Phase F. It’s NASA’s clinical term for the end of a mission that has an appropriately clinical definition: The purpose of Phase F is to implement the systems decommissioning and disposal planning and analyze any returned data and samples. This succinct summary—one as cold as the vacuum through which a satellite soars—does not do justice to the time and effort required to ensure a mission’s success. And by any measure, NASA's Terra mission has been a resounding success. For more than 20 years and more than 100,000 orbits of the planet, the five sensors aboard this school bus-sized satellite have amassed a climate data record unmatched by a single spacecraft.
Aside from incidents including emergency maneuvers to avoid space debris and the occasional micrometeoroid strike, Terra’s 20th century software and hardware are, with few exceptions, working fine and all sensors are collecting science-quality data. Overall, Terra is still going strong, and could keep going. And going. And going. . . .
And, at least for the next several years, Terra’s instruments will continue to collect data and add to an ever-growing climate data record. But the eventual end of this groundbreaking mission is on the horizon. While Terra still has years of science ahead of it, planning is underway to ensure that Terra’s end will be as successful as the almost quarter-century it has spent in space.
Terra’s evolving mission
Terra is the flagship of NASA’s Earth Observing System (EOS) series of satellites, and launched on December 18, 1999, with 340 kilograms of hydrazine fuel and a straightforward mission: Gather data to better understand how Earth is changing and to identify the consequences for life on Earth due to these changes. The YouTube video at right was created to celebrate Terra's 20th anniversary and provides a wealth of information about this amazing mission.
The durability of the spacecraft and its instruments turned what was slated to be a six-year mission into a more than 20-year voyage of discovery. “Terra is in what’s called Phase E, and that’s the operations phase of the mission,” says Dr. Kurt Thome, the Terra project scientist. “We’ve been in Phase E and in extended operations for many, many years.”
A key achievement of the Terra mission team has been the maintenance of Terra’s equator crossing time. Terra was placed in a Sun-synchronous polar orbit 705 km above the planet, an orbit it shares with NASA’s Aqua (launched in 2002) and Aura (launched in 2004) EOS spacecraft. Terra’s initial orbital parameters enabled it to pass over the equator at 10:45 am, Mean Local Time (MLT). Two years into the mission, several maneuvers changed Terra’s position within this orbit to shift its equator crossing time by 15 minutes, and Terra has crossed the equator at 10:30 am, MLT, for more than 20 years with a variance of no more than two minutes. This precision enables Terra’s instruments to collect data at similar Sun angles every day, every year to form a climate data record. Detailed information about how Terra instrument data are processed and how NASA’s Earth Observing System Data and Information System (EOSDIS) archives and distributes these data is available through the links at the end of this article and on the Terra mission website.
While Terra is still in its 705 km orbit, the last orbital inclination burn occurred in February 2020. The spacecraft is now drifting and its equator crossing time is slowly becoming earlier and earlier. “We’ve not changed our orbit; what we’ve changed is where we are in that orbit,” Thome says. “We’ve left our two-minute box of time control.”
While a change in equator crossing time means a change in Sun angle when data are collected, this does not mean the end of Terra science operations. All instruments are working and will continue to collect science-quality data. In fact, as Thome points out, members of Terra’s instrument science teams have plans for observations that can be conducted at these new Sun angles and crossing times.
And for the next few years, the science team and Terra data users will reap the benefits of new data acquired using this changing Sun angle and open new doors of discovery into Earth processes. But a spacecraft can only drift for so long, and Terra has approximately 26.1 kg of hydrazine fuel left.
F is for fuel
“Not many people get to go to work and fly a spaceship for a living,” says Terra Mission Director Dimitrios Mantziaras. Mantziaras oversees the day-to-day operations of the Terra platform, including making sure all systems are working. And a major part of mission operations is fuel management.
Terra left Earth with a full tank of hydrazine, but it can’t pull over to top off the tank. The fact that for more than 20 years Terra has maintained fuel reserves to accomplish all planned maneuvers—along with a few unplanned maneuvers (such as fuel expended to avoid debris from a destroyed Chinese satellite in 2007)—is a significant space achievement.
Fuel is used for three primary types of maneuvers: inclination adjustment maneuvers to control the platform’s equator crossing MLT; drag makeup maneuvers (DMUs) to control orbital altitude, which is how high above Earth (apogee) and how close to Earth (perigee) the spacecraft goes; and risk mitigation maneuvers (RMM)/debris avoidance maneuvers (DAMs) that are used to protect the platform from collisions with known on-orbit debris or other spacecraft, which are constantly tracked by the U.S.-led multinational Combined Space Operations Center and updated three times a day.
