Herschel Probe Index Page

The Herschel Space Observatory is the largest infrared space observatory launched to date. Equipped with a 3.5 metre diameter reflecting telescope and instruments cooled to close to absolute zero, Herschel observes at wavelengths that have never previously been explored. After a roughly 50-day journey from Earth, Herschel entered its operational orbit around the second Lagrange point of the Sun-Earth system (L2), for a nominal mission lifetime of three years.

The European Space Agency's Herschel Space Observatory (formerly called Far Infrared and Sub-millimetre Telescope or FIRST) has the largest single mirror ever built for a space telescope. At 3.5-metres in diameter the mirror will collect long-wavelength radiation from some of the coldest and most distant objects in the Universe. In addition, Herschel is the only space observatory to cover a spectral range from the far infrared to sub-millimetre.

Infrared astronomy is a young and exciting science. In recent decades infrared astronomers have unveiled tens of thousands of new galaxies, and have made surprising discoveries such as the huge amounts of water vapour that fill our Galaxy. Yet scientists know there is still much more to discover. Objects such as other planetary systems, or processes like the birth of galaxies in the early Universe, can best be studied with infrared telescopes situated in space and therefore freed from the restrictions imposed by the Earth's atmosphere. This is the reason ESA has constructed the Herschel observatory.

The Spacecraft

The Herschel spacecraft is approximately 7.5 metres high and 4 × 4 metres in overall cross section, with a launch mass of around 3.4 tonnes. The spacecraft comprises a service module and a payload module. The service module houses systems for power conditioning, attitude control, data handling and communications, together with the warm parts of the scientific instruments. The payload module consists of the telescope, the optical bench, with the parts of the instruments that need to be cooled, i.e. the sensitive detector units and cooling systems. The payload module is fitted with a sunshield, which protects the telescope and cryostat from solar visible and infrared radiation and also prevents Earth straylight from entering the telescope. The sunshield also carries solar cells for the electric power generation.

The Telescope and Instruments

The Herschel telescope is a Cassegrain design with a primary mirror diameter of 3.5 metres, the largest single mirror ever built for use in space. The three scientific instruments are:

HIFI (Heterodyne Instrument for the Far Infrared), a very high resolution heterodyne spectrometer

PACS (Photodetector Array Camera and Spectrometer) - an imaging photometer and medium resolution grating spectrometer

SPIRE (Spectral and Photometric Imaging Receiver) - an imaging photometer and an imaging Fourier transform spectrometer

The instruments have been designed to take maximum advantage of the characteristics of the Herschel mission. In order to make measurements at infrared and sub-millimetre wavelengths, parts of the instruments have to be cooled to near absolute zero. The optical bench, the common mounting structure of all three instruments, is contained within the cryostat and over 2000 litres of liquid helium will be used during the mission for primary cooling. Individual instrument detectors are equipped with additional, specialised cooling systems to achieve the very lowest temperatures, down to 0.3 K for PACS and SPIRE.

Herschel Capabilities

Herschel is the only space facility ever developed to cover the far infrared to sub-millimetre parts of the spectrum (from 55 to 672 µm). It will open up an almost unexplored part of the spectrum, which cannot be observed well from the ground.

The Questions Herschel Will Answer

The questions that Herschel will seek answers to include:

How galaxies formed and evolved in the early Universe

How stars form and evolve and their interrelationship with the interstellar medium

Herschel will also investigate the chemistry of our Galaxy and the molecular chemistry of planetary, cometary and satellite atmospheres in the Solar System.


 Fast Facts Herschel
Launch date: 14-May-2009 13:12 UT
Last observation: 29-Apr-2013 (liquid helium coolant exhausted)
Launch vehicle: Ariane 5 ECA
Launch mass: 3400 kg
Mission phase: Routine operations
Orbit: Lissajous orbit about the second Lagrange point of the Sun-Earth system (L2)
  • Study the formation of galaxies in the early universe and their subsequent evolution
  • Investigate the creation of stars and their interaction with the interstellar medium
  • Observe the chemical composition of the atmospheres and surfaces of comets, planets and satellites
  • Examine the molecular chemistry of the universe


Joint Launch

Herschel was carried into space by an Ariane 5 ECA launcher on 14 May 2009 and about sixty days after launch reached its orbit around L2. For reasons of cost effectiveness, ESA decided to launch Herschel together with Planck, a mission to study the cosmic microwave background radiation. The two spacecraft separated soon after launch and are being operated independently.

