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With the Landsat 9 launch scheduled in less than 18 months, this question about Landsat 7 was posed to the Landsat Science Team (LST) during its June meeting at the Earth Resources Observation and Science (EROS) Center.

What should be done with the old bird?

Logo of Landsat 7 20th Anniversary
Logo for the Landsat 7 20th Anniversary.

L7 has been acquiring images of Earth for 20 years now. It has a scan line corrector that’s not working. There are single point failures on the spacecraft that weren’t there at launch. And sometime in mid-2021, its orbit will fall outside of the observational window it needs to fulfill its science mission.

“We have questions,” says Keith Alberts, the Landsat Mission Manager at EROS. “We’re hoping the LST can help us come up with some answers.”

Originally, the orbit of the aging satellite was to be lowered by as many as 20 kilometers when its science mission ended to help prepare it for decommissioning, Alberts said. But that plan changed when it was decided to extend the mission until after the expected launch of Landsat 9 in December 2020 in case issues arose with L9 after launch. Then a second factor was added in when NASA and the USGS decided L7 was the client for NASA’s Restore-L robotic refueling mission scheduled for launch in early 2023.

As of now, L7 is orbiting the planet at 705 kilometers in a constellation with other Earth-observing satellites. Once the science mission ends, L7’s orbit will be lowered to 697 kilometers to accommodate the Restore-L refueling, though the timeline to do that remains uncertain, Alberts said.

The satellite doesn’t need to be lowered right away since Restore-L isn’t happening until 2023, Alberts said. “The main reason we’d want to lower is to get out of the constellation so we’re not causing a risk to other missions in that constellation,” he said. “But if everything’s working the way it is today, that risk isn’t really changing, so we could still collect data” even if L7 is out of the observational window for its science mission.

Today, L7 needs to cross the equator on each pass at 10 a.m. plus-or-minus 15 minutes Mean Local Time (MLT) to fulfill that science mission. Whether it is crossing the equator over Indonesia, the Congo, or Brazil, it must do so at roughly 10 a.m. each time because scientists doing research want the sun’s angle to be the same image to image, season to season, and year to year for the land surface phenomena they are studying.

The 10 a.m. plus-or-minus 15 minutes has been the standard operational mandate for L7’s two decades in space. On occasion, its thrusters have been fired in a maneuver called a Delta-I to counter the gravitational forces at the equator that cause it to slowly drift back toward 9 a.m. MLT as it circles the planet. The 20th and final such Delta-I maneuver for L7 was Feb. 7, 2017. From a fuel perspective, the thruster burns that day exhausted the satellite’s ability to do any more such maneuvers.

Based on LST advice, the USGS decided L7’s science mission would end as gravitational drift takes it past the 9:15 MLT boundary in about July 2021. That’s assuming, of course, the satellite stays healthy until then.

As part of their information gathering in preparation for making recommendations to the L7 Program Decision Board about what to do with the satellite, Alberts asked the LST if it could see any non-traditional uses for L7 data if its orbit stays at 705 kilometers but breaches the 9:15 boundary.

Similarly, is the data useful at all if the orbit is lowered to 697 kilometers? At 705 kilometers, data acquired by Landsat sensors are catalogued by Path and Row numbers under a global system called the Worldwide Reference System (WRS). If the satellite is moved out of the WRS to a lower orbit, are there still any non-traditional science uses that would be of value?

Those are answers Alberts hopes to get from the LST within the next 6 to 12 months. There’s a third question, too: What happens if Restore-L is successful?

“That one is far enough out that the answer is really going to be dependent on what the answers are to the first two questions,” Alberts says. “If the answer to both of the first two questions is, ‘No, there’s no use for the science data in those situations,’ then even if Restore-L is successful, we’ll probably just use the extra fuel to shorten the time required for Landsat 7’s re-entry into the atmosphere.”

As they started to ponder what the possibilities might be, the LST had plenty of its own initial questions and observations. Among them:

  • Is there some way to simulate what data will look like at the lower orbit, or outside of the operational timeframe, to help decide if it will be useful or not?
  • Is there a way to understand what the ground track will look like at 7 to 8 kilometers lower?
  • Will there be any focus issues at the lower altitude?
  • Is there a way to compensate for shadow angles that will be different at the lower altitude?
  • If L7 is successfully refueled and is still healthy, is there any value in it remaining in the 705-kilometer orbit?

They are all good and important questions, Alberts said. A year from now, with help from the LST, the answers should be a lot clearer.


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