Two ambitious science missions—one circling Earth closer than any other research satellite, the other orbiting a position in space more than 930,000 miles (1.5 million km) from us—will breathe their last breath this week as commands are transmitted by the European Space Agency (ESA) to shut down the Gravity Field and Steady-State Ocean Circulation Explorer (GOCE) and the Planck deep-space observatory. Launched within weeks of each other, back in early 2009, the missions could hardly have been more different in their scientific targets, objectives, and operating locations, but both are kindred spirits in that they have accomplished far more and have survived far longer than could have ever been anticipated.
GOCE’s “End of Days” has drawn inexorably closer over the course of the last year, partly due to its remarkably low altitude and also due to a steadily dwindling supply of propellant for its ion engine. Launched in March 2009 atop a Rokot booster from the Plesetsk Cosmodrome in northern Russia, the spacecraft formed part of ESA’s Living Planet program and sought to map Earth’s gravitational field with a highly sensitive Electrostatic Gravity Gradiometer (EGG) of three accelerometers. To achieve its mission and achieve the maximum possible resolutions, the arrow-shaped GOCE spacecraft was injected into a low orbit of about 160 miles (260 km), and its ion engine—carrying 88 pounds (40 kg) of xenon—was tasked with compensating for the continuous deceleration induced by atmospheric drag. Ion propulsion was also regarded as a better alternative to chemical propulsion, which could trigger disturbances and impair the spacecraft’s observations.
And those observations have centered on making high-resolution measurements of terrestrial gravity gradients along three axes. A Global Positioning System (GPS) receiver and laser retroreflector also helped to compensate for non-gravitational forces upon GOCE and enabled it to be tracked by ground-based laser systems. As well as determining gravity field anomalies at spatial resolutions of better than 62 miles (100 km), the spacecraft sought to make measurements of the “geoid”—the shape of the “ideal” global ocean at rest, which is considered critical for accurate estimations of ocean circulation and sea-level change—with an accuracy of just 0.4-0.8 inches (1-2 cm). Since launch, GOCE has proven its worth in the exploration of volcanic regions, the measurement of polar ice-sheet thickness, and the understanding of ocean behaviour, including currents and heat-transport processes. Two years ago, in March 2011, it became the first orbiting instrument to detect the devastating Tōhoku earthquake and subsequent tsunami which hit Japan.
However, it has not been entirely smooth sailing for GOCE. In early 2010, it endured a computer fault and a serious communications system malfunction, which caused the spacecraft to stop downlinking scientific data to receiving stations. Nevertheless, ESA successfully restored normal operations in September 2010, and a few weeks later the mission was extended until the end of 2012. GOCE’s xenon tank was expected to be empty about 20 months after launch, but unusually low solar activity produced a calmer atmosphere and less severe drag on the spacecraft and the mission was extended far longer than intended. During its final year of activity, steps were taken to steadily reduce the altitude of GOCE’s orbit from 158 miles (255 km) to just 146 miles (235 km) to enhance its ability to generate images of smaller-scale oceanic features, including eddy currents, but it was recognized that xenon levels were only sufficient to keep it operating for a few more months. Earlier this year, in May 2013, its orbit was reduced yet further to 142 miles (229 km), placing it “at the lowest altitude of any research satellite” currently in operation.
Finally, on 18 October, Christoph Steiger, ESA’s GOCE Operations Manager at the European Space Operations Centre (ESOC) in Darmstadt, Germany, revealed that the end of the mission was in sight. “An important milestone has just been reached,” he wrote, “with the pressure in the fuel system of GOCE’s ion engine dropping below 2.5 bar, which is the nominal operating pressure required to fire the engine.” Estimating that about 0.7 pounds (350 g) of xenon remained, Steiger added that the exact onset of “orbital decay” would depend upon the performance of the engine beyond its normal pressure range. “If it kept functioning down to a pressure of 0 bar, corresponding to a completely empty tank, the orbital decay would start around 26 October,” he continued. “However, we expect the ion engine to terminate quite some time before that.”
At length, on Monday, 21 October, ESA announced that “the mission came to a natural end when it ran out of fuel” and noted that “the satellite is expected to re-enter Earth’s atmosphere in about two weeks.” It was noted that data acquisition and operations will continue during these two weeks, until such time that harsh environmental conditions at low altitude cause GOCE’s systems to stop working. At this stage, the spacecraft systems will be shut down. “This innovative mission has been a challenge for the entire team involved: from building the first gradiometer for space to maintaining such a low orbit in constant free-fall, to lowering the orbit even further,” said Volker Liebig, ESA’s Director of Earth Observation Programmes. “The outcome is fantastic. We have obtained the most accurate gravity data ever available to scientists. This alone proves that GOCE was worth the effort and new scientific results are emerging constantly.”
Alongside the end of operations of GOCE, ESA also plans to shut down its long-serving Planck space observatory today (Wednesday, 23 October), after more than four years completing no fewer than five all-sky surveys of the Cosmic Microwave Background (CMB) radiation—the “afterglow” of the theorized Big Bang—and refining estimates of the age of the Universe and its rate of expansion. Named in honor of German physicist Max Planck (1858-1947), who won the Nobel Prize for Physics in 1918 for his work on quantum theory, the space observatory was boosted into space atop an Ariane 5 rocket from the Guiana Space Centre in Kourou, French Guiana, in May 2009. Two months later, it reached its operating location in a so-called “Lissajous orbit,” circling the Earth-Sun L2 Lagrangian Point, about 930,000 miles (1.5 million km) from the Home Planet. At this location, Planck occupied a region at which the gravitational influences of Earth and the Sun are balanced and was far away from the disruptive effects of our planet on its sensitive instruments.
Planck’s instruments consist of a High Frequency Instrument (HFI) and a Low Frequency Instrument (LFI), which were intended to perform at least two all-sky surveys of CMB radiation, but had actually completed no less than five surveys by January 2012. At that point, the HFI’s supply of helium-3 refrigerant was exhausted, making it inoperable, but this did not prevent Planck investigators from utilizing the LFI to execute three more surveys for data refinement. Science operations finally ended in early October 2013.
The mission follows in the footsteps of two previous NASA missions, the Cosmic Background Explorer (COBE)—whose science team won the Nobel Prize in Physics in 2006—and the Wilkinson Microwave Anisotropy Probe (WMAP). Operating at microwave and infrared wavelengths, Planck was described by participating JPL scientist Krzysztof Gorski as “the Ferrari of cosmic microwave background missions.” Gorski stressed that the spacecraft fine-tuned the technology from COBE and WMAP to achieve more precise results. “For a car, that can mean an increase in speed and winning races,” he said. “For Planck, it results in giving astronomers a treasure trove of spectacular data and bringing forth a deeper understanding of the properties and history of the Universe.”
The age of that Universe, and its estimated rate of expansion (known as the “Hubble constant”), have been refined by Planck. An all-sky map of CMB distribution was publicly released in March 2013, with data suggesting that the cosmos is around 100 million years older than previously thought. Slight temperature fluctuations pointed to the formation of “ripples” when the Universe was only 370,000 years old, which may have given rise to galactic clusters and the formation of dark matter. Planck’s data pegged the estimated age of the cosmos at 13.798±0.037 billion years and offered new insights into its expansion, providing a revised rate of 67.15±1.2 km/sec/megaparsec. This is a little less than previous estimates from both the Hubble and Spitzer Space Telescopes.
“The data also show there is less dark energy and more matter in the Universe than previously known,” explained NASA’s Jet Propulsion Laboratory (JPL) of Pasadena, Calif., which contributed to both the Planck mission design and the instruments. “The map, based on the mission’s first 15.5 months of all-sky observations, reveals tiny temperature fluctuations in the Cosmic Microwave Background, ancient light that has traveled for billions of years from the very early Universe to reach us. The patterns of light represent the seeds of galaxies and clusters of galaxies we see around us today.”
In spite of its enormous success, it was realized that the steady depletion of Planck’s helium coolant would eventually spell the end of the mission. This has proven considerably complex, for ESA understood the significance of the L2 Lagrangian Point for the emplacement of future scientific spacecraft and was keen to remove Planck from the region and dispose of it appropriately. “This includes ‘passivating’ the spacecraft,” said Steve Foley, ESA’s Spacecraft Operations Manager at ESOC in Darmstadt, “and placing it onto a disposal trajectory that will keep it in a parking orbit around the Sun, well away from the Earth-Moon system for hundreds of years.”
Together with the highly successful Herschel space telescope—which exhausted its supply of coolant and was deactivated earlier this year—Planck is one of only two ESA missions to be located at L2. Andreas Rudolph, responsible for astronomy mission operations at ESOC, noted that the “scientifically valuable” nature of the L2 location meant that it was “important that we set a positive precedent as to how we dispose of missions there.” On 9 October 2013, Planck performed a two-day maneuver to move itself away from L2 and commence a slow drift away from Earth. On Monday, 21 October, mission controllers fired Planck’s thrusters to empty its fuel tanks and ensure that it ends its mission “in a permanently safe configuration.” The final shutdown of the spacecraft is expected today (Wednesday), with Planck’s on-board software reprogrammed in a such a way that it will be unable to automatically reactivate its transmitters. Batteries will be disconnected and on-board protection mechanisms will be disabled by mission controllers.
“The final step,” explained Steve Foley, “will be the simple act of switching off the transmitters. We will witness the silencing of Planck and we will never receive a signal from her again. This is important because we cannot cause radio interference for any future mission.” According to ESA, the final command will be sent Wednesday by Planck’s Project Scientist Jan Tauber. “While the end of this outstanding scientific mission was always foreseen with the exhaustion of helium coolant,” said Paolo Ferri, ESOC’s Head of Mission Operations, “it seems fitting that we have a colleague from the science team to send the final command that once and for all silences the Planck spacecraft.”