Mantziaras notes that Terra will be allowed to continue to drift in orbit until its equator crossing time slips to 10:15 am, MLT, which is expected to occur in the fall of 2022. At this point Terra will exit the science constellation. “Once we’ve reached our mission and MLT requirement we plan to lower our orbit so we can get out of the way of the other 705 km missions [like Aqua and Aura],” he says. “We’re going to do two large burns similar to the inclination burns we did throughout the mission to get us five or six kilometers below the 705 km altitude. From there, we’re just going to drift our orbit.”
At this lower orbit, Terra will be less of a threat to any other satellites and will still have fuel reserves to execute RMM and DAM as needed. The instruments will remain on as atmospheric drag begins to pull Terra back to Earth over the next several years. Fuel remaining at this mission milestone: approximately 12 kg.
The return home
Someday over the next several years, probably after Terra exits the science constellation, NASA Headquarters will send a letter to the Terra mission operations and science teams that it’s time to initiate Phase F and terminate the mission. Like all NASA missions, Terra has a formal plan for Phase F that is constantly updated to ensure proper steps are followed. Thome notes that the full Phase F process will take two or three years. “At the end of that process everything is done, instruments are off, data have been archived, and it has been assured that people can still get access to these data,” he says.
Thome and the science teams will take care of passivating the five instruments. Once the instruments are powered down, Mantziaras and the operations team will take over to shut down the platform. “We’ll use the remaining fuel to lower our perigee and then we have a series of procedures and commands that we run to turn out the lights on Terra,” Mantziaras says. “We want to make sure we get all the fuel out. We deplete the batteries and put everything in a safe mode before we turn out the lights.”
And then, as Thome points out, things get pretty simple. “It’s fairly large and it’s fairly well-behaved, so folks who are tracking Terra will know what it is and who owned it originally; they do not have to worry about someone all of a sudden flipping a switch and doing a maneuver,” he says. “And this is where the communication comes in. By communicating with all of the other international organizations and maintaining that communication, everybody can avoid [Terra].”
But the platform still has to return to Earth. Again, this will be a simple process. After Mantziaras and his team lower the platform’s perigee as much as possible, atmospheric drag will take over to lower the spacecraft to a point where it will burn up in the atmosphere. Don’t worry about scheduling watch parties anytime soon—Terra’s final, fiery reentry is not expected to occur until more than 40 years after the platform is switched off.
Thome and Mantziaras both note that reentry risk mitigation is a major component of the Terra End of Mission Plan and any threat from the satellite’s reentry is minimal. Not only is Terra constructed of materials designed to burn up in the atmosphere, its polar orbit takes it over some of the most sparsely populated areas of the planet. “If a [shoebox-sized] CubeSat maintained its size coming in it would do much more damage than Terra could ever do in its current configuration,” Thome points out.
Data never die
Data preservation is a critical aspect of Phase F. Even after the end of the mission, all Terra data will continue to be archived, enhanced, maintained, and available without restriction through NASA’s EOSDIS. “Terra has great quality data,” says Thome. “While there might be some newer instruments, they can’t collect data in 1999.”
Thome further observes that the science team has constantly worked to enable Terra instrument data to be used with data collected by next generation instruments. Terra data will continue to remain relevant as a climate data record as NASA launches its upcoming Earth System Observatory series of missions. The NASA and competitively awarded commercial missions comprising the Earth System Observatory will build upon the EOS climate data record and provide key information to guide efforts related to studying climate change, mitigating disasters, fighting forest fires, and improving real-time agricultural processes.
“The legacy of Terra and Aqua and Aura is that we set the standard for doing a big mission with multiple instruments and doing it successfully,” says Thome. “Once we understand the data from the Earth System Observatory missions, then data from Terra and Aqua and Aura and all the data from the early 2000s will be critical to help fill in those time gaps.”
An epic legacy
Thome conservatively estimates that more than 5,000 people have been involved in Terra, many of whom, like himself and several of the instrument leads, have been part of the mission since the beginning. This does not include the millions of global data users who constantly download Terra data products using NASA Earthdata Search, cite Terra data in thousands of publications, or who benefit tangentially from the many discoveries enabled by Terra data.
“The greatest joy of my scientific career was being named the Terra project scientist,” says Thome. “To be part of [the mission] at all was a great story; to be part of its end will be bittersweet, but it will be good to see it end on good terms.”
Mantziaras also has strong feelings about the next phase of Terra, and observes that Terra accomplished everything that was asked of it—and much more.
“When you’re in charge of keeping a satellite safe it becomes special to you, kind of like a pet. It will definitely be sad to see [Terra] come to an end. Also very different because you’re changing gears from protecting it and keeping it safe to figuring out how to power it down for good,” he says. “It is absolutely an amazing feat of engineering. The folks at Lockheed Martin Valley Forge that built the platform built us an amazing spacecraft.”