goes smoothly — as one must to approve any mission — the refurbishment will enable Hubble to continue its ground-breaking astronomical research for another five years, until 2013, and perhaps beyond. If Hubble’s stunning record of achievement to date is any indication, those five extra years will be worth the cost.

NASA officials put the refurbishment mission’s price tag at $900 million, roughly half of which already has been spent on the replacement hardware and to keep the Hubble engineering team together since 2004. Coming up with the remaining $450 million will not be painless — it will come largely at the expense of other science programs. But Hubble is a proven science workhorse, and in reinstating the servicing mission NASA is in fact heeding the wishes of the astronomy community.

NASA also has answered U.S. lawmakers who were not ready to let go of Hubble, particularly Sen. Barbara Mikulski (D-Md.), for whom the telescope is an important source of jobs. Having gotten their way, Mikulski and others who pushed for the servicing mission are now obligated to spare no effort in securing the extra $1 billion in emergency funding for NASA next year proposed by Mikulski and Sen. Kay Bailey Hutchison (R-Texas).

Hubble, launched in 1990, is without question NASA’s most productive science program ever. Practically useless until the first repair mission back in 1993 corrected a faulty set of optics, Hubble has since generated thousands of scientific papers, directly observed a planet outside the solar system, provided astronomers with new insights into the origins of the universe and phenomena such as black holes, and offered breathtaking views of distant galaxies and supernovas to the public. The latter point should not be taken lightly: It is a good bet that many of today’s young or aspiring astronomers chose that career because of what they had seen from Hubble.

The repair mission is not merely to extend the observatory’s on-orbit life. As the National Academy of Sciences noted in its 2004 report on Hubble repair options, the telescope’s capabilities have grown with each successive servicing mission, and the 2008 upgrade will be no exception. The addition of the Cosmic Origins Spectrograph, for example, will give scientists their first detailed look at the mysterious matter that forms the underlying structure of the universe — known as the cosmic web.

When then-NASA Administrator Sean O’Keefe announced in January 2004 that the agency would not undertake the mission, he gave astronaut safety in the post-Columbia era as the reason. In short, astronauts servicing Hubble would not be able to inspect their shuttle for launch damage as thoroughly as they would if it were docked at the international space station; nor could they take refuge aboard the station should such an inspection reveal irreparable damage to the orbiter’s hull.

Under the circumstances, and given what was known at the time, Mr. O’Keefe’s decision was not unreasonable. But it was premature. Not long after it was announced, for example, retired U.S. Navy Adm. Harold Gehman, who chaired Columbia Accident Investigation Board, said complying with the safety recommendations contained in the panel’s report did not preclude a Hubble repair mission.

One of the first things Mr. Griffin did upon taking over as NASA administrator in April 2005 was to direct the Goddard Space Flight Center to begin preparations for a final Hubble repair mission. Mr. Griffin clearly recognized the unpopularity of ditching Hubble, but rather than reinstate the servicing mission outright he took the prudent approach: lay the necessary groundwork but await a thorough analysis of post-Columbia space shuttle flights before making a final decision.

During the press conference announcing that decision, Mr. Griffin said inspection and repair techniques demonstrated by the crews of the three post-Columbia shuttle flights helped convince him that the higher risk associated with the Hubble repair mission is manageable. Moreover, NASA is taking the extra step of making a second shuttle available to launch on short notice to retrieve the repair crew should their orbiter be deemed unsafe for a return from orbit.

Having a shuttle on standby will occupy well into 2008 the launch pad that NASA intends to use for its planned Ares-1 crew launcher, and the servicing mission likely will delay the first test launch of the new vehicle. But Mr. Griffin said the overall Ares-1 program will not be delayed, and in any event it is a relatively small price to pay given the science at stake with Hubble.

Mr. Griffin appears to have carefully considered the full range of risks and costs involved in going back to Hubble one more time, and carefully weighed them against the prospective benefit: another five years of research and exploration that could literally change the way humankind views the universe and its place therein.

In ultimately choosing to accept some risk and financial burden in the name of science and discovery, Mr. Griffin revalidated what NASA can and should be all about.