Mission Lifetime

Herschel has a nominal routine operational lifetime of three years, with a possible extension of one year. About 7000 hours of science time will be available per year. Herschel is a multi-user observatory accessible to astronomers from all over the world.

Herschel's Predecessors

In 1983 the US-Dutch-British IRAS satellite inaugurated infrared space astronomy by mapping 250 000 cosmic infrared sources and large areas of extended emission.

In November 1995 ESA launched its Infrared Space Observatory, ISO, which has allowed a much more detailed study of the infrared sky. ISO observed in the wavelength range from 2.5 to 240 µm and achieved an one thousand fold increase in sensitivity and a one hundred fold improvement in angular resolution (at 12 µm) compared to IRAS. ISO's operational lifetime was one year longer than planned, ending in May 1998.

The Spitzer Space Telescope (formerly SIRTF, the Space Infrared Telescope Facility) was launched on 25 August 2003. During its nominal 2.5-year mission and subsequent extended operations, Spitzer obtained infrared images and spectra in the wavelength range 3 to 180 µm with its 0.85 metre telescope and three science instruments operating at cryogenic temperatures. Rather than operating at L2, as Herschel does, Spitzer is in an Earth trailing heliocentric orbit.

Our solar system contains three zones: the inner, rocky planets; the gas giant planets; and the Kuiper Belt. Pluto is one of the largest bodies of the icy, “third zone“ of our solar system. The National Academy of Sciences placed the exploration of the third zone in general - and Pluto-Charon in particular - among its highest priority planetary mission rankings for this decade. New Horizons is NASA's mission to fulfill this objective.


The Herschel science payload comprises three instruments that perform a combination of spectrometry, imaging spectrometry and imaging photometry covering a wavelength range from 55 to 672 µm.

Herschel's primary objectives are to:

  • Study the formation of galaxies in the early Universe and their subsequent evolution
  • Investigate the creation of stars and their interaction with the interstellar medium
  • Observe the chemical composition of the atmospheres and surfaces of comets, planets and satellites
  • Examine the molecular chemistry of the Universe

With its ability to observe across the far infrared and sub-millimetre wavelengths, Herschel furnishes observation data that has previously been unobtainable.

The instruments have been provided by collaborative efforts between scientific institutes in ESA member states, Canada and the USA. Principal Investigators in different countries led the nationally funded collaborations during the development of the respective instruments, and continue to lead the instrument consortia during the operational phase of the mission.


Herschel Instruments

Instrument Name

Instrument Description

Principal Investigator

Heterodyne Instrument for the Far Infrared (HIFI) Very high resolution heterodyne spectrometer Frank Helmich, SRON (Groningen, The Netherlands)
Photodetector Array Camera and Spectrometer (PACS) Imaging photometer / integral field line spectrometer Albrecht Poglitsch, MPE (Garching, Germany)
Spectral and Photometric Imaging Receiver (SPIRE) Imaging photometer / imaging Fourier transform spectrometer Matthew Griffin, University of Wales (Cardiff, United Kingdom)

The instrument payload has been conceived and optimised with the prime science goals in mind, but in addition it offers a wide range of capabilities for the general observer.

Instruments in Brief

The Herschel scientific instrument complement comprises three instruments, two cameras (PACS and SPIRE) with additional imaging spectroscopy capabilities, and a very high-resolution heterodyne spectrometer (HIFI). The instruments were provided by three consortia led by their respective Principal Investigator.

Herschel Principal Investigators


Frank Helmich, SRON (Groningen, The Netherlands)


Albrecht Poglitsch, MPE (Garching, Germany)


Matthew Griffin, University of Wales (Cardiff, United Kingdom)

Heterodyne Instrument for the Far Infrared (HIFI)

HIFI is a very high-resolution heterodyne spectrometer. The heterodyne detection principle involves translating the frequency range of the astronomical signal being observed to a lower frequency where it is easier to perform the required measurements. This is done by mixing the incoming signal with a very stable monochromatic signal, generated by a local oscillator, and extracting the difference frequency for further processing. HIFI observes in seven bands covering 480 to 1910 Ghz, or the wavelength range 157-625 µm. Bands one to five, which give continuous coverage from 480 to 1250 GHz, use superconductor-insulator-superconductor (SIS) mixers. Bands six low and six high cover 1410 to 1910 GHz and use hot electron bolometer (HEB) mixers.

