For Jeff Privette, a scientist at the National Oceanic and Atmospheric Administration's National Climatic Data Center, 2011 was not a great year.

Privette keeps track of climate sensors on satellites as part of his job, and one of the climate sensor programs he follows was having issues.

This sensor, currently mounted on a satellite mission known as SORCE, tracks how much energy from the sun hits the top of Earth's atmosphere, a measure known as total solar irradiance. Climate scientists need to monitor total solar irradiance because changes in that energy quantity could affect the Earth's climate.

"The sun is a major energy input to the Earth system," Privette said. "If we don't measure the sun ... there is no chance of understanding climate change and variability."

SORCE launched in 2003, and it's now on its last legs. Even in 2011, scientists like Privette were eager for its replacement to get up and running.

But in spring 2011, the launch of a satellite called Glory, which carried the newest version of the solar irradiance sensor, failed, and Glory plummeted into the Pacific Ocean, carrying the sensor with it.

That wasn't Privette's only problem. After Glory, the next satellite scheduled to carry a total solar irradiance sensor had been part of the National Polar-orbiting Operational Environmental Satellite System (NPOESS), the over-budget, never-completed satellite effort that was canceled in 2010.

And the program put in place to replace NPOESS, known as the Joint Polar Satellite System (JPSS), was years away from launching a solar irradiance sensor.

"Because the JPSS is delayed, our ability to get a [total solar irradiance] instrument up and flying to overlap SORCE became a huge concern," Privette said. "We didn't know what to do."

Last-minute improvisation
If SORCE stopped working before another, similar instrument got up and going, scientists would have a big problem.

To ensure it is working correctly, a new instrument's measurements have to be compared with the existing ones, while both are in orbit. Especially because changes in solar irradiance are very small, this is the best way for scientists to be sure they are getting accurate data. They can tweak the new instrument, if necessary.

Without instrument overlap, there is no good way to test the new sensors. In 2011, it looked to Privette, and others in the field, like 35 years of continuous data might come to an end as SORCE slowly decayed, alone in space.

In early 2012, though, Privette's luck changed. It turned out the Air Force had some extra room on a satellite it was planning to launch in fall 2013. Now NOAA just needed a spare sensor.

That's where Greg Kopp came in. Kopp's a senior research scientist at the University of Colorado's Laboratory for Atmospheric and Space Physics. His lab had made the sensor on SORCE and had another sensor it thought could fit on the Air Force satellite.

"So they did a very quick engineering study, and by late spring of last year they realized, on paper, this looks like it could work," Privette said.

By the standards of satellite instrument procurement and launch, this was a lightning-fast transition. Privette called it "phenomenal."

This sensor, once it is launched, should provide a bridge between the SORCE sensor and its planned follow-up on the polar satellites, slated to launch in 2017. Yet while Privette and others worried about the solar sensors are understandably relieved, in many ways, the high-drama saga is more a cautionary tale than a celebratory one.

Risky business gets more so
Big satellite programs, at least in recent history, have suffered from long delays and budget overruns. These problems threaten the continuity of not just solar climate data records, but many other satellite-based monitoring systems that play a key role in human understanding of how the Earth and its weather and climate work.

"If our nation's spacecraft programs are responsible for providing important long-term records requiring continuity, maintaining planned schedules is critical," Kopp said.

Graeme Stephens, the director of the Center for Climate Sciences at NASA's Jet Propulsion Laboratory, chaired a National Academy of Sciences committee that evaluated how NOAA planned to bridge the gap between its solar irradiance sensors.

"Monitoring the whole Earth climate system continuously without gaps is fundamental," Stephens said. Yet, he added, the United States' historic approach to this has been "piecemeal" and "ad hoc."

Many satellites are reaching the end of their life span, and few are being built to take their place, observers say.

In February, the Government Accountability Office warned of impending gaps in weather satellite data and put weather satellite systems on its high-risk list due to the failure of NPOESS.

And although Landsat 8, the U.S. Geological Survey-operated satellite that monitors changes on the Earth's surface, recently launched successfully, a follow-up mission, which will take years to build, is not yet in the works. On Thursday, the National Research Council issued a report saying current management practices and the "historical pattern of chaotic programmatic support" have made the program's mission unsustainable.

"We have no real concerted or visionary approach to monitor the climate system over the long haul," Stephens added.

A crucial flip-turn in space
A quirk in how the new solar irradiance sensor will work illustrates how cobbling together last-minute solutions leads to imperfect outcomes.

Known as the TSI Calibration Transfer Experiment (the acronym, TCTE, is pronounced "tecate"), the instrument must face the sun in order to measure solar irradiance.

Yet the Air Force STPSat-3 satellite carrying it is otherwise full of sensors that gaze earthward. So once a week, the satellite will flip around and face the sun for a full orbit, capturing solar irradiance.

Once-weekly readings are not as good as continuous ones. But they are good enough to maintain a consistent record until the full-time sensor goes up, likely in 2017.

In a time of budget cuts, some in the aerospace industry are arguing for a change in the entire satellite planning and purchasing process to smaller, nimbler systems.

Colorado's Kopp, who has built small, single-purpose satellites to monitor solar irradiance, thinks that approach could be a viable way forward.

"To reliably monitor long-term inputs affecting the Earth's climate, small dedicated missions have large advantages in cost and schedule over 'do-everything' spacecraft programs," he said.

But whether satellites are big or small, the nation still needs a plan for how it monitors the Earth, NASA's Stephens pointed out. Otherwise, there will be more situations like that of the solar sensor's.

"It comes down to the need to develop a kind of coherent strategy that includes big systems, and little systems, and mini-systems, and micro-systems -- the whole sort of portfolio," Stephens said. "And we don't have that."

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500