More than a year since the roar of powerful Merlin 1D+ engines last rolled across the West Coast of the United States, a brand-new Falcon 9 booster took flight from Space Launch Complex (SLC)-4E at Vandenberg Air Force Base, Calif., early Saturday to deliver the long-awaited Sentinel-6A ocean-monitoring satellite into orbit. Named in honor of the late Dr. Michael Freilich, former head of NASA’s Earth Science Division, it is one of two identical spacecraft which will bring a battery of radar altimeters, microwave radiometers and other instrumentation to bear on our planet’s oceanic circulation, weather and sea-levels.
After lifting off smoothly at 9:17 a.m. PST (12:17 p.m. EST), the B1063 core completed the third Return to Launch Site (RTLS) at Vandenberg, touching down on Landing Zone (LZ)-4 a few minutes later, as its second stage propelled Sentinel-6A into orbit. Sentinel-6A was deployed into space about 53 hours after launch. “The satellite will begin solar panel deployment about one hour and seven minutes post-launch,” NASA noted in its pre-flight update, “and is expected to make first contact about 25 minutes after that.”
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 will be joined by its twin, Sentinel-6B, in about 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 three decades. First was TOPEX/Poseidon, a 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 Falcon 9 on only its second Vandenberg mission 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 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 sent to the IABG analysis and test center in Ottobrunn, near Munich, for six months of mechanical, thermal and acoustic testing, which ended in May 2020.
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 tests 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 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 told Associate Administrator of the Science Mission Directorate (SMD), 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.
A month after his passing, the 2,600-pound (1,190 kg) spacecraft was loaded aboard an Antonov An-124 cargo transport at Munich Airport and airlifted across ten time zones to reach Vandenberg on the Pacific Coast. Containers holding Sentinel-6A and its critical support equipment were then offloaded onto trucks and driven 15 miles (25 km) to the SpaceX Payload Processing Facility.
During the course of the next several weeks, fueling and pressurization of the spacecraft were completed and earlier this month it was encapsulated in its bulbous Falcon 9 payload fairing. Completion of the Flight Readiness Review (FRR) on 16 November was followed by the rollout of the 230-foot-tall (70-meter) Falcon 9 booster for a customary Static Fire Test of the nine Merlin 1D+ first-stage engines last Tuesday. It was then returned to the Horizontal Integration Facility (HIF) for installation of the payload fairing and returned to the pad late Friday.
And the booster itself has been the source of a recent spate of delays. During an attempt to fly another Falcon 9 mission with the GPS III-04 payload in early October, a dramatic abort and engine shutdown in the final seconds before liftoff prompted a raft of fleetwide inspections. This led to two engines being replaced on B1063 and a resultant hit on Sentinel-6A’s targeted launch date, which slipped from 10 November until the 21st.
With an 80-percent probability of acceptable weather, this morning’s launch occurred on time, marking the fifth occasion in 2020 that as many as three Falcon 9s have flown in a single calendar month. It was SpaceX’s 16th mission out of Vandenberg since September 2013 and its first in more than a year. The SLC-4E site last echoed to the roar of nine Merlin engines in June 2019 with the launch of the three-spacecraft Radarsat Constellation Mission (RCM). B1063 performed admirably on its maiden outing, burning smoothly uphill for the first 2.5 minutes, before separating and returning smoothly to a “land” landing.
Since “Just Read the Instructions”, previously assigned to West Coast recovery operations, was transferred to the East Coast earlier this year in response to the growing tempo of missions from Florida, there was no ASDS availability for the Sentinel-6A launch. As such, the returning B1063 core was directed to Landing Zone (LZ)-4 at Vandenberg, the repurposed SLC-4W pad which previously supported 93 Atlas-Agena and Titan rocket launches between July 1963 and October 2003.
More than a decade later, the site was leased by SpaceX as a landing pad. Including this morning’s landing, only three Falcon 9s—the others being the SAOCOM-1A booster in October 2018 and last year’s RCM booster—have returned to alight at LZ-4.
With its first and only Vandenberg mission of 2020 thus accomplished, a full plate awaits SpaceX on the West Coast next year, although only NASA’s Double Asteroid Redirection Test (DART) has a definitive launch date of 22 July. SpaceX was awarded the $69 million launch services contract for this important mission last year and DART seeks to demonstrate the capability of a spacecraft to “deflect” asteroid by impacting it at kinetic velocities.
After launch, DART will intercept the potentially hazardous near-Earth asteroid Didymos, which is part of a binary system with its tiny “moon” Dimorphos. It is this latter object, measuring only 560 feet (170 meters) across, that DART will impact in late 2022.
Other Vandenberg launches planned for SpaceX next year include a pair of SmallSat “rideshare” payloads in June and December and a pair of DigitalGlobe WorldView Legion Earth-imaging satellites in the second half of 2021. The latter are expected to double DigitalGlobe’s ability to collect high-resolution 12-inch (30 cm) imagery and triple the available capacity over high-demand areas.
Additional Vandenberg missions include the first members of Germany’s SARah fleet of reconnaissance satellites and the classified NROL-87 for the National Reconnaissance Office. SpaceX won NROL-87 in early 2019, as part of a $297 million, three-mission contract. Under the terms of that contract, NROL-87 is required to be placed into orbit by December 2021.