HIFI Frequency Bands


Mixer type

Lower freq.

Upper freq.



480 Ghz

636 Ghz



634 Ghz

804 Ghz



799 Ghz

961 Ghz



949 Ghz

1122 Ghz



1108 Ghz

1280 Ghz



1426 Ghz

1703 Ghz



1703 Ghz

1907 Ghz

The difference signal from the heterodyne process is passed to the instrument spectrometers housed in the service module.

There are four HIFI spectrometers, two Wide-Band acousto-optical Spectrometers (WBS) and two High Resolution autocorrelation Spectrometers (HRS). One of each spectrometer type is available for each polarization (vertical and horizontal polarization). All spectrometers can be used either individually or in parallel.

HIFI Spectrometers


Wide band (2x)

High resolution (2x)




High Resolution




Bandwidth (GHz)




Resolution (MHz)




Velocity resolution (ms-1)

680 - 220

170 - 90

110 - 80

Photodetector Array Camera and Spectrometer (PACS)

PACS is an imaging photometer and integral field line spectrometer.  The instrument comprises two sub-instruments which offer the two basic and mutually exclusive modes:

In imaging dual-band photometry mode, PACS images a field of view of 1.75 × 3.5 arcminutes simultaneously in two bands, one of either 60 - 85 µm or 85 - 125 µm together with 125 - 210 µm, with full beam sampling in each band. The detectors for this mode are two bolometers arrays.

In integral field spectroscopy mode, PACS performs spectroscopy between 51 and 220 µm over a field of view of 47 × 47 arcseconds, resolved into 5 × 5 pixels. An image slicer employing reflective optics is used to re-arrange the two-dimensional field-of-view along a 1×25 pixels entrance slit for a grating. This PACS mode provides a resolving power between 1000 and 4000 (i.e. a spectral resolution of about 75-300 kms-1) depending on wavelength, with an instantaneous velocity coverage of about 1500 kms-1.  The detectors for this mode are two germanium/gallium photoconductor arrays.

Spectral and Photometric Imaging Receiver (SPIRE)

SPIRE comprises a three band imaging photometer and an imaging Fourier transform spectrometer. SPIRE employs arrays of spider-web bolometers with neutron transmutation doped (NTD) germanium temperature sensors as its detectors.

The photometer images a 4 × 8 arcminute field of view on sky in three bands simultaneously.

SPIRE Photometer Characteristics

Centre Wavelength (µm)





~ 3

~ 3

~ 3

Number of detectors




Detector array size (mm)

45 × 23

45 × 23

45 × 23

The photometer has three observing modes:

  • Point source photometry
  • Small area map; for sources or area with diameters smaller than 5 arcminutes
  • Large area map; for covering large areas of sky or extended sources larger than 5 arcminutes across

The SPIRE spectrometer is based on the Mach-Zehnder configuration. One input port receives the incoming beam from the telescope while the second port accepts a signal from a calibration source. The two output ports each have a detector array, one for 194-313 µm (37 detectors) and the other for 303-671 µm (19 detectors). The spectrometer is operated in continuous scan mode. The spectral resolution can be adjusted in the range between 0.04 and 0.8 cm-1, corresponding to λ/Δλ of 50 to 1000 at 250 µm. The SPIRE spectrometer has a circular field of view 2.6 arcminutes across.

Mission Name

Herschel, originally named FIRST (Far InfraRed and Sub-millimetre Telescope), was renamed in honour of Sir William Herschel, who in 1800 demonstrated the existence of infrared light. Both he and his sister Caroline Herschel were pioneering and successful astronomers.

Mission Objectives
  • Study the formation of galaxies in the early Universe and their subsequent evolution
  • Investigate the creation of stars and their interaction with the interstellar medium
  • Observe the chemical composition of the atmospheres and surfaces of comets, planets and satellites
  • Examine the molecular chemistry of the Universe

With its ability to observe across the far infrared and sub-millimetre wavelengths (55 - 672 µm), Herschel will furnish observation data that has previously been unobtainable.


