With less than six weeks to go before its scheduled launch atop a Falcon 9 booster out of Vandenberg Air Force Base, Calif.—the first SpaceX mission from the West Coast in more than a year—the Sentinel-6A Michael Freilich ocean-monitoring satellite is ready for its long-awaited shipment to the United States. Prime contractor Airbus Defence and Space recently announced that the 2,600-pound (1,190 kg) spacecraft will be loaded aboard a cargo aircraft at Germany’s Munich Airport later this week to begin its transcontinental journey across ten time-zones to reach Vandenberg on the Pacific Coast. Liftoff of the mission, which forms a constituent part of Europe’s Copernicus Earth observation program, is currently targeted for 11:31 a.m. PST (2:31 p.m. EST) on 10 November.
Part of a collaborative effort between NASA, the National Oceanic and Atmospheric Administration (NOAA), the Centre national d’études spatiales (CNES, the French national space agency) and the European Space Agency (ESA), together with the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) and the European Commission, Sentinel-6A is one of two identical oceanography missions to be launched over the next five years.
Its twin, Sentinel-6B, is targeted to fly in 2026. Equipped with radar altimeters and microwave radiometers and supported by Global Navigation Satellite System (GNSS) precise-orbit determination receivers, Doppler tracking antennas, laser reflector arrays and radio-occultation instruments, the twins will examine changes in sea-level, weather and ocean circulation, as well as climatic variability manifested through such phenomena as El Niño and La Niña.
“Global sea-level rise is, in a way, the most complete measure of how humans are changing the climate,” said Dr. Josh Willis, the mission’s project scientist at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif. “If you think about it, global sea-level rise means that 70 percent of Earth’s surface is getting taller: 70 percent of the planet is changing its shape and growing. It’s the whole planet changing.”
Originally designated Jason-CS (“Continuity of Service”), the Sentinel-6 twins follow in the footsteps of four U.S./European oceanography missions stretching back almost three decades. First was TOPEX/Poseidon, a collaborative U.S/French endeavor, launched in August 1992, which remained operational for over 13 years and was described by one scientist as the most successful ocean experiment ever conducted.
Next came Jason-1, launched in December 2001 and deactivated in July 2013. More recently, Jason-2 rose from Earth in June 2008 and was decommissioned last fall. And currently in active service, Jason-3—which roared aloft atop a SpaceX Falcon 9 booster in January 2016—is approaching the end of its minimum five-year lifespan.
Decades of space-based and ground-based observations have revealed that Earth’s surface temperature continues to rise at an alarming rate, according to NASA. Our oceans play a key role in stabilizing the climate by absorbing 90 percent of trapped heat from excess “greenhouse gases”, like carbon dioxide, which have been emitted into the atmosphere in copious quantities since the start of the Industrial Revolution. As the oceans heat up, their volume expands, increasing the volume of water and also melting ice sheets and glaciers, which contributes further to sea-level rises.
This has continued unabated for over a quarter-century and TOPEX/Poseidon and the three Jasons demonstrated that Earth’s oceans are rising by an average of 0.1 inches (3.2 mm) annually, with evidence in the last few years of a global increase to almost 0.2 inches (4.8 mm). This has forced humanity to face the inevitable consequences: from the effects of flooding to coastal erosion and storm hazards to negative impacts upon marine wildlife. The Sentinel-6 twins will continue these ongoing measurements, examining the oceans down to scales on the order of millimeters, through at least 2030, to afford scientists a 40-year continuous “set” of data.
But the mission’s emphasis is not solely upon sea-level measurements. The twins will also assist with weather predictions, assess atmospheric change and collect high-resolution vertical profiles of temperature and humidity. Their radar altimeters will map up to 95 percent of Earth’s ice-free oceans every ten days, examining currents, wind speeds and wave heights for maritime safety and the protection and management of increasingly busy coastal regions. And they will afford scientists the capability to understand smaller oceanic features (such as complex currents), which carry important benefits for navigation and worldwide fishing communities.
Airbus Defence and Space received the 177-million-euro ($208 million) contract to build Sentinel-6A back in December 2014, with an expectation that the spacecraft would be inserted into a near-polar orbit some 825 miles (1,330 km) high. By September 2019, Sentinel-6A was structurally complete at Airbus’ facility in Friedrichshafen, Germany, and was despatched to the IABG analysis and test center in Ottobrunn, near Munich, for several months of mechanical, thermal and acoustic testing.
For the spacecraft, the acoustic tests were particularly jarring. Four 60-second blasts of sound were fired by huge loudspeakers with increasing intensity. At their peak, they reached 140 decibels, significantly higher than the ear-splitting level of 110 decibels produced by a pneumatic drill or chainsaw.
The spacecraft’s final months on Earth have been further complicated by the worldwide march of the COVID-19 coronavirus pandemic, which meant far fewer engineers were present in the IABG “clean room” and most of the mission personnel were forced to follow the progress of the testing remotely. “Remarkably, we have reached an important milestone completing the acoustic vibration tests, which simulate the noisy environment of liftoff and ascent through the atmosphere,” said Pierrik Vuilleumier, ESA’s Copernicus Sentinel-6 mission project manager. “This just shows how the team is determined to meet the launch date in November, despite the difficult circumstances.”
Last January, Sentinel-6A was officially renamed in honor of ocean physicist Dr. Michael Freilich, who retired in August 2019 as head of NASA’s Earth Science Division. In a touching tribute, NASA Administrator Jim Bridenstine related the heartache of learning of Dr. Freilich’s cancer diagnosis. “We’ve got to do something quickly,” he remembered telling Associate Administrator of the Science Mission Directorate, Dr. Thomas Zurbuchen. “I want him to know that all the work he’s done is extremely meaningful.” Although Dr. Freilich lived long enough to learn that Sentinel-6A would be named for him, he sadly died in August.
“This honor demonstrates the global reach of Mike’s legacy,” said Mr. Bridenstine in January. “We are grateful for ESA and the European partners’ generosity in recognizing Mike’s lifelong dedication to understanding our planet and improving life for everyone on it. Mike’s contributions to NASA and to Earth science, worldwide, have been invaluable and we are thrilled that this satellite bearing his name will uncover new knowledge about the oceans for which he has such an abiding passion.”
“It would be an honor if we could name that spacecraft after you,” Dr. Zurbuchen recalled asking Dr. Freilich. “Would you accept that?”
“I’ll be selfish,” replied Dr. Freilich, undoubtedly with a glint in his eye. “Absolutely!”