As NASA marked the 10th birthday of its Terra environmental monitoring satellite this past December, agency officials and climate scientists likely were counting their blessings as well. Launched in 1999 on what was supposed to be a five- to seven-year mission, Terra continues to collect reams of data: All of the $1 billion flagship’s five main instruments remain fully operational with the exception of one, which has lost only some functionality.

Terra was the vanguard of NASA’s Earth Observing System, which was envisioned as an armada of spacecraft whose data would enable scientists to distinguish between natural and human-induced causes of climate change. But as is so often the case with highly ambitious space programs everywhere, NASA’s eyes were bigger than its bank account, and the agency was forced to abandon plans to launch three rounds of dedicated spacecraft to gather a 15-year data record and rely instead on a civil-military weather satellite system to pick up where the initial missions left off.

But the National Polar-orbiting Operational Environmental Satellite System (NPOESS) has been delayed significantly by technical and management problems, and the launch of the first satellite, once scheduled for 2008, is still four years away. A precursor mission also is behind schedule, with launch now scheduled for late 2011.

NPOESS prime contractor Northrop Grumman says the program’s technical problems are now behind it, citing as evidence the delivery of a key instrument that was blamed for much of the trouble — the Visible Infrared Imager Radiometer Suite — for integration with the precursor craft. This is good news, even if that sensor won’t be able to collect ocean color data, one of 22 capabilities it was originally supposed to have. Northrop Grumman says subsequent versions of the sensor will have that capability, which is important for monitoring the biological health of the world’s oceans.

But it remains to be seen whether the NPOESS program will avoid further difficulties down the road, or how long Terra and its smaller sister satellites — Aqua and Aura, launched in 2002 and 2004, respectively — will continue to operate. NASA officials warn that even though Terra appears healthy, it could fail at any time, and the agency in any event plans to take the spacecraft out of service in 2017.

All one can do at this point is hope that Terra can hang on long enough to ensure a relatively smooth transition to the follow-on systems without leaving an irreconcilable data gap. Good engineering, execution and luck can occasionally overcome questionable planning, but only up to a certain point.