  Description Principal Investigator
HIFI (Heterodyne Instrument for the Far Infrared) Very high resolution heterodyne spectrometer Frank Helmich, Space Research Organization Netherlands (SRON) (Groningen, The Netherlands)
PACS (Photodetector Array Camera and Spectrometer) Imaging photometer / medium resolution grating spectrometer Albrecht Poglitsch, Max-Planck Institut für Extraterrestrische Physik (MPE) (Garching, Germany)
SPIRE (Spectral and Photometric Imaging Receiver) Imaging photometer / imaging Fourier transform spectrometer Matthew Griffin, University of Wales (Cardiff, United Kingdom)
Orbital Insertion

Herschel was launched on an Ariane 5 ECA rocket together with ESA's Planck spacecraft on 14 May 2009, at 13:12:02 UTC. The two spacecraft separated after launch and were directly injected towards the second Lagrange point of the Sun-Earth system, L2. About sixty days after launch, Herschel entered a Lissajous orbit around the L2 point at a distance of around 1.5 million km from Earth, on Earth's nightside.  The spacecraft's orbit around L2 has an average amplitude of about 700 000 km and a period of about 178 days.

Operations Centre

Herschel's Mission Operations Centre (MOC) is located at ESA's European Space Operations Centre (ESOC) in Darmstadt, Germany and is responsible for the daily operations, health and safety of the spacecraft. For communication with the spacecraft ESA's New Norcia (close to Perth, Australia) and Cebreros (close to Avila, Spain) deep space antennas are used, with New Norcia serving as the main ground station. In the phase immediately after launch the Kourou (French Guiana) and Villafranca (Spain) ground stations were also used.


The Herschel science operations team is situated in the Herschel Science Centre (HSC) at ESA's European Space Astronomy Centre (ESAC) in Villanueva de la Cañada in Spain.

The Herschel instrument control centres for monitoring and optimising the instruments' performance are located at:

for PACS: the Max Planck Institute for Extraterrestrial Physics, Garching, Germany;
for SPIRE: the Rutherford Appleton Laboratory, UK;
for HIFI: SRON Netherlands Institute for Space Research, the Netherlands.
Launch Information

Herschel was carried into space on 14 May 2009, at 13:12:02 UTC, by an Ariane 5 ECA launcher, from the Guiana Space Centre, Kourou, French Guiana. Herschel was launched together with ESA's Planck spacecraft, in the launch configuration as depicted on the right. The two spacecraft separated within 30 minutes after launch and proceeded independently to different orbits about the second Lagrange point of the Sun-Earth system (L2).

The upper stage of the Ariane 5 ECA launcher injected first Herschel and then Planck into their individual transfer trajectories bound for L2. Upon separation, Herschel was three-axis stabilised.

After a journey lasting about sixty days, Herschel entered its operational orbit, a large Lissajous orbit around L2, 1.5 million km away from the Earth, on Earth's night-side. The orbit has a period of about 178 days and an average amplitude of about 700 000 km around L2.


Herschel was carried into space on 14 May 2009, at 13:12:02 UTC, by an Ariane 5 ECA launcher, from the Guiana Space Centre, Kourou, French Guiana. Herschel was launched together with ESA's Planck spacecraft.

Within 30 minutes after launch, and about two minutes from each other, the two spacecraft were released and each placed on their individual escape trajectory toward L2. Upon separation, Herschel was three-axis stabilised.

The next day, 15 May 2009, a trajectory control manoeuvre (TCM) was performed as planned to fine-tune Herschel's trajectory.

The Herschel spacecraft took about sixty days to reach its orbit around L2, the second Lagrange point of the Sun-Earth system, 1.5 million kilometres away from the Earth in the anti-Sun direction.

Location of L2 (not to scale)

At L2, Herschel entered a large Lissajous orbit about the Lagrange point. Lissajous orbits are the natural motion of a satellite around a collinear libration point in a two-body system and require less momentum change to be expended for station keeping than halo orbits, where the satellite follows a simple circular or elliptical path about the libration point.

Herschel's orbit around L2 has an average amplitude of about 700 000 km, and a period of about 178 days. The chosen orbit takes Herschel about 500 000 kilometres above and below the plane of the ecliptic with a maximum azimuthal excursion of around 800 000 kilometres either side of the Lagrange point. The Earth to spacecraft distance varies from approximately 1.2 to 1.8 million kilometres. No insertion manoeuvre is needed to achieve this orbit.

Orbits about L2 are dynamically unstable; small departures from equilibrium grow exponentially with a time constant of about 23 days. Herschel will use its propulsion system to perform orbit maintenance manoeuvres roughly once each month.

