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Resupply missions in space: Why does NASA want to refuel Landsat 7? And what does it mean?
Landsat 7 is the seventh satellite of the Landsat program, launched on April 15, 1999. Its main goal is to update the global satellite image archive and provide the latest, cloud-free imagery. The program is managed and operated by the United States Geological Survey (USGS). Through the NASA WorldWind project, users can freely navigate and view 3D imagery from Landsat 7 and other sources. The satellite's companion satellite, Earth Observing-1, followed into the same orbit one minute after launch, but in 2011 its orbit began to degrade as it ran out of fuel.
Landsat 7 was designed to last five years, but has been in operation for more than two decades, demonstrating its resilience and continued functionality.
Landsat 7 was built by Lockheed Martin Space Systems and has the capacity to collect and transmit 532 images per day. It operates in a 705 km polar, sun-synchronous orbit, scanning the entire Earth's surface. The satellite weighs 1,973 kg, is 4.04 meters long and has a diameter of 2.74 meters. Unlike its predecessor, Landsat 7 has 378 Gbits of solid-state storage, equivalent to about 100 images. Its main instrument is the Enhanced Thematic Mapper (ETM+), an oscillating-mirror scanner for image detection.
Instrument Characteristics
Landsat 7 is equipped with a variety of instruments, including:
- Full-color channel with 15-meter spatial resolution (Channel 8)
- Visible light channels with 30m spatial resolution (Channels 1-5, 7)
- Thermal infrared channel (Channel 6) with 60m spatial resolution
- Full aperture, 5% absolute radiometric calibration
Scan line corrector failure
On May 31, 2003, the Scan Line Corrector (SLC) of the ETM+ instrument failed. The function of SLC is to compensate for the forward motion of the satellite in order to obtain parallel scanning images. Without the help of SLC, the instrument images in a zigzag manner, potentially resulting in some areas being imaged repeatedly and some areas not being imaged at all. This means that approximately 22% of the Landsat 7 scenes were missing when the SLC was not functioning properly.
Despite the data gaps, an assessment conducted by USGS showed that the imagery can still be used for many scientific applications.
Applications of satellite images
In August 1998, NASA commissioned EarthSat to produce Landsat GeoCover, an accurate dataset of orthorectified Landsat imagery covering most of the Earth's landmass. The contract is part of NASA's Science Data Acquisition Program and is managed by NASA's John C. Stingness Space Center. This dataset was later upgraded to EarthSat NaturalVue, a natural color simulated Landsat 7-derived imagery that is still used as the basis for many web mapping services.
Orbital Lowering and Future Possibilities
Landsat 7 requires regular orbit maneuvers to ensure that the Land Time (LMT) for data collection is maintained within the required range. The last orbital maneuver took place on February 7, 2017, after which the satellite's orbit began to slowly decrease. By 2021, its ground transit time has been postponed from 10:00 AM to approximately 9:15 AM. With the launch of Landsat 9 on September 27, 2021, Landsat 7 is scheduled to be retired.
ConclusionNASA is currently exploring the possibility of using the OSAM-1 robotic servicing mission to refuel Landsat 7 to ensure its successful decommissioning and transform it into a calibrated instrument to extend its scientific utility.
As Landsat 7 nears the end of its service, it will be critical to see whether other satellites will continue its three-decade mission of imaging, and more importantly, how that data will help us better understand and respond to climate change. Challenges?
