Taking the pulse of our planet's health and detecting clues needed for weather forecasts have been the chief tasks for Earth-orbiting weather observatories over the past four decades, and that legacy will be extended this week when the latest spacecraft blasts off from California.

Workers install the first half of the Delta rocket's nose cone around NOAA-N at the launch pad last week. Photo: Russ Underwood/Lockheed Martin

The NOAA-N satellite is slated for liftoff at 1022 GMT (6:22 a.m. EDT; 3:22 a.m. PDT) from Space Launch Complex-2 West at Vandenberg Air Force Base atop a Boeing-built Delta 2 rocket.

"When it launches, NOAA-N will not only be our eyes above the Earth, but our eyes into the future," said Gregory Withee, assistant administrator for the NOAA Satellite and Information Service.

"Because it will strengthen our understanding about what the environment around the world is doing, not just here in the U.S., NOAA-N will bring us one step closer to truly global coverage of Earth's complex processes," added NOAA Administrator Conrad Lautenbacher, Jr.

The 3,130-pound spacecraft -- to be renamed NOAA-18 once safely in orbit -- is the fourth in the current series of five Polar Operational Environmental Satellites with improved imaging and atmospheric sounding capabilities that will operate to the end of this decade. The program has a heritage that dates back to the dawn of the space program.

After entering service later this summer, the Lockheed Martin-made satellite will replace an aging sister-craft, NOAA-16, launched in September 2000, ensuring an uninterrupted flow of data such as imagery, temperature measurements and atmospheric profiles that are the building blocks of weather forecasts.

"Today, satellites provide more than 99 percent of the observations used in NOAA's operational weather and climate prediction numerical model runs. Recognizing that, we can't understate the importance of satellite data in the success of these forecast models," said Louis Uccellini, director of the NOAA Centers for Environmental Prediction.

"Since NOAA-N will be operational by late summer, it will help us to develop the outlook for the upcoming fall and winter."

The satellites also build long-term databases for climate monitoring and global change studies.

"Data from NOAA's polar-orbiting satellites are essential to the success of our weather and seasonal forecasts and El Nino and La Nina forecasts," said Uccellini.

While meteorologists use the data gathered by the craft to generate weather predictions, agricultural scientists need the information for drought management and monitoring vegetation and soil moisture, and the aviation community uses NOAA satellites to detect and track volcanic ash plumes and re-route aircraft as needed.

This newest spacecraft becomes the catalyst to developing the Global Earth Observation System of Systems, Withee said.

"GEOSS was established by an agreement signed by 60 nations just recently, including the United States and European Commission. (The program) commits to linking existing environmental monitoring technology into one system to better predict a whole host of issues to the benefit to mankind, including weather, climate and natural catastrophe," he said.

"The challenge before us is to connect with scientific dots and technology dots an integrated, international, comprehensive Global Earth Observation System of Systems so that we have a complete picture of the global environment. NOAA satellites, including NOAA-N, will play a significant role."

This illustration depicts a NOAA polar-orbiting weather satellite in space. Photo: NASA

Like its predecessors, NOAA-N is fitted with the international Search and Rescue Satellite-Aided Tracking System, called COSPAS-SARSAT. Over the past 23 years, spacecraft equipped to hear distress signals from emergency beacons are credited with saving almost 5,000 people in the United States, and more than 18,000 worldwide.

In the first four months of 2005, NOAA said 36 people were rescued in the U.S. thanks to the vital link between the satellites and beacons carried by airplanes, boats and hikers. Last year, Alaska had the most rescues, with 37, while Florida had 36.

"These beacons help save lives," Withee said. "As temperatures get warmer, and outdoor activities increase in remote areas -- where cell phones don't work -- having one of these emergency beacons is a good safety practice."

"Beacons remain one of the most reliable means of signaling a distress to search and rescue personnel," said Lieutenant Commander Jay Dell from the Coast Guard's Office of Search and Rescue. "The timeliness and accuracy of SARSAT alerts are extremely valuable to search and rescue planning and response."

The weather satellites detect a beacon's transmission and alert ground controllers in Suitland, Maryland. From there, the signal is forwarded to a Rescue Coordination Center operated by the U.S. Coast Guard for maritime emergencies or the Air Force for land search and rescue situations, according to NOAA.

Wednesday's launch will mark the first NOAA weather satellite to ride a Delta 2 rocket into space. Recent spacecraft in the series flew aboard refurbished Titan 2 Intercontinental Ballistic Missiles that were originally built to carry nuclear bombs.

The Titans lacked the necessary thrust to deliver the satellites into their 540-mile high orbits. In fact, the Cold War relics deployed the satellites on sub-orbital trajectories, forcing the craft to carry onboard solid-fueled kick motors to generate the critical final boost to achieve orbit.

"There is tremendous benefit for using the Delta 2 because the Delta 2 is a rocket that was specifically designed to deliver a satellite to orbit. That is what we are getting from the Delta 2. Previously, we launched on rockets that were trying to deliver weapons," said Karen Halterman, NASA POES project manager at Goddard Space Flight Center.

"Once we separated from the Titan, we had to have our own solid rocket motor to fly the satellite into orbit. It is risky and excessive. So we believe that the reliability of NOAA-N is much improved by going to a Delta 2, which is a very proven launch vehicle."

NOAA-N is the 119th flight of the workhorse Delta 2 rocket, which debuted in 1989. The booster has performed successfully for the past 63 consecutive launches since 1997 and 116 times overall in its history.

The construction of NOAA weather satellites, getting them launched into space and performing the initial on-orbit checkout falls under NASA's responsibility. Control of the spacecraft is handed from the civilian space agency to NOAA about three weeks after liftoff.

The NOAA-N mission is valued at $341 million, which includes $160 million for the satellite structure, $71 million for the instruments and $65 million for the Delta 2 rocket.

You can follow Wednesday's early morning countdown and the ascent to orbit in our live Mission Status Center.