Why L2?

The Sun, Earth and Moon are intense sources of both straylight and thermal modulation, and reducing their effects drove the choice of orbit for Herschel. Near Earth orbits were eliminated mainly because the large thermal influx would render it extremely difficult to reach temperatures below 100 K in the spacecraft's focal plane, or to achieve the required thermal stability. The nearest far-Earth orbit possible would have been that around one of the Lagrange points of the Earth-Moon system; such an orbit however (which shares the Lunar motion around the Earth) suffers from the fact that the Earth or the Moon are often not very far from the telescope line-of-sight.

The optimal choice of orbit for Herschel, resulting from a trade-off of the various payload requirements, several spacecraft technical constraints (most importantly related to telecommunications to ground), and the transfer-to-orbit cost, is a Lissajous orbit around the L2 Lagrange point of the Sun-Earth system. At this location the Sun, the Earth, and the Moon are all easily shielded from the payload.

Why Observe in the Infrared?

Large parts of the Universe are too cold to radiate in the visible wavelength range or at shorter wavelengths. Study of these cooler objects is only possible by observing in the infrared spectrum or at even longer (sub-millimetre) wavelengths. Bodies with temperatures between five and fifty Kelvin have radiative emission peaks in the wavelength range observed by Herschel, and gases with temperatures between ten and a few hundred Kelvin exhibit their brightest molecular and atomic emission lines at these wavelengths.

Additionally, many objects of great interest to astronomers are concealed within or behind clouds of gas and dust. In the early stages of their formation, stars and planets are surrounded by the gas and dust clouds from which they are being created. Galactic cores and most of the remnants of the early Universe are also hidden from view by dust clouds. The dust particles in these clouds are comparable in size to the wavelength of visible light and are therefore efficient at scattering or absorbing radiation at these wavelengths. Infrared radiation is less affected by these clouds - the longer the wavelength, the thicker the dust cloud that it can penetrate.

Why Observe in Space?

Water vapour in the Earth's atmosphere absorbs radiation across large parts of the infrared and sub-millimetre wavebands, making ground based observations at these wavelengths impossible. Limited observations can be made using techniques such as high altitude balloons but a space-based observatory is the only truly satisfactory solution to this problem.

By orbiting at L2, some 1.5 million kilometres from Earth, Herschel is not troubled by any atmospheric absorption. In addition, the spacecraft avoids any problems caused by thermal infrared radiation from the Earth interfering with observations. The L2 orbit also prevents the occurrence of temperature changes due to the spacecraft moving in and out of eclipse in an Earth orbit, which are a particular problem for infrared instruments requiring extreme thermal stability.

Mass About 3400 kg at launch
Dimensions 7.5m high, 4m × 4m overall cross section
Launcher Ariane 5 ECA from Guiana Space Centre
Mission Lifetime 3 years nominal from end of commissioning phase
Wavelength Infrared and sub-millimetre: 55 to 672 µm
Telescope Cassegrain, 3.5m primary and 0.3m secondary mirror
Other News
Other News

Jan 22, 2014- News Release--Scientists using the Herschel space observatory have made the first definitive detection of water vapor on the largest and roundest object in the asteroid belt, Ceres.

Plumes of water vapor are thought to shoot up periodically from Ceres when portions of its icy surface warm slightly. Ceres is classified as a dwarf planet, a solar system body bigger than an asteroid and smaller than a planet.

Herschel is a European Space Agency (ESA) mission with important NASA contributions.

“This is the first time water vapor has been unequivocally detected on Ceres or any other object in the asteroid belt and provides proof that Ceres has an icy surface and an atmosphere,” said Michael Küppers of ESA in Spain, lead author of a paper in the journal Nature.

The results come at the right time for NASA's Dawn mission, which is on its way to Ceres now after spending more than a year orbiting the large asteroid Vesta. Dawn is scheduled to arrive at Ceres in the spring of 2015, where it will take the closest look ever at its surface.

“We've got a spacecraft on the way to Ceres, so we don't have to wait long before getting more context on this intriguing result, right from the source itself,” said Carol Raymond, the deputy principal investigator for Dawn at NASA's Jet Propulsion Laboratory in Pasadena, Calif. “Dawn will map the geology and chemistry of the surface in high resolution, revealing the processes that drive the outgassing activity.”