Officials with the Landsat 7 Earth-watching satellite program have now spent almost two months struggling with a problem that significantly degrades the scientific value of images from the observatory.

An artist's concept of Landsat 7. Credit: Lockheed Martin

First discovered shortly before 2200 GMT (6:00 p.m. EDT) May 31, the failure of a crucial component on the craft's primary instrument -- the Enhanced Thematic Mapper Plus -- has left data from all of June and most of July unavailable to scientists and other users that have taken advantage of such images in the past.

The instrument's scan line corrector likely experienced a mechanical or electrical failure. Engineers have been running tests on a ground model of the device at Raytheon's Santa Barbara Remote Sensing facility, the builder of the ETM Plus instrument on Landsat 7, which has a resolution of up to about 49 feet.

The scan line corrector assembly compensates for the satellite's rapid forward motion as it collects images from orbit over 400 miles high. Without the corrector, or SLC, the forward motion remains uncorrected, Landsat 7 spokesman Ron Beck explained.

Without the help of the SLC, individual image scans overlap and also leave large physical gaps near the edge of each picture. Only portions of the image near the center are left completely unfettered and valid. Overall, about 30 percent of the total image is missing in each downlinked picture.

Controllers immediately suspended normal Landsat 7 operations and limited activity to just spacecraft housekeeping and operations related to the anomaly investigation and recovery effort.

Throughout June and most of July, teams have been tasked with two principal objectives -- to identify the cause of the SLC failure and to explore the scientific validity of data without an operational scan line corrector. Officials hope testing of a ground model of the SLC assembly will lead to a possible recovery of the instrument.

"The anomaly investigation team will look at recovery options based on the entire information gathered to date, as well as supplemental test data. It is yet not clear what the recovery may entail or when it will occur," Beck said. Testing of the ground SLC is expected to be completed by early August.

The other focus of the investigation efforts has been to look into the usage of images without the scan line corrector in case a recovery is not possible. Engineers have been able to fill in large portions of missing data in a process called interpolation.

Partial Landsat 7 image of Railroad Valley, NV showing pre-anomaly (top), post-anomaly without interpolation (middle), and post-anomaly with interpolation (bottom). Credit: USGS

Ground stations have recently collected more data, but current processing techniques cannot be applied to the affected images. The U.S. Geological Survey, which controls flight operations of Landsat 7 from the EROS Data Center in Sioux Falls, South Dakota, will archive this data for potential processing and distribution in the future.

"Our initial findings indicate that processing the data with the SLC off is possible," Beck told Spaceflight Now. "However, extensive system changes are necessary to support the data."

Once processed and on the market, however, another hurdle still remains. Images artificially enhanced still may not be usable to certain users. Scientists and others have relied on such data for a number of applications that include monitoring crops, urban areas, and natural disasters, among a number of other uses.

Applications that involve small, heterogeneous study areas are particularly affected because gaps in images can cause important losses in such cases, officials say. Small agricultural study areas and coral reef monitoring were given as possible examples.

Meanwhile, other researchers say the flawed data is still useful for their applications, especially those that involve studies of a global nature. USGS officials say they will pursue other ways to expand the scientific validity of the images.

While the Landsat 7 satellite and its valuable data remain out of reach of users, the USGS has pointed those looking for similar images to Landsat 5 and the EO-1 satellite's Advanced Land Imager -- launched in March 1984 and November 2000, respectively. An instrument aboard NASA's Terra satellite could also stand in for some Landsat 7 applications, a USGS web site says.

Costing close to $800 million total, Landsat 7 launched into polar orbit aboard a Delta 2 rocket on April 15, 1999, from Vandenberg Air Force Base, California. Now just over four years into its five-year design life, this is the "fourth incident during the life of the mission that has had the potential to jeopardize the spacecraft's ability to meet its performance requirements," the USGS said in its June monthly update.

"Each of the other incidents was investigated and resolved without significant impact to the Landsat 7 mission."

Landsat 7 continues the over 30-year tradition of Landsat satellite operations dating back to 1972 with the launch of Landsat 1. Today, Landsat 7 and 19-year old Landsat 5 operate in tandem with a number of other Earth observatories. Landsat 6 suffered a launch failure and did not reach orbit in 1993.

Officials are looking forward to the next evolutionary step in the Landsat program. Called the Landsat Data Continuity Mission, NASA and the USGS are teaming up to evaluate proposals from the private sector to provide Landsat data for at least five years past 2006.

Launch of a commercially-built, owned, and operated satellite is expected in late 2006. Formulation study contracts were awarded to Resource21 and satellite-operator DigitalGlobe in 2001 to explore designs and operations architectures. An implementation contract to a single company is expected soon.

Asked if this latest development with the scan line corrector would put the Landsat 7 follow-on mission on the fast-track, Beck said little could be done to compress the mission timeline.