All good things come to an end and the European Space Agency’s Herschel Space Observatory mission is no exception. After more than three years in orbit, the most powerful infrared telescope ever flown in space has ceased scientific operations after the last of the liquid helium used to supercool its instruments ran out.
Launched from the Guiana Space Center in French Guiana on 14 May, 2009 atop an Ariane 5 rocket, the Herschel was sent into what is called a Lissajous orbit 1,500,000 kilometers (930,000 miles) from Earth. In such an orbit, the spacecraft circles, not the Earth, but the Lagrangian point L2 – a spot where the gravitational forces of the Earth, Moon and Sun balance out.
M31 - Andromeda Galaxy in infrared, x-ray and optical (Image: ESA/Herschel/PACS/SPIRE/J.Fritz, U.Gent/XMM-Newton/EPIC/W. Pietsch, MPE/R. Gendler)
The 3,300 kilogram (7,300 lb) unmanned observatory boasts a 3,500 mm (140 in) Ritchey-Chrétien telescope with a focal length of 28.5 meters (94 ft) and operates on far-infrared and submillimeter wavelengths from 60 to 670 µm.
The Herschel’s main instruments consist of the Heterodyne Instrument for the Far Infrared (HIFI), which is a very high-resolution heterodyne spectrometer that works in seven bands covering wavelengths between 157 and 625 µm. The second is the Photodetector Array Camera and Spectrometer (PACS) and rounding out the trio is the Spectral and Photometric Imaging Receiver (SPIRE), which contains another camera and spectrometer working on a different range of infrared wavebands from the PACS.
What all three of these instruments have in common is that they need to be cooled to a temperature of -271°C (-456 F) inside a cryostat filled with 2367 liters (625 gal) of liquid superfluid helium that made up nearly 10 percent of the Herschel spacecraft’s original weight. These allowed Herschel to see into the universe’s distant past by making the detectors extremely sensitive. In addition to the liquid helium, Herschel’s solar panels formed a sunshade. The paradox of space is that even though the Sun can quickly bring the temperature of an object up to above boiling, putting it into shade sends it into hundreds of degrees below zero, though not nearly cold enough for Herschel’s instruments.
Water lines toward low-mass protostars in the NGC 1333 star-forming region (Image: ESA and the HIFI consortium; L.E. Kristensen for the WISH Key Programme. Background image: NASA/JPL-Caltech/R. Gutermuth (Harvard-Smithsonian CfA))
The helium was expected to eventually evaporate away and finally ran out on Monday afternoon (April 29, 2013) as Herschel was starting its daily communication session with a ground station in Western Australia. A clear rise was seen in all the instruments, which soon stopped working.
The primary objective of the Herschel telescope was to study star formation both in our galaxy and out to the limits of observation on the edge of the visible universe and dating some 14 billion years ago. In addition to surveying thousands of galaxies, it also was used study planet formation in systems closer to Earth, molecules in interstellar space, as well as asteroids and comets in our Solar System.
According to ESA, Herschel did its job well. “Herschel has exceeded all expectations, providing us with an incredible treasure trove of data that that will keep astronomers busy for many years to come,” said Prof. Alvaro Giménez Cañete, ESA’s Director of Science and Robotic Exploration.
Among its accomplishments, Herschel produced evidence that galaxies in the first billions of years of the universe contributed many more stars than previously thought, shone new light on the part played by dark matter and filaments in the formation of galaxies, suggested that supermassive black holes may affect star formation by removing gases from galaxies by means of radiation pressure. It also studied phenomenon such as the debris halo around the star Fomalhaut, which resembles our Solar System’s Kuiper Belt and may give clues to the formation of the planets.
Furthermore, Herschel’s study of nebulae and dust disks around protostars indicate the existence of enough water in individual examples to fill the Earth’s oceans a million times over.
“Herschel has offered us a new view of the hitherto hidden Universe, pointing us to a previously unseen process of star birth and galaxy formation, and allowing us to trace water through the Universe from molecular clouds to newborn stars and their planet-forming discs and belts of comets,” said Göran Pilbratt, ESA’s Herschel Project Scientist..
The Herschel Space Observatory has returned over 35,000 scientific observations, more than 25,000 hours of data from about 600 observing programs and 2000 hours of calibration observations. According to ESA, this archive is so extensive that it is expected to produce more discoveries over time than during the mission itself. Meanwhile, Herschel will remain in communication with Earth for a few weeks while tests are carried out it. The spacecraft is then to be propelled to a stable parking orbit around the Sun, where it will remain for centuries.
The video below shows the construction of the Herschel Space Observatory.