SUMMERLAND KEY, FLA. — NASA’s four-year-old ICESat spacecraft had been scheduled to end its mission by the close of 2007, but program officials have asked top NASA science managers to extend the mission by two to three years provided the craft’s temperamental laser altimeter remains healthy enough, ICESat officials and a NASA science manager said.

ICESat has been documenting the retreat of Earth’s ice cover by bouncing laser beams off Earth’s surface and measuring the signal’s time of return. As the ice melts or subsides, the signals take slightly longer to get back to ICESat.

Top-level NASA managers will consider the ICESat proposal as part of a review of ongoing Earth science missions under way this month. ICESat managers are scheduled to make their pitch at NASA headquarters April 26. Michael Freilich, director of NASA’s Earth Science Division, said he has not yet seen the ICESat proposal.

The ICESat team was buoyed by the recent release of the first decadal survey of Earth science priorities. The scientists who conducted the survey recommended that NASA launch a successor to ICESat, called ICESat 2 , saying the technology for ICESat 2 could be ready by 2010.

Freilich has informed Congress that the decadal survey would be the agency’s “primary guide” as it selects future missions. That blessing could make it difficult for NASA to end the current Ice Cloud and Land Elevation Satellite mission if its laser system is still returning meaningful data, a NASA official said.

“It’s way premature and ill advised to prejudge the outcome of a formal peer review analysis process that we’ve set up in the senior review,” Freilich said.

Glaciologists and other scientists who rely on ICESat are hoping for either no gap or a small gap in coverage between the current mission and the proposed ICESat 2 mission, which the decadal survey expects to cost $300 million.

“It’s important to have a time series of data,” Bob Schutz of the Center for Space Research at the University of Texas in Austin, said. Schutz heads the 20-person ICESat science team. Much of the science of ice revolves around separating short-term, seasonal changes from long-term trends, which can only be done by gathering data consistently over a long period of time, he said. “Otherwise, the short-term changes will be merged into long term,” Schutz added.

When ICESat was launched in 2003 scientists expected to receive years worth of non-stop coverage from the satellite’s three lasers. They were to be operated in succession as each wore down. That plan was abandoned when the first ICESat laser fizzled after just 36 days of operation. An independent panel disassembled a flight spare of the laser system and discovered that a manufacturing flaw in the electronics caused the first laser to burn out.

ICESat managers followed the panel’s recommendation to operate the remaining lasers at lower temperatures at the cost of some atmospheric readings. ICESat managers also agreed to turn the system on intermittently for short “campaigns” of 30 days or so. The idea was to conserve the lasers long enough to return batches of data over a five-year time span.

The strategy has worked, but ICESat is down to its last laser with the exception of some power left in its second laser. “It looks like the laser energies are holding up well enough to get another five or six ops periods,” said Jim Abshire, the ICESat laser instrument science manager.

The proposal to extend the mission calls for turning ICESat’s data on twice a year, he said, which would equate to two to three more years of the mission. Abshire expects the lasers to fade away rather than burn out, but he said there remains some uncertainly over their rate of decline.

ICESat eventually will lose its ability to see the coastal regions of Greenland and Antarctica because the atmosphere at the coasts is cloudier than it is above the center of the ice, Abshire said. ICESat officials point to the scientific discoveries the spacecraft has made in the area of climate change as a justification to continue the mission.

Most recently, scientists led by geophysicist Helen Amanda Fricker of the Scripps Institution of Oceanography in San Diego studied images of Antarctica derived from ICESat’s altimetery readings. Fricker’s team found that patches of Antarctic ice streams have sunk by as much as 9 meters.

Ice streams are similar to glaciers in that they carry ice slowly to the sea, but they are not technically glaciers because they are not squeezed between rocky valleys.

Fricker concluded that reservoirs of water perhaps 30 meters deep flow from pool to pool at the bottom of the kilometer-deep ice streams. As a reservoir loses water, the ice on top of it subsides, and the resulting divot shows up in the ICESat maps.

“The significance is that there’s a whole new system existing underneath the ice that we didn’t know about before,” Fricker said of her paper, “An Active Subglacial Water System in West Antarctica Mapped from Space,” in the March 16 issue of Science. “If you have water underneath an ice stream it’s going to flow faster. It’s lubricating it.”

That finding could have a big impact on predictions of how fast Antarctica will lose its ice under various global warming scenarios. Fricker’s finding “is an example of the kind of serendipitous discoveries that we only could have made with ICESat,” said Seelye Martin, program scientist for cryospheric science at NASA headquarters.