[Paper Review] Herschel Space Observatory - An ESA facility for far-infrared and submillimetre astronomy

G. L. Pilbratt, J. R. Riedinger+9 more|arXiv (Cornell University)|May 28, 2010

Astrophysics and Star Formation StudiesPhysics and Astronomy12 references1,884 citations

TL;DR

Herschel Space Observatory, a European Space Agency mission with significant NASA participation, is a far-infrared and submillimetre space telescope launched in 2009, featuring a 3.5 m passive telescope and three science instruments—PACS, SPIRE, and HIFI—enabling unprecedented observations of star formation, interstellar medium physics, and cosmic evolution. The mission achieved full operational status with performance matching pre-launch predictions, including 2" pointing accuracy and a nominal lifetime of 3.5–4 years, with over 20,000 hours of guaranteed and open observing time allocated to the global astronomical community.

ABSTRACT

Herschel was launched on 14 May 2009, and is now an operational ESA space observatory offering unprecedented observational capabilities in the far-infrared and submillimetre spectral range 55-671 μm. Herschel carries a 3.5 metre diameter passively cooled Cassegrain telescope, which is the largest of its kind and utilises a novel silicon carbide technology. The science payload comprises three instruments: two direct detection cameras/medium resolution spectrometers, PACS and SPIRE, and a very high-resolution heterodyne spectrometer, HIFI, whose focal plane units are housed inside a superfluid helium cryostat. Herschel is an observatory facility operated in partnership among ESA, the instrument consortia, and NASA. The mission lifetime is determined by the cryostat hold time. Nominally approximately 20,000 hours will be available for astronomy, 32% is guaranteed time and the remainder is open to the worldwide general astronomical community through a standard competitive proposal procedure.

Motivation & Objective

• To provide a comprehensive overview of the Herschel Space Observatory as a fully operational facility for far-infrared and submillimetre astronomy.
• To detail the mission’s design, including its 3.5 m passive telescope, cryogenic instruments, and orbit at the Sun-Earth L2 point.
• To document the in-flight performance and validation of the observatory’s optical, pointing, and instrument systems.
• To outline the observing strategy, including guaranteed time (32%) and open time (68%) allocation via competitive proposals.
• To present the mission’s scientific rationale, focusing on star formation, interstellar medium processes, and cosmic evolution across cosmic time.

Proposed method

• The observatory employs a 3.5 m diameter Cassegrain telescope cooled passively to 160 K using a silicon carbide structure and a superfluid helium cryostat.
• Three instruments are used: PACS and SPIRE are direct detection cameras/medium-resolution spectrometers, while HIFI is a high-resolution heterodyne spectrometer.
• The spacecraft operates in a large-amplitude quasi-halo orbit around the Sun-Earth L2 point, minimizing thermal and radiation interference.
• Observations are conducted using a combination of scan map, pointed, and target-of-opportunity modes, with real-time pointing accuracy monitored via star trackers.
• The mission uses a competitive proposal system to allocate open-time observing, with data processed through the Herschel Science Ground Segment using tools like HIPE and HCSS.
• In-flight validation includes performance checks on pointing, instrument response, and cryostat helium consumption to predict mission lifetime.

Experimental results

Research questions

• RQ1 How does the Herschel Space Observatory’s telescope and instrument design enable high-sensitivity observations in the far-infrared and submillimetre range?
• RQ2 What is the in-flight performance of the observatory’s pointing system, and how does it compare to pre-launch predictions?
• RQ3 How is the mission lifetime determined, and what is the predicted duration based on cryostat helium consumption?
• RQ4 What is the allocation strategy for observing time, and how is the open-time proposal process structured?
• RQ5 What scientific capabilities does Herschel offer for studying star formation, interstellar medium processes, and cosmic evolution?

Key findings

• The Herschel Space Observatory achieved a pointing accuracy of approximately 2", in line with pre-launch predictions, with a minor restriction in the solar aspect angle from 120° to 110° relative to the telescope boresight.
• The first direct measurement of remaining helium in the cryostat, conducted on 25 November 2009, indicated a predicted mission lifetime between 3.5 and slightly over 4 years.
• HIFI was successfully reactivated on 10 January 2010 using redundant warm electronics after a cooperative failure investigation, restoring its science operations.
• The observatory’s optical, instrument, and pointing systems performed as expected during the early mission phase, validating the operations concept.
• Over 20,000 hours of observing time are available, with 32% allocated as guaranteed time and the remainder open to the global astronomical community via competitive proposals.
• The mission’s first results symposium in May 2010 featured around 200 scientific presentations and approximately 150 papers in final acceptance, indicating strong scientific productivity.