For the past eight years, a NASA science satellite 1.5 million kilometers from Earth has been monitoring the solar wind 24 hours a day, giving forecasters at the National Weather Service’s Space Environment Center here about an hour’s advance warning when gusts of fast-moving, charged particles with the potential to create geomagnetic storms strong enough to knock out power grids are headed Earth’s way.

The satellite, the Advanced Composition Explorer (ACE), is more than three years past its design life. While ACE has enough fuel to last past 2020, there is no telling how much longer the satellite and its solar-wind sensors will remain in service. With no approved plan to replace ACE’s live solar-wind-monitoring capability, space weather forecasters here worry that their ability to issue timely solar-wind alerts could suddenly cease with no advance warning to users.

“Solar-wind data drive the alerts and warnings we produce for magnetic storms almost hand-in-glove,” said Joseph Kunches, the Space Environment Center’s chief of space weather operations.

While solar wind constantly buffets the Earth’s magnetic field, abrupt fluctuations in the solar wind can produce geomagnetic storms with the potential to knock out power grids.

The Space Environment Center receives transmissions from a variety of satellites that help the 41-person operation monitor solar-storm activity, according to Kunches, but only ACE has the right sensors and is properly positioned out beyond the bow shock of the Earth’s protective magnetosphere to provide early warning of incoming bursts of solar wind.

To better plan for ACE’s eventual demise, the National Oceanic and Atmospheric Administration (NOAA) — which operates the National Weather Service — is seeking public feedback on what the loss of the solar-wind data would mean. A 45-day public comment period opened April 3 and runs through May 18.

“We know these solar-wind data are important. About one million ACE solar wind files are downloaded from NOAA servers every month by nearly 25,000 unique customers,” Kunches said. “[The Space Environment Center’s] public Internet serves 4.8 million real-time solar-wind data display files every month. We need to hear from these customers about potential impacts when these data are no longer available.”

In the first week of the 45-day comment period, NOAA received 122 responses, according to NOAA spokesman Greg Romano. Approximately 90 responses were received in the first two days, he said.

NOAA has no firm plans to replace ACE’s real-time solar-wind-monitoring capability, which it paid to add to the NASA science satellite. However, NOAA has taken the first, tentative steps toward a possible replacement.

NOAA released a so-called Broad Agency Announcement in July 2005 seeking ideas for meeting its solar-wind-monitoring requirements. Two study contracts worth $200,000 to $300,000 apiece went to teams le d by Lockheed Martin Space Systems and Space Services Inc. , a Houston-based firm pursuing a commercial solar sail mission. Since those studies were completed earlier this year, NOAA officials have been evaluating them for fiscal and budgetary feasibility.

Patricia Mulligan, the space weather requirements team lead at the NOAA Environmental Satellite, Data and Information Service in Silver Spring, Md., said in a telephone interview she would be presenting her recommendations to NOAA senior management April 18. Among those scheduled to receive Mulligan’s briefing are Greg Withee, NOAA assistant administrator for satellite and information services, and D.L. Johnson, NOAA assistant administrator for weather services.

Mulligan said the Lockheed Martin team, which includes the San Diego-based Scripps Institute of Oceanography, looked at two main options for delivering the real-time solar-wind data the Space Environment Center needs: building a dedicated solar-wind-monitoring microsatellite or refurbishing the Deep Space Climate Observatory, the orphaned NASA Earth-observing satellite formerly known as Triana.

The Deep Space Climate Observatory was at one point slated to launch aboard the Space Shuttle Columbia on the fateful STS-107 research mission, but the satellite was bumped from the manifest and never found another NASA-sponsored ride. NASA notified the Deep Space Climate Observatory’s principal investigator in December that the cash-strapped space agency could no longer continue to fund the project in hope that a launch opportunity would come along.

The Deep Space Climate Observatory’s principal investigator, Scripps researcher Francisco Valero, said in a recent interview that the fully assembled satellite already includes multiple solar-weather-monitoring instruments and has always had the requirement to get solar weather data to the Space Environment Center within five minutes of its collection.

A low-energy particle instrument Space Environment Center officials would like to see included could be added to the Deep Space Climate Observatory for $2 million, according to Valero.

Like ACE, the Deep Space Climate Observatory was built to operate at a gravitationally stable location 1.5 million kilometers from Earth known as Lagrange Point 1. Valero said the costs to complete the Deep Space Climate Observatory and operate it for five years are “very reasonable” — much less, in fact, than what it would cost to launch the satellite on a dedicated rocket.

For that reason, Valero wrote NASA earlier this year proposing launching the Deep Space Climate Observatory on the same rocket as the Lunar Reconnaissance Orbiter, which is slated to launch in October 2008 on either at Atlas 5 or Delta 4 launch vehicle. NASA turned down Valero’s ride-share suggestion and announced April 10 that it intends to use the 1,000 kilograms of excess capacity to fly a $73 million lunar impactor mission proposed by NASA Ames Research Center, Moffett Field, Calif.

Valero said April 12 that even though no launch aboard the Lunar Reconnaissance Orbiter is available, he still believes the Deep Space Climate Observatory represents a cost-effective way to satisfy NOAA’s solar-wind data requirements.

Space Services Inc. President Charles Chafer did not return a telephone call placed April 12. However, Chafer has said in previous interviews that he has proposed meeting NOAA’s solar-wind data needs by adding an appropriate payload to a commercial solar sail spacecraft serving a variety of customers.

NOAA has made no commitment to funding any of the proposed mission concepts and there is no guarantee Mulligan’s recommendation will make it any further than Withee and Johnson.

At the Space Environment Center, where Kunches said he does not care how he gets the solar-wind data as long as he continues to get it, he and other forecasters are keeping their fingers crossed that ACE remains in service as a valuable early-warning system.

Ron Zwicki, the Space Environment Center’s acting director, said ACE would not be the first satellite to far exceed its design life. But while the solar sentinel could still have many productive years left, Zwicki said some instruments are showing signs of aging, making it anyone’s guess how much longer the satellite can last.

“The bottom line is the probability grows each year that something can go wrong,” he said. “We have two key instruments on there that if they go, that’s it. There’s no replacement. There is no other satellite up there right now [and] there is none in the U.S. on the board at the moment.”