ESA’s Herschel space observatory has exhausted its supply of liquid helium coolant, ending more than three years of pioneering observations of the cool Universe.
The event was not unexpected: the mission began with over 2300 litres of liquid helium, which has been slowly evaporating since the final top-up the day before Herschel’s launch on 14 May 2009.
The liquid helium was essential to cool the observatory’s instruments to close to absolute zero, allowing Herschel to make highly sensitive observations of the cold Universe until today.
The confirmation that the helium is finally exhausted came this afternoon at the beginning of the spacecraft’s daily communication session with its ground station in Western Australia, with a clear rise in temperatures measured in all of Herschel’s instruments.
“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,” says Prof. Alvaro Giménez Cañete, ESA’s Director of Science and Robotic Exploration.
Herschel has made over 35 000 scientific observations, amassing more
than 25 000 hours’ worth of science data from about 600 observing
programmes. A further 2000 hours of calibration observations also
contribute to the rich dataset, which is based at ESA’s European Space
Astronomy Centre, near Madrid in Spain.
The archive will become the legacy of the mission. It is expected to
provide even more discoveries than have been made during the lifetime of
the Herschel mission.
“Herschel’s ground-breaking scientific haul is in no little part down to
the excellent work done by European industry, institutions and academia
in developing, building and operating the observatory and its
instruments,” says Thomas Passvogel, ESA’s Herschel Programme Manager.
“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,” says Göran Pilbratt, ESA’s Herschel Project
Scientist.
Copyright: ESA/Herschel/SPIRE/PACS/D. Arzoumanian (CEA Saclay) for the “Gould Belt survey” Key Programme Consortium.
Star birth
Herschel’s stunning images of intricate networks of dust and gas
filaments within our Milky Way Galaxy provide an illustrated history of
star formation. These unique far-infrared observations have given
astronomers a new insight into how turbulence stirs up gas in the
interstellar medium, giving rise to a filamentary, web-like structure
within cold molecular clouds.
If conditions are right, gravity then takes over and fragments the
filaments into compact cores. Deeply embedded inside these cores are
protostars, the seeds of new stars that have gently heated their
surrounding dust to just a few degrees above absolute zero, revealing
their locations to Herschel’s heat-sensitive eyes.
Following the water Trail
Over the first few million years in the life of newborn stars, the
formation of planets can be followed in the dense discs of gas and dust
swirling around them. In particular, Herschel has been following the
trail of water, a molecule crucial to life as we know it, from
star-formation clouds to stars to planet-forming discs.
Herschel has detected thousands of Earth ocean’s worth of water vapour
in these discs, with even greater quantities of ice locked up on the
surface of dust grains and in comets.
Closer to home, Herschel has also studied the composition of the
water-ice in Comet Hartley-2, finding it to have almost exactly the same
isotopic ratios as the water in our oceans.
These findings fuel the debate about how much of Earth’s water was
delivered via impacting comets. Combined with the observations of
massive comet belts around other stars, astronomers hope to understand
whether a similar mechanism could be in play in other planetary systems,
too.
Copyright: ESA–C. Carreau/C. Casey (University of Hawai'i); COSMOS
field: ESA/Herschel/SPIRE/HerMES Key Programme; Hubble images: NASA,
ESA
Galaxies across the Universe
Galaxies across the Universe
Herschel has also contributed to our knowledge of star formation on the
grandest scales, spanning much of cosmic space and time. By studying
star formation in distant galaxies, it has identified many that are
forming stars at prodigious rates, even in the early years of the
Universe’s 13.8 billion-year life.
These intense star-forming galaxies produce hundreds to thousands of
solar masses’ worth of stars each year. By comparison, our own Milky Way
Galaxy produces the equivalent of only one Sun-like star per year on
average.
How galaxies can support star formation on such massive scales during
the first few billions of years of the Universe’s existence poses a
crucial problem for scientists studying galaxy formation and evolution.
Herschel observations are hinting that when the Universe was young,
galaxies had much more gas to feed from, enabling high rates of star
formation even in the absence of the collisions between galaxies
normally needed to spark these spectacular bouts of star birth.
“Although this is the end of Herschel observing, it is certainly not the
end of the mission – there are plenty more discoveries to come,” says
Dr Pilbratt.
“We will now spend the next few years making our data accessible in the
form of the best possible maps, spectra and various catalogues to
support the work of present and future astronomers. Nevertheless, we’re
sad to see the end of this phase: thank you, Herschel!”
Notes for Editors
ESA’s Herschel space observatory was launched on 14 May 2009 and, with a
primary mirror 3.5 m across, is the largest, most powerful infrared
telescope ever flown in space.
Its two camera/imaging spectrometers, PACS (Photoconductor Array Camera
and Spectrometer) and SPIRE (Spectral and Photometric Imaging Receiver),
together covered wavelengths of 55–670 microns.
A third science instrument, HIFI (Heterodyne Instrument for the Far
Infrared), a very high resolution spectrometer, covered two wavelength
bands, 157–212 microns and 240–625 microns. All three instruments were
cooled to –271ºC inside a cryostat filled with liquid superfluid helium.
The mission finally exhausted its coolant today.
Herschel will continue communicating with its ground stations for some
time now that the helium is exhausted, during which a range of technical
tests will be performed.
Finally, in May, it will be propelled into its long-term stable parking orbit around the Sun.
In addition to the legacy of the scientific data, the mission resulted
in a number of technology advances applicable to future ESA missions.
Herschel saw the development of advanced cryogenic systems, the
construction of the largest mirror ever flown in space, and the most
sensitive direct detectors for light in the far-infrared to millimetre
range.
Manufacturing techniques have already been applied to the next
generation of ESA’s space missions, including Gaia and the James Webb
Space Telescope.
Markus Bauer
ESA Science and Robotic Exploration Communication Officer
Tel: +31 71 565 6799
Mob: +31 61 594 3 954
Email: markus.bauer@esa.int
Göran Pilbratt
ESA Herschel Project Scientist
Tel: +31 71 565 3621
Email: gpilbratt@rssd.